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Demography, dispersal and colonisation of larvae of Pacific giant salamanders (dicamptodon tenebrosus.. Ferguson, Heather Margaret 1998

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D E M O G R A P H Y , D I S P E R S A L A N D C O L O N I S A T I O N O F L A R V A E O F P A C I F I C G I A N T S A L A M A N D E R S (DICAMPTODON TENEBROSUS, G O O D ) A T T H E N O R T H E R N E X T E N T O F T H E I R R A N G E by Heather Margaret Ferguson H o n s . B . S c , T h e U n i v e r s i t y o f Toronto 1995 A T H E S I S S U M B I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S Department o f Z o o l o g y v W e accept this thesis as c o n f o r m i n g Jx> the(TeT3t}hpd standard T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A N o v e m b e r 1998 © Heather Margaret Ferguson, 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of zealot The University of British Columbia Vancouver, Canada Date DE-6 (2/88) Abstract T h e Paci f ic G i a n t Salamander, (Dicamptodon tenebrosus G o o d ) is red-listed i n B r i t i s h C o l u m b i a , the northern extent of the species' range. L i t t l e is k n o w n about the demography o f these populations and their abil ity to recover f r o m disturbance by recolonisat ion. I conducted a f ie ld experiment to measure the co lonis ing ability o f larval P a c i f i c G i a n t Salamanders at 4 streams w i t h i n the C h i l l i w a c k V a l l e y o f B r i t i s h C o l u m b i a . I also estimated basic survival , growth and dispersal rates for these larvae. These rates were c o m p a r e d to others f r o m O r e g o n where this species is not considered threatened. Mark-recapture i n 120 m reaches i n four streams i n 1996 and 1997 revealed (a) l o w e r larval densities, (b) l o w e r annual growth rates and (c) s imi lar annual s u r v i v a l o f these larvae i n comparison to those i n s imi lar O r e g o n streams. D u e to s lower growth rates, I hypothesise the the larval period i n B r i t i s h C o l u m b i a is 2-3 times longer than i n O r e g o n . T o study colonisat ion, larvae were removed f r o m a 2 5 - 4 0 m section w i t h i n each 120 m reach and the recolonisation o f each section was monitored for 13 months. D e p l e t e d reaches were repopulated s l o w l y by larval dispersal and more q u i c k l y by adult reproduction. F e w larvae m o v e d more than 4 m . F u l l recolonisat ion o f these reaches was predicted to take 6-42 months. P r o v i d e d terrestrial adults are available, l o c a l reproduction appears to be a more effective means of repopulating an area than larval i m m i g r a t i o n . L a r v a l dispersal was not inf luenced by larval density, biomass, substrate, wetted width , depth, or pool-rif f le composi t ion. L o g g i n g - i n d u c e d habitat shifts may thus have little consequence to larval dispersal as movement was u n i f o r m l y l o w through a variety o f m i c r o - habitats. A l t h o u g h logging and other disturbances may increase the rate o f l o c a l ext inct ion o f D. tenebrosus'm B r i t i s h C o l u m b i a , these populations are not unusually susceptible to disturbance. Despite hav ing lower density and growth rates than i n other parts o f their range, larvae i n B r i t i s h C o l u m b i a exist w i t h i n the survival and growth bounds o f other non-threatened stream-dwel l ing salamanders. M o r e importantly, recruitment can facilitate rapid recovery f r o m small-scale disturbances. Conservation efforts should focus on terrestrial as w e l l as aquatic habitat and dispersal routes. Table of Contents Abstract i i Table o f Contents i v L i s t of Tables v i L i s t o f Figures x Acknowledgements x i v D e d i c a t i o n x v C H A P T E R 1 General Introduction 1 C H A P T E R 2 Life history and demography of Pacific Giant Salamander larvae in five streams at the northern limit of its range 7 Introduction 7 M e t h o d s 11 Results 17 Discuss ion ; . 2 0 Conclus ions 2 6 C H A P T E R 3 Determinants of Dispersal in Pacific Giant Salamander larvae 45 Introduction 4 5 M e t h o d s 49 Results 54 D i s c u s s i o n 58 Conclus ions 6 0 C H A P T E R 4 Colonising ability of Pacific Giant Salamander larvae 78 Introduction 78 M e t h o d s 81 Results 89 Discuss ion 92 Conclus ions 99 iv C H A P T E R 5 General Conclusions 116 Bibliography 123 Appendix 1 132 Appendix 2 133 Appendix 3 134 List of Tables Table Page 2.1 A g e and category o f forest surrounding streams used i n demographic analysis o f larval D. tenebrosus populations. 27 2.2 D u r a t i o n and frequency o f sampling o f five sites used i n demographic study. Sites were sampled once a week. 28 2.3 Dates of sampling for estimation o f larval abundance and survival . O n l y one sampling period was possible at F o l e y R as the efficiency o f capture became so l o w i n A u g u s t o f 1996 that m o n i t o r i n g was discontinued. Larvae were st i l l present i n this site i n 1997 but at very l o w density. 29 2.4 M e a n larval abundance and density at the f ive study sites. N u m b e r s i n brackets represent the number o f density estimates used to calculate the mean value. 3 0 2.5 M e a n larval density i n streams running through different forest types. Clearcut sites had signif icantly lower density than those running through y o u n g second growth ( A N O V A , F 2 , i 4 = 4 .593, p= 0. .029). 31 2.6 M e a n body length ( + S D ) o f larvae at each site. T u k e y ' s H S D test was used to compare sites. Sites that share the same H S D letter are statistically indistinguishable at p = 0.05. 32 2.7 Jo l ly -Seber (J-S) estimates o f D. tenebrosus larval survival and disappearance rates ( + S E ) . J-S s u r v i v a l estimates are for one month i n the summer o f 1996 and for 9 months (September - June) over the winter o f 1996-1997. Disappearance rates have been scaled to reflect change over a month p e r i o d i n both s u m m e r and winter. 33 2.8 T h e relationship between change i n snout-vent length and number o f days between capture o f D. tenebrosus larvae d u r i n g the active season. P-values are the probabi l i ty that the slope o f the change i n length vs. t ime relationship is indist inguishable f r o m zero. 34 vi Table Page 2.9 Table 2.9: Demographic variables collected for larva l D. tenebrosus and the c losely related D. ensatus throughout their range. Subscripts represent the source o f information: a) this study, b) K e l s e y 1995, c) C o r n & B u r y 1989, d) N u s s b a u m & C l o t h i e r 1973, e) K e s s e l & K e s s e l 1943 & f) H a y c o c k 1991. W h e r e possible, data are stratified by forest type w i t h logged sites be ing less than 10 years o l d and forested sites older than 10 years. D a t a presented i n the m i d d l e o f a b o x are not habitat specific. 35 2.10 S u m m a r y table o f demographic variables and their relationship to larval density and logging history. F o r each variable, sites are ranked f rom highest value to lowest (highest = left, lowest = right). O f the four demographic variables, t ime since harvest correlates o n l y w i t h dai ly growth rate (negative). L a r v a l density correlates w i t h none o f the four demographic variables. P r o B H = P r o m o n t o r y B H , P r o 3 a = P r o m o n t o r y 3a, C e n = Centre H F , F o l R = F o l e y R and T a m C = T a m i h i C - D S . 36 3.1 N a m e and location o f four D. tenebrosus larvae streams used i n study. A l l populations were studied f r o m June to O c t o b e r i n 1996, and i n June and September o f 1997. 62 3.2 Macrohabitat and environmental features measured at each site. 63 3.3 Substrate definit ion and size classes. S ize designation refers to the longest longitudinal axis o f the stone. 64 3.4' Percentage o f movers and non-movers at each site. T h e larvae at Centre H F were significantly more l iable to m o v e than those at a l l other sites (Chi-squared test for t w o w a y tables, % 2 = 10.348, p < 0.025). 65 3.5 M e d i a n distance m o v e d by larvae at each site. T h e m e d i a n cumulat ive distance m o v e d at Centre H F was s ignif icantly higher than at any other site ( B r o w n - M o o d M e d i a n test, % 2 = 18.42, d f = 3, p < 0.001). 66 vii Table Page 3.6 M o v e m e n t behaviour and habitat properties o f four D. tenebrosus larvae populations. V a l u e s i n brackets (x,y) represent the number o f samples used in each analysis, wi th " x " indicating the number o f times the site was visited and " y " the number o f samples taken on each visit. T h e + symbol denotes w h i c h habitat variables were significantly different between sites., w i t h different letter superscripts being significantly different f r o m one another ( T u k e y ' s H S D test). The * s y m b o l indicates w h i c h habitat properties dist inguish the most mobi le populat ion, Centre H F , f r o m al l the others. 67 3.7 A n a l y s i s o f covariance results f r o m study of loca l movement (10 m reach scale), habitat characteristics and site effects. Sixteen data points were used i n each A N C O V A (4 f r o m each site). M a i n effect results refer to the relationship between a continuous habitat variable and the l o g ( x + l ) transformed n u m b e r o f immigrants and emigrants respectively. Interaction effects test the hypothesis that the slopes o f the relationship between a habitat variable and movement are s imi lar at a l l sites. 68 4.1 Periods when colonisation by D. tenebrosus larvae were m o n i t o r e d . 101 4.2 S ize and location o f removal zones w i t h i n the 120 m study reach. 102 4.3 N u m b e r o f larvae detected and removed f r o m experimental stream reaches. T h e number o f detected larvae does not i n c l u d e indiv iduals - that were k n o w n to have dispersed out o f the r e m o v a l zone before the start o f the experiment based on their locat ions i n subsequent weekly surveys, or those o n the verge o f transformation. R e m o v a l efficiency is the total number removed d i v i d e d by the total number detected. 103 4.4 N u m b e r s o f D. tenebrosus larvae estimated to have c o l o n i s e d each removal zone annually under the Conservat ive , Statistically Probable and L i b e r a l M o d e l s . 104 4.5 N u m b e r o f larvae captured i n 120 m study area and the percentages o f these that were colonisers. 105 viii M a g n i t u d e of local density reduction caused by experimental removal o f larvae i n four study reaches and the expected times for these depletions to be replenished by colonisation. T h i s prediction o f recolonisation time is based on the assumption that rates measured dur ing the 13 months o f this experiment w o u l d remain constant through t ime. I d i v i d e d the total length of the depleted reaches by their estimated recovery t ime to predict the rate at w h i c h stream reaches w i t h s imi lar density reductions w o u l d be recolonised. List of Figures Figure Page 2.1 Est imated abundance of D. tenebrosus larvae at four study sites i n 1996 and 1997. Bars represent one standard error 37 2.2 S ize frequency distributions o f D. tenebrosus larvae at 5 sites. Distributions are p o o l e d over a l l captures f r o m June-September 1996, and June and September 1997 (one record per indiv idual ) . 38 2.3a Seasonal changes i n size structure o f larvae at the Centre H F site. T h e x-axis represents total body length (mm) and the y-axis is proportion i n sample. 39 2.3b Seasonal changes i n size structure o f larvae at the Promontory 3a site. A x e s are the same as i n F i g u r e 2.3a. 4 0 2.3c Seasonal changes i n size structure o f larvae at the Promontory B H site. A x e s are the same as i n F i g u r e 2.3a. 41 2.3d Seasonal changes i n size structure o f larvae at the T a m i h i C - D S site. A x e s are the same as i n F i g u r e 2.3a. 4 2 2.4 Seasonal changes i n larval size structure i n 4 D. tenebrosus populations in 1997. A x e s are the same as i n F i g u r e 2.3a. 43 2.5 Predicted change i n snout-vent length ( m m ) i n D. tenebrosus larvae f rom four different streams. These rates apply only between June and September (active season). T h e slope o f these relationships d i d not signif icantly differ between sites ( A N C O V A , Site effects F 3 , 2 i o = 0.332, p = 0.802) . 44 3.1 Seasonal differences i n the average v o l u m e o f each 120 m reach . T h e error bars represent one standard error. A v e r a g e water v o l u m e was significantly higher i n the early season at Centre H F (T-test, t = 2.814, p = 0.002, d f = 8), P r o m o n t o r y 3a (t = 4.146, p = 0 . 0 0 4 , d f = 8) and Promontory B H (T-test, t= 3.334, p = 0.0125, df =7). N = 6 for dark bar, N = 4 for l ight bar. 69 3.2a Seasonal changes i n wetted w i d t h at four streams. 7 0 3.2b Seasonal changes i n depth at four streams. 7 0 x Figure Page 3.3 Seasonal differences in the mean percentage o f pool habitat i n each stream w i t h standard errors. T h e transformed values o f mean percentage o f pool habitat were significantly different between the late season 1996 and early season 1997 at Centre H F (T-test, t = -3.924, p = 0.006, df=7) and at T a m i h i C - D S (T-test, t= -4.082, p = 0.004, d f = 8 ) . S a m p l e sizes as i n F i g . 3.1. 71 3.4a Differences i n mean air temperature between late season 1996 and early season 1997. E a c h site mean was based on 8-18 observations. 72 3.4b Differences i n mean water temperature between late season 1996 and early season 1997- E a c h site mean was based on 8-18 observations. 72 3.5 Cumulat ive distance traveled by D. tenebrosus larvae early and late i n their active season. These data are pooled f r o m al l o f the four streams. E a r l y season refers to movements made i n June and J u l y 1997 (n = 30) and late season to movements made i n August and September 1996 (n = 76). There was no significant difference between these two distributions ( K o l m o g o r o v - S m i r n o v test, p = 0.985). 73 3.6 Proport ion o f larvae that m o v e d (displacement greater than 0.5 m) and d i d not move at two different periods throughout the active season. T h e proportion o f movers was not s ignif icantly different between these two periods. 74 3.7 M e a n larval density and standard error at four D. tenebrosus larvae streams. 75 3.8 Relat ionship between the number o f larvae m o v i n g into and out o f a 10 m reach o f stream during a 13 m o n t h experiment. The, number of larvae m o v i n g into a zone was not significantly/related to the number m o v i n g out ( A N C O V A , , i m m i g r a t i o n effect F i , n = 3.201, p = 0.101). There were no interactions between site interactions ( A N C O V A , site effects F 3 , n = 0.618, p = 0.618). 76 3.9 The relationship between body length and cumulat ive distance traveled, pooled over al l sites (n = 2 4 1 ) . T h e dark l ine is the l ine o f best fit between these two variables . T h i s regression was not significant (r 2 = 0.007, p = 0.192). T h e relat ionship between these variables is described as : Dis tance traveled (m) = 0 . 0 6 1 * B o d y length(mm) + 4.81. 77 4.1 Steps taken to determine the n u m b e r o f larval D. tenebrosus colonists under the Statist ical ly Probable M o d e l . 107 xi Figure Page 4.2 Percent recovery o f art i f ic ial ly depleted stream reaches one year after disturbance under 3 different colonisat ion models. Percent recovery is the number of colonists after one year d i v i d e d by the number o f larvae detected i n each removal zone before manipulat ion. R e c o v e r y values greater than 1 0 0 % indicate that the pre-disturbance abundance was exceeded. 108 4.3 Percentage of colonisers i n 120 reach as a function o f mean larval density. 109 4.4a Percentage of in-stream colonisers i n source areas as a funct ion of mean population density. A n in-stream coloniser is one that was o r i g i n a l l y captured in the source areas and then dispersed into the r e m o v a l zone. 110 4.4b Percentage of recruits i n the removal zone as a function of mean populat ion density i n the source areas. 110 4.5 O r i g i n o f colonists that colonised the r e m o v a l zone. In-stream dispersers are larvae that entered the r e m o v a l zone f r o m up or d o w n stream. Recruits are young-of-the-year larvae that were l i k e l y deposited into the removal zone by adult dispersal and ovipos i t ion. I l l 4.6 Expected distribution of mean snout-vent length ( S V L ) i n a r a n d o m l y selected group of 37 non-colonis ing larvae and the observed mean S V L i n 37 k n o w n colonisers. 112 4.7 Expected distribution of mean snout-to-ventral length ( S V L ) i n a randomly selected group o f 7 n o n - c o l o n i s i n g larvae and the observed mean S V L in 7 k n o w n in-stream colonisers . There is no significant difference i n the mean length o f larvae that act ively co lonised the removal zone and those that d i d not. 113 4.8 Distr ibut ion of mean cumulat ive distance travel led by 7 non-colonis ing larvae f r o m the source areas o f each site (based on 1000 trials). T h e mean value o f this distr ibut ion, 10.7m, was l o w e r than the mean va lue m o v e d by the 7 c o l o n i s i n g larvae : 26.1 m (p = 0.056). 114 Figure Page 4.9 Distr ibut ion of the expected number o f downstream movements i n a randomly selected group of 7 non-colonis ing larvae (all sites pooled, based on 1000 trials). T h e mean of the expected distr ibut ion, 3.1 , was not significantly different f r o m the observed number o f downstream movements made in the group o f 7 c o l o n i s i n g . larvae, p = 0.224. 115 xiii Acknowledgments I cannot imagine a more supportive, insightful and helpful supervisor than I found i n D r . W i l l i a m N e i l l . I thank h i m for our many hours o f discussion regarding m y thesis and the wonders o f nature in general, his patience and bel ie f i n me when I encountered challenges w i t h m y work, and his sense o f humour. It has truly been a pleasure to w o r k w i t h D r . N e i l l and his advice has been invaluable. I w o u l d also l ike to thank m y committee members D r . Jamie S m i t h and D r . J o h n Richardson. Their careful review o f m y research has been instrumental i n shaping m y in i t ia l ly rough tangle of ideas into coherency. I greatly appreciate their help. I w o u l d also l i k e to thank D r . D a n H a y d o n for his statistical advice. I was very pr iv i leged to have an excellent team of dedicated f ie ld assistants: D i a n e C a s i m i r , Leonardo F r i d , M a r k K r a u s e , Shelagh Parken, A l e x Skrepnik , and C a r o l y n n Stephenson. M a n y thanks to them for their hard w o r k and unflagging pursuit o f salamanders i n rain or shine. A very special thanks goes to K a r l M a l l o r y for his advice on site selection, capturing techniques, and assistance w i t h f ie ld supplies and housing. A d d i t i o n a l thanks go to the many other volunteers w h o came out to C h i l l i w a c k for a few days to help w i t h m y f ie ld w o r k . T h i s project w o u l d not have been possible without the generous f inancia l support o f Forest R e n e w a l B . C . and the W o r l d W i l d l i f e F u n d o f Canada. I w o u l d also l i k e to thank N . S . E . R . C . w h o provided personal f inancia l support through a P G S A scholarship. I must also thank the many people whose fr iendship and sense o f h u m o u r kept me on track. Special thanks go to Chr is t ine , A n n e , B e a , Christ iarine, K r i s t a , Stephanie, Michel le ,<Tony, D a v i d C . and W e n d y for " d r a g g i n g " me for a beer, keeping me company at the R a i l w a y C l u b , and enduring Braveheart a record 10 times. F i n a l l y , I w o u l d l i k e to each and every one o f the 600+ P a c i f i c G i a n t Salamanders that had to endure m y handl ing throughout the course o f this experiment. I sincerely hope the information I gained through this endeavour w i l l prove useful to the maintenance o f this e lusive and beautiful creature w i t h i n the C h i l l i w a c k V a l l e y . xiv For Irene and Tom, Dulcius ex Asperis Chapter 1: General Introduction In 1989, the Paci f ic Giant Salamander {Dicamptodon tenebrosus G o o d ) was declared vulnerable by the Committee on the Status o f Endangered W i l d l i f e i n Canada ( C O S E W I C ) . T h i s species is red-listed and classified as "at r i s k " by the p r o v i n c i a l government o f B r i t i s h C o l u m b i a . Despite this designation o f the highest level o f risk (Br i t i sh C o l u m b i a M i n i s t r y o f E n v i r o n m e n t , L a n d s & Parks 1993), the true status o f this species i n C a n a d a and the extent to w h i c h it is threatened are u n k n o w n . M u c h o f the evidence supporting D. tenebrosus' l i s t ing comes f r o m observation o f perceived threats such as l o g g i n g ( H a y c o c k 1991), and the assumption that be ing on the margin o f the species' range makes populations i n B r i t i s h C o l u m b i a inherently more vulnerable to extirpation. T h i s assumption is based on field evidence f r o m other taxa that shows peripheral populations have a greater probabi l i ty o f loca l ext inct ion ( L o m o l i n o & C h a n n e l 1995, Nathan et a l . 1996) than those i n the centre due to l o w e r populat ion densities ( H e n g e v e l d 1990, L a w t o n 1993), l o w e r surv iva l ( R a n d a l l 1982, Rogers & R a n d o l p h 1986), and l o w e r fecundity (Caughley et a l . 1 9 8 6 ) . W h i l e these factors may warrant concern, there has been no thorough demographic examination o f D. tenebrosus populations i n B r i t i s h C o l u m b i a to demonstrate that they are truly imperi l led. T h e scientific criteria required to assess whether a species is at r isk are not w e l l defined. M a n y different schemes o f assessment have been proposed ( A y e n s u 1981, M a c e 1991, Spellerberg 1992, P r i m a c k 1993, C a u g h l e y & G u n n 1996), but no standard methodology has been adopted. It is generally agreed however , that evaluation o f requires k n o w l e d g e o f l o c a l demography, the impact o f human activities and the general abi l i ty o f the species to respond to disturbance (Soule & K o h n 1989, P r i m a c k 1993, E l l i s & Seal 1995). U s i n g these issues as a guide, several key questions can be formulated to assess whether D. tenebrosus populations i n B r i t i s h C o l u m b i a merit special conservation attention: a) Local Demography • A r e population densities signif icantly reduced i n regions where the species is considered threatened i n comparison to areas where they are not? • A r e populations dec l in ing i n regions where they are considered threatened? • A r e vital demographic rates such as surv iva l , fecundity and/or growth s ignif icantly lower i n regions where the species is considered threatened i n comparison to areas where it is not? • A r e D. tenebrosus' v i tal demographic rates uncharacteristically l o w i n compar ison to other non-threatened stream d w e l l i n g salamanders? b) Impact of human activities • A r e population densities, surv iva l , fecundity and/or growth rates reduced i n regions that have been logged, the primary form o f human disturbance i n D. tenebrosus' range? • D o the specific habitat changes caused by logging compromise D. tenebrosus' dispersal and recolonisation abil ity? c) General ability to recover from disturbance • H o w q u i c k l y can D. tenebrosus recolonise sites o f loca l extinctions? Some o f these questions are addressed i n this thesis. In a two year study such as this one, long term population trends cannot be assessed. H o w e v e r , basic l i fe history informat ion and seasonal demographic rates can be estimated. T h i s informat ion is a crucia l first step for identi fy ing whether these allegedly vulnerable populations behave differently f r o m those i n Oregon, W a s h i n g t o n and C a l i f o r n i a where the species is not a major conservation concern. These basic rates can also be compared to those i n other, non-threatened stream-dwel l ing salamanders to reveal whether this salamander is intrinsical ly less viable than other species. Such fragi l i ty w o u l d indicate an increased susceptibility to extinction, even i n the absence o f disturbance. 2 T o obtain reliable demographic estimates, I repeatedly sampled a few populations (5) instead o f less intensively sampling a large number o f sites. T h i s study is the first to produce robust estimates of larval density, survival , growth, dispersal and c o l o n i s i n g abil i ty for several populations o f D. tenebrosus. M a n y of these parameters such as surv iva l and growth have never before been rigorously estimated for D. tenebrosus in B r i t i s h C o l u m b i a . Others such as co lonis ing abil ity have never been measured for this species anywhere i n its range. In addition to the estimation of basic v i ta l rates, I examined i f the more recently logged sites displayed distinct demographic properties. A l t h o u g h not rigorous, this qualitative analysis may generate prel iminary hypotheses o f h o w habitat influences larval persistence. T h i s species' general ability to recover f r o m disturbance, a f inal indicator o f vulnerabi l i ty , was studied experimentally. In B r i t i s h C o l u m b i a , logg ing is presumed to increase the frequency of loca l ext inct ion (Fair 1989, H a y c o c k 1991), yet almost nothing is k n o w n o f Dicamptodon's abil ity to recover by colonisation. I simulated point extinctions w i t h i n larval populations and monitored the speed of recolonisation. I also studied larval dispersal i n dif fering stream m i c r o - habitats to determine i f recolonisation is habitat-dependent. I) Natura l H i s t o r y Pac i f i c Giant Salamanders are an important component o f the vertebrate fauna in the forests o f the Pac i f i c Northwest. In the streams where it occurs, this salamander is often the dominant predator and can constitute up to 9 9 % o f the total vertebrate b iomass ( M u r p h y & H a l l 1981). Larvae prey pr imar i ly upon benthos (Parker 1994) although large indiv iduals can also eat T a i l e d F r o g (Ascaphus truei) larvae, smal l f ish and smal ler conspecif ics (Nussbaum et a l . 1983). A d u l t salamanders consume large prey i n c l u d i n g m i c e , shrews and l izards ( B u r y 1972). W i t h 3 larvae occurr ing at densities o f up to 3 per m 2 i n some P a c i f i c Northwest streams ( K e l s e y 1995), they may regulate numbers o f their prey species. In B r i t i s h C o l u m b i a , these animals spend at least two years as aquatic larvae (Richardson & N e i l l 1995). Larvae reside pr imar i ly i n c o o l , fast f l o w i n g headwater streams although some have been found i n larger streams and lakes. Af ter the larval per iod, D. tenebrosus either transform into sexually mature terrestrials or remain i n streams i n neotenic f o r m . A d u l t s can grow up to 35 c m i n total length, m a k i n g this species the largest semi-aquatic salamander i n N o r t h A m e r i c a (Nussbaum et a l . 1983). II) Dis t r ibut ion P a c i f i c Giant Salamanders are found along the western coast o f N o r t h A m e r i c a f r o m northern C a l i f o r n i a to southern B r i t i s h C o l u m b i a . In Canada, D. tenebrosus is found only i n the C h i l l i w a c k R i v e r drainage basin and a few adjacent smal l tributaries that drain directly into the Fraser R i v e r . W i t h i n the C h i l l i w a c k V a l l e y watershed, D. tenebrosus is distributed patchily w i t h many unexplained gaps i n its distribution. Survey w o r k i n C h i l l i w a c k detected D. tenebrosus i n only 21 o f 59 seemingly habitable streams (Richardson & N e i l l 1995). It is possible that many o f these currently barren streams have experienced l o c a l ext inct ion. III) R o l e o f larvae i n Urodele populat ion d y n a m i c s M y research was conducted solely o n larvae o f D. tenebrosus. W h i l e no concrete prediction o f species' persistence can be d r a w n f r o m the study o f one l i fe history stage, larval ecology is an essential component o f populat ion b i o l o g y . Furthermore, there is reason to bel ieve that larvae may be the only stage i n recently disturbed habitats. Terrestrial adults may suffer great mortality i n clearcuts due to increased desiccation a n d freezing i n exposed habitats (Richardson 4 1994). Under such a scenario, depopulated areas would rely on larval propagules from undisturbed stream reaches for recolonisation. Although this hypothesis has not yet been tested, it suggests that studies of larvae are vital to judge the recovery potential of this species. IV) Organisation of Thesis This thesis is composed of three research chapters and a general conclusions section. The research chapters will address the following three topics: 1) D. tenebrosus larval demographic rates in British Columbia, 2) habitat-dispersal associations and 3) recolonising ability. In combination, these studies will provide new insights into the resilience of this species and whether it is currently compromised in British Columbia. Each chapter is based on two summers of research within the Chilliwack Valley. The major goals and hypotheses of each chapter are outlined below. 1) Life history and demography of Pacific Giant Salamander larvae in five streams at the northern limit of its range The primary aim of this chapter is to provide base-line demographic information on larval D. tenefcrosw^populations in British Columbia and compare rates with those collected in more southerly, non threatened populations. I also examine the influence of forest age from 5 to 60 years on demography and the impact of larval population density on survival and growth. 2) Determinants of Dispersal in Pacific Giant Salamander Larvae In this chapter, I examine the influence of abiotic and biotic factors on dispersal of D. tenebrosus larvae. As dispersal is the key means by which populations re-establish in disturbed 5 areas, a knowledge o f the environmental and demographic factors that l i m i t movement may be useful for management. 3) Colonising Ability of Pacific Giant Salamander larvae In this experiment I measured h o w q u i c k l y artif icial ly depleted stream reaches were recolonised by D. tenebrosus larvae. I tested whether larvae are capable o f recovering f r o m s m a l l disturbances w i t h i n a short period o f t ime (< 1 year). I also tested whether this recolonisat ion is l i m i t e d by source population density, size structure and location. 4) General Conclusions The general conclusions section summarises the major f indings o f this research. T h i s section discusses the implicat ions o f this research to the evaluation o f D. tenebrosus'' current status and future persistence i n B r i t i s h C o l u m b i a . 6 Chapter 2: Life history and demography of Pacific Giant Salamander larvae in five streams at the northern limit of its range Introduction: A s the number o f species exposed to human disturbance increases, there is a great need to assess the potential impacts on population persistence. A l t h o u g h no definit ive criteria exist, it is wide ly accepted that information on local demography is v i ta l to evaluate the v iabi l i ty o f populations (Soule & K o h n 1989, Caughley & G u n n 1996, P r i m a c k 1993). B a s i c l i fe history information is useful both to understand habitat requirements and suggest the mechanisms that l i m i t populations in different areas. Reductions in demographic rates such as surv iva l , reproduction and growth between habitats can also suggest causes o f decl ine and loca l ext inct ion (Caughley & G u n n 1993). Conversely i f demography is unaffected by habitat change, there is little reason for concern about the persistence o f such populations under s i m i l a r events. In this chapter I present detailed demographic analyses o f larval P a c i f i c Giant Salamanders (Dicamptodon tenebrosus) i n f ive sites i n the C h i l l i w a c k V a l l e y o f B r i t i s h C o l u m b i a , the northern extent of this species' range. In B r i t i s h C o l u m b i a , populations o f this red-listed species are thought to be l i m i t e d by c o o l temperatures and threatened by logg ing ( H a y c o c k 1991). V e r y little demographic analysis has been conducted to support these c la ims. I examined larval survival , recruitment, metamorphosis and growth. M y aims were to: 1) Provide base-line demographic information for larvae i n B r i t i s h C o l u m b i a ; 2) E x a m i n e the relationship between logging history and larval demographic parameters; 3) E x a m i n e the relationship between surv iva l , growth and larva l density. 7 1) Pac i f i c Giant Salamander life history - a review G i v e n the secretive nature o f D. tenebrosus, much of the basic l i fe history o f this animal remains u n k n o w n . W h a t is k n o w n about this species' ecology and response to disturbance comes f r o m studies i n Washington, Oregon, and Cal i fornia . V e r y little research has been conducted i n B r i t i s h C o l u m b i a . The f o l l o w i n g summarises what is k n o w n about D. tenebrosus' l i fe history and the major questions that remain. a) Reproduction Whereas D. tenebrosus in Oregon and C a l i f o r n i a are bel ieved to have distinct breeding periods i n Spring and F a l l (Nussbaum et al . 1983), prel iminary evidence f r o m B r i t i s h C o l u m b i a suggests the t iming of breeding is variable ( H a y c o c k 1991). I f this is true, D. tenebrosus i n B r i t i s h C o l u m b i a w o u l d be one o f the few temperate amphibian species to display asynchronous breeding. (Duel lman & Trueb 1986). A l t h o u g h the t i m i n g of breeding m a y be variable, it is potential ly concentrated i n specific months or seasons. In this study, mark-recapture throughout the active seasons o f 1996 and 1997 was used to test for seasonality i n recruitment o f D. tenebrosus. Observations i n aquaria and the f i e l d show that eggs take approximately 200 days to hatch (Nussbaum et a l . 1983) into larvae 33-35 m m i n total length ( N u s s b a u m & C l o t h i e r 1973). N e w l y hatched larvae remain buried i n the substrate and attached to their y o l k sac for three to four months before appearing i n streams at 45-51 m m i n length ( N u s s b a u m & Clothier 1973). C o m b i n i n g these periods, I assume that larvae are first detectable 9-11 months after they are spawned. U s i n g this cr i terion, the t i m i n g o f breeding i n m y streams was back-calculated as 9-11 months f rom the first appearance o f s m a l l , 45-50 m m l o n g larvae. 8 b) Transformation After 2-4 years as larvae, D. tenebrosus either transform into terrestrial adults or remain i n the stream i n neotenic form ( D u e l l m a n & Trueb 1986). T h e t i m i n g o f transformation varies considerably between populations ( N u s s b a u m & C l o t h i e r 1973), but is bel ieved to occur between June and August . I f so, the frequency o f large larvae should decl ine over this per iod. I tracked changing size structure throughout the active season to pinpoint the t i m i n g o f transformation i n C h i l l i w a c k streams. D e s c r i b i n g the chronology o f natural abundance fluctuations i n larval populations, either by recruitment or metamorphosis, w i l l help biologists distinguish change caused by life history processes and change caused b y extrinsic factors. c) Survival L i t t l e is k n o w n about larval survival i n D. tenebrosus and h o w it varies seasonally and spatially. Scientists have identified several sources o f mortal ity i n D. tenebrosus, but not their net effect on survival . C h i e f agents o f mortal i ty i n this species are thought to be c a n n i b a l i s m , predation, and desiccation (Nussbaum & C l o t h i e r 1973). P r e l i m i n a r y research i n the C h i l l i w a c k region suggests that larval survival varies throughout the year. M o r e larvae "disappear" over the summer than they do over the winter ( N e i l l & R i c h a r d s o n 1998). It is unclear whether this increased summer loss rate is due to transformation or higher mortal i ty . I attempted to separate these alternatives by correcting summer s u r v i v a l rates for loss due to transformation. Seasonal differences in survival were then assessed by c o m p a r i n g this less biased summer rate to the winter disappearance rate. 9 d) Growth Working in one British Columbia stream, Haycock (1991) found that first year larvae increased between 0.5 to 3.2 mm in snout-vent length (SVL) per month during the active season. It is unclear whether these rates vary between streams, habitats and regions. I measured larval growth at four sites within the Chilliwack Valley. Mean growth at these sites was compared to estimates from larvae in Oregon, the centre of D. tenebrosus' range, to examine i f northern populations showed signs of depressed growth 2) The Impact of Logging on Life History Parameters Most studies of D. tenebrosus in the Pacific Northwest have inferred logging effects by correlating larval density to the age of the surrounding forest. Results of these studies have been mixed, with some finding reduced density in logged stands (Bury 1983, Bury & Corn 1988, Connor et al. 1988, Corn & Bury 1989, Cole et al. 1997), others finding no effect (Hawkins et al. 1983, Kelsey 1995) and still others finding increased density in logged areas (Murphy et al. 1981, Murphy & Hall 1981). Without examining demographic rates, it is difficult to interpret why abundance varies, increasing or decreasing, in logged areas. I investigated growth and survival in different aged stands in addition to larval density to determine i f they varied with forest age. Only a small number of sites were investigated in this analysis so my ability to detect habitat-specific population trends is low. However i f recently logged habitat is of poorer quality to D. tenebrosus larvae than mature forest, I expected to find some correlation between larval demographic rates and forest practices. If logging is highly detrimental to these animals, I predicted that either larval density, survival and/or growth should increase with forest age. 10 Demographic analysis may also reveal whether one o f the proposed benefits o f l i v i n g i n a recently logged stream, increased growth ( M u r p h y et al . 1981, M u r p h y & H a l l 1981, H a w k i n s et al . 1983), is v a l i d . Temperature and primary productivity o f streams often rise after l o g g i n g , possibly enhancing the food supply and length o f g r o w i n g season o f D. tenebrosus ( M u r p h y et al . 1981, C o r n & B u r y 1989). H i g h e r growth rates c o u l d increase the fitness o f larvae i n clearcut streams by shortening the length o f time they spend exposed to size-dependent canniba l i sm and predation. I tested whether forest age related to larval growth rate. 3) B i o t i c Regulat ion in D. tenebrosus L a r v a l Populat ions In addition to forest habitat, I also examined the influence o f larval density on demography. S o m e studies o f larval salamanders indicate s u r v i v a l and growth are p r i m a r i l y a function o f populat ion density (Kusano 1981, Petranka & S i h 1986, B u s k i r k & S m i t h 1991). It is therefore possible that larval demography is more inf luenced by density than forest habitat. I examined whether survival and growth decreased w i t h increasing larval density at four sites. A l t h o u g h the number o f sites used in this analysis is l o w , i f strong density dependence was acting these trends should be evident. Methods: Site Selection F i v e headwater streams in four watersheds o f the C h i l l i w a c k R i v e r drainage basin were selected for study. Sites were selected both on the basis o f accessibi l i ty by logging road and larval abundance. O n l y sites at w h i c h at least one larvae was detected w i t h i n a prel iminary thirty minute searching per iod were used. Sites differed i n their l o g g i n g history (Table 2.1). F o u r o f 11 the f ive sites were used in a colonisation experiment (Chapter 4) and were sampled intensively throughout the active season i n 1996 and 1997. A t these sites, larvae were r e m o v e d f r o m a central portion of the stream. This manipulation should not influence this analysis as a l l demographic estimates were gathered f rom larvae l i v i n g outside o f the r e m o v a l zone. T h e fifth site, F o l e y R, was studied only for a few months i n summer 1996. A s a consequence of reduced s a m p l i n g frequency at this site, estimates o f survival and growth were not c o m p a r e d to those at other sites. Abundance was calculated for this site and used i n the analysis o f logging history and larval density. Mark-Recapture A 120 m reach o f stream was selected at each site. L a r v a e l i v i n g w i t h i n these reaches were routinely sampled using mark-recapture. Part ial removals o f 2 5 - 4 0 m i n length were conducted on four o f these f ive streams. O n l y the remaining 8 0 to 95 m o f unmanipulated (non- cleared) reach was used i n this analysis (Table 2.1) S a m p l i n g was conducted weekly (Table 2.2). W i t h the exception of F o l e y R, a l l sites were sampled at least twenty t imes. T h i s frequency of sampling ensured that the opportunity to recapture animals was h igh . O n each visit , the entire 120 m reach was systematical ly searched. A l l large rocks and debris were turned over and the substrate inspected for larvae. A l l overturned material was returned to its or ig inal posi t ion. L a r v a e were detected by sight or touch and captured i n s m a l l d ip nets. T h e i r location was recorded to the nearest ha l f meter and m a r k e d by ty ing fluorescent f lagging tape to a rock. Captured larvae were he ld i n d i v i d u a l l y i n 1 L plastic jars during subsequent processing. 12 U n m a r k e d larvae were anaesthetised i n a 0.33g L " 1 solution o f M S 2 2 2 (tricaine methanesulfonate). W h i l e anaesthetised, larvae were marked either by toe c l i p p i n g or the insertion o f a Passively Induced Transducer (P.I .T.) tag ( A V I D M i c r o chips , M U S I C C 21-23). E a c h tag emits a distinct electromagnetic field w h i c h can be p i c k e d up by a hand h e l d reader ( A V I D P o w e r Tracker II) and translated into a unique identity code. A n i m a l s wi th a total length < 100 m m were toe c l ipped. A unique c o m b i n a t i o n o f one or two toes was removed f rom these animals wi th a scalpel . Toes that appeared to be r e g r o w i n g on subsequent capture were c l ipped again. L a r v a e > 100 m m were g i v e n a P . I .T . tag. T o insert a tag, a smal l inc is ion was made anterior to the h i n d leg on the animal ' s side. A disinfected tag was then inserted by hand (wearing medica l gloves) under the first layer o f s k i n . T h e w o u n d was disinfected with antibacterial ointment and sealed w i t h V e t Bond™, a veterinary surgical adhesive. O n recapture, al l larvae were examined for toe loss and scanned w i t h a hand-held P.I .T . tag reader. T h e total body and snout-vent length o f each larva were recorded to the nearest mi l l imetre . A n i m a l s were also weighed on a portable electronic balance (Ohaus Inc.) accurate to 0.1 g . A n i m a l s were returned to their in i t ia l point o f capture after they had regained their s w i m m i n g ability. Larval Abundance and Density Estimation I used a c losed mark-recapture m o d e l to estimate larva l density throughout this study. C l o s e d models assume that no birth, death or dispersal into or out o f the study area have occurred during the period when the mark-recapture data were col lected. A s such, data must be gathered over a short per iod o f t ime to m i n i m i s e bias due to non-closure ( P o l l o c k et a l . 1990). 13 Mark-recapture data were split into four periods: S p r i n g 1996, F a l l 1996, S p r i n g 1997 and F a l l 1997 (Table 2.3). W i t h the exception of F a l l 1997, each p e r i o d consisted o f data f r o m four mark-recapture episodes over four weeks. A four-week per iod was thought to be the longest span of time that w o u l d meet the assumptions o f a c losed populat ion m o d e l . In F a l l 1997, density was estimated on the basis o f two instead o f four sampl ing periods. T h e program C A P T U R E ( B u r n h a m et a l . 1994) was used to calculate larval abundance dur ing each 4 week interval (Spring 96, F a l l 96, S p r i n g 97). F r o m inspect ion of capture records, it was evident that some animals were more l i k e l y to be caught than others. A s a consequence, the assumption of equal probabi l i ty .of capture was v io lated. T o account for this, the data were fitted to a specific model w i t h i n C A P T U R E ( B u r n h a m & Overton 1979) k n o w n as M ( h ) that compensates for heterogeneity in capture probabi l i ty ( B u r n h a m & O v e r t o n 1979). T h i s m o d e l requires more than two sampling occasions to determine the amount o f variat ion i n capture probabi l i ty between animals. W i t h only two sampl ing intervals, F a l l 1997 abundance c o u l d not be estimated by the M ( h ) model . A b u n d a n c e at this t ime was calculated using the L i n c o l n - Peterson method ( L i n c o l n 1930). C h a p m a n ' s M o d i f i c a t i o n of the L i n c o l n - P e t e r s o n method was used to offset bias caused by l o w recapture probabi l i ty ( C h a p m a n 1951) ( A p p e n d i x 1). B y ignoring heterogeneity i n capture probabil it ies between indiv idua ls , this model m a y underestimate abundance i n comparison to M ( h ) ( P o l l o c k et a l . 1990). L a r v a l density (individuals per m 2 ) was estimated at each site by d i v i d i n g the estimated abundance by the area o f the study reach (length of unmanipulated stream searched m u l t i p l i e d by the average wetted width) (Table 2.1). 14 Size Structure Variation: Recruitment, metamorphosis and logging impacts Measurements of total length on first capture were pooled over all time periods and used to generate a cumulative length-frequency histogram at all five sites. I used Kolmogorov- Smirnov tests to examine i f larval size-frequency distributions varied between sites with different logging history. Additionally I examined the mean body size of larvae at each site. Statistical differences in larval size between sites were evaluated using a Kruskal-Wallis test. To investigate temporal trends in recruitment and metamorphosis, I examined how size structure changed throughout the larval active season at four sites. A t each site, histograms of larval total length were computed for each month of the study period: June - September 1996, June and September 1997. Kolmogorov-Smirnov tests were used to determine whether the shape of the length distribution changed significantly through time. The data were examined for evidence of a sudden appearance of larval recruits at some point during the active season and for a sudden disappearance of large larvae due to transformation. Larval Disappearance Rates and Survival Open population mark-recapture models give relatively robust, unbiased estimates of disappearance rates between sampling intervals (Pollock et al. 1990). Disappearance does not necessarily reflect the amount of death, as animals may also leave the study area by dispersal. From this point forward, I wi l l use the term "disappearance" to refer to the percentage of animals that leave the study area over a given period, while "mortality" is used only when the actual death rate is implied. I calculated and compared larval disappearance rates over one 15 month i n the active season (mid J u l y to m i d August) and over winter (September - M a y ) at four sites. T h e P r o g r a m J O L L Y was used to estimate these rates ( H i n e s 1991). I also recorded information on dispersal and transformation to assess h o w strongly these processes inf luenced summer disappearance rates. B y measuring distances travel led between captures, I was able to characterise larval dispersal distances and estimate the probabi l i ty o f migrat ion into or out o f the study zone between s a m p l i n g periods. C a l c u l a t i n g the percentage of loss due to transformation was more diff icult . F r o m m y observations i n the C h i l l i w a c k area, most larvae > 130 m m i n total length showed signs o f i m m i n e n t transformation, i.e. considerable reduction of g i l l size and the appearance o f m a r b l i n g on the s k i n . U s i n g a cut-off o f 130 m m total length, I calculated the percentage o f larvae large enough to be o n the verge o f transformation in each S p r i n g sample. I f this fraction was h i g h , I interpreted disappearance rates f rom S p r i n g to F a l l as be ing significantly influenced by metamorphosis. Growth G r o w t h between captures was defined as the change i n snout-vent length ( S V L ) . A s many larvae lose part o f their ta i l , poss ib ly as a result o f fights, S V L is a m o r e accurate measure of skeletal growth than total body length. T h e difference i n S V L length between first and last capture dur ing the active season (June - September) was plotted as a funct ion o f the number o f days between captures. A l inear regression was used to determine the strength o f this relationship. A m p h i b i a n growth is probably best described by a c u r v i l i n e a r rather than l inear relationship, w i t h rates s l o w i n g d o w n w i t h age. H o w e v e r I was e x a m i n i n g size changes only over a few months o f the active season and not between years. I assume l inear analysis is sufficient to describe this short term growth. 16 T o examine how age influences growth, I calculated dai ly growth rates for larvae < 100 m m (small) i n total length and > 100 m m ( large) . A l t h o u g h there is no def init ive means o f ageing larvae, m y successive 1996-1997 mark-recapture suggests larvae 100 m m i n length are at least one year o ld . Thus this analysis attempts to look at differences between larvae i n their first year, and those older (2-4 years?). I used analysis o f covariance to determine i f dai ly growth was affected by body size. I f body size strongly affected growth rate, comparisons between sites were stratified by size. Results Abundance and Density Estimation L a r v a l density i n the f ive study streams varied between 0.46 and 1.31 larvae m " 2 (Table 2.4). W i t h the exception of F o l e y R, mean density at each site was based on four estimates o f abundance. A s only one measurement o f density was taken at F o l e y R , this site was exc luded f rom statistical analysis of between-site density differences. M e a n density varied signif icantly between the remaining four sites but not between S p r i n g and F a l l ( T w o - w a y A N O V A , site effects: F 3 , 8 = 9.642 p = 0.005, season effects: F i , 8 = 0.419, p = 0.535). L a r v a l density was highest i n the young second growth site, but not s ignif icantly so (Table 2.5). In the four sites monitored for two summers, larval abundance s h o w e d moderate seasonal and annual fluctuations (Figure 2.1). S p r i n g densities i n 1997 were a lways s l ight ly lower than F a l l 1997 estimates. S i m i l a r l y i n 1996, almost a l l S p r i n g density estimates were equal or less than the F a l l values. 17 Size Structure S u m m e d across a l l sampling dates, larval body length at a l l sites varied f r o m 40-160 m m (Figure 2.2). M e a n larval size varied signif icantly between sites (Table 2.6). L a r v a l size was significantly higher in the young second growth site ( K r u s k a l - W a l l i s , % 2 = 29.152, p < 0.001) . G i v e n that only one site was i n this habitat category, this result c o u l d be due to random site variat ion and not to forestry treatment. Size-structure fluctuated between months i n the summer o f 1996 (Figure 2.3 a,b,c,d) and 1997 (Figure 2.4). In F a l l 1997, the size structure at Promontory 3a and T a m i h i C - D S was significantly shifted towards small larvae i n the 4 0 - 5 0 m m length range ( K o l m o g o r o y - S m i r n o v test, p < 0.001). A s imi lar inf lux o f s m a l l larvae appeared at the P r o m o n t o r y B H and Promontory 3a i n A u g u s t 1996. T h e proport ion o f larvae larger than 130 m m T L consistently dec l ined f r o m June to September at P r o m o n t o r y B H and P r o m o n t o r y 3a. In 1996, signif icant decreases i n large larvae were evident as early as J u l y ( K o l m o g o r o v - S m i r n o v test, p < 0.01). S i m i l a r decreases were seen in 1997, but as no J u l y sample was taken i n this year it is diff icult to pinpoint the start o f this decl ine. Larval Disappearance Rates and Survival Before summer and winter rates were compared, they were scaled to the same t ime period. A one-month winter disappearance rate was extrapolated f r o m the nine month rate in the f o l l o w i n g way: 1 M o n t h W i n t e r Disappearance Rate = (9 month W i n t e r Disappearance R a t e ) 1 ' 9 B a c k calculated monthly winter disappearance rates were not consistently higher or l o w e r than monthly summer rates (Table 2.7). S u m m e r disappearance rates d i d not appear to be related to 18 forest practices, wi th rates being s imi lar ly l o w i n the oldest and youngest site (Promontory B H and T a m i h i C - D S ) . T h e same is true for winter rates that had no specif ic association w i t h the logging history o f sites. Summer and winter disappearance rates were not c lear ly associated w i t h larval density, with the two streams most s imi lar in density (Centre H F and P r o m o n t o r y B H ) hav ing the most divergent rates. O n average, larval disappearance over a one month per iod i n the summer was about 12%. A s D. tenebrosus larvae are poor dispersers (Chapter 3), I have assumed dispersal does not s ignif icantly impact month to month disappearance rates i n a 120 m reach. Transformat ion, however, c o u l d account for a more significant loss o f indiv iduals over the summer months. F o r example, i n the first sampling period o f 1996, 18% (34/192) o f a l l larvae were large enough to be close to transformation and 8 0 % of these same i n d i v i d u a l s (n = 34) were never caught again. T h e mean capture probabil i ty at a l l sites var ied between 15-20% per occasion. G i v e n that each stream was sampled an additional 15 times, these large indiv iduals should have been recaptured at least once during the remainder o f the study i f they were st i l l i n the reach i n larval f o r m . T h e fraction of the monthly summer disappearance due to transformation can be approximated as the percentage o f larvae >130 m m i n an area at the start o f a summer month m u l t i p l i e d by their observed disappearance rate over the same one month per iod. C o m b i n i n g data f rom a l l m y study sites, this value equals approximately 1 0 % ( 1 3 % of larvae > 130 m m T L x 8 0 % disappearance rate o f these larvae). T h e percent o f larvae that actually die over a one month period i n the summer can be estimated as the percentage o f total disappearances minus the percentage o f disappearances due to metamorphosis : 1 2 % - 1 0 % = 2 % . T h i s value sl ightly underestimates mortality as it assumes a l l disappearance o f larvae > 130 m m was due to transformation and not death. T a k i n g this value as a l o w e r extreme, I assume that between 2 % - 19 5 % of larvae die over a one month per iod i n the summer, the rest o f the loss be ing due to transformation. A s transformation does not occur i n winter, winter disappearance rates are l i k e l y to be a g o o d reflection o f mortal i ty . I thus conclude that larval mortal i ty is l o w e r throughout the summer ( 2 - 5 % per month) than it is i n winter (mean disappearance o f 12% per month) . C o m b i n i n g these estimates of summer mortality with winter disappearances rates, mean annual surv iva l o f larvae was approximated to be between 3 0 % and 3 5 % . Growth In the 4 sites where growth was studied, smal l larvae (< 100 m m total length) grew faster than large larvae ( > 100 m m total length) but not s ignif icantly so ( A N C O V A , F i > 2 n = 1.485, p = 0.224). Pool ing;across a l l body sizes, growth was described at a l l sites (Table 2.8). M e a n dai ly larval growth rate throughout the active season was 0.06 m m ( 9 5 % C L : 0.04 - 1.11 m m per day). G r o w t h at the o n l y clearcut site, T a m i h i C - D S , was almost twice as fast as other sites (Figure 2.5) although the trend was not significant. L a r v a l density d i d not influence variat ion i n growth. Discussion: Larval Demography in British Columbia a) L a r v a l Density In this study, mean larval density was 0.88 + 0.09 i n d i v i d u a l s per square meter o f stream. M y study sites were chosen because they had relatively h i g h larval densities and therefore they reflect m a x i m u m densities w i t h i n the C h i l l i w a c k area. 2 0 b) Reproduct ion T w o sites experienced a sharp increase i n young-of-the-year larvae (Promontory 3a & T a m i h i C - D S ) i n the late active season (August - September). U s i n g N u s s b a u m et al . ' s (1983) developmental data as a guide, breeding at these two sites must have been concentrated i n September-October o f 1996 to give rise to a recruitment pulse i n the F a l l o f 1997. A t the other two sites, density increased i n A u g u s t and September but there was no increase i n the frequency of smal l larvae. It is diff icult to interpret this mixture o f results. A t both T a m i h i C - D S and Promontory 3a, the increase o f recruits i n F a l l o f 1997 c o u l d have been the result o f a s ingle c lutch hatching. In both cases, the appearance o f hatchlings was concentrated w i t h i n a 10 m reach of stream. M y results c o u l d thus be expla ined by the existence o f one female at each of the two sites lay ing eggs at approximately the same t ime, and not seasonally restrictive breeding. Direct study of adults at many different streams is needed to* c lari fy seasonal trends i n reproduction. Unfortunately radio-tracking o f 20+ adult D. tenebrosus i n the C h i l l i w a c k region by Johnston (1998) and L . F r i d (pers. c o m m ) have fa i led to y i e l d any information o n reproduction. c) Transformation T h e size structure throughout 1996 (Figure 9a) shows a loss o f large larvae ( > 130 m m T L ) between June and A u g u s t at both the P r o m o n t o r y 3a and P r o m o n t o r y B H site. N o change i n the frequency of large larvae was found at either T a m i h i C - D S or Centre H F . It is possible that larvae i n the latter sites were more prone to neoteny than those at P r o m o n t o r y 3 a and B H . 21 Losses at the Promontory sites were most visible between June and July, suggesting that transformation may peak in the early stages of the active season. d) Survival After correcting the mean survival rate across all sites for transformation loss, monthly survival of larvae was found to be higher in summer than in winter. Harsh climatic conditions over the winter, including snowfall and the freezing of streams, may be responsible for reduced survival during this season. Extrapolated over a year, these mean survival rates suggest that only 30-35% of larvae survive each year. Survival throughout a 2-4 year larval period (as suggested by Duellman & Trueb (1986) for this species) could thus vary from 1-12%. e) Growth Larval D. tenebrosus in my study sites grew between 1.3 mm and 3.2 mm in SVL per month from June through September. I found no significant difference between the growth of larvae < 100 mm TL and > 100 mm TL, and thus estimate and all subsequent values were based on the pooled set of all larvae, regardless of body size. My growth rates are similar to those reported by Haycock (1991) who found that first year larvae in one Chilliwack stream grew between 0.5 mm and 3.2 mm SVL per month (mean = 1.3 mm). The above growth calculations are for the active season only (June - September). As rates likely slow during winter months, theseestimates cannot be extrapolated to predict annual growth. At each site, a few larvae were recaptured in successive summers and their annual growth could be calculated. These individuals were not used in my growth analysis as their inclusion would have violated the assumption of linear regression that every value on the x-axis 22 has a measurable value of y (Zar 1984). A s larvae were only sampled i n summer, t ime (the x - axis) was a continuous variable only throughout the active season and not between years. T e m p o r a r i l y ignoring this statistical concern, I inc luded annual growth information f r o m these larvae calculated a growth rate o f 7.3-10.6 m m S V L a year. A s s u m i n g the same amount o f length is added every year, it c o u l d take 4-6 years for larvae i n m y study areas to g r o w f r o m their S V L when first detectable (= 25 m m ) to their S V L at metamorphic size (70 m m +). Comparison of Demographic Rates with other areas in D . tenebrosus' Range G i v e n the paucity of data f r o m other parts o f D. tenebrosus' range, it is di f f icult to make robust geographic comparison between larval demography i n B r i t i s h C o l u m b i a and demography i n W a s h i n g t o n , Oregon and C a l i f o r n i a where the species is not considered threatened. W h a t is k n o w n about this species and its closest relative, Dicamptodon ensatus, is presented i n Table 2.9. A few general geographic trends i n demography are evident, although i n a l l instances these patterns require more replication to be conf i rmed. M e a n density o f larvae i n forested streams i n Oregon was 2.3 larvae per square meter ( C o r n & B u r y 1989), almost three times the m a x i m u m density recorded i n this study. T h e difference i n larva l density between W a s h i n g t o n and B r i t i s h C o l u m b i a is not nearly so pronounced. T h e mean density o f larvae reported i n this study exceeded that f rom Washington. H o w e v e r the data f r o m W a s h i n g t o n was based on a random sampling o f sites whereas data i n this study was d r a w n f r o m streams k n o w n to have reasonably high densities of larvae. A s such it should not be c o n c l u d e d that abundance is generally higher i n B r i t i s h C o l u m b i a , but rather that these areas l i k e l y d o not differ greatly i n larval density. N u s s b a u m & Clothier (1973) estimated annual surv iva l o f first year D. tenebrosus larvae i n one Oregon stream to be 4 3 % , s l ight ly greater than the 3 0 - 3 5 % estimated i n this study. 23 A n n u a l surv iva l does not appear to vary m u c h between these regions, h o w e v e r the length o f the larval p e r i o d does. A c c o r d i n g to m y analysis, larvae i n m y four study streams c o u l d take 4-6 years to reach metamorphic size (130 m m T L +). L a r v a e i n two O r e g o n streams were estimated to grow 2-3 times faster than larvae i n m y study, and are bel ieved to have a larva l per iod o f o n l y two years (Nussbaum & Clothier 1973). E v e n i f annual survival was the same i n Oregon and B r i t i s h C o l u m b i a , net survival through the larval per iod w i l l be lower i n B r i t i s h C o l u m b i a . F o r example i f annual survival was 4 0 % i n both regions, surv iva l throughout the entire larval p e r i o d w o u l d be 16% i n Oregon (2 year larval period), and only 0 . 5 - 3 % i n B r i t i s h C o l u m b i a (4-6 year larval period). T h i s difference in net larval survival may help expla in w h y densities o f D. tenebrosus are lower in B r i t i s h C o l u m b i a than in the centre o f its range. H o w e v e r , many m o r e populations i n both B r i t i s h C o l u m b i a and Oregon need to be studied before any geographic trends i n survival can be confirmed. The Impact of Logging on Life History Parameters T h e l o w number o f sites used i n this study makes it di f f icult to examine the influence o f logging on D. tenebrosus. A l t h o u g h m y sites differed i n logg ing history f r o m recently clearcut (< 5 years) to mature second growth (+ 6 0 years), there was almost no repl icat ion o f part icular forest age classes. A s such, I cannot ascertain whether variat ion i n demographic rates is due to logging effects or random site variation. H o w e v e r even w i t h a s m a l l number o f sites, it is useful to examine i f the more recently logged sites display distinct demographic properties. A c r o s s m y f ive study streams, o n l y larval growth appeared to be associated wi th forest practices (Table 2.10). A l t h o u g h not statistically s ignif icant, larva l growth rate at the clearcut site was almost twice as fast as i n any o f the c losed canopy sites. T h i s observation is c o m m o n i n 24 fisheries research, where growth is frequently found to increase i n clearcut streams (Hartman & Scrivener 1990). M y results suggest this phenomenon also occurs i n D. tenebrosus larvae. I f both larvae and adults o f D. tenebrosus have greater fitness i n clearcut streams due to increased growth however, it is unclear w h y these areas are sometimes found to have the lowest densities o f larvae (Corn & B u r y 1989, W e l s h 1991, C o l e et a l . 1997). Further research i n recently logged and unharvested areas is needed to determine whether growth enhancement is a constant feature o f larvae i n streams draining clearcuts. It is possible that this phenomenon occurs only under certain altitude, product iv i ty and c l imat ic condit ions. R e g i o n a l differences i n these variables may explain why studies o f D. tenebrosus throughout its range have found var ied associations between logging history and larval density ( M u r p h y et a l . 1981, M u r p h y & H a l l 1981, B u r y 1983, H a w k i n s et a l . 1983, B u r y & C o r n 1988, C o n n o r et a l . 1988, C o r n & B u r y 1989, K e l s e y 1995, C o l e e t a l . 1997). Biotic Regulation o / D i c a m p t o d o n tenebrosus Larvae Populat ion density was not correlated w i t h i n d i v i d u a l growth or surv iva l across m y four study sites. G i v e n that D. tenebrosus are aggressive and cannibal ist ic , the lack o f a relation between density and survival is surprising, especial ly as density-dependent surv iva l has been . found i n other stream d w e l l i n g salamanders (Shoop 1974, Petranka & S i h 1986). It is possible that cooler c l imatic conditions experienced by larvae l i v i n g i n B r i t i s h C o l u m b i a l i m i t populations f r o m attaining densities at w h i c h resources become scarce and competi t ion/cannibal ism occur. Conclusions: 25 The mode of larval regulation in my study areas still remains uncertain. There is weak evidence that logging may influence larval growth but not density or annual survival. On the basis of my investigation, I propose that D. tenebrosus larvae living at the northern extent of their range in British Columbia are limited by regional climatic conditions. This is supported by reduced growth, density and survival (as a consequence of a longer larval period) at my sites in comparison to those from Oregon, the centre of the species' range. B y elevating stream temperature, logging may enhance larval growth rates. Studies of more clearcut areas are required to confirm if growth enhancement is a universal feature of these habitats, and i f so, what the long term implications of this phenomenon are on population processes. 26 c« S CM W o J: G u eu - o sa 5 I e £ CB -M H •M EC 2 o o Mil T3 C o o CD to bJQ| C 3 O a, X) C O o CD c/2 T J O >-'. W)| T3 C o o CD M Cfl V >• M « JS cu CJ C cu E H CN 53 CD CD U i u u fc 03 C O $ c o s o d a o 6 o CL, © '1 s 155 I K CM r - O N O N VO O N O N h ON ,2 a X> 5T oo oo r- CN CN .3 *2 0) e CM or oo K a e U O N VO O N O N O N •4-* 3 bO 3 < c 3 o U Pi >~. Oi 1——I O fc 00 CN VO ON ON r- ON ON ON X> ON 2 o <1> •=> 00 , <u c 3 o CN 5T oo CN u, ti X> e o> • 5T oo c3 C O O •*-» c o 00 CN ON ON VO ON ON ON ON g- ON C CD -t—» ti 00 CD in 5P CN oo CD i • CN C CN c CD M CD 2 00 PQ >-> i_ o +-> c o B o l-l co CN VO ON ON CN r - CD ON ON ON CN X) e . . & „ ti C O oo CD 3 w 7 « i CN © s i 8 1 00 Q I U oo Ov OS Ov Ov u |co C S 3 Ml c u , a so o\ Ov h o •*-» o. 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Ov « —' 'lo "H3 T J co CO co *e « Sv W fi o =y « - H T J CO C co rt M rt to T J W O 35 CO CU ca cn I o Ul CM C U u E ca H pa o PH ca u CM PQ o u CM '•a tf ca PQ o , *->' Cu ca c U C/l CU U E ca H c CD U E ca H s .S3 ca 51 ' f c 3 co | u X! C CD p-J "ca •*-» o H c ca CD CD X! -u» O u o 'ca Q ca "3 E c X PQ o CO C IQ l a fc , ca M £- E o ca PU H c CD u PQ cn o o u u CM CM CO CD ca CD O e co CD CD CD -C o -5. CD E 3 X> co ca CU U ' Centre HF • • • Spring 1996 Spring 1997 Promontory BH 100 • 50 Spring 1996 Spring 1997 250 • 200 • 150 • 100 • 50 • 0 • Spring 1996 Promontory 3a Spring 1997 250 • 200 • 150 • 100 • 50 • 0 • Tamihi C-DS Spring 1996 Spring 1997 Figure 2.1: Est imated abundance o f D. tenebrosus larvae at four study sites i n 1996 and 1997. B a r s represent one 5 standard error. F o l e y R SO 100 120 140 T o t a l L e n g t h (m m ) 160 1 8 0 0 .3 0 .2 + 0.1 0 T a m ih i C - D S B l 1 n | Pro | mi | real f feeji | 60 8 0 100 120 140 160 180 T o t a l L e n g t h (m m ) 0 .4 ti g 0.3 3 I it 0 2 C e n tre H F 80 100 1 2 0 140 T o t a l L e n g t h ( m m ) 16 0 18 0 P r o m o n to ry B H 80 100 120 140 160 180 T o t a l L e n g t h (m m ) P r o m o n t o r y 3a Centre H F June 1996 0.5 0.4 0.3 - - 0.2 - - 0.1 0 -I | | | | | "a | ™ | | | m | m | m \ m \ | O O O O O O O + •3- co oo o CMTJ- co o T - T — T — T — 00 0.5 0.4 - - 0.3 - - 0.2 - 0.1 | M | — | B July 1996 o , o CO T - T - 00 o o o o . o  , o + CO CO O O J CO o August 1996 September 1 996 0.5 T 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 Figure 2.3a: Seasonal changes i n size structure o f larvae at the Centre H F site. The x-axis represents total body length ( m m ) and the y-ax is is proport ion i n sampl Promontory 3a June 1 996 July 1996 0.5 -r- 0.4 - - 0.3 - o o o o o o o + CO CO O CM CD O 0.5 August 1 996 H H o co + o oo September 1 996 H h o co + o oo Figure 2.3b: Seasonal changes in size structure of larvae at the Promontory 3a site. Axes are the same as in Figure 2.3a. 40 Promontory B H June 1 9 9 6 Ju ly 1 9 9 6 A u g u s t 1 9 9 6 0 .5 +—H H o CM H 1 h o o CO + o oo S e p t e m b e r 1 9 9 6 Figure 2.3c: Seasonal changes in size structure of larvae at the Promontory B H site. Axes are the same as in Figure 2.3a. Tamihi C-DS July 1996 0.5 0.4 0.3 0.2 0.1 0 o H — I — H o CO 0.4 0.3 0.2 0.1 0 o r̂ 0.5 0.4 0.3 + 0.2 0.1 + 0 o H 1—H o CD + o CO August 1996 H — I — H o CD ^ — i — h o CO + o oo September 1996 H—I—I—h o CD O CO + o CO Figure 2.3d: Seasonal changes in size structure of larvae at the Tamihi C-DS site. Axes are the same as in Figure 2.3a. CO CD CD C 3 10 CO CM i - O d d d d d J - s" O) T — fgi \_ 02 CD n E_ E , ™ . 0) Si Q . n i ' CD CO — i — i — i — i — 091 oei. Ofr LO CO CM T-_ o d d d d d CD o CD c 3 091- 021. LO •>3- CO CM i - O • d d d d d LO -o- CO CM . i - o d o d o d * ffl N i - . O d d d d LO ̂  n CM t - o d d d d d cq CM t - ; o d d d d H—I—I—h 10 cq ou o d d d d d c .u 3 .o PH co O _ O ' X2 4-* o u- OO 2 o 3 ^ CD „ § M < CD •xs O 2? w y <D E—1 ca -cs 5a I i cj to >S oo x! -a £> £ Ci „̂ •—i c •s ^ > = *n CD S CD co JO CO CD C CD CD X) >H .<D C ca O .5 ,>» its CD C bfl O . ca x> X! DH c oo ca CD co ti CD CJ a PH ca ui CD CO CD i n H cu g fa E 3 ca •8f> S to tS T 3 T J X! O 00 § d "3 11 co m CD m -C <-s Chapter 3: Determinants of Dispersal in Pacific Giant Salamander Larvae Introduction: T h e pr incipal a i m of this study was to examine the influence o f abiotic and biotic factors on the dispersal and movement o f D. tenebrosus larvae. A s dispersal is a key means by w h i c h populations can re-establish i n sites o f local extinctions, a knowledge o f the environmental and demographic factors that enhance movement may be useful for management. I studied movement by D. tenebrosus larvae at two different spatial scales i n four streams i n the C h i l l i w a c k V a l l e y . A t each scale I examined the relationships between larval dispersal, habitat, and populat ion density. T h i s two-tiered approach was taken to determine whether micro-habitat features (< 10 m) or the general state o f the stream (mean condi t ion i n 120 m reach) were a better predictor o f larval movements. In addit ion to addressing spatial var iat ion, I also tested for temporal shifts i n m o b i l i t y i n response to seasonal changes i n stream temperature, water v o l u m e and abundance o f p o o l habitat. S o m e scientists have argued that habitat disturbance, speci f ical ly b y logg ing , is particularly detrimental to Paci f ic Giant Salamanders as they are adapted to the historical ly stable, temperate rainforests o f the Pac i f i c Northwest ( W e l s h 1991). H o w e v e r very little is k n o w n about D. tenebrosus' abi l i ty to survive through disturbance and/or disperse i n response to local ly adverse conditions. In this study, I describe the median values and general range o f distances D. tenebrosus larvae are capable o f m o v i n g over two summers and examine whether dispersal is elevated in habitats w i t h physica l attributes s imi lar to logged streams (i.e. open canopy, h igh silt) . T h i s information w i l l demonstrate the capacity o f larvae to respond to disturbance by dispersal. 45 Components of larval habitat and their potential impacts on dispersal A l m o s t nothing is k n o w n about the stream attributes that influence dispersal by D. tenebrosus and facilitate its re-entry into streams after loca l ext irpat ion. In this study, I examined 13 different environmental factors that might influence larva l dispersal (Table 3.2). These variables are frequently referred to i n the literature as important components o f stream- d w e l l i n g amphibian and fish habitat (Southerland 1986, T u m l i n s o n et a l . 1990, W a l l s et a l . 1992, M u r p h y 1995, W e l s h & L i n d 1996, Slaney & M a r t i n 1997). T h e variables f a l l into f ive categories: 1) H y d r o l o g y (stream depth, w i d t h , v o l u m e and percent p o o l ) , 2) G e o m o r p h o l o g y (slope and substrate composit ion) , 3) C l i m a t e (mean air and water temperature), 4) Disturbance history (time since harvest and percent canopy coverage) and 5) F o o d avai labi l i ty (average macrobenthos abundance). Possible effects o f these variable p h larval ecology and dispersal are discussed below. 1) Hydrology L a r v a l Pac i f i c G i a n t Salamanders are predominantly f o u n d i n pools ( H a y c o c k 1991). It is unclear whether higher abundance i n pools is due to lower mortal i ty , increased i m m i g r a t i o n , or adult preference for ov ipos i t ion i n these areas. Stream depth and w i d t h are also good predictors o f larval salamander abundance, wi th abundance frequently decreasing wi th increasing wetted width (Richardson & N e i l l 1995) and increased stream depth (Southerland 1986, T u m l i n s o n 1990). B y f o l l o w i n g movement into and out o f reaches o f different w i d t h , depth and p o o l composi t ion, I tested whether dispersal c o u l d account for abundance patterns associated w i t h these variables. 46 2) Geomorphology The abundance of stream dwelling salamanders is often correlated with substrate type (Tumlinson 1990, Welsh & Lind 1996). In Western Washington and Oregon, the abundance of D. tenebrosus was positively correlated with the number of substrate crevices and cover objects available (Hall et al. 1978, Murphy & Hall 1981, Connor et al. 1988). To maintain their position against a current, D. tenebrosus larvae must be able to grip the substrate. Gravel and pebble substrates are easier for larval salamanders to grip onto than fine sediment (Holomuzki 1991). Thus alteration of stream sediment size may change displacement rates. In the field I tracked movement rates of larvae on a variety of different substrate types. If displacement increases on silty substrates, influxes of fine sediment into streams after logging (Murphy 1995) may trigger a net loss of larvae from these reaches. 3) Climate Larval activity is often reduced at high temperatures (Maurer & Sih 1996). In Chilliwack, D. tenebrosus larvae become sluggish and easy to catch at stream temperatures > 20 C (W. Nei l l , pers. comm.). In this study, I compared movements by larvae at four sites differing in mean air and water temperature. If increasing temperature reduces movements, warming of streams may significantly lower dispersal between stream reaches and connected tributaries. Summer water temperatures in streams draining clear cuts in coastal Oregon were up to 1 0 C higher than in those under a closed canopy (Beschta et al. 1987). Even in the absence of other habitat change, increased temperature in logged streams could limit larval dispersal. 47 4) Food avaUabUity The abundance of aquatic invertebrates in streams often increases for a few years after logging. Clearcut sites in Oregon had an average of 1.5 to 2.3 times as many benthic invertebrates in June and August as those in forested sites (Murphy et al., 1983). Such increases may reduce the need to make extended foraging trips in recently logged streams. I thus predicted larval salamanders should move more frequently and perhaps over greater distances in sites with low aquatic invertebrate abundance. Density Dependent Determinants of Movement and Dispersal In my second analysis, I investigated whether the density of resident larvae influences the number of dispersers an area produces or absorbs. If it does, the reduction or removal of one high density population could alter the flow of individuals to or from surrounding areas. As D. tenebrosus larvae interact aggressively and prey on smaller conspecifics (Nussbaum et al. 1983, Connor et al. 1988, Mallory 1996), I predicted that greater mortality and/or emigration should occur from high density reaches, and that movement frequency should increase within high density areas. These predictions are based on the assumption that neighbour-neighbour physical contacts increase with density, and should trigger agonistic displacements to other areas of the stream. I tested these predictions at two scales. The first prediction was tested by studying dispersal in and out of a series of 10 m reaches with differing densities, and the second by monitoring the numbers and lengths of movements made within four 120 m reaches of different density. 48 Body size and dispersal A final factor that may influence larval m o b i l i t y is body size. B o d y size determines h o w easily a larva can be displaced either by the stream current or other conspeci f ics (Bruce 1986, M a l l o r y 1996). In a stream mesocosm experiment, larval body size was the most important determinant o f displacement probabil ity ( M a l l o r y 1996). S m a l l larvae were routinely displaced or eaten by larger larvae i n M a l l o r y ' s experiment. Consequently I predicted that movement w o u l d decrease with increasing body size. Methods: Abiotic Determinants of Dispersal I) Habitat and Movement: 120 m Reach Scale L a r v a l movement and habitat associations were studied at four streams differing i n logging history i n the summer of 1996 and 1997 (Table 3.1). A t each stream a 120 m reach was chosen and thirteen measures o f stream habitat were col lected (Table 3.2). Af ter appropriate statistical transformation, the mean value o f each variable was c o m p u t e d for each site. O n e way analysis o f variance was then e m p l o y e d to test for between-site differences i n habitat. A l l percentages (i.e. % p o o l habitat, canopy cover and substrate c o m p o s i t i o n ) were arcsine square root transformed and al l counts (i.e. number o f benthic invertebrates i n one sample) were square root transformed to better approximate a n o r m a l distr ibution before analysis. Mark-recapture censuses were conducted w e e k l y at each site f r o m June to October 1996, and i n June and September 1997 to study larval movement (see Chapter 2). A l l larvae were uniquely marked, either by toe c l i p p i n g or a P .I .T . tag. O n each vis i t , the locat ion o f every larva was recorded to the nearest 0.5 m . L a r v a l dispersal w i t h i n each 120 m reach was 49 characterised by two variables: 1) the proportion of m o v i n g and stationary larvae and 2) the cumulative length of movements. A movement was considered to be any displacement > 0.5 m f r o m the capture point. Information on altitude and t ime since harvest was taken f r o m forest cover maps o f the C h i l l i w a c k A r e a (1: 250000). A l l other variables were measured i n the f ie ld . B e n t h i c invertebrate abundance was assessed f r o m approximately twelve samples col lected at each site i n J u l y and August 1996. Samples were col lected i n riffles every 2 0 to 30 m a long the study reach using a 30 c m x 30 c m Surber sampler (250 u.m mesh). T h e substrate w i t h i n the 9 0 0 c m 2 quadrat was v igorously raked for one minute. A l l material drift ing up f r o m the substrate was captured i n the drift net and stored i n 3 3 % ethanol. In the lab, this material was sorted through a 1 m m sieve and counted under a dissecting microscope. T h e percent o f p o o l habitat i n each 10 m section o f the 120 m study reach was estimated visual ly on four occasions in the F a l l o f 1996 (August-September) and on s ix occasions i n the Spr ing o f 1997 (May-June) . P o o l s were identi f ied as areas o f stream approximately 9 0 0 c m 2 or greater i n area o f st i l l water. T h e percent p o o l habitat i n the entire 120 m reach was calculated by averaging a l l estimates f r o m the 12 consecutive 10 m estimates. These means were averaged over al l sampl ing days to give a grand mean for the percent o f p o o l habitat i n each site over the course o f the study. T h e percent o f canopy closure w i t h i n each of 12 consecutive 10 m sections was estimated using a hand he ld densiometer w i t h a 10 x 10 gr id . A s canopy coverage d i d not vary m u c h throughout the study, this variable was measured o n l y once. T h e slope f r o m the start o f each 10 m reach to the end was estimated us ing a c l inometer . These twelve estimates were averaged to give the mean slope o f each stream reach. 50 O n the first day of habitat sampling, the widest point i n each 10 m sampling reach was determined and marked with a wooden stake. W e e k l y measurements o f wetted w i d t h , m a x i m u m and mean depth were taken at these points (one per 10 m reach) i n August-September 1996 and June 1997. M e a n depth was based on the average o f six equally spaced measurements o f depth taken along a transect perpendicular to the stream bank. W e t t e d w i d t h and depth measurements were c o m b i n e d to estimate the volume of water i n each 10 m section us ing the f o l l o w i n g equation: Vol. of Water in 10m Reach (m) = (Wetted Width at Point) x (Mean Depth at Point) x 10 F o r each sampling day, the total water o f v o l u m e i n the study site was calculated as the s u m of volumes f r o m al l twelve 10 m reaches. Substrate composi t ion at each site was described i n four randomly chosen 10 m reaches. Stream substrate was classified into f ive different categories on the basis o f particle size (Table 3.3). F i n a l l y , air and water temperatures were measured at each 120 m reach on every s a m p l i n g occasion. JJ) Habitat and L o c a l Dispersal : 10 m R e a c h Scale F o u r non-contiguous 10 m segments were r a n d o m l y chosen f r o m each 120 m reach. Seven environmental variables that varied between segments were measured i n each 10 m zone: percent coarse substrate (substrate > 64 m m across the longest longi tudinal axis), percent silt substrate, percent canopy coverage, percent p o o l habitat, slope, m a x i m u m wetted w i d t h and m a x i m u m stream depth. Measurements were taken at each reach on several occasions 51 throughout the summer of 1996 and 1997. All percentages were arcsine square root transformed for analysis. I used analysis of covariance to determine if there was a relationship between each habitat variable and the number of larvae moving into or out of a reach (log(x + 1) transformed) and whether this relationship varied between sites. Seasonality and Movement: Variation in Time Water volume, percent pool and temperature (air and water) show strong seasonal fluctuations in streams. Mean values of these variables were computed for August-September 1996 (late active season) and May-June 1997 (early active season). I examined whether seasonal changes in these variables were mirrored by corresponding differences in larval movement. Biotic Determinants of Dispersal I) Larval Density and Movement: 120 m Reach Scale In this analysis I examined the association of movement frequency, movement length to larval density within the four 120 m study reaches. Mark-recapture data collected in 1996 and 1997 were used to estimate the larval density at each of the four study streams. I calculated the mean density of larvae at each site throughout two summers of study (see chapter 2 for estimation methods). I assessed whether the frequency and median length of movements made at each site increased with larval density. 52 II) L a r v a l D e n s i t y and L o c a l Dispersal : 10 m Reach Scale F o u r non-contiguous 10 m reaches were randomly chosen f r o m each o f the four study sites. F r o m repeated mark-recapture sampling, the total number o f resident animals l i v i n g i n each reach f r o m June 1996 to September 1997 was recorded. A resident was defined as an animal that was only ever caught w i t h i n the 10 m reach. O v e r this same per iod, the number o f immigrants and emigrants into each zone was recorded. A n immigrant was a larva in i t ia l ly caught outside the 10 m focal reach that subsequently dispersed into it. A n emigrant was a larva ini t ia l ly found w i t h i n the 10 m reach that dispersed out. A per capita i m m i g r a t i o n rate for each 10 m reach was calculated for the 13 month period f r o m June 1996 t i l l September 1997 by d i v i d i n g the number o f larvae that m o v e d into the zone by the size o f the resident populat ion. A per capita emigrat ion rate was s imi lar ly calculated by d i v i d i n g the number o f larvae that left each reach by the number o f residents. I used analysis o f covariance to test i f loca l i m m i g r a t i o n and emigrat ion rates depended on resident density, .and whether these relationships varied between sites. A s imi lar analysis was used to determine i f the biomass o f resident larvae w i t h i n a 10 m reach was related to local i m m i g r a t i o n and emigrat ion. Body size and dispersal F i n a l l y I tested the hypothesis that larval dispersal is negatively related to body size. I used a linear regression to relate larval body size and distance travel led by larvae w i t h i n a 120 m study reach at a l l sites. A t the 10 m scale, I used a chi-squared test to compare the proportions o f large (> 100 m m total length) vs. s m a l l larvae (< 100 m m total length) that dispersed. 53 Results: Abiotic Determinants of Dispersal I) Habitat and M o v e m e n t : 120 m R e a c h Scale L a r v a e at 3 o f the 4 study sites were h ighly sedentary. L a r v a e at Centre H F had the highest probabil i ty of m o v i n g (Table 3.4). N i n e t y three percent o f larvae m o v e d at this site i n comparison to 7 1 - 8 0 % at the other streams. T h e median distance m o v e d by Centre H F larvae, 8 m , was also greatest (Table 3.5), f o l l o w e d by T a m i h i C - D S , P r o m o n t o r y B H and P r o m o n t o r y 3a. The distribution o f distances travelled b y larvae at Centre H F was s ignif icantly different f r o m the two Promontory sites ( K o l m o g o r o v - S m i r n o v test, p < 0.01), but not f r o m the T a m i h i C - D S site ( K o l m o g o r o v - S m i r n o v test, p = 0.34). F o u r habitat variables dist inguished Centre H F : percent pebble, percent gravel , water volume and wetted w i d t h (Table 3.6). Centre H F had less gravel and pebble than any o f the : other three sites. Substrate at this site was m a i n l y composed o f sand/silt (33%) and large boulders (37.2%). Centre H F was also the narrowest stream w i t h a mean m a x i m u m wetted width o f just over 1 m (Table 3.6). Water depth was sl ightly but not s igni f icant ly l o w e r at this site. T h e total water v o l u m e contained i n the 120 m reach was also s igni f icant ly l o w e r at Centre H F , l i k e l y as a result o f its narrow mean wetted w i d t h . II) Habitat and L o c a l Dispersa l : 10 m R e a c h Scale N o n e o f the 7 measured microhabitat variables were, related to larval dispersal i n 10 m stream reaches (Tables 3.7a, b). In every case, there was a signif icant interaction between the slope o f movement-habitat relationship and the site at w h i c h data were col lected. A l t h o u g h 54 reach scale attributes may influence larval dispersal, the nature o f this relationship l i k e l y varies between sites and no general predict ion can be made f r o m knowledge o f the selected habitat variables alone. Seasonality in Dispersal Stream hydrology and temperature var ied significantly between the late active season i n 1996 and the early active season i n 1997. A t three sites, water v o l u m e i n the early season was double or more o f that in late season (Figure 3.1) and both wetted w i d t h and depth decreased (Figures 3.2 a & b). The amount o f p o o l habitat also increased throughout the active season (Figure 3.3). W i t h the exception of T a m i h i C - D S , air and water temperature changes between sampl ing periods in the late season 1996 and early season 1997 were modest (Figures 3.4 a & b). T h e mean difference in water temperature between these two periods d i d not exceed 3 C at any site. M e a n air temperature at T a m i h i C - D S fe l l by 8.3 C between s a m p l i n g periods i n the late season o f 1996 and early season o f 1997. Despite large hydrolog ica l changes and moderate temperature changes, there were no differences in larval movement between early and late season. T h e distribution o f distances travelled by larvae i n the early summer was s imi lar to that in late summer ( K o l m o g o r o v - S m i r n o v test, p = 0.985) (Figure 3.5), and the frequencies o f movements were nearly identical between the two periods (Figure 3.6). Peak flow i n small headwater streams o f the C h i l l i w a c k va l ley usual ly occurs w i t h snow melt i n A p r i l or M a y . Capture eff iciency is very l o w at these times because o f c o o l water temperatures ( < 5 C ) and poor v i s i b i l i t y o f larvae i n fast f l o w i n g currents. L a r v a l dispersal 55 c o u l d increase during this time i n response to f l o w , but this possibi l i ty was not tested i n this study. If larval dispersal fluctuates seasonally, it does so outside o f the June - September active season. Biotic Determinants of Dispersal I) L a r v a l Densi ty and M o v e m e n t M e a n larval density varied significantly over the four study sites (Figure 3.7) but this variat ion was not related to either measurement o f movement. L a r v a l densities at P r o m o n t o r y B H and Centre H F were significantly higher than the two other sites. Despite this s imi lar i ty i n larval density, these two streams displayed very different movement patterns. A l m o s t a l l larvae at Centre H F m o v e d at least once and when they d i d , ha l f travel led at least 8 m . In contrast more than a quarter o f the larvae at Promontory B H fa i led to m o v e and those that d i d general ly stayed w i t h i n 2-3 m o f their or ig inal point o f capture. A c r o s s these four streams, there is no evidence that D. tenebrosus' density influenced movement. II) L a r v a l Densi ty and L o c a l Dispersa l T h e number o f resident larvae i n a 10 m reach d i d not s ignif icantly affect loca l immigrat ion ( F U 1 = 1.683, p = 0.101, r 2 = 0 . 1 8 0 ) o r emigrat ion ( F U i = 1.576, p = 0 .235 , r 2 = 0.153). There were no were no interactions between site and loca l i m m i g r a t i o n or emigrat ion ( A N C O V A , immigrat ion site effects: F 3 i n = 0.329, p = 0 .804; emigrat ion site effects: F 3 i U = 0.199, p = 0.895). There was almost an identical number o f immigrants and emigrants i n each reach (Figure 3.8). T h i s correlation suggests that the tendencies to immigrate and emigrate at the 10 m scale are not independent. L a r v a e that i m m i g r a t e d into a reach were more l i k e l y to 56 leave it after a few months than those that were established i n the area at the beginning o f the experiment. F r o m a total o f 41 larval immigrants, 14 later emigrated ( 3 4 % o f total) whereas only 15 % of larvae i n each reach at the start of the study later emigrated. T h e total biomass of resident larvae had no effect on immigrat ion ( A N C O V A F i , n = 1.878, p = 0.198, r 2 = 0.0217) or emigrat ion rates ( A N C O V A F U i = 1.381, p = 0.265, r 2 = 0.155) and there were no significant site effects ( immigrat ion site effects F 3 , n = 606, p = 0.625; emigration site effects F 3 , ] i = 0.350, p = 0.790). A s i m m i g r a t i o n and emigrat ion rate were not related to either the density or biomass o f residents i n 10 m reaches o f stream, it seems u n l i k e l y that biotic interactions have a strong influence on l o c a l dispersal o f larvae. Body Size and Dispersal L a r v a l body size was not strongly correlated to dispersal distance w i t h i n a 120 m reach o f s treanr .Body size was posit ively, but not signif icantly, correlated wi th cumulat ive distance travelled by larvae (Figure 3.9). A t the 10 m scale, the proport ion o f large larvae (> 100 m m total length) dispersing was significantly greater than for s m a l l larvae ( x 2 = 4 .831 , p < 0.05, 1 df). Contrary to m y predict ion, smal l larvae were s l ight ly m o r e sedentary than their larger conspecifics. A s s u m i n g that size is the most important predictor o f larval displacement, these results suggest dispersal was not due to the involuntary displacement of s m a l l larvae by larger indiv iduals or a strong current. 57 Discussion Abiotic Determinants of Movement A c r o s s m y 3 o f m y 4 study sites, the rates and lengths o f larval movement were s imi lar . O n l y one site exhibited different movement behaviour, Centre H F , where larvae tended to m o v e more frequently and further than at the other three streams. Centre H F differed i n substrate and wetted w i d t h f rom the other sites, but w i t h such little variation i n movement amongst streams there is no way of correlating these habitat differences to variat ion i n dispersal . In fact, the lack of association between these variables and movement at the 10 m scale suggests they have no effect on movement. A t the 10 m reach scale, none o f the 7 measured habitat variables was associated w i t h larval mobi l i ty i n D. tenebrosus. T h i s pattern suggests that the pos i t ive associat ion between larval density and p o o l habitat, decreasing wetted w i d t h and some substrate classes is not created .by dispersal into preferred areas. I f larvae are found at higher densities i n pools or narrow reaches, it is because either adults selectively oviposit and/or larvae survive better i n these areas. If shifts in the habitat variablesT studied affect larval demography, they do so by changing survival and not dispersal. T h e lack o f association between larval movement and a l l other measured habitat variables c o u l d also be a function o f inappropriate measurement scale. P r i o r to this study, little was k n o w n about larval dispersal by D. tenebrosus. F i e l d study o f other stream d w e l l i n g larval salamanders found that they can m o v e up to 10 m in one day ( H o l o m u z k i 1991). I thus chose to partit ion and describe habitat i n 10 m units, assuming that larvae were capable o f m o v i n g between reaches of this length in response to l o c a l condit ions. H o w e v e r , most larvae m o v e d less than 5 m oyer a season. Consequently, larvae may be capable o f selecting o n l y amongst habitats w i t h i n a few 58 meters o f their or ig in. Therefore I can only conclude that pool habitat, water depth and v o l u m e do not expla in movement between r e a c h e s o f 10 m or greater. Density Dependent Determinants of Dispersal L a r v a l density had no influence on movement by D. tenebrosus larvae at 10 m and 120 m reach scales. This observation contradicts m y or ig inal predict ion o f density-dependent regulation as a result o f intraspecific aggression and cannibal ism. G i v e n the host i l i ty that characterises most larval interactions i n the laboratory ( M a l l o r y 1996), it is surpris ing that density had no impact on movement. It is possible however that m y study was conducted o n too large a scale to detect local effects o f density. M a l l o r y ' s (1996) study o f larval interactions was conducted in pools and riffles a few metres i n length. Results observed at the 1-5 m scale m a y not explain movement over larger areas. Al ternat ive ly the cannibal ism and antagonism noted b y M a l l o r y may not reflect interactions i n natural settings. It is also possible that density may b e l more important i n more southerly parts o f the range where larval densities and biomass are higher than i n B r i t i s h C o l u m b i a ( M u r p h y & H a l l 1981, K e l s e y 1995). Body size and dispersal T h e fact that one third o f a l l immigrants into 10 m reaches later became emigrants suggests that a sub-section o f larvae are more m o b i l e than the rest o f the populat ion . I f this is so, one might ask what differentiates a disperser f r o m a resident. I in i t ia l ly expected smaller individuals to be more vulnerable to displacement by the stream current or other larvae (Bruce 1986, M a l l o r y 1996). Contrary to this expectation, I found that large larvae were s l ight ly more mobi le than smaller individuals , perhaps because large larvae face lower risks when travel l ing. 59 C a n n i b a l i s m risk decreases wi th body size and large larvae may be less l i k e l y to be attacked whi le m o v i n g than their smaller conspecifics. These results suggest that movements throughout the stream are not forced by dominant conspecifics. Conclusions M y main conclusion is that D. tenebrosus larvae exhibit h igh site f idelity and extremely l i m i t e d dispersal. M o s t larvae fai led to m o v e more than 5 m over 13 months. O f those that d i d move , ninety percent stayed w i t h i n 2 0 meters o f their or ig inal capture point. A l t h o u g h variat ion existed between sites, movement was generally conservative i n t ime and space. T h e correlation between i m m i g r a t i o n and emigration at the 10 m scale suggests that although most larvae are sedentary, a smal l number o f transient animals travel frequently throughout the stream. It is unclear w h y these individuals are transient. A s size was a weak predictor o f movement length, this behaviour cannot be ascribed to a part icular age group or to dominance interactions. Larvae seem i l l -equipped to disperse i n response to habitat changes. A n y l o c a l and lethal impact that c o u l d not be avoided by a movement o f less than 2 0 m w o u l d l i k e l y k i l l 9 0 % o f a l l larvae. It should be cautioned, however, that is c o n c l u s i o n is based on the observation o f larvae w i t h i n relatively stable environments. Other than m y mark-recapture surveys and seasonal shifts i n c l imate and stream f l o w , there were no disturbances or drastic habitat changes w i t h i n each stream during this study. It is possible that larval dispersal is elevated i n more rapid ly changing or highly disturbed environments than used i n this study. P o o r larval dispersal abi l i ty is not unique to D. tenebrosus. Other species o f stream- dwel l ing salamanders exhibit s imilar behaviour. Desmognathus fuscus, Desmognathus ochrophaeus and Ambystoma barbouri have s m a l l h o m e ranges o f 1.44, 1, and 1 m 2 60 respectively (Ashton 1975, H o l o m u z k i 1982, 1991). W h i l e larvae o f D. tenebrosus certainly have l i m i t e d dispersal capabilities, this trait does not dist inguish the species. L o w dispersal of larvae does not necessarily make this species vulnerable to ext inct ion. Recent evidence suggests that adults are not s imi lar ly l i m i t e d i n movement. Seasonal dispersal distances > 100 m were recorded i n some radio-tracked adults i n the C h i l l i w a c k drainage (Johnston 1998). H o w e v e r disturbances such as l o g g i n g may put adults at greater r isk than larvae by increasing their probabil i ty o f desiccation w h i l e on land (Blaustein et a l . 1994). If adult mortality is h igh, the site f idelity of larvae may hasten loca l ext inct ion. U n t i l the exact demographic effects o f logging on both adults and larvae are k n o w n , the consequences o f l o w larval m o b i l i t y on population persistence are u n k n o w n . H o w e v e r this study suggests that logging-associated habitat changes such as increased silt , temperature and riffle habitat do not trigger loca l emigrat ion. 61 3 o l-l O C o o co 00 cn 3 O rt co .Si -4—» c o 6 o t/3 o co c o OH o p- 3 .52 t— lc ON — OS c —« rt T3 O & «~ S2 co § e fa <D OJ rt u. 0> to <*> CO T3 C > rt ca jo rt c co 3 3 >-> £ - s C c rt vo" Ov Ov 3 c O rt <*-< l_ rt 40 o x> c 2 .2 ^ o o o rt m •a e c 3 rt !—> E o rt z rt, .. 00 ^ rt H I o o 3 03 X) co «i CU | 2 V* CO cu rs £3 CD 1/3 X i 3 co •3 C = ".£ .S O u. u o CU o & t £ OS +-» C/3 x> 3 CO cu r̂ : C M CO t £ c£ CO CU O. O E 03 CO E .S 03 <U c/3 3 « > O u O CU CM t5 o l-P O E ID "53 > < TJ O i£ E H cu u, H a I - 03 CU o c 03 TJ c 3 CU » 03 >-X) cu > Substrate Class Size Designation Boulder > 256 mm Cobble 64 mm - 256 mm Pebble 16mm- 64mm Gravel 2 mm- 16 mm Sand/Silt < 2 mm Table 3.3: Substrate definition and size classes. Size designation refers to the longest longitudinal axis of the stone. 64 a o CD > O 6 o C o ^ co a o . v »5 |S CO e Ti O tf O "t3 I S CU ca -4—* IES ca ĉ  O CD tf CU •4—» o o\ r- (N N H <n O <—• ON CO f- o m TJ- TJ- ON m ON m a G CU U cSpQg & & , o o c o 6 o CM CM u CO oo CO ON CN ca o H o ' c 6 0 Site Median distance moved (m) Range(m) n Centre HF 8.0 0.5 - 111.5 83 Promontory 3 a 1.5 0.5 - 62.5 44 Promontory BH 2.0 0.5 - 104.0 70 Tamihi C-DS 4.0 0 - 34.0 45 All sites combined 3.8 0.5- 111.5 231 Table 3.5: Median distance moved by larvae at each site. The median cumulative distance moved at Centre HF was significantly higher than at any other site (Brown-Mood Median test, X 2 = 18.42, df= 3, p< 0.001). 66 + o „ m o - CN o d PQ o Ov CN 03 i O CN OO Q l U S , ^ 2 c U CO ' 1 03 l> •4—» C • B 3 , ^ Q o o o CN m o o od CO -a CO > o £ i CO co co X T3 U 13 H m o VO O o o o d h o VO 00 o m • * d o o o d o o o | tn| o o v o o o PQ o VÔ CN + O VO O VO in Ov VO h o CN cn od CN o co K CN CN CN |oq CO CN VO CO Iqi in CN in 03 VO^ CN h o o VO CN Ov d CN Ov CO CN in q in © v d I co co v d oo Q U m o o >n vo d CN CN- O d CN o " in CO CN VO CN 00 oq CO oq co co| CN CN Ov 2 » c U © vo CN CO| d CN in CO CN oT Ov CN ^ . 1 r- co vol CN v q in in co oo co in co C 03 l> rt C . « CO CO —̂> 03 co CO > e CO (0 s-c O tU rt o x: rt c CO PQ 1=8: c 03 (0 U I rt| CO i<5 c 03 u £1 3 03 X. ~o o c u £1 CO 21 3 O PQ $1 x o U * co X X co PL, C# £ 10 > 03 u- a s i c 03 00 CN CN CN CN en2 CN 00 U CO Ov CM CO X CO 03 t i rt co X S x H "5 co s co > CO co rt X co X5 co .s 13 u > -a . co co „E E * O co •J3 CO * £ 3 -43 S ° to CO rt X rt 3 B S co *a 3 S3 oo . 3 CO "5 X T3 £ £ §T § *co bp , M co e * » Q £ 00 CO ffi no rt co •i-"1 CO CO > dZ O -a B c« -43 C rt Su co *••» • *-« . T3 Q .5 <-< t ,0 3 CO ! > CO p-,rt^ O rt B o , a rt "fj •R ^ •9 w •fi . £ •o S rt co u 3 3 «> O oo 1 1 •fi s B O CO >-i 6 x > | ° 3 <-< CO ^ g CU co >— oo X J - « H 2 rt w •td >-i 1 1 X O -g § 2 B 00 X rt £ o 5 £ S O S u E <G C <L> — e 2 oo "c c c fi rt o s s ci •f i 0 3 o . cS " -s ja •2 . s ix co O •f i Ml £ y E u u 2 c - ° c O d fi c rt a co * c "£3 •^ « rt co X u, CO 6 w c £ \-i rt rt B • « CO C rt rt <D « rt co ° rt co i - E '43 00 S « X C 00 £ rt fr 3 ^ 2 B T3 &, 67 CO o CD » ca x: U I T J c 03 C o 'ii u (50 03 > u 03 J la co d CN m m d o d o fe pq c _o » o o3 u <D c o PH oo Tt |M3 0 0 CM2 o fe W G 'o3 PH CN'I '•9 CL) 03 i- cn Xi 3 CO (U co i— 03 i 3 I* > o u cT 5T Q u CD » 03 3 £ 03 CO co O X ! u ta « T j C C o Ui 0 0 P-l "ca > 13 fe PH c o 03 u CD —̂» C o fe PH c 'o3 PH PH CO CD > O U CM co CO X i T3 T J CD > —̂» CD £ C D | . £ X ! co C CO .2 C o co CD •c K CD CD ' X i o 03 X i u O ,CD « H ca '—' x: 3 ^. CO u V CD CD . t i CD ^ CH -ti lo rt CD u c. ca CD CO *H X i U & .£ ™ 3 6 S 9 I? C X ! CD CD E CD > O ca Tt © < CO < , <£3 CD , £ .c 3 T3 CD co t- p CD co " 2 3 O 0 co T J C £ © 03 « « O 3 £ U T J 0 0 c ' £ 0 3 1 -2 .£ xi <~ .2 CD > E .ti 3 X ) C 03 CD ca E 3 a CD O CD 2 x i CD £ «> •c * ca co o .£ o o o ca .2 ta co -rj ^ C ca CD C CD • • co X) o r- fe + 1 & § 0 0 -5 o ca CD g X ! ~ cD £ u X ) CM ca o H co 03 CO ri co "ca CD X ! co O O CM 3 £ ^ <D § 5 CO co 03 H CD CD T J C ca ca CD 3 ~ CD ^ CD fe X ) CD 68 c o CO ca CD CO c o 00 CO CD CO C3 LU CO • C D T f C v J O C O C D T j - O J O ipeey WQZl UJ aiun|OA Jeie/w eBejeAV 5 -° £ I * ,o U CL, HH X3 ^ Tj- & ° ° II d IS <A - ca o p -is t=: o J3 § II u 8 & £ I-, OO CN Z iS I 5Q -C c-i >n • x q CD © P c 6 u CD CM o CN CD * 8 £ i i •4) co "ft x> S ~ PQ c CD O O CD s 3 > CD ca CD 0 0 ca ^ CD t i .5 CD c/3 p co ca II ,4> CO c+_ ^ * ""3 fe £ - T J TT ~ 2 © S £ 9 co O ca CD CO cn v >H s CD CD > ca cn t 250 > 200 T J | 150 © 5 a) g 100 -- ^ ^ " ^ I * n . _ u • Early Season @ Late Season CD o 2 Centre HF Promontory 3a Promontory BH Tamihi C- DS Site Q . CU Q E £ 55 0> O) CD w CU > < 18 16 14 . . 12 - 10 -• 8 6 4 2 0 b) I Early Season I Late Season Centre HF Promontory Promontory Tamihi C- 3a BH DS Site Figures 3.2a & b: Seasonal changes i n wetted w i d t h and depth i n four streams containing D. tenebrosus larvae. E a r l y season measurements were col lected i n June and J u l y 1997. Late season measurements were taken i n A u g u s t and September 1996. Sample sizes as i n F i g . 3.1. 70 c o CO CO CD (fi >^ TZ CO LU c o CO CO CO ty) CD CO I • X O o E o i Q_ O CD O O O CO O O J ipeey LUQI. u; |ood juaojad sBejaAV o co *L 13 tu -a CO co cn E c £ U co C +2 03 03 ^ O N tu H B - S co O co o 8 rt b 8 JS CO hj IH s o CO N T 3 'co S3 * - T3 o3 "3 co CO N O O N ON 03 CO X CO B o co 03 CO co CO rt rt CO X 03 CO OO " S t O O d 03 rt 1 X o rt ~ rt CO C N co oo £ q II. co 00 C3 rt B co B co B co t-i .CO CO rt 00 ~ Q S ' o3 rj & l » rt ts x c 'S ba B B 03 CO E co X co CO co s co I— .CO -a 13 B O co 03 CO oo CU u S OB 03 T 3 S 03 x r- x <a o o vo C M © rt q O o CO II s °< P H < ? § II co rt J—1 HH tu o co co a) £ o fl) O) i = 11 CD ca 3 fc a> <s a> cu _ a. oi H < 1 c cs a SI o 10 8 + 2 + ^ 0 °2 -8 + -10 Centre HF Pro3a ProBH TamC-DS Site b) o u _ 5 :§ a) a. •9 E 4) CO = £ I I > co C 10 — cn 0) O) Ol T-c = 5 co o 5 i 4 3 + 1 + 0 + -2 + -3 = -4 + Centre HF Pro3a ProBH I H . HI TamC-DS Site Figure 3.4a& b: Differences in the mean air and water temperature between late season 1996 and early season 1997 sampling. Each site mean was based on 8-18 observations. 72 Cumulative Distance Travelled by Pacific Giant Salamander Larvae (m) Figure 3.5: Cumulative distance travelled by D. tenebrosus larvae early and late in their active season. These data are pooled from all of the four streams. Early season refers to movements made in June and July 1997 (n = 30) and late season to movements made in August and September 1996 (n = 76). There was no significant difference between these two distributions (Kolmogorov-Smirnov test, p = 0.985). 73 • Movers ID Non Movers Early Season Late Season Time during the active season Figure 3.6: Proportion of larvae that moved (displacement greater than 0.5 m) and did not move at two different periods throughout the active season. The proportion of movers was not significantly different between these two periods. J 74 I Figure 3.7: Mean larval density and standard error at four D. tenebrosus larvae streams. 75 CO 14.0i C O E 12.0. in T— c 10.0. o CQ CD 8.0. cr E o 6.0. CO o 4.0. o CD c •> 2.0. o E CO CO 0.0. £ CO -J -2.0, * * * • SITE * TamC-DS • ProBH * Pro3a • CenHF -2.0 . 0.0 2.0 4.0 6.0 8.0 10.0 12.0 # Larvae moving into a 10 m Reach in 15 months Figure 3.8: Relationship between the number of larvae moving into and out of a 10 m reach of stream during a 13 month experiment. The log (x + 1) transformed number of larvae moving into a zone was significantly related to the log (x + 1) transformed number moving out (ANCOVA, F U i = 5.619, p = 0.037). There were no site interactions (ANCOVA, site effects F 3 , u = 0.333, p = 0.802). 76 C 'S O • • • — I — I — I — I — h - o o o o o o CN O 00 VO "3- CN E S en a <u o o H o CN + CD c« (UI) UIB3JJS UI p3||3ABJX 33UBJSIQ H x E .22 X 77 Chapter 4: Colonising Ability of Pacific Giant Salamander Larvae Introduction: T h e abil i ty of salamander populations to recover f r o m loca l disturbances has been debated by conservation biologists (Petranka et al . 1993, A s h & B r u c e 1994, Petranka 1994). Spec i f ica l ly , herpetologists have argued over whether amphibians can compensate for increased rates o f habitat disturbance by rapid recolonisation. In the P a c i f i c Northwest , habitat loss is p r i m a r i l y due to logging and development. Several studies have found that populat ion densities o f aquatic salamanders are l o w e r i n streams draining through clearcuts than i n undisturbed stands ( B u r y & C o r n 1988, C o n n o r et a l . 1988, C o r n & B u r y 1989, W e l s h 1991, C o l e et a l . 1997). T h e s m a l l populat ion p a r a d i g m tells us that smal l populations are more vulnerable to local ext inct ion than large populations (Caughley 1994). Thus by decreasing population density, logging may increase the probabi l i ty o f loca l extinction. If dispersal can facilitate rapid recolonisation o f disturbed areas, an increased frequency o f loca l extinction may have no l o n g term affect on salamander persistence. H o w e v e r , i f larval and adult dispersal are weak, regional extinction m a y ensue when entire landscapes are disturbed. It is generally bel ieved that amphibians are poor dispersers ( D u e l l m a n & T r u e b 1986, Blauste in et a l . 1994) and it has even been suggested that amphibian communit ies are inf luenced more by dispersal ability than by specif ic habitat tolerances or competit ive interactions (Cortwright 1986). A twenty year translocation study i n western Indiana f o u n d that the semi- aquatic T w o - L i n e d Salamander, Eurycea cirrigera, c o u l d survive i n many areas outside its traditional range, but it had been exc luded f r o m those areas b y poor dispersal abi l i ty ( T h u r o w 78 1996). Thus even within undisturbed, tolerable habitat, dispersal may limit amphibian distribution and community composition. Pacific Giant Salamanders and disturbance In this study, I used experimental techniques to measure the colonising ability of larval Pacific Giant Salamanders (Dicamptodon tenebrosus). In Canada, this provincially red-listed species is restricted to the Chilliwack River drainage basin where it is distributed patchily. Survey work in this area detected D. tenebrosus in only 22 of 59 seemingly habitable streams within this area (Richardson & Neill 1995). It is possible that many of these currently barren streams experienced local extinction in the past. Logging has occurred on much of D. tenebrosus' Canadian range, and may increase the frequency of local extinction (Haycock 1991). Though little is known of this species' ability to respond to local extinction by colonisation, larvae reappeared in one Washington stream only two years after it had temporarily dried (Nussbaum & Clothier 1973). It is unknown whether these animals were dispersers from nearby areas or survivors that took refuge in subsurface waters during the drought. I tested the hypothesis that colonisation of barren stream reaches by D. tenebrosus is rapid (< 1 year) and is accomplished by larval dispersal. I did this by simulating reach extinctions at four stream sites. I removed larvae from 25-40 m stream sections and then monitored recolonisation of these areas for a year. As colonisation implies the establishment of animals in an unoccupied habitat, this phenomena could not be studied by monitoring immigration into populated reaches. If movements are influenced by the presence of conspecifics, as found in D. tenebrosus larvae in the lab by Mallory (1996), dispersal rates into 79 populated and depopulated reaches will vary significantly (Stenseth & Lidicker 1992). Thus to model the natural process of colonisation, experimental removals had to be conducted. In addition to measuring the speed of recolonisation, this experiment yielded information about the relative contribution of larval dispersal and adult reproduction to the repopulation of barren areas. Both larvae and terrestrial adults are potential dispersal agents in this species. Terrestrial adults are more mobile than larvae, with some radio-tracked individuals travelling up to 305 m from their capture site between July and October (Johnston 1998). Although larvae are more limited in their dispersal capabilities, they are much more numerous than adults and can move > 50 m during the active season (Neill 1998). Consequently they may be the most efficient colonisers in stream reaches experiencing frequent, small-scale disturbances due to debris torrents or rock slides. Such disturbances are relatively common in headwater streams of the Pacific Northwest and their frequency is increased by logging (Lamberti 1991). I examined the size structure of the colonists of my removal zones to assess which life history stage, adult or larvae, had added the most individuals. Even if larvae are not efficient colonisers, there are reasons for studying their movements. Larvae may be the only viable dispersal stage in logged habitats, because terrestrial adults may suffer high mortality in clearcuts due to an increased risk of desiccation and freezing. (Richardson 1994). Under such a scenario, depopulated areas could be recolonised only by larval propagules from undisturbed stream reaches. It has not yet been tested, however this hypothesis suggests studies of larvae are vital forjudging recovery potential. Although removal studies have been widely used to estimate dispersal rates in other taxa (Stenseth & Lidicker 1992), these methods have seldom been used on amphibians (Bruce 1995). My study is one of the first to use removal techniques to estimate colonisation in salamanders. 80 Removal techniques have several shortcomings (Appendix 2). In this experiment I have employed a mixture of field and statistical techniques to reduce the impact of the five most serious biases noted by Stenseth and Lidicker (1992) (Appendix 2). Although none of these corrections completely eliminates bias, they provide more accurate measures of colonisation. Because conservation decisions often rely upon the recovery potential of a species, it is essential that these estimates be as accurate as possible. Methods: Measuring Colonisation in the Field The colonising ability of D. tenebrosus larvae was studied in four headwater streams in the Chilliwack River Valley: Centre HF, Promontory 3a, Promontory BH and Tamihi C-DS. The location and age of surrounding forest habitat at each site is detailed in Chapter 2 (Table 2.1). At each site, a 120 m long reach of stream was set aside for study. Colonisation was studied by removing all larvae from a 25-40 m central section of this study reach 1) Pre-Removal Sampling Each 120 m reach was searched intensively each week in June and July 1996 (Table 4.1) to identify all larvae that might later act as colonists. Larvae were captured by hand and individually marked (see Chapter 2) before being returned to their location of capture. All sites were sampled between 5 to 8 times to enumerate the larval population before removals began (Table 4.1). Even after this effort, some unmarked individuals were found within the study reach suggesting not all resident larvae may have been marked or some dispersal from beyond the study reach took place. 81 2) Creat ing R e m o v a l Zones After the init ial marking period, removal zones were created i n the m i d d l e o f each 120 m reach. T h e length and area o f the removal zone at each site varied between 25-40 m l o n g a n d 26-75 m 2 (Table 4.2). The size o f the r e m o v a l zones var ied because o f a p r i o r dec is ion not to remove more than one third o f the larval populat ion at any site. T h i s fract ion was chosen to ensure that there were more than enough indiv iduals i n the adjacent reaches to f u l l y recolonise m y removal areas. In two sites, larvae were heavi ly clustered i n the m i d d l e o f the study reach and only 25 m c o u l d be cleared w h i l e at the other t w o sites, larvae were distributed more uni formly and the middle 4 0 m was cleared. R e m o v a l s were conducted on a dai ly basis at each site i n late J u l y and early A u g u s t 1996. M e s h fences (1 m m 2 ) were bui l t to obstruct dispersal into or out o f the area d u r i n g the r e m o v a l period. A l l captured salamanders (larvae, neotene or adults) were taken out o f the stream and housed i n artificial stream channels. T h e number o f larvae removed varied between sites (Table 4.3). Searching was stopped when no larvae were captured i n the r e m o v a l zone on two consecutive days. A t this t ime the dispersal fences were removed and the reach was opened for colonisation. 3) M o n i t o r i n g Colonisat ion E a c h 120 m site, i n c l u d i n g a r e m o v a l zone and up and downstream source reaches, was monitored on a weekly basis unt i l late September 1996, when water temperature dropped b e l o w 6 C and larvae c o u l d no longer be detected. W e e k l y s a m p l i n g resumed f r o m June to m i d J u l y 1997 and again for two weeks i n September 1997. N o further removals occurred after monitoring started i n 1996. T h e identity and locat ion o f each larva found inside and outside o f 82 the removal zone were recorded. A l l newly found larvae were uniquely marked so that their future dispersal c o u l d be fo l lowed. A l l larvae captured w i t h i n the removal zone after c lear ing were categorised as potential colonists. B y continuing to mark individuals i n areas outside the removal zone, I estimated larval abundance i n the adjacent reaches. I used the program C A P T U R E to calculate larval abundance i n the source areas i n September 1996 and June 1997 ( B u r n h a m et a l . , 1994). Estimates for these periods were based on four weekly mark-recapture surveys. P o p u l a t i o n sizes i n the adjacent areas were also calculated i n September 1997. A s o n l y two surveys per site were conducted i n this month, populat ion size c o u l d not be estimated by C A P T U R E (insufficient sampling intervals). Instead a s imple L inco ln-Peterson model was used to estimate the size o f the September 1997 populat ion. Detai ls o f both this m o d e l and the C A P T U R E estimate are inc luded i n Chapter 2. Statistical Models of Colonisation Colonisat ion was f o l l o w e d for just over a year at a l l sites. A l l colonisat ion rates represent the number o f colonists entering the removal zone i n a one year per iod (the exact period that each site was monitored is s h o w n i n T a b l e 4.1). T w o issues made the enumeration of colonists diff icult . T h e first p r o b l e m was that not a l l animals found in the removal zone after c learing were marked. It was thus unclear whether these larvae had dispersed into the zone or were residents that had not been removed. Second, many potential colonists were captured only once i n the removal zone; G i v e n the l o w capture probabi l i ty o f these animals, it is uncertain whether these indiv iduals were transients or colonists that remained undetected i n the zone. I dealt with these problems by ca lculat ing colonisat ion under three different models: a) 83 Conservative b) Liberal and c) Statistically probable. The conservative and liberal rates were calculated to establish the range of values within which the true per annum colonisation rate of larvae lies. The statistically probable model incorporates information on site-specific trapping efficiency and capture history to estimate a likely number of colonists. The assumptions and methods used to derive each estimate are detailed below: a) Conservative Colonisation Only larvae that were initially marked outside the removal zone and then captured within it were considered colonists. Unmarked animals found within the zone were considered missed residents except for several small, unmarked larvae ( < 60 mm total length) found in the removal zone in September 1997. These animals were too young to have been present in the stream when the manipulations were taking place. They were considered to be colonists as their presence was most likely due to post-removal reproduction. These animals will be referred to as recruited colonists. To separate transient dispersers from true colonists, this model also required evidence that dispersing larvae had settled in the removal zone. All larvae dispersing into the zone had to be captured at least twice in the zone to be considered colonists. b) Liberal Colonisation Under this model, all larvae captured within the removal zone after clearing were counted as colonists. Any animal that was caught once within the removal zone, whether marked or not, was added to the coloniser pool. This method definitely overestimates colonisation as removals were not 100% successful at any site. Several larvae were found in each removal zone that had been marked prior to manipulation but had not been successfully 84 cleared. Although these animals were excluded from this estimate of colonisation rate, their presence indicates that at least some of the unmarked larvae found in the removal zone were missed residents. c) Statistically Probable Colonisation Unmarked larvae found in the removal zone after clearing were divided into two categories: those hatched before the manipulation and those that hatched after it. Any small larvae (< 60 mm total length) found in the removal zone in September 1997 were considered recruited colonists as described above. As I could not determine the origin of the other unmarked larvae in the removal zone, I used a site-specific removal efficiency rate to infer how many were likely missed residents. To do this I compiled a list of all larvae captured in the removal zone before manipulation. I subtracted from this list the number of larvae known to have dispersed out of the zone or that I suspected to have transformed before clearing. Larvae larger than 130 mm in total length that showed signs of gill resorption or skin mottling were classified as probable transformers. I divided the number of larvae that were removed at each site by the corrected number detected before clearing to obtain a removal efficiency rate for each stream. I used this efficiency index to calculate the number of unmarked larvae found in the removal zone that were likely missed residents. For example if the efficiency rate of a particular clearing was 75%, then 25% of larvae known to be present in the removal zone prior to manipulation were not successfully cleared. Thus if 20 unmarked larvae were later found in the removal zone, I inferred that 25% of them (5 larvae) had been present before the clearing and were not true colonists. By applying this correction, I divided the total number of unmarked 85 larvae into missed residents and immigrants. The number of unmarked larvae inferred to be immigrants was added to the number of marked animals known to have immigrated to calculate the total number of larvae that dispersed into the removal zone over the course of the experiment. The number of dispersers does not necessarily equal the number of colonists as some dispersers may have later emigrated or died. For each disperser into the removal zone, I calculated its probability of remaining undetected in the area for the balance of the experiment. If this probability was greater than 50%, I assumed this animal was still within the zone. These animals were designated as colonists. If the probability of non-detection was less than 50% and I never caught it again, I assumed it had died or dispersed. The specific methodology used to estimate this probability is described in Appendix 3. A schematic diagram detailing the steps taken in the model is given in Figure 4.1. Percent replacement of removed individuals by colonists The number of colonists predicted under each of the three models was calculated. These numbers were divided by the number of animals initially taken out of the removal zone to estimate what percent of the removed individuals were replaced by colonists in a year. Density Dependent Colonisation A per capita colonisation rate was calculated for each site by dividing the predicted number of colonists by the total number of larvae marked in the source areas above and below the removal zone. This rate represents the proportion of larvae in an undisturbed reach that 86 were capable o f local recolonisation. O n l y the number o f colonists estimated under the Statistically Probable M o d e l was used i n this and al l further analyses. This per capita colonisation rate was also used to examine the relationship between larval density in source areas and speed o f recolonisation. I f density dependence was operating on dispersal, I predicted that per capita colonisat ion at each o f four sites w o u l d increase w i t h increasing density o f the source populat ion. Origin of Colonists T h e co lonis ing group at each site was composed o f two types o f animals: second year or older larvae and young-of-the-year recruits. Young-of-the-year recruits were < 6 0 m m total body length i n the second year of the experiment. These animals were the product o f breeding i n F a l l 1996 and w o u l d not have hatched unt i l the summer o f 1997. A l l other c o l o n i s i n g larvae w o u l d have been i n the stream at the t ime o f the removals and w o u l d only be found i n the removal zone i f they had dispersed i n . B y compar ing the number o f indiv iduals i n each category, I c o u l d infer the relative contributions o f reproduction versus larval dispersal to the colonisat ion process. Body Size and Colonisation I wished to determine whether c o l o n i s i n g larvae were a r a n d o m sub-set o f indiv iduals f r o m the source areas or a unique size class. T o do this I s u m m e d the total number o f k n o w n colonists (37) f r o m all sites, i n c l u d i n g both larval colonists and adult-dispersed recruited colonists. A n equal number o f indiv iduals was then randomly selected f r o m the set o f a l l larvae that d i d not colonise the removal zone d u r i n g the year o f m o n i t o r i n g (pooled across a l l sites, n = 619). The mean snout-vent length ( S V L ) o f these indiv iduals was calculated. T h i s procedure 87 was repeated 1000 times to generate a distribution o f the expected mean snout-vent length i n a group o f 37 randomly selected non-colonis ing larvae. I then compared the observed mean S V L of c o l o n i s i n g larvae to this distribution. If the observed value fe l l w i t h i n the outer f ive percent o f values i n the expected distribution, the body size o f colonisers was considered signif icantly different f r o m resident larvae (alpha = 0.05). A Pascal-based randomisat ion test program was written by D r . D . H a y d o n for this and al l subsequent resampling analyses. Distance Travelled By Colonisers A n o t h e r resampling analysis was conducted to determine i f larva l colonisers exhibited distinct movement distances f rom non-colonisers. T h e cumulat ive distance travelled by a l l non- co lonis ing larvae throughout the experiment was calculated, after e x c l u d i n g larvae that d i d not move. N o n - m o v e r s were excluded as a compar ison of indiv iduals that by definit ion must m o v e (colonisers) w i t h those that often do not ( 8 - 2 5 % of larvae remained stationary, Chapter 3) w i l l y i e l d the obvious result that colonisers are more m o b i l e . Instead, I w i s h e d to k n o w whether colonisat ion proceeded by the short-distance dispersal characteristic o f most larvae (Chapter 3), or long-distance dispersal of a few atypical ly m o b i l e individuals . F r o m the pooled data set o f a l l n o n - c o l o n i s i n g yet m o b i l e larvae (those that m o v e d > 0.5m, n = 213), a number o f indiv iduals equal to the number o f in-stream larval colonisers (n = 7) was randomly selected 1000 times. Af ter each selection, the mean cumulat ive distances travelled by the group was computed. A n expected distr ibution of cumulat ive distance travelled by non-colonis ing larvae was generated f r o m these values. T h e observed mean distance travelled by co lonis ing larvae was compared to this distr ibution to determine i f they were m a k i n g statistically longer movements than those i n the source areas. 88 Direction of Colonisation A f inal analysis was undertaken to determine i f colonisation was direct ional ly biased. T o do this I first examined the net direct ion of movements made by n o n - c o l o n i s i n g larvae i n the source areas o f each stream. E a c h l a r v a that made a net downstream m o v e m e n t over the course o f the experiment was assigned a direct ion code of " 0 " , and each larvae that m o v e d upstream received a "1". Larvae that made no net movements were not i n c l u d e d i n the analysis. D i r e c t i o n records f r o m al l four sites were p o o l e d into one data set (n = 213). F r o m this data set, a number o f individuals equal to the number o f in-stream larval colonisers (n = 7) was randomly selected 1000 times. Af ter each selection, the number o f upstream movements i n the group was calculated by s u m m i n g direction codes of a l l individuals to produce an expected distr ibution of the number o f upstream movements. I then s u m m e d the net direct ion codes of in-stream larval colonisers and compared it to the expected distr ibution. Results: T h e numbers o f colonists at each site was calculated under 3 different co lonisat ion models (Table 4.4). Conservative estimates o f co lonisat ion varied between 0 and 5 larvae per year. These estimates are l o w because at least 10 unique indiv iduals were detected i n each removal zone after disturbance. B y assuming that none o f the u n m a r k e d animals found i n the removal zone were colonisers, this estimator excludes a significant proport ion of dispersal . T h e l iberal model predicted f u l l replacement o f r e m o v e d larvae at three o f four sites (Figure 4.2). T h e liberal estimates undoubtedly overestimated colonisat ion. T h e biggest f law i n this model is the assumption that a l l unmarked indiv iduals f o u n d i n the r e m o v a l zone post-manipulat ion were 89 dispersers. F r o m the efficiency index, I estimated that at least 2 5 % of larvae ini t ia l ly detected i n the removal zone post-clearing were missed residents. A n y local recovery predictions based on this model w i l l be optimistic. The Statistically Probable model consistently produced estimates m i d w a y between the conservative and l iberal models. Percent replacement of removed individuals by colonists T h e percentage o f removed indiv iduals that were replaced by colonisat ion var ied amongst sites (Figure 4.2). U n d e r the Statist ical ly Probable M o d e l , 29 to 2 1 0 % o f the larvae removed f r o m each site were replaced i n one year. W h i l e T a m i h i C - D S recovered fu l ly , colonisat ion had replenished only 2 9 - 7 7 % o f the r e m o v e d p o o l at the other three sites. Per Capita Colonisation O n l y a small percentage o f a l l larvae caught w i t h i n each 120 m s tudy zone were colonists (Table 4.5). A c r o s s the three forested sites, (Centre H F , P r o m o n t o r y 3a and P r o m o n t o r y B H ) , the per capita colonisat ion rate was remarkably u n i f o r m , ranging between 3 to 5 percent o f a l l captured indiv iduals per year. In contrast, 13% percent o f captures at T a m i h i - C D S were colonists. A s w i l l be discussed be low, the h i g h per capita rate at T a m i h i C - D S is l i k e l y due to loca l ly higher recruitment at this site. Density Dependent Colonisation There was no relation between the per capita co lonisat ion rate at a site and the mean larval density o f larvae i n the source reaches (Figure 4.3). A possible density association was observed when the per capita rate was split into t w o values, one for co lonisat ion by larval 90 dispersal and another for colonisation by recruitment (Figures 4.4 a & b). There was a trend for colonisation by larval dispersal to increase wi th density. H o w e v e r this trend is based on differences o f one or two dispersing indiv iduals between sites and c o u l d easily be due to chance. Origin of Colonists T h e percentage o f colonisat ion that was due to reproduction varied considerably between sites. Colonisers were pr imari ly dispersing larvae at two sites, and p r i m a r i l y recruits at the remaining two (Figure 4.5). The only site to complete ly recover f r o m the r e m o v a l , T a m i h i C - D S , was restocked entirely by recruits. A t this site, sexually mature animals must have bred i n the removal zones a few months after the removal had taken place. Promontory 3a was also exc lusive ly colonised by recruits, but had only replaced 2 9 % o f its previous inhabitants by the end o f the experiment. Body Size and Colonisation The expected distribution o f mean S V L s i n 37 n o n - c o l o n i s i n g larvae is shown i n F i g u r e 4.6. C o l o n i s i n g larvae were significantly smaller than non-colonis ing individuals . T h i s result is not due to greater dispersal by smal l larvae, but to higher recruitment i n the depopulated zones. A s shown i n Chapter 3, body size had little inf luence on movement i n any o f the four study streams. T h e only weak trend observed was an increase i n m o v e m e n t w i t h body size, contradicting the notion that recruits are the most m o b i l e . I f the removal zones h o l d more recruits than the source areas, it is because m o r e eggs were deposited and/or successfully hatched within them. 91 E x c l u d i n g recruits f r o m the sample, I tested whether the body s i z e o f larval colonisers was significantly different f rom non-colonis ing individuals. The mean S V L o f in-stream colonisers, 53.6 m m , was not significantly different f r o m the mean S V L o f 7 randomly selected non-colonisers (expected mean = 53.4 m m , p = 0.446, F i g u r e 4.7). Distance Travelled By Colonisers T h e expected mean distance travelled by non-colonis ing larvae was l o w e r than that m o v e d by the in-stream colonists, but not s ignif icantly so (Figure 4.8). T h e mean distance travelled by in-stream colonis ing larvae was more than twice the mean recorded i n the source areas. Direction of Colonisation P o o l i n g across a l l sites, the ratio o f downstream to upstream movements was 42: 58. j T h i s slight preference for upstream movement was reflected i n the randomisat ion tests, w h i c h predicted an average of 3.9 net upstream movements i n a group o f 7 dispersing larvae; In contrast wi th this value, six out o f seven c o l o n i s i n g larvae m o v e d upstream into the removal zone. A l t h o u g h this result is not s ignif icantly different f r o m expected (p = 0.224, F i g u r e 4.8), it proves larvae are capable o f m o v i n g upstream against the current into a new area. Discussion L o c a l recovery i n D. tenebrosus populations was variable dur ing the first year f o l l o w i n g a disturbance. F u l l recovery occurred in only 1 o f 4 sites w i t h co lonisat ion replenishing only 29- 7 7 % of removed individuals at the remaining three streams. G i v e n the s m a l l area o f these 9 2 removals and the high abundance o f larvae i n nearby source reaches, it is surpris ing that f u l l recovery d i d not occur at al l sites. It is unclear whether larvae lacked the abi l i ty to co lonise at a faster rate or s imply had no cause to m o v e f r o m where their i n i t i a l locat ion (i.e. no density dependence or destructive habitat change forc ing movement) . F u l l repopulation w i t h i n a year after removal was achieved only at T a m i h i C - D S , the sole stream running through a recent clear cut. M e a n air temperature at this site was higher than at the other three streams (Table 3.6). T h e mean abundance of macrobenthos at this site was less than half that o f the forested sites. It is not k n o w n h o w or i f these variables affect co lonisat ion speed, but they d i d not appear to e x p l a i n the variat ion i n dispersal rates between four unmanipulated streams i n Chapter 3. W i t h no replicate clear cut sites, it is imposs ib le to determine whether this is a habitat or site effect. A s m y study is one of the first removal experiments to be conducted on amphibians, the closest taxonomic comparison I can m a k e is to other aquatic vertebrates. S u c h comparisons show the recolonisation ability o f D. tenebrosus larvae to be poor. F o r example , several species o f fish removed f r o m 40-100 m reaches i n an I l l inois stream regained 9 0 % o f their or ig ina l abundance w i t h i n 10 days (Peterson & B a y l e y 1993). M u c h variat ion, however , exists among fish species, wi th some predicted to recolonise w i t h i n a few weeks ( L a r i m o r e 1959), others a few months (Matthews 1986) and others up to a year ( G u n n i n g & B e r r a 1969). In almost a l l o f these studies, the experimental reaches cleared were larger than i n m y experiment. I f recolonisation proceeds at the rates observed i n m y experiment, f u l l numerica l recovery at the three unsaturated sites s h o u l d take 6-42 months (Table 4.6). I d i v i d e d the total length of each depleted reach by its predicted recovery t ime to estimate h o w fast reaches experiencing s imi lar reductions c o u l d be replenished (Table 4.6). F o r example, m i l d 9 3 disturbances that caused density reductions o f 0.1 larvae m 2 (magnitude o f m y depletion at P r o m o n t o r y 3a) w o u l d be recolonised at a rate o f 2 0 m per year. A l t e r n a t i v e l y , severe disturbances that caused depletions o f 1.1 larvae m" 2 (Centre H F ) , a value w h i c h w o u l d cause complete extirpation at many streams, w o u l d be recolonised at a s lower rate o f 7.1 m per year. I n o w use these simple predictions to estimate the t ime required for recolonisat ion i n stream reaches running through a clearcut ( m a x i m u m length o f 4 0 0 m). I f l o g g i n g triggered o n l y moderate depletions o f 0.1-0.3 larvae m" 2 , larval recolonisation o f a 4 0 0 m x 1 m reach c o u l d take 8-20 years. H o w e v e r i f logging triggered an almost f u l l extirpation o f larvae (depletion o f > 1.1 larvae m" 2 ), recolonisation o f this stream reach c o u l d take more than 55 years. E i g h t to fifty f ive years for the f u l l recolonisat ion o f a stream running through a cut- b l o c k agrees wi th other estimates for salamanders i n logged habitats. P lethodont id salamanders i n eastern N o r t h A m e r i c a were estimated to take 20-25 to 50-70 years to return to pre-harvest density i n cutblocks ( A s h & B r u c e 1997). H o w e v e r other species o f amphibians are faster colonisers. In Spain, an o l d l ignite mine site was recolonised by several amphibian species w i t h i n only two years of abandonment (Galan 1997). S i m i l a r l y art i f ic ial ponds i n a B a v a r i a n experiment were colonised by the newt Triturus alpestris w i t h i n a year (Joly & G r o l e t 1997). Var ia t ion i n recovery speed is l i k e l y a result o f species-specific colonisat ion abil i ty, the magnitude o f depletion caused by the disturbance, and dispersal barriers i n the landscape. A l t h o u g h the above extrapolation o f m y small-scale results to larger areas provides a quick comparison to other species, these calculat ions are not accurate enough to i n f o r m management decisions. M y study provides a detai led description o f larval co lonisat ion, but there was no study o f adults. I have shown that reproduct ion increases l o c a l density more rapidly than 94 larval dispersal. Thus, understanding the colonis ing abi l i ty o f adults is p ivota l to estimating the speed o f recovery by D. tenebrosus after large disturbances. Extrapolat ion of small-scale results to large areas is also r isky as rates measured at one scale do not always predict behaviour at another. T h r u s h et a l . (1997) f o u n d that co lonisat ion speed for some benthic marine organisms decreases signif icantly w i t h increasing plot size. R e a l disturbances often act over a m u c h wider area than any experimental plots and must be restocked by a proportionately smaller colonist p o o l . A s a consequence, co lonisat ion rates measured i n smal l areas w i l l l i k e l y overestimate the recovery speed of large areas. T h e type o f disturbance applied in this experiment may also y i e l d overly optimistic colonisat ion rates. Depopulat ion was achieved by r e m o v i n g indiv iduals experimental ly and not by destructive habitat change. T h i s experiment d i d not e x p l i c i t l y consider the role o f habitat on colonisat ion. A s larvae were found i n a l l sites pr ior to manipulat ion, the habitat was suitable to larvae. Habitat clearly affects amphibian colonisation i n addition to intrinsic dispersal abi l i ty (Hecnar & M c C l o s k e y 1997, S k e l l y & M e i r 1997). M y experiment has s h o w n only h o w q u i c k l y larvae can recolonise acceptable habitat. In the field, even i f D. tenebrosus can reach a depopulated area quick ly , they may avo id settling i n it or die w i t h i n it i f the habitat is unsuitable. It is thus uncertain h o w much or i f m y rates w o u l d vary under different types o f habitat change. H o w e v e r , the h igh speed of colonisat ion i n the one clearcut site suggests l o g g i n g does not necessarily deter movement. F i n a l l y i n m y study, recolonisation refers o n l y to the numerical replacement o f individuals and not to biomass recovery. Pre and post-removal larva l b iomass c o u l d not be compared as the number o f colonists was statistically inferred and hot direct ly enumerated. A s such, the precise identity o f each colonist was not k n o w n and thus their total b iomass c o u l d not be calculated. 95 T h i s omiss ion may optimistical ly bias the rate o f recovery at the T a m i h i C - D S . A l t h o u g h this site exceeded its pre-removal abundance w i t h i n a year, the colonisers were p r i m a r i l y smal l individuals (< 60 m m T L ) . Larvae found i n the removal zone o f this site before clearing were generally large individuals (> 100 m m T L ) . T h e discrepancy i n size between the pre and post- removal occupants o f this zone suggests f u l l biomass recovery was not achieved at this site. Life History and Colonisation A s mentioned above, fu l l recolonisation occurred only at T a m i h i C - D S where colonists were exclusively recruits. This colonisation was l i k e l y achieved entirely by adults breeding i n the removal zone. Colonisat ion by larval dispersal occurred at t w o sites, but never added as many individuals to the removal zone as reproduction. L a r v a l dispersal never contributed more than 13 individuals to any removal zone. A d u l t females can carry between 85-200 eggs (Nussbaum 1969). U n l e s s egg-to-larvae mortal ity is greater than 9 0 % , one c lutch o f eggs c o u l d provide just as many colonists to a stream reach as loca l larval dispersal. Egg-to-larvae surv iva l i n D. tenebrosus is u n k n o w n , but was 2 2 % i n one populat ion o f the related Ambystoma maculatum (Shoop 1974). I f this rate is s imi lar to that i n D. tenebrosus, one reproduct ive event c o u l d increase local density i n depopulated areas m u c h more effectively than larval immigrat ion f r o m adjacent reaches. A l t h o u g h m y one-year study is informative, the f inal outcome o f the colonisat ion process cannot be judged f r o m observation on this t ime scale. Re-establishment o f D. tenebrosus in m y removal zones w i l l depend on the surv iva l o f larvae to sexual maturity, a process that c o u l d take 2-6 years (Chapter 2). S tudying only larval colonists without consideration o f their surv iva l to sexual maturity may overestimate the speed o f colonisat ion. A l t h o u g h indiv iduals may have 96 higher survival and/or growth i n the absence o f conspecifics, it is st i l l problematic to assume a l l larval colonisers w i l l survive to adulthood. F o r example, one intertidal study f o u n d that defaunated areas were q u i c k l y recolonised by a polychaete w o r m species. M o s t c o l o n i s i n g polychaetes, however, d ied without contributing to the long term recovery o f the plots (Thrush et al. 1996). A l t h o u g h the number o f larval colonists entering a depopulated area may be a g o o d indicator o f future occupancy, cont inued monitor ing is required to ensure their presence leads to the long-term survival of indiv iduals . Size of Colonisers The mean size o f larvae w i t h i n the removal zones was s ignif icantly l o w e r than outside, reflecting that most new recruits were located i n the removal zones. Recrui ts at P r o m o n t o r y 3a and T a m i h i C - D S were clustered heavi ly i n and around the removal zone and were not spread evenly throughout the rest o f the stream. T h i s m a y be the r a n d o m outcome o f a single c lutch at each site being coincidental ly deposited i n or immediate ly adjacent to the r e m o v a l zone. Alternat ively there may be a selective advantage to being hatched i n depopulated areas. In support o f the latter hypothesis, C o n n o r et a l . (1988) found densities o f first and second year D. tenebrosus larvae to be twenty times greater i n stream sections i n w h i c h older salamanders and fish were absent. Recruits c o u l d be selectively concentrated i n the removal zone i f adults chose to lay eggs in areas o f l o w larval density, or i f hatchlings were more successful i n the absence o f conspecifics. T h e first o f these scenarios, selective ov ipos i t ion , has been recorded i n other species o f stream d w e l l i n g salamanders (Kats & S i h 1992). T h e second hypothesis, that hatchling survival is greater i n l o w density areas, is also feasible for D. tenebrosus. Pac i f i c G i a n t 97 Salamanders are k n o w n to prey on smal l conspecif ics i n the lab ( M a l l o r y 1996) and I observed some instances o f cannibal ism i n the f ie ld. N e w recruits are the most vulnerable to intraspecif ic predation and conceivably may survive best i n the absence of conspecifics. I f this hypothesis is true, this benefit may favour dispersing adults and promote recolonisation i n disturbed landscapes. Dispersal Behaviour of Larval Colonists O b v i o u s l y larval colonisers had to m o v e some distance to enter the r e m o v a l zone, however their mean dispersal distance was twice that o f mobi le , non-colonis ing larvae. M a n y potential colonists were clustered just o n the boundary o f the r e m o v a l zone (1-5 m away). H a d they dispersed into the removal zone, the mean distance travelled by c o l o n i s i n g indiv iduals w o u l d be no different f r o m that o f non-colonisers. H o w e v e r colonisat ion d i d not proceed by gradual range expansions o f these fringe animals but by a few long distance movements (4 - 63 m , mean = 26.1 m ) by highly m o b i l e individuals . Colonisers may have been behavioural ly predisposed to movement and encountered the r e m o v a l zone by chance. T h i s i d e a is supported by f indings i n Chapter 3 that suggest i n populated stream reaches, the larval populat ion is composed of a large number o f h ighly sedentary larvae and a smal l number o f transient individuals. O f the seven co lonis ing larvae, s ix m o v e d upstream into the zone. T h e dominant upstream direction o f colonisat ion suggests that it was not a lways forced by the stream current. T h i s contradicts previous c la ims that most co lonisat ion occurs by downstream drift o f larvae (Bruce 1985, 1986). S e c o n d growth forest d o w n s t r e a m o f d is turbed reaches m a y therefore be a more important source o f colonists than o l d growth stands located upstream. These second 98 growth areas may be more vulnerable to forest harvest and development than upstream sources. T h i s activity c o u l d have more a destructive influence on D. tenebrosus metapopulation dynamics than the harvest o f o l d growth. Conclusions: M y experiment provides new insights into the response o f the P a c i f i c G i a n t Salamander to disturbance in B r i t i s h C o l u m b i a . N u m e r i c a l recovery f r o m small-scale "ext irpat ions" occurs between 6-42 months after disturbance. Ext irpat ions throughout streams the length o f clearcuts ( m a x i m u m length o f 400 m) l ike ly take significantly longer to be ful ly recolonised. T h i s suggests that D. tenebrosus i n recent clearcuts (< 5 years) are more apt to have s u r v i v e d through the logging event than to have recolonised after a loca l extirpation. T h e i r presence suggests surv iva l through logging is possible provided the stream remains intact. A l t h o u g h depopulated areas can be restocked by both in-stream dispersal o f larvae and adults, dispersal and oviposi t ion by adults appears to be the most rapid means o f recolonisat ion. Conservation efforts should therefore be directed p r i m a r i l y at adult dispersal capabil i t ies and habitat requirements. F i n a l l y , it is obvious f rom the above discussion that the measurement o f colonisat ion i n the f ie ld is complicated. Accurate estimates are hampered by biases due to s m a l l sample size, l o w recapture rates, and restricted spatial scales. W h i l e the colonisat ion rates I have prov ided are a potentially useful management tool , they should be used wi th caution. 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C 8J O H U 104 Site # Unique Larvae Caught in 120 m Study Reach % Captures that Were Colonists Centre H F 162 4.9 P r o m o n t o r y 3 a 133 3.0 Promontory B H 239 5.4 T a m i h i C - D S 145 13.0 Table 4.5: N u m b e r o f larvae captured i n 120 m study area and the percentages o f these that were colonisers. 105 b 2 U ca ~3 ^ > C O  E, 3 c o -a o *4J o n is e  o O w h i d u e o n is e  d CN vd CN d vn 4J ca CD u co l CD CD a Pi d en  C CD X> CD fo r X> fo r o CO u im e 4-» >> im e X! 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A n in-stream coloniser is one that was originally captured in the source areas and then dispersed into the removal zone. b) (Bottom): Percentage of recruits in the removal zone as a function of mean population density in the source areas. 110 CO CD ® , CO d E CO CD Ct • 0) c CD > o E 0> tr c o o o "co > 6 CO CO co £• o £ 1 o C o O l I O N I D I f l ^ n C l I r O O O O O O O O O O |OOd jazjuo|oo IBJOJ. jo uoijjodojd 11  113 T3 CD > o 6 I J co o 1— s o CO cd > C 03 CD 1Z X! 2 e i_i 03 ^ X CD * - 0 3 Jo I % £ O CO ^ '2 J2 2 o i > V o c ^ § c i - -2 >, g x x T3 'S CD to TJ ^ > CO CD O C 03 •«—» CO CD > 2 3 CJ c a CD o CD 3 > C c3 CD 2 CD VD X ° * CO "c3 V O C N 8 § 2 "5 C rt x o ~- c rt CD 5 .52 "3 co Q x '2 w o CD " o 8 N, CO b i CD CD 3 c3 X a « ~ 1— rt *< O X 114 CO ' cO ca ° ,2 CM " O o m'r> T3 O ^ CD \Q CM S i I 13 is 6 0 d co cD •a 1 5 -g c o £ •~ - - 1 * *> « s <u t i CO <MJ 3 CD O C E CD > O e > E ca o CD CD C > FJ e 2 E s a g — r CD E H ia ca • co I 1 o S 3 2 CD 1 2 8 2 g CD O = 1 "I a co "o « " -e CD O O ^ O cD CD CD •S "a P o 3 ca" ca co bQ f—I CO •• 'E ©\ o T t o .1. § r- E o c CD u-,W •3 C N C N c d fic  II 'E CM bo ' c o ca 4^ > no  lar  1 Chapter 5: General conclusions T w o categories of r isk must be addressed when evaluating a species' status: the l ikel ihoods o f stability and o f persistence. Stability refers to the probabi l i ty that abundance w i l l remain constant, and persistence to the probabil i ty o f ext inct ion w i t h i n a g iven t ime per iod (Connel l & Sousa 1983). G i v e n the diff iculty o f identifying 'stable' equi l ibr ia and dist inguishing uncharacteristic declines f r o m natural variat ion, it is often most useful to study factors w h i c h influence extinction probabil ity and recovery potential ( C o n n e l l & Spusa 1983). B y e x a m i n i n g the short-term population b i o l o g y o f larvae, m y thesis has focused on factors that may influence D. tenebrosus persistence i n B r i t i s h C o l u m b i a . O n l y long term m o n i t o r i n g o f populat ion trends w i l l show whether this species is numerical ly stable i n this province. In the introductory chapter, I presented three general areas o f investigation f r o m w h i c h information on D. tenebrosus populat ion viabi l i ty and continued persistence can be drawn: studies o f local demography, the impact of human activities, and the abi l i ty to recover f r o m disturbance. A l t h o u g h I d i d not r igorously explore a l l o f these issues i n this thesis, m y research on larval demography and co lonis ing abil ity bears on each issue. Af ter brief ly rev iewing m y findings as they relate to these three areas, I w i l l discuss whether the s u m total o f m y research supports the notion that this species is at r isk i n B r i t i s h C o l u m b i a . I. R e v i e w o f major results a) L o c a l demography Comparison of larval demography between threatened and non-threatened areas I found the mean larval density in m y 5 sites, 0.88 + 0.09 m ' 2 , was just over a third o f that reported in Oregon, the centre o f the species' range ( C o r n & B u r y 1989). T h e difference 116 between m y density estimates and those f rom Western Washington, a neighbouring region where they are not endangered, is not nearly so pronounced. K e l s e y (1995) calculated the mean larval density i n unharvested stands i n Western Washington to be 1.1 m" 2 , only s l ight ly greater than i n this study. L o w e r densities i n B r i t i s h C o l u m b i a suggests that these populations differ i n one or more key demographic rates f r o m those i n Oregon. A n n u a l survival does not appear to vary m u c h between these regions, however the length of the larval period does. N u s s b a u m & C l o t h i e r (1973) estimated annual larval survival i n one Oregon stream to be 4 3 % , only s l ightly higher than the 3 0 - 3 5 % mean annual rate I estimated. A c c o r d i n g to m y analysis, larvae i n m y four study streams c o u l d take 4-6 years to reach metamorphic size (130 m m T L +). L a r v a e i n two Oregon streams were estimated to g r o w 2-3 times faster than larvae i n m y study, and are bel ieved to have a larval per iod o f only t w o years (Nussbaum & Clothier 1973). E v e n i f annual survival was the same i n O r e g o n and B r i t i s h C o l u m b i a , net survival through the larval per iod w i l l be l o w e r i n B r i t i s h C o l u m b i a . F o r example i f annual survival was 4 0 % i n both regions, survival throughout the entire larval per iod w o u l d be 16% i n Oregon (2 year larval period), and only 0 . 5 - 3 % i n B r i t i s h C o l u m b i a (4-6 year larval period). This difference in net larval surv iva l may help explain w h y densities o f D. tenebrosus are lower in B r i t i s h C o l u m b i a than i n the centre o f its range. H o w e v e r , m a n y more populations i n both B r i t i s h C o l u m b i a and Oregon need to be studied before any geographic trends i n survival can be confirmed. Comparison of D . tenebrosus larval demography with other salamanders L a r v a l survival varies markedly between species and habitats and no typical value can be identified for stream d w e l l i n g salamanders. H o w e v e r it is useful to note that larval surv iva l in D. 117 tenebrosus is s imilar to that i n other species. I approximated annual survival o f D. tenebrosus larvae to be 3 0 - 3 5 % (corrected for transformation loss). B a s e d on these rates, surv iva l o f D. tenebrosus through a 4-6 year larval per iod w o u l d be 0 . 5 - 3 % . T h i s range is s i m i l a r to that o f both Ambystoma barbouri and Ambystoma texanum whose survival through a 6 0 day l a r v a l per iod is 0.5-12.5% and 1-4% respectively (Petranka & S i h 1986, H o l o m u z k i 1991). In one N o r t h C a r o l i n a stream, Gyrinophilus porphyriticus was found to have an annual surv iva l o f 2 1 % (Beachy 1997), w h i c h w o u l d y i e l d a net surv iva l o f 0 . 2 % through its 4 year larval per iod . T h u s survival o f D. tenebrosus larvae i n B r i t i s h C o l u m b i a is s i m i l a r to that o f other s tream-dwel l ing species. T h e growth rates I found for D. tenebrosus larvae are sl ightly l o w e r than recorded i n other temperate aquatic salamander species. I estimated D. tenebrosus larvae i n m y study streams w o u l d grow between 7.3-10.6 m m S V L per year. A t s imi lar latitudes i n A l b e r t a and Quebec, larvae o f the p o n d d w e l l i n g Ambystoma macrddactylum and Ambystoma maculatum grow approximately 15 m m S V L although there is considerable variation (Flageole & L e C l a i r 1992, R u s s e l l et al . 1996). Y e a r l y increases o f 12-20 m m S V L have been reported i n stream dwel l ing Eurycea wilder ae and Hynobius kimurae larvae (Beachy 1997, M i s a w a & M a t s u i 1997), but as these studies were conducted i n m o r e southern locations, c o m p a r i s o n c o u l d be confounded by latitude effects. A l t h o u g h these between-species comparisons are useful , they may be confounded by differences i n body size. B i g g e r species w i l l l i k e l y have greater absolute growth even though their proportionate rate o f increase c o u l d be l o w e r than i n smal l species. The species I have discussed here have s l ightly smal ler larvae (1-2 cm) than D. tenebrosus. I have shown that despite h a v i n g reduced growth rates in comparison to populat ions i n the centre o f the species' range, D. tenebrosus i n B r i t i s h C o l u m b i a has larval demography s imi lar 118 to other stream d w e l l i n g salamanders. A n n u a l survival and growth rates i n D. tenebrosus larvae are comparable to those i n other, non-threatened species. A l t h o u g h growth may not be m a x i m a l i n B r i t i s h C o l u m b i a , larvae i n these populations are not unusual w i t h respect other stream- dwel l ing species. b) T h e impact o f human activities T h e l o w number o f sites used i n this study makes it dif f icult to examine the influence o f logging on D. tenebrosus. W i t h almost h o repl icat ion o f forest age classes, I c o u l d not test whether variation i n larval demography was due to l o g g i n g or r a n d o m site variat ion. H o w e v e r I found no relation between forest age and larval density across m y f ive study sites. T h i s neutral result has also be found by H a w k i n s (1983) and K e l s e y (1995), but contradicts the posit ive association between density and forest age found by B u r y (1983), C o n n o r et a l . (1988), C o r n & B u r y (1989), (Cole et a l . 1997) and the negative association found by M u r p h y et a l . (1981) and •; M u r p h y & H a l l (1981). I also noted that larval growth i n m y only clearcut site was twice as fast as i n m y second growth sites. F r o m these observations, I speculate that c learcutt ing can reduce the density o f larvae but that survivors may benefit f r o m increased growth i n disturbed habitats. F i n a l l y I found that local larval dispersal (more than 10 m) was not inf luenced by any o f 7 stream habitat variables i n c l u d i n g substrate type, pool-r i f f le c o m p o s i t i o n , wetted w i d t h and depth. Dispersal was u n i f o r m l y l o w through a w i d e variety o f micro-habitats. M o v e m e n t i n m y clearcut site was indist inguishable f r o m that i n m y second growth sites. B l a u s t e i n et. a l . (1994) suggested that anthropogenic habitat alteration exacerbates a m p h i b i a n populat ion ext inct ion by hampering recolonisation. M y results suggest that logg ing- induced habitat shifts i n streams have 119 little consequence for the local dispersal o f D. tenebrosus larvae. It is not k n o w n , however , whether these habitat changes influence the longer distance movements o f larvae between confluent streams or the overland movement o f terrestrial adults. c) General abi l i ty to recover disturbance T o predict the l i k e l i h o o d of persistence, it is necessary to have in format ion o n a populat ion's capacity to increase f r o m l o w numbers either by recruitment or i m m i g r a t i o n (Blaustein et a l . 1994). T h e speed o f recolonisation varied between sites but was predicted to take 6-42 months to repopulate reaches o f 25-40 m (26-75 m 2 ) . A s s u m i n g the rates I observed i n 13 months o f study remained constant through t ime, moderate depletions o f 0.1-0.3 larvae m"" i n headwater streams running through clearcuts (approximately 4 0 0 m x l m ) c o u l d take 8-20 years to be ful ly recolonised by larvae. Alternat ively i f logging caused an almost complete extirpation of larvae, f u l l recolonisation o f reaches running through a 4 0 0 m cutblock c o u l d take approximately 55 years. T h e average l i fe span of D. tenebrosus i n the w i l d is not k n o w n , however s i m i l a r l y s ized aquatic salamanders can l i v e approximately 25 years in captivity ( D u e l l m a n & Trueb 1986). If this value obtains i n the f ie ld , recolonisation o f stream reaches < 4 0 0 m after moderate to severe disturbances c o u l d be achieved i n one or two populat ion turnovers. T h u s p r o v i d e d source populations are nearby and habitat is suitable for breeding, numerica l recovery can occur over short ecological t ime spans (less than 2 generations). Exper imenta l ly defaunated stream reaches were repopulated both by larval dispersal and adult reproduction. L o c a l reproduction appears to be a m u c h more eff icient means o f repopulating an area than larval i m m i g r a t i o n . O n l y 4 - 5 % o f larvae i n reaches adjacent to m y 120 removal zones became colonists and this dispersal never contributed more than 13 i n d i v i d u a l s to any o f m y plots i n 13 months. In contrast, c lumps o f 15-20 young-of-the-year, p o s s i b l y a l l f r o m the same c lutch, were found at two sites i n the summer o f 1997. T h i s suggests that i n one breeding attempt, an adult female c o u l d provide an equal or greater number o f colonists than supplied by neighbouring reaches w i t h 100-200 larvae. II. Implications for assessment o f D. tenebrosus' status i n B r i t i s h C o l u m b i a A l t h o u g h logg ing and other disturbances may increase the rate o f l o c a l ext inct ion, m y research suggests that D. tenebrosus populations i n B r i t i s h C o l u m b i a are not unusual ly susceptible to disturbance. A l t h o u g h they are found at l o w e r densities than i n other parts o f the species' range, larvae i n these populations exist w e l l w i t h i n the surv iva l and growth bounds o f other non-threatened stream-dwell ing salamanders. Furthermore, the c o m b i n e d influences o f recruitment and larval recolonisation can facilitate rapid recovery f r o m small-scale disturbances. Consequently, any argument o f vulnerabi l i ty must be based on the action o f extrinsic factors such as logging. In the absence o f conclus ive p r o o f that l o g g i n g increases l o c a l ext inct ion rate b e y o n d that w h i c h can be balanced by recolonisat ion, it is uncertain whether D. tenebrosus i n B r i t i s h C o l u m b i a are truly imperi l led . I caut ion, however , that m y results are d r a w n f r o m small-scale manipulations with l i m i t e d repl icat ion. T h e i r abil i ty to describe the dynamics o f a l l populations o f D. tenebrosus i n B r i t i s h C o l u m b i a and their response to disturbance is therefore l i m i t e d . M y colonisation rates were measured under o p t i m a l habitat condit ions and i n the presence o f source areas containing many potential colonists . T h i s situation is not l i k e l y to occur i n the f ie ld , especially i f potential disturbances such as l o g g i n g occur frequently enough to d i m i n i s h source 121 populations. T o conf i rm m y conclusions about the status o f D. tenebrosus in B r i t i s h C o l u m b i a , future research should examine the colonisation of larger areas w i t h a l o w e r avai labi l i ty o f potential dispersers and the co lonis ing abil ity o f terrestrial adults. Bibliography A s h , A . N . 1997. Disappearance and return o f Plethodontid Salamanders to clearcut plots i n the Southern B l u e R i d g e Mounta ins . Conservat ion B i o l o g y 11: 983-989. A s h , A . N . & R . C . B r u c e . 1994. Impacts of t imber harvesting on salamanders. Conservat ion B i o l o g y 8: 300-301 . A s h t o n , R . E . 1975. A study o f movement, home range, and winter behavior o f Desmognathus fuscus (Rafinesque). Journal o f Herpetology 9: 85-91. A y e n s u , E . S . 1981. Assessment of threatened plant species i n the U n i t e d States. In H . Synge (editor): T h e b i o l o g i c a l aspects o f rare plant conservation. J o h n W i l e y & Sons, Chichester. B e a c h y , C . K . 1997. Effect o f predatory larval Desmognathus quadramaculatus o n growth , survival , and metamorphosis of larval Eurycea wilderae. C o p e i a 1997: 131-137. B e r y e n , K . A . I 9 9 0 . Factors affecting populat ion fluctuations i n larval and adult stages o f the w o o d frog (Rana sylvaticd). E c o l o g y 71: 1599-1608. Beschta , R . L . , R . E . B i l b l y , G . W . B r o w n , L . B . H o l t b y & T . D . Hofstra . 1987. Stream temperature and aquatic habitat: fisheries and forestry interactions. In E . O . Salo & T . W . C u n d y (editors): Streamside management: forestry and fishery interactions. C o n t r i b u t i o n N o . 7, Institute o f Foresty Resources, U n i v e r s i t y o f W a s h i n g t o n , Seattle. B lauste in , A . R., D . B . W a k e & W . P . Sousa. 1994. A m p h i b i a n Decl ines : J u d g i n g stability, persistence, and susceptibility o f populations to l o c a l and g lobal extinctions. Conservat ion B i o l o g y 8: 60-71. 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Prentice H a l l , N e w Jersey. 131 Appendix 1: Chapman's Modification of the Lincoln-Peterson Method N=(r+ l)(n + 1) -1 (m + 1) where N = estimated population size, r = number of animals caught, marked and released in the first sample, n = the total number of animals caught in the second sample, and m = the total number of marked animals caught in the second sample (Chapman 1951). l~ (r + 1) (n + 1) ( r - m) (n - m 1 1/2 L (m + l)2(m + 2) J 132 I f rt I to CO e © , o ro CU rt a £ CU co s o N T3 P O CD o £ fi i rt CO O PH T3 c I. 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CO 1 P y •a -5 i 3 co P b •= CO rt 3 o X a rt rt co ° £ s CO 3 co X) CO X co < •o 3 rt ,— co x co CO O CD l - l co C CO CO x £ £ ~ CD CD c R--.3 CD rt co •-H I H s g 7. § 5 t3 co DH fi CO 3 I H rt 3 "" o co S3 x CD O . rt •x '55 CO o O H CO rt co £ 33 CO rt rt co PH CD 3 <s rt CD CO £ 3 * CO CO co CO £ 3 rt > _ 2j» co rt cd £ £ CO r - H O co X X > x : co <U rt >- • H rt 4 - 1 CO x̂  x rt ^ rt rt _ » H CO 3 " 3 .2 PH * c i co rt •3 4> o c o § CO CO 'co S C O CO CD - O T3 CO CD - O CD 5 u •s 1 p rt 25 3 CD Xi — " 3 rt co 3 1 l H CD PH X CD £ CO I X co CD fc O co PH E •*—* -w c3 '1 / — \ c s O N O N 3 1 ^ £ o co CO '1 rt > O £ p CO CO CO rt 3 O £ £ o U ri '1 CU a a CO CU I CO "3 > o S CO CS IS 3 CD > fi - 3 "3 rt 3 £ 3 rt 3̂ ^ 3 . t i CD rt co 2" rt Ĉ H H 0 o CO 3 O 'rt rt PH O O H co fi 3 O 00 co 3 CD Q co rt CO '3 o u rt o CD Q S o 133 Appendix 3 Estimating the Probability of one-time capture in the removal zone T h i s technique was used to estimate the probabi l i ty that a larvae caught o n l y once i n the r e m o v a l zone remained resident but undetected unt i l the end o f the experiment. A larvae was assumed to have colonised the removal zone on the first day it was captured i n this area. Between this date and the end o f the experiment there were n possible s a m p l i n g occasions i n w h i c h it could be recaptured g iven it was al ive and w i t h i n the study area. T h e program C A P T U R E was then used to estimate the mean per occas ion capture probabi l i ty o f larvae at each site ( B u r n h a m et a l . 1994). T h i s probabi l i ty was used to calculated an expected number o f recaptures given the a n i m a l remained al ive and i n the removal zone unti l the end o f the experiment. F o r example let us assume that a larvae was first caught i n the removal zone i n the 10 th mark-recapture after manipulat ion but never again i n the remaining four sampling intervals L e t us further assume that the mean capture probabi l i ty o f larvae over this time per iod 0.15 per occas ion. T h e probabi l i ty o f the a n i m a l being present on a l l subsequent sampling days but not detected was calculated as f o l l o w s : P ( never detected i n 4 occasions I present) = ( 1 - 0 .15) 4 = 0.52 ( E q n . 1) Thus there is a 5 2 % probabil i ty this animal remained i n the zone after first capture but was not captured again. It was arbitrarily dec ided that any larvae w i t h a greater than 5 0 % probabil i ty o f non-detection w o u l d be considered a colonist under the Statist ical ly probable model . 134

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