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UBC Theses and Dissertations

Population dynamics of Microtus townsendii in a linear habitat Calvert, Mary Frances Wargo 1976

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POPULATION DYNAMICS OP MICROTUS TOWNSENDII IN A LINEAR HABITAT *y MARY FRANCES CALVERT B.Sc, Texas Tech University, 1968 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE DEPARTMENT OF ZOOLOGY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February, 1976 In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree l y ava i l ab le for reference and study. I fu r ther agree that permission for extensive copying of th i s thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that Copying or pub l i ca t i on of th is thes i s for f i nanc ia l gain sha l l not be allowed without my wr i t ten permiss ion. Department of The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 D a t e ~??W<^  ^ / / / / i i ABSTRACT The population dynamics of Mlcrotus townsendll l n a l i n e a r habitat were monitored weekly for one year (June, 1971-August, 1972). Demographic c h a r a c t e r i s t i c s such as l o c a t i o n on study area, sex, weight (age), reproductive condition, and s u r v i v a l were recorded weekly. An Intensive study of early Juvenile s u r v i v a l was also undertaken. The population was observed to decline l n the spring of 1972 from peak densities the previous summer and early spring. The decline did not appear to r e s u l t from decreased reproductive e f f o r t or d i s p e r s a l , but could be explained s o l e l y on the basis of mortality. Several observations were made which did not follow the usual demographic trends l n mlcrotine population biology. Extremely high densities with small home ranges for individuals were found on the l i n e a r study area as compared with areas of continuous habitat. This population became sexually mature at much lower body weights than other populations of Mlcrotus  townsendll l n the area and average body weights were lower, although growth rates were comparable. 1 i i i TABLE OF CONTENTS Introduction 1 Methods k Study Area k Trapping Schedule 7 Dispersal 8 Juvenile Study 9 Introduction Experiment 10 Results 11 Population Density 11 Reproduction 16 Growth and Body Size 25 Mortality 33 Dispersal 45 Home Ranges 50 Discussion 52 Literature Cited 6 0 iv LIST OF TABLES Table 1. Maximum t r a p p a b i l i t y during 4 periods 15 Table 2 . Percentages of adults, subadults and juveniles l n breeding condition during the three seasonal periods 21 Table 3» Median body weight at sexual maturity f o r Mlcrotus townsendll. Males and females grouped 23 Table 4 . Mean growth rates (% per day) adjusted by regression to a standard 35 S vole for the three time periods of study 27 Table 5. Minimum s u r v i v a l rates per 14 days. Sample si z e i n parentheses 35 Table 6 . Estimates of early Juvenile production and s u r v i v a l f o r three seasonal periods 38 Table 7. Mean density and recruitment rate f o r three subsections of the study area. Recruitment was measured by the number of new young recruited per l a c t a t l n g female. A l l values expressed f o r an area of one acre ( . 40 ha) 40 Table 8. Number of Juveniles present one and four weeks a f t e r introduction onto densely populated and uninhabited areas 42 Table 9 . Number of Mlcrotus townsendll colonizing the removal area and the number of tagged animals disappearing from the control area. 46 Table 10. Number of animals abruptly moving t h e i r home range over 100 yards 49 Table 11. Average range lengths l n feet f o r males and females according to season. Sample sizes i n parentheses •*• 51 V LIST OF FIGURES Figure 1. A e r i a l photograph of study area at the in t e r s e c t i o n of Highway 499 and Steveston Highway i n Richmond, B r i t i s h Columbia. Linear habitat shows up as a dark s t r i p between l i g h t d i r t roads. Letters indicate sections of the study area 5 Figure 2. Map of study area 6 Figure 3. Population density of Mlcrotus townsend11 on l i n e a r study area. Winter months are shaded. Note that females always outnumber males. 12 Figure 4. Weight (age) d i s t r i b u t i o n of newly captured Mlcrotus towns end 11 on bi-weekly basis 13 Figure 5. Bi-weekly percentages of females pregnant throughout study 18 Figure 6. Bi-weekly percentages of l a c t a t i n g adult and subadult females 19 Figure ?. Bi-weekly percentages of males i n breeding condition. 20 Figure 8. Number of pregnancies per female 24 Figure 9. Mean body weights of male and female Mlcrotus townsendll. Winter months are shaded 26 Figure 10. Growth rates {% per day) of male Mlcrotus  townsendll, adjusted to a standard 35 S vole. Winter months are shaded. 28 Figure 11. A few selected growth curves f o r male Mlcrotus townsendll 29 Figure 12. Body weight d i s t r i b u t i o n for male Mlcrotus townsendll. 30 Figure 13. Instantaneous r e l a t i v e growth rates (#/day) of introduced juveniles 32 Figure 14. Mean s u r v i v a l rate per 14 days f o r male and female Mlcrotus townsendll from 1971-72. Horizontal l i n e i s an a r b i t r a r y d i v i s i o n at a s u r v i v a l rate of 0.71; s u r v i v a l rates below th i s l i n e represent losses of more than 50$ of the population every 4 weeks 34 ACKNOWLEDGEMENT I wish to thank Dr. Charles Krebs and Dr. Dennis Chltty f o r t h e i r support and assistance throughout th i s study and f o r t h e i r h e l p f u l comments on the manuscript. I'm es p e c i a l l y g r a t e f u l to my husband, Glen, who helped with the f i e l d work and gave me moral support and encour-agement throughout the years that i t has taken to complete t h i s degree. 1 INTRODUCTION The c y c l i c a l nature of mlcrotlne population fluctuations has been described and debated by many over the past 50 years (Chitty, I960; Krebs, 1966). Ty p i c a l l y , rodent populations increase from low numbers to a peak, and then decline to low numbers again over a 3-4 year period. These fluctuations are not d l r e e t l y related to known changes l n the physical environ-ment of the population (Chitty, I960; Krebs, 1966). Various demographic parameters such as adult and juvenile s u r v i v a l , length of breeding season, age at f i r s t reproduction, growth rates, and s i z e of individuals have been found to change i n a systematic way with the d i f f e r e n t phases of the population cycle. Anoexcellent review of the past l i t e r a t u r e and current theories of population cycles i n small mammals has recently been made (Krebs and Myers, 1974). The general problem of understanding c y c l i c fluctuations i n small rodent populations can be broken down Into several components. One, which has received considerable attention, has been the question of what factor or factors prevent these animal populations from i n -creasing i n d e f i n i t e l y . Chitty ( i960, 1967, 1969) has suggested that genetic changes are a necessary component of the conditions leading to numerical f l u c t u a t i o n s . Spacing behavior may be under some genetic control, and s e l e c t i o n operating through spacing be-havior may be a d r i v i n g mechanism fo r the population cycles. Aggressive behavior within high density populations could be 2 responsible f o r cessation of breeding among some adults, low s u r v i v a l of the young, and f a i l u r e of the young to mature rap i d l y and breed. These effects have been experimentally demonstrated i n populations of deermice (Sadlelr, 1965; Healey, 1967). Aggressive behavior acting i n the context of t e r r i t o r i a l behavior possibly causes d i s p e r s a l of individuals^ and drives subordinates into marginal habitats, where s u r v i v a l and breed-ing success are sharply reduced. It i s very d i f f i c u l t to study immigration and emigration i n an area of continuous habitat, and next to impossible to determine home range si z e with any accuracy i n the continuous grassland which i s t y p i c a l habitat f o r many voles (Van Vleck, 1 9 6 9 ) . Almost a l l of the studies carried out on mlcrotines have been done i n f i e l d s within which a part of the population was studied on a g r i d . The effects of immigration and emigration were f e l t to be s i g -n i f i c a n t , however. Krebs et a l . (1969) fenced i n grids so that no immigration or emigration was possible, and found that popu-l a t i o n processes were so greatly altered that numbers became abnormally high. Myers and Krebs (1971) looked at the animals dispersing from a population and found s i g n i f i c a n t differences i n behavioral as well as genetic c h a r a c t e r i s t i c s of the immigrants. The purpose of t h i s study was to describe the population dynamics of Mlcrotus townsendll i n a l i n e a r habitat. This species had not been studied before and there have been no i n -tensive studies of vole populations i n a l i n e a r habitat. I 3 planned to monitor individuals i n terms of b i r t h place, home range, d i s p e r s a l , reproductive success, and s u r v i v a l . Special emphasis was placed upon studying Juvenile s u r v i v a l and re-cruitment into the adult population, since t h i s i s an area about which l i t t l e Is known l n populations of mlcrotine rodents. METHODS Study Area Most studies of vole populations have been carried out ln areas of extensive habitats; this one was carried out in a linear habitat. The study area (Pigs. 1 & 2) was a fence row with a 10-15 foot wide strip of grass dividing fields of potatoes in Richmond, British Columbia. In most places the grass fence row was elevated 1-2 feet above the level of the fields/and a plowed d i r t road, approximately 10 feet wide, separated It from the f i e l d s . The vegetation in the fence row was an old f i e l d pasture mix of perennial grasses. The linear habitat was 1900 feet (579 m) long and had an area of .566 acres (0 .23 ha). Variation ln the habitat along the fence row is indicated in Fig. 2. However, in general, differences were in the width of the fence row (6-22 f t ) , the height above the adjacent fields (and therefore susceptibility to winter flooding)^and the presence or absence of bushes. One section of the area (D 17-27) was covered with dense bushes for 200 feet and was unsuitable habitat for Mlcrotus townsendll. It proved to be an effective barrier against casual extension of home ranges, although a 1-2 foot strip of grass on one side provided a pathway for migrating animals. 5 FIG. 1. A e r i a l photograph of study area at the in t e r s e c t i o n of Highway 499 and Steveston Highway i n Richmond, B r i t i s h Columbia. Linear habitat shows up as a dark s t r i p between l i g h t d i r t roads. Letters indicate sections of the study area. 6 FIG. 2. Map of study area. (ssEsa study areas grass potatoes d i r t roads 7 Trapping Schedule I attempted to count a l l the Mlcrotus townsendll l i v i n g l n the l i n e a r habitat. I trapped intensively with Longworth l i v e traps at weekly Intervals f o r a period of 52 weeks (June 1971-June 1972), with follow-up trappings at 3-week interv a l s throughout the summer of 1972. I set 150-200 traps at 20-foot inter v a l s along the l i n e a r habitat f o r a minimum of 3 periods (usually 2 nights and 1 day) each week, at a l l times using more traps than the maximum population of a given area, and s e t t i n g two traps at each s t a t i o n l n periods of high density. I pre-baited the traps with r o l l e d oats for two weeks p r i o r to the beginning of the study, and l e f t the traps locked open and containing oats and cotton bedding during the week-long i n t e r -v a l between trappings. I scraped the tunnels and nest boxes of traps which were esp e c i a l l y d i r t y before s e t t i n g them each time. To avoid death from heat l n the summer, I trapped at night and i n the early morning, and locked the traps open during the day. I also placed a piece of apple i n each trap to provide moisture during hot weather. The traps were covered with plywood boards (10" x 12" x 3/4") to provide i n s u l a t i o n against heat and cold, and were placed i n a c t i v e runways within 2 feet of the s t a t i o n p o s i t i o n . During some trapping periods, alternate traps were set on d i f f e r e n t nights ( i . e . I# 3 t 5 ; 2 , 4 , 6 ) to avoid the d i f f i c u l t y that movements were probably r e s t r i c t e d by an animal's tendency to enter the trap nearest i t s burrow. When 8 determining the home ranges of p a r t i c u l a r i n d i v i d u a l s , I would occasionally close down an entire block of traps to see i f the animal moved into the next block. I trapped Sections A and I at Irregular Intervals l n order to follow the movement of immigrants and emigrants, and I also set traps l n the plowed roadway between the grass s t r i p and the f i e l d as well as i n the potatoes to see i f animals were leaving the grassy s t r i p . No voles were ever caught out l n the potato f i e l d s . The following basic demographic data were collected for each animal at each trapping period: l o c a t i o n on study area, sex, weight, reproductive condition, ( i . e . s c r o t a l or non-s c r o t a l ; or, pregnant, perforate, or l a c t a t l n g , as Judged by nipple condition). Dispersal To study d i s p e r s a l I used two methods. The f i r s t was to determine the home range of each i n d i v i d u a l and to note sudden changes i n the home range l o c a t i o n . Home ranges were deter-mined by c a l c u l a t i n g the area of the fence row between farthest points of capture. However, since the width of the fence row varied and since i t was Impossible to determine i f an i n d i -vidual was u t i l i z i n g the entire area, I decided that the adjusted range length (range length plus half the distance to the next s t a t i o n on either end) would more accurately measure the range of individuals (Van Vleck, 1969). 9 The second method used to study d i s p e r s a l was to esta b l i s h an area (Area D, F i g . 2) from whioh a l l resident and l a t e r on a l l immigrant voles were removed. The l a t t e r could then be characterized according to o r i g i n , sex, s i z e , and reproductive condition. Juvenile Study To study juvenile s u r v i v a l and recruitment into the trap-pable population, I proceeded as follows: a) By toe-clipping a l l l i t t e r s born i n the traps, I deter-mined s u r v i v a l and recruitment into the trappable population; b) By using home range data and reproductive data from the females, I was able ( i n 60$ of the cases) to determine the probable female parent and l i t t e r mates of the juveniles captured. c) By placing pregnant females that were within a few days of d e l i v e r y i n an enclosure, I was able to get further inform-ation on the s u r v i v a l of l i t t e r s . The females were housed i n standard laboratory mouse cages equipped with water b o t t l e s , oats, and carrots or apples. These cages were placed near a window i n an unheated storage b u i l d i n g . L i t t e r s were toe-clipped and returned to the f i e l d . The procedure f o r r e i n t r o -ducing the nursing female and her l i t t e r was to lock them i n the trap with bedding, oats|and pieces of apple/and to place the closed trap back i n the f i e l d at the pos i t i o n from whioh It had been removed. The trap was opened as q u i e t l y as pos-s i b l e several hours l a t e r and was not disturbed u n t i l the following week. 10 Introduction Experiments I studied the s u r v i v a l of Juveniles i n the absence of adults i n two introduction experiments. In these, I released 4-5 week old juveniles, which were born l n a laboratory colony of Mlcrotus townsendll at University of B r i t i s h Columbia, onto removal Area D, and also onto a control Area B, with i t s na t u r a l l y dense population of Mlcrotus. The Juveniles from 4 l i t t e r s In each experiment were divided so that equal numbers of each sex and of each l i t t e r were placed l n the experimental and oontrol groups. In early March 1972, ten juveniles (6 males; 4 females, ave.wt. 2 9 . 5 g) were introduced at random along the length of each of the grids, and were l e f t f o r a period of one month. During t h i s time, t h e i r s u r v i v a l , d i s -persal, reproductive condition, and growth rates were deter-mined. The experiment was repeated l n A p r i l 1972 with the re-lease of seven juveniles (5 males, 2 females, ave.wt. 30 g) onto the experimental area and s i x (4 males, 2 females) onto the control area. 11 RESULTS Population Density During the 52 weeks In whioh th i s study was carried out, 348 Mlcrotus townsendll were captured 3119 times. As can be seen i n F i g . 3, t h i s population began increasing at a rate of 11.4$ per week as t h i s study began i n the summer of 1971. reaching a high of 68 individuals i n mid-July. Trapping was well established by t h i s time, and I f e l t that most residents were enumerated. This i n i t i a l Increase (Fig. 3) was possibly just a trapping phenomenon as voles became used to the traps. From Aug. to Oct. 1971. the population was s t a b i l i z e d at approximately 55 i n d i v i d u a l s , and then i n Nov. and Dec. rose again to 68 (instantaneous rate of increase, r * .09 per week). Females throughout t h i s time outnumbered males^and the sex r a t i o ranged from 31% to 43# males during the f a l l and winter. In Jan. 1972 the population began increasing again at the rate of 9% per week, reaching a high of 80 animals i n early Feb. In terms of a continuous habitat, t h i s would have rep-resented an average of 166 voles/acre. Density did vary i n d i f f e r e n t sections of the study area with area B-C consistently £ having the highest density (the equivalent of 366 voles per acre during Mar.). Most of the recruitment during Feb. and Mar. consisted of juveniles and subadults, which were probably born on the study area during the winter. From the peak i n l a t e Feb. 1972, the population declined s t e a d i l y at the rate 12 FIG. 3. Population density of Mlcrotus townsendll on l i n e a r study area. Winter months are shaded. Note that females always outnumber males. 12a 250 100 + i i i i i i i I i l i t | i i ) i i i i i i i i i i i i JUL SEP NOV J A N MAR MAY JUL 1971 1972 Y E A R 13 PIG. 4. Weight (age) d i s t r i b u t i o n of newly captured Mlcrotus  townsendll on bi-weekly basis. 13a CO UJ _J O > Q LU Q: H CL < o hi d 2 25 201 15 10 JUL SEP 1971 B Juvcn i le s<30g • Sub-adults 30-42 g lu lts ^ 4 3 g Sad NOV JAN YEAR MAR 1972 MAY JUL 14 of 5 to Q% per week, with only occasional recovery, reaching a low of eight animals l n Aug. 1972. There was some juvenile recruitment l n the spring of 1972, but from l a t e May u n t i l Aug., no juveniles were recruited, and the population consisted almost e n t i r e l y of new adult and subadult Immigrants rather than old residents (Fig. 4). Even though the study area was over a mile long and separated by the removal area and the section of bushes along D, and high water along C i n the winter, a l l sections of the area were synchronous i n t h e i r fluctuations l n population density. Each section was analyzed to see i f It was out of phase with other sections, but no differences were found. The decline i n 1972 was common to my whole study area, although another population of M. townsendll 5 miles to the south-west was reaching peak numbers l n 1972 (LeDuc, 1974). Areas A and I adjacent to the study area were trapped at monthly Intervals and were also found to be synchronous with the r e s t of the study area. Trappablllty Maximum t r a p p a b l l l t y f o r t h i s population was calculated by the summation of a l l animals a c t u a l l y caught at time t divided by the number of animals known to be a l i v e at that time (Krebs et a l . 1969). Trappablllty l n thi s study (Table 1) was high l n the summer of 1971 and the spring of 1972 (79-93%) but was only $k% during the months of Dec. and Jan. It 15 TABLE 1. Maximum t r a p p a b i l i t y during 4 periods. Sample Size Trappability June-Nov. 1971 1300 86.8# Dec.-Jan. 1971-72 398 5**7% Feb.-April 1972 1137 79*1% May-June 143 93.9% 16 i s f e l t that t h i s was probably owing to the unusual 15 days of snow cover we had during these two months, and the very cold weather accompanying i t . Reproduction Reproductive output varies seasonally and with population density i n voles (Krebs and Myers, 1974). I s h a l l consider three components of the reproductive e f f o r t of this population of Mlcrotus townsendll. Length of Breeding Season This population was breeding during the entire study except for a 3-4 week period In l a t e Nov. and early Dec. 1971. Cessation of breeding was Indicated by a decrease i n the number of perforate females and absence of pregnant females during th i s time, and absence of l a c t a t i n g females i n mid-Dec. and early Jan. (Figs. 5 & 6). Males did not go out of breeding condition as thoroughly as the females did, as only 20% of those formerly breeding became non-scrotal (Fig. 7). Breeding resumed i n early Jan. and continued throughout the length of the study. Intensity of Breeding Table 2 divides the information on breeding condition by season among the three weight (age) classes. There appear to 17 be no differences In the percentages of males In breeding con-d i t i o n between the peak summer of 1971 and the spring decline of 1972. Neither i s there any difference i n the percentage of females a t t a i n i n g sexual maturity i n 1971 and 1972 as evidenced by the presence of perforate vaginas. One notice-able difference i s a drop i n the percentage of l a c t a t l n g adult females from an average of kk% i n the peak summer to 27% i n the decline. However, the percentage of l a c t a t l n g subadult females was almost the same during the decline as It had been during the peak (about 20%). 18 FIG. 5. Bi-weekly percentages of females pregnant throughout study. 18a 100 J U L SEP NOV JAN MAR MAY JUL 1971 1972 YEAR 19 FIG. 6. Bi-weekly percentages of l a c t a t l n g adult and sub-adult females. 19a 100 adul t s o subadu l t s JUL S E P ° 1 N O V ^ J A N 1971 YEAR MAR MAY JUL 1972 2 0 F I G . 7 . Bi-weekly percentages of males i n breeding condition. 21 TABLE 2 . Percentages of adults, subadults and juveniles l n breeding condition during the three seasonal periods. Scrotal Perforate Lactating Males Females Females ad. sub. Juv. ad. sub. Juv. ad. sub. juv. Summer 1971 100 96 46 97 95 57 44 18 0 Winter 1971-72 97 75 26 87 49 4 24 8 0 Summer 1972 98 93 36 96 90 63 27 21 0 22 Size at Sexual Maturity Table 3 gives the median body weight at sexual maturity for voles during 1971 and 1972. Median weight at maturity was calculated according to the technique of L e s l i e et a l . (1945). Maturity i n males was judged by the presence of large testes and i n females by the presence of perforate vaginas or medium to large size mammary glands, or open pubic symphyses, or a l i t t e r i n the l i v e - t r a p . When analyzed sepa-r a t e l y , the data showed that there were no differences between the two sexes. Weight at maturity was higher i n winter than i n summer, but the major conclusion i s that maturity was achieved at very small sizes i n thi s population (only 17-27 S in summer 1972). Leduc and Krebs (1975) report median weights at maturity of 38-47 g for a spring population of M. townsendll only 5 km. away. Number of Pregnancies Because of the intensive trapping program i n thi s study, I was able to determine pregnancies In ind i v i d u a l females from t h e i r weight changes and l a c t a t i o n history. F i g . 8 gives the frequency d i s t r i b u t i o n of the number of pregnancies observed In a l l females which reached subadult size and were known to be a l i v e one month or longer. Twelve females were found never to be pregnant. Note that the number of pregnancies per i n d i -v idual f a l l s o f f smoothly from the modal value of one pregnancy. 23 TABLE 3 . Median body weight at sexual maturity for Mlcrotus townsendll. Males and females grouped. Time Period Sample Size Median Weight 95$ Confidence Limits June-Aug. 1971 142 25 23-27 Sept.-Nov. 144 24 22-26 Dec.-Feb. 257 35 3^-36 Mar.-May 1972 102 21 17-27 FIG. 8. Number of pregnancies per female. 40 30 20 10 h o i — i 0 1 2 3 4 5 6 7 8 9 10 NO. OF PREGNANCIES/FEMALE 25 Growth and Body Size Trapping once a week provided a detailed record of changes in body weight. F i g . 9 gives the mean body weights In grams for males and females In thi s study. For each pair of weights obtained on an in d i v i d u a l , an Instantaneous r e l a t i v e growth rate was calculated according to the formula given i n Brody (19^5). For each 4-week period, a regression was calculated of growth rate on average body weight, and the growth rate de-termined f o r a standard 35 S vole. Pregnant females were excluded from these data. Growth rates for Mlcrotus townsendll are highest i n the spring, decline during the summer and f a l l , and are lowest during the winter (Leduc, 1974). Table 4 groups these growth data and shows that growth rates were lower i n both males and females In 1972, such that the summer growth rates of 1971 are above the spring growth rates of 1972. F i g . 10 gives the growth rates (% per day) of males In this population adjusted to a standard 35 g vole. F i g . 11 i l l u s t r a t e s a few selected growth curves f o r males. Peak populations of many microtine rodents are character-ized by individuals of high body weight (Krebs and Myers, 1974). F i g . 12 gives the body weight d i s t r i b u t i o n f o r males i n my study. Voles were lar g e r i n 1971 than In 1972. Unfortunately I do not have data f o r the spring of 197lj^when differences might be maximal (Chitty and Chltty, 1962), but i n June 1971, voles up to 84 g were caught. In March 1972 the large s t male FIG. 9. Mean body weights of male and female Mlcrotus townsendll. Winter months are shaded. 27 TABLE 4. Mean growth rates (% per day) adjusted by regression to a standard 35 g vole for the three time periods of study. Time Period Males Females Growth Rate S.E. : Growth Rate S.E. Summer 1971 1.30 0.12 1.25 0.13 Winter 1971-72 0.51 0.12 0.23 0.10 Summer 1972 0.95 0.23 0.99 0.16 28 FIG. 10. Growth rates (% per day) of male Mlcrotus  townsendll. adjusted to a standard 35 g vole. Winter months are shaded. 28a FIG. 11. A few selected growth curves f o r male Mlcrotus  townsendll. M A L E BODY WE IGHT (GM.) ro 4> 0) Co O O O O O PIG. 12. Body weight d i s t r i b u t i o n f o r male Mlcrotus townsendll. 30a IL_0 n 13 -n-Tl-n _ n - i n- — i r~i—i—i i — i -, r~n—, rri — 1 1 1 1 1 1 11 H T T r x b 4—H 1 1 1 1 j 1 h O O O 00 O CD O o CM 13 12 > o CL LU cn < LU >-O) S H V d O Nl 1 H 9 I 3 M 31 caught weighed 62 g, and by June 1972, only one i n d i v i d u a l reached 70 g, a l l the rest being below 62 g. Some individuals were recaptured over a long period of time, and i t i s possible to obtain further d e t a i l s of Indi-v i d u a l growth. Iverson and Turner (197*0 described substan-t i a l weight losses during the winter months fo r Mlcrotus  pennsylvanlcus l n Manitoba, and i t would be i n t e r e s t i n g to look f o r t h i s e f f e c t i n Mlcrotus townsendll. In t h i s study f i v e adult males weighing 50-60 g i n the f a l l of 1971 l o s t an average of 18% of t h e i r body weight during the winter. Two of these same individuals plus two others regained the l o s t weight l n the spring and then i n May l o s t an average of 19$ of t h e i r body weight. Fig . 13 plots the instantaneous r e l a t i v e growth rates of the juveniles l n the introduction experiment against t h e i r o r i g i n a l body weights. It i s Interesting to note that the growth rates of the A p r i l introductions were s i g n i f i c a n t l y higher (ave. 1.48^/day) than these i n March (ave. ,42^/day). Two factors could be responsible: a poorer q u a l i t y food l n March than l n A p r i l when the grass resumed growth; and/or the e f f e c t of adult density upon growth of the juveniles. The o v e r a l l density of the study area was d e c l i n i n g rapidly from early March to late Apr11 j however, there were no differences between growth rates of Juveniles introduced onto the removal area and those onto the densely populated area l n the same month. Juvenile s u r v i v a l on the rest of the study area was so poor during these two months, that there are only two 31a resident Juveniles In March and one In A p r i l f o r growth rate comparisons. March growth rates of resident juveniles are s i m i l a r to^the Introduced juveniles^and the growth rate of the single A p r i l juvenile Is confounded by pregnancy. FIG. 13. Instantaneous relative growth rates ($/day) of Introduced juveniles. DAY i n t roduced in: o © M a r c h ^3 -0 O A p r i l O < X ^2.0 0 cr. 0 08 OO ° J t O m & u h- ' £ o ® o — I — 1 ®-—I - 0 10 2 0 3 0 4 0 I N IT IAL B O D Y WT (GM.) 33 Mortality Mortality l n a l i v e - t r a p p i n g study Is equated with d i s -appearance from the trappable population and therefore Includes emigration. In this study, I was Interested l n temporal changes l n s u r v i v a l rates, measured by d i r e c t enumeration. Four components are discussed: s u r v i v a l l n the trappable popu-l a t i o n ; early juvenile s u r v i v a l ; s u r v i v a l of juveniles l n introduction experiments; and s u r v i v a l of l i t t e r s born l n the l i v e - t r a p s . Survival l n the Trappable Population Survival rates were measured by d i r e c t enumeration of marked animals. Survival rates compare the number of i n d i v i -duals caught i n a given trapping period with the number of the same individuals known to be a l i v e i n the next trapping period, which l n t h i s study was one week l a t e r . These s u r v i v a l rates were then squared to convert to s u r v i v a l over a 14-day period so that they would be comparable with those already found l n the l i t e r a t u r e (Krebs, 1966; Krebs e t . a l . , 1969) . F i g . 14 presents s u r v i v a l rates f o r males and females. These rates were grouped Into summer and winter periods and a mean s u r v i v a l rate calculated f o r each period. These data are presented i n Table 5. Throughout the study males had lower s u r v i v a l rates than females. In both males and females there were s t a t i s t i c a l l y s i g n i f i c a n t differences l n s u r v i v a l rates among the three 34 FIG. 14. Mean s u r v i v a l rate per 14 days f o r male and female Mlcrotus townsendll from 1971-72. Horizontal l i n e Is an ar b i t r a r y d i v i s i o n at a s u r v i v a l rate of 0 . 7 1 ; s u r v i v a l rates below th i s l i n e represent losses of more than 50% of the popu-l a t i o n every 4 weeks. 34a JUL SEP 1971 NOV JAN MAR MAY J U L 1972 YEAR TABLE 5* Minimum sur v i v a l rates per 14 days. Sample size i n parentheses. Weight cutoffs* juveniles <30 g subadults 30 - 42 g adults ^ 4 3 g Season MALES FEMALES Adults Subadults Juveniles Total Adults Subadults Juveniles Total Summer 1971 Winter 1971-72 Summer 1972 .73 (261) .83 (93) .59 (139) * .64 (56) .84 (60) .42 (34) #* .51 (39) .83 (33) .30 (11) .68 (353) .84 (177) .55 (208) #* .77 (286) .87 (104) .68 (195) .76 (110) .88 (114) .61 (123) * .83 (45) .95 (41) .54 (19) .77 (441) .90 (244) .66 (375) **• 36 periods. Survival l n the peak population during the winter of 1970-72 was high, and dropped quite low l n the spring de-c l i n e which followed. In past studies on Mlcrotus, differences have been found between s u r v i v a l rates of the adult, subadult, and juvenile components of the population during various phases of the popu-l a t i o n cycle (Chitty, 1952; Krebs, 1964, 1966; Krebs e t . a l . , 1969). I therefore divided my data Into these weight-age classes and looked at s u r v i v a l rates summed over the three periods. Although no s t a t i s t i c a l l y s i g n i f i c a n t differences were found between the three components of the population at any time i n the study, some Interesting trends can be noted. During the summer peak of 1971t the male subadults and the Juveniles had lower s u r v i v a l rates than did adult males. In females, however, subadults and Juveniles had s u r v i v a l rates greater than or equal to those of adults. In the winter of 1971-72 a l l classes of males and females survived equally well; then, during the spring decline, the young and subadults of both sexes survived worse than the adults. It should again be mentioned here that i t was impossible to t e l l the difference between s u r v i v a l and emigration. Early Juvenile Survival Since Juvenile Mlcrotus townsendll do not reach a trap-pable s i z e u n t i l they are 4-6 weeks old, It i s usually d i f f i -c u l t to determine early Juvenile s u r v i v a l . Since trapping was 37 conducted on a weekly basis i n thi s study, i t was possible to close l y monitor the reproductive condition of the females by actual observation of the advanced stages of pregnancy and l a c t a t i o n or by Inference from the body-weight data. Owing to the nature of the l i n e a r habitat, i t was also possible to determine which i n d i v i d u a l Juveniles belonged to which l i t t e r s . In 60% of the l i t t e r s i t was possible to determine the female parent of each l i t t e r which survived to trappable s i z e . Table 6 summarizes the information obtained on early Juvenile s u r v i v a l during the 62 weeks i n which t h i s study was conducted. From an average of 5 young per l i t t e r , i t appears that approximately 70% of the Juveniles are l o s t between b i r t h and weaning at the age of 2-3 weeks. Between weaning and the time at which the young reach trappable s i z e at 4-6 weeks of age, another 16% disappear, leaving only lk% to enter the trap-pable population. When this early Juvenile s u r v i v a l was broken down into the summer and winter periods, the recruitment rates given i n Table 6 were found. The s u r v i v a l of Juveniles from weaning u n t i l they entered l i v e traps (30-38 g ave. weight at f i r s t capture) was constant at 40-45$ i n a l l three periods. By contrast, s u r v i v a l of Juveniles between b i r t h and weaning was variable and very poor i n the declining population of 1972. Juveniles were recruited from 3k% of the l i t t e r s produced i n 1971» but from only 20% of the l i t t e r s produced i n 1972. Other l i v e trapping studies on voles have used an index of early Juvenile s u r v i v a l to estimate Juvenile losses (Krebs, 1966). This Index i s the number of small voles recruited into 38 TABLE 6. Estimates of early juvenile production and s u r v i v a l for three seasonal periods. Time of Year June-Oct. 1971 Summer Nov.-Feb. 1971-72 Winter March-June 1972 Spring Totals Number of advanced pregnancies i n week t (data from 9k females) 115 18 92 225 Number of l a c t a t i n g females i n week t * i 80 (70%) 14 (78*) 47 (51*) 141 (63*) Number of l a c t a t i n g females l n week t+2 46 (40*) 7 (39*) 15 (16.3*) 68 (30*) Number of l i t t e r s from which young were recruited 42 15 18 75 Number of Juveniles captured 94 30 39 163 Number of Juveniles recruited per pregnancy .81 1.67 .42 .72 Number of juveniles captured per l i t t e r r ecruited 2.24 2.0 2.17 2.17 39 the population divided by the number of l a c t a t l n g females two weeks before. For Mlcrotus townsendll, small voles are those less than 40 g and from my data I obtained these indices: Summer 1971 1.2 Winter 1971-72 2.6 Summer 1972 1.0 A 20% drop In early Juvenile s u r v i v a l i s thus indicated between the summer of 1971 and 1972, In agreement with the more detailed data given In Table 6. Mean density and recruitment rate for three subsections of the study area were determined (Table 7). Recruitment was measured by the number of new young (36 g or less) recruited per l a c t a t l n g female. A l l values are expressed f o r an area of one acre, although i n a c t u a l i t y , Area B-C i s 2680 s q . f t . ; Area E, 5440 sq . f t . ; and Area F, 8640 s q . f t . Area F consis-ten t l y had the lowest density, and i n the summer of 1971 did have the highest recruitment rate (.8); but thi s rate d i f f e r e d only s l i g h t l y from the recruitment rate of .7 i n Area E, which had a density twice that of Area F. In the winter of 1971-72, densities were very high i n Area B, yet the recruitment rate was also high (2.3). and Area F, with i t s low density had a lower recruitment rate (1.8). During the decline. Area B also showed higher recruitment than did the other two areas with lower d e n s i t i e s . TABLE 7. Mean density and recruitment rate for three subsections of the study area. Recruitment was measured by the number of new young recruited per l a c t a t i n g female. A l l values expressed for an area of one acre (.40 ha).* Area B-C Area E Area F Mean density Recruitment rate Mean density Recruitment rate Mean density Recruitment rate Male Female Male Female Male Female Summer 1971 65 126 0.4 81 92 0.7 26 36 0.8 Winter 1971-72 74 206 2.3 88 115 0.8 36 41 1.8 Summer 1972 42 112 0.8 41 74 0.3 16 26 0.5 •Area B-C i s 2680 sq.ft.? Area E, 5^40 sq.ft.» Area F, 8640 s q . f t . 41 Survival of Juveniles In Introduction Experiments To study s u r v i v a l of juveniles under densely populated and sparsely populated conditions, I conducted two introduction experiments (Table 8). In the f i r s t , i n March, 1972, s u r v i v a l d i f f e r e d considerably between the densely populated area (40$ a f t e r 4 weeks) and the removal area (70%), but i n the second i n A p r i l , i t did not (66$, 71$). Table 8 summarizes the percent of juveniles surviving one week and four weeks a f t e r introduc-t i o n . Also included i n the table are the numbers of s c r o t a l males and breeding females l i v i n g on Area B at the s t a r t and f i n i s h of the introduction experiments. The disappearance of most of the introduced juveniles occurred during the f i r s t week, regardless of the presence or absence of adults. Their success at establishing themselves may depend upon how quickly they fi n d cover and become f a m i l i a r with the area. In a high density population the presence of aggressive adults might prolong or i n h i b i t t h i s . Ideally these experiments would have been conducted during a peak population when numbers were r e l a t i v e l y stable rather than at a time when numbers were decreasing as ra p i d l y as they were i n the second experiment. During the time these experi-ments were conducted, the population on Area B dropped from 23 animals (equivalent to 946/ha or 383/acre) to 7 animals (133/ acre or 328/ha). I t i s possible that the decrease i n numbers improved the s u r v i v a l rate of the Introduced Juveniles i n the second experiment. 42 TABLE 8. Numbers of juveniles present one and four weeks a f t e r Introduction onto densely populated and uninhabited areas. Date March 1972 A p r i l 1972 Experimental Area Area D Area B Area D Area B Males Fe-males Males Fe-males Males Fe-males Males Fe-males No. of juveniles released .6 4 6 4 5 2 4 2 No. present a f t e r 1 week 5 2 4 1 4 2 4 1 No. present a f t e r 4 weeks 5 2 3 1 3 2 3 1 Adult popu-l a t i o n at time of Introduction 0 0 7 9 0 0 6 8 Adult popula-t i o n at end of 4-week period 0 0 6 8 0 0 2 5 43 Survival of l i t t e r s born In traps Twenty-six l i t t e r s were born In the traps with an average l i t t e r s i z e of 4.9±.3. In the week following the b i r t h of these l i t t e r s , only 9 of the 26 females (35$) were found to be l a c t a t l n g , as opposed to 66$ In the population as a whole. Three of these 9 l i t t e r s entered the trappable population. Since t h i s represented only 11.5$ of the l i t t e r s born In the traps, It Is possible that s u r v i v a l of the young to trappable age was considerably lowered by b i r t h of the l i t t e r In a trap. This was possibly because some females may have had t h e i r young s l i g h t l y prematurely under the stress of being captured. Some abandoned t h e i r young a f t e r being released from the trap, and others, even i f i t was the females that removed them from the trap, may have encountered d i f f i c u l t y i n l o c a t i n g a suitable nest s i t e . One female had three consecutive l i t t e r s exactly 21 days apart i n the traps. Xoung were recruited from one of these l i t t e r s . Possibly the young which were cared for i n the trap f o r several days before being moved or disappearing were much more vulnerable to predatlon by weasels or other predators when t h e i r mother was out than they would have been i n a burrow nest s i t e . The practice of holding pregnant females u n t i l a f t e r the b i r t h of t h e i r l i t t e r s was discontinued a f t e r a two-month period (July-Sept.) when I realized how few survivors there were of those born i n the traps. Of the eight females held u n t i l l i t t e r i n g , none raised t h e i r young to trappable age. 44 We must now ask how trapping affected the success with which females raised t h e i r young to weaning. It i s possible that many young died i n the nest because t h e i r mothers were being held i n traps anywhere from 5 minutes to 12 hours before being released. Since I trapped f o r three 12-hour periods i n each week, i t i s possible that an in d i v i d u a l could spend up to 36 hours l n a trap depending upon how quickly i t entered the trap a f t e r i t was set or reset. The only way i n which I com-pensated f o r thi s i n my study was to r e f r a i n from s e t t i n g a trap i n a given area i n which I knew a l a c t a t i n g female was i n residence. However, the f i r s t trapping period of each week would endanger a l l l i t t e r s . The extent of the danger would of course depend upon the age of the l i t t e r and need f o r frequent nursings as well as the presence of predators. Some of the loss indicated l n Table 6 could thus be a r e s u l t of my trapping, but there i s no reason to suspect that t h i s e f f e c t would d i f f e r between 1971 and 1972. 45 Dispersal In t h i s study, there were two groups of animals which may be la b e l l e d "dispersers". The f i r s t group were those Immi-grants onto the removal area; the second were those which abruptly and permanently moved t h e i r home ranges more than 100 yd (91.4 m) from the centers of t h e i r o r i g i n a l home ranges. Removal Experiment After the f i r s t 10 weeks of trapping, I removed a l l the residents of area D on the grid ( Fig. 2) and began removing any Immigrants into t h i s area. From Sept. u n t i l early Dec. 1971, I removed 17 voles from the removal area (Table 9). Thirteen of these were males with an average weight of 38.5 S and four were females (three Juveniles and one a d u l t ) . Twelve of these 17 voles were tagged and had moved onto th i s area from other sections of the study area; f i v e were caught there f o r the f i r s t time. These 12 tagged Individuals represented 16$ of the 76 animals which disappeared from the trappable popula-t i o n during t h i s period. From Dec. u n t i l Feb. 1972 no animals colonized the removal area. High water standing In section C of the grid ( Fig. 1) and snow cover during this time of year probably account In part f o r this lack of movement onto the removal area. From Feb. u n t i l June 1972, during the population decline, an a d d i t i o n a l 20 immigrants were removed from area D. Of these Immigrants, 13 (65$) were males and seven females. 46 TABLE 9. Number of Mlcrotus townsendll colonizing the removal area and the number of tagged animals disappearing from the control area. Month No. of tagged voles d i s -No. of voles removed No. of tagged voles removed appearing from, study area males females males females June 1971 5 July 43 August 35 September 20 3 1 2 -October 28 7 - 6 -November 28 3 3 3 1 December 11 - - - -January 1972 2 - - - -February 22 4 - 2 -March 51 5 4 2 -A p r i l 26 3 2 3 -May 28 1 1 - mm June 14 mm - - -July 21 - - - -Totals 334 26 11 18 1 47 Eight were tagged and had moved from other sections of the study area, while 12 were captured there f o r the f i r s t time. The eight tagged animals represented 6$ of the 127 animals which disappeared from Feb. to June 1972. In summation, a f t e r i n i t i a l c l e a ring of a l l residents, 37 animals were removed from area D during the course of the study. Nineteen were tagged, and had moved from some other area on the g r i d . Eighteen had not been captured previously and were either a) immigrants from outside the study area, b) animals born on the study area which moved onto the removal area before being trapped and tagged i n t h e i r area of b i r t h , or c) animals born on the removal area. Since the removal area was located In the center of the fence row, from 200-400 yd. from the perimeters of the study area, i t i s l i k e l y that many of the Immigrants were from category (a). Intensive re-moval should have eliminated animals i n category (c), which suggests that most of the immigrants were probably from category (b). Nine of the untagged immigrants were adults and nine were subadults or Juveniles. Of the 37 animals removed, 13 (35$) were adults; 17 (46$) subadults; and 7 (19$) Juveniles. Eleven were females and 26 (70$) were males. 48 Home Range Movements During t h i s study 35 animals (35/3^8 = 10* of the total) moved t h e i r home ranges more than 100 yards from the center of th e i r o r i g i n a l range: 26 (74*) males and 9 (26*) females. Eighteen of these 35 voles moved onto the removal area and were themselves removed. They have therefore also been counted i n the removal dispersers. Abrupt movements of home ranges occurred primarily during two periods: Sept.-Nov. and Feb.-(Table 10). F a l l movements coincided with Increased s u r v i v a l of Juveniles; spring with the s t a r t of the decline. Since both groups of dispersers had moved over 100 yd and since the sex and age-weight classes were s i m i l a r , I have pooled the data from both groups. Forty-seven animals were considered to be dispersers. This represented 13*5* of the t o t a l population. Fourteen were females (30*) and 33 (70*) were males. A l l of the males were l n breeding condition. Half of the females were juveniles which had not begun to breed, and half were subadults i n breeding condition. Snap traps were set i n a one^/quarter acre portion of the f i e l d adjacent to study area A during the decline i n March and A p r i l to see i f some of the disappearance could be explained by emigration. One hundred traps were prebalted f o r 2 days, set for 3 days, and nights and checked 4 times during each 24 h period. Only 3 voles were captured l n 300 trap nights and none of these were tagged emigrants from the study area. A p r i l , with very few movements during other months. TABLE 10. Number of animals abruptly moving t h e i r home range over 100 yards. Males Females June 1971 1 _ July - mm August 1 September 1 October 5 3 November 2 1 December - -January 1972 1 mm February -March 2 2 A p r i l 3 mm May •* 1 June - -July -Totals 26 9 50 Home Ranges Home ranges were calculated for each Individual captured more than 5 times. Since the habitat was linear, I decided to measure home ranges In terms of adjusted range length, which is the observed range length plus half the distance to the next station on each end (Table 11). The adjusted range lengths of 80 males varied from 40 feet (12.2 m) to 460 feet (140.2 m), with one individual ranging 740 feet (225.6 m) regularly within 24 hours. The median of the male range lengths was 140 feet (42.7 m) and the average length was 184 feet (56.1 m). Females had a much shorter range length. One hundred and seven females had range lengths varying from 40 feet (12.2 m) to 320 feet (97.5 m) with a median of 100 feet (30.5 m) and an average of 108.6 feet (33»1 nt) • Assuming that these voles travelled the entire width of the fence row, we find the aver-age area of the range to be .036 acre (.015 ha) for males and .024 acre (.0097 ha) for females. Table 11 shows that males have longer (and therefore larger) ranges than females In a l l seasons; that both males and females had smaller ranges in winter than summer, and that both had larger ranges during the decline, when densities were lower, than during the peak. 51 TABLE 11. Average range lengths In feet f o r males and females according to season. Sample sizes In parenthes es. Males Females Summer 1971 Winter 1971-72 Summer 1972 190+145 (39) 147± 93 (3D 232±l40 (14) 107±51.8 (49) 97±38 (42) 134+61 (27) 52 DISCUSSION Population densities vary because of changes l n rates of reproduction, mortality, and d i s p e r s a l . I w i l l now examine these various c h a r a c t e r i s t i c s l n th i s population to see If they can account f o r the observed fluctuations l n density, and then compare my res u l t s with those obtained f o r other populations of Mlcrotus. Population Density It Is unfortunate that trapping i n th i s study did not be-gin In early 1971 so that the c h a r a c t e r i s t i c s of the spring 1971 population could be compared with those of the decline l n the spring of 1972. If thi s population had followed the pattern of the other M. townsendll populations being studied l n t h i s area (LeDec, 1975; Krebs, MS l n prep.). It would have been high l n the spring of 1971t dropping s l i g h t l y u n t i l May and June when thi s study began. The presence of heavy adults and high densities during the summer of 1971 suggests that 1971 and early 1972 were peak periods f o r this population. Peak densities averaged about 262-3^9 voles per ha (109-l 4 l per acre) and as high as 948 per ha l n section B of the study area at Its maximum density. These density figures com-pare with peak densities of 136-148 per ha f o r M. pennsylvannlous and 86-99 per ha f o r M. ochrogaster (Krebs, et a l . , 1969); 6l7-642/ha for M. c a l l f o r n l c u s (Krebs, 1966); and l62-298/ha with one population reaching 625/ha f o r M. townsendll (LeDuc, 53 1975; Krebs MS In prep.). Since other populations of M. townsendll have reached high densities s i m i l a r to those found In thi s study. It appears that the high densities I observed were not an a r t i f a c t of the l i n e a r habitat. Reproduction Reproductive changes are par t l y responsible f o r changes occurring In population d e n s i t i e s , thereby influencing c y c l i c trends. I now look at the d i f f e r e n t components of reproduc-t i v e e f f o r t to see i f any can help to explain the observed f l u c t u a t i o n In numbers. This population of Mlcrotus  townsendll d i f f e r e d In two reproductive trends (length of breeding season, and age at sexual maturity) normally seen i n peak and dec l i n i n g populations of microtines. Winter breeding among Mlcrotus has been noted by many workers and most frequently occurs during the increase phase of the population cycle. The breeding season often ends ab-normally early i n the peak year (Krebs, 1964b; K e l l e r and Krebs, 1970; and Krebs and Myers, 1974), and the populations begin de-c l i n i n g i n l a t e winter or early spring. In 11 of the 12 studies of Mlcrotus reviewed by Schaffer and Tamarin (1973). there was a shortening of the breeding season of the peak year and no change i n the 12th study. I t i s therefore unusual to find a population of Mlcrotus, such as this one, with only a b r i e f cessation of breeding In the winter of a peak year, f o l -lowed by Increased breeding and good s u r v i v a l In l a t e winter, and the decline s t a r t i n g i n March. 54 Most workers seem to agree that pregnancy rate does not vary l n r e l a t i o n to the cycle (Krebs and Myers, 1974) and t h i s population also showed l i t t l e difference l n reproductive In-tensity between the peak and decline phases. K e l l e r and Krebs (1970) show that median weight at sexual maturity was higher l n peak populations of M. pennsylvanlcus and M. ochrogaster but equal In increasing and declining popu-l a t i o n s . In t h e i r general review of the l i t e r a t u r e , Krebs and Myers (1974) found that i n most studies, animals matured l a t e r i n the peak year than during the increase and decline. The r e s u l t s l n t h i s study were contrary to the above trends. The median weight at sexual maturity did not change su b s t a n t i a l l y from the peak summer of 1971 to the spring decline of 1972. In a l l cases weights at maturity were much lower i n th i s population than i n other populations of M. townsendll l n this area. Maturity weights were s l i g h t l y lower during the decline than the peak, but since growth rates were also s l i g h t l y lower, the rates were probably about the same. In summary, the decline i n t h i s population cannot be ex-plained on the basis of changes i n the reproductive c h a r a c t e r i s t i c s . 55 Growth Rates and Body Weights This population of Mlcrotus showed the same sequence of changes In mean body weight as recorded f o r other c y c l i c mlcrotlnes by Chltty and Chltty (1962) and Krebs (1964 and l a t e r papers). Growth rates are affected by seasonal v a r i a -t i o n and f o r t h i s study the mean growth rate was lowest during the winter (.5l#/day)« Comparing summer growth rates, how-ever, I found that t h i s population had a lower growth rate (•95#/day) during the decline than during the peak (1.3$/day). It appears that body weights In this population are lower than In other populations of Mlcrotus townsendll In thi s area. Not only Is the body weight at sexual maturity lower, but also the average body weight f o r the d i f f e r e n t phases of the popula-t i o n cycle Including the maximum male body weight during the peak. Maximum body weights observed for males In this popula-t i o n were 84 g with an average of 48 g; while other nearby populations had body weights as high as 105 g with averages of 60 g during peak. The high body weights In other populations of M. townsendll were recorded In March and A p r i l of peak years and no data are ava i l a b l e on the body weights of thi s popula-t i o n during the corresponding period. Average male body weights f o r t h i s population were 48.6 g during the summer of 1971. 40.7 g during the winter of 1971-72, and 48.8 g during the spring decline of 1972. These are paralle l e d by corres-ponding figures of 60 g, 45 g, and 56 g reported by LeDuc (1975). 56 Mortality The decline l n numbers of t h i s population can be explained s o l e l y on the basis of low s u r v i v a l rates during the spring of 1972. The s u r v i v a l rates dropped from 84* per two weeks i n the peak population of winter 1971 and early 1972 to 55* during spring. The decline i n s u r v i v a l occurred at a time when the grass had resumed growing so that food was abundant. No weasels were captured at t h i s time (they are usually caught i n the Longworth traps, i f present) and there was no reason to suspect an unusual amount of predatlon by owls, hawks, racoons, or f e r a l cats l n the spring of 1972. The farmer did not s t a r t plowing and planting his f i e l d s u n t i l May, so there were no a g r i c u l t u r a l practices such as the spraying of herbicides which could have contributed to the decline i n numbers. Survival rates during this decline are s i m i l a r to those found i n other Mlcrotus declines(Krebs etyal., 1969) and those of Mlcrotus  townsendll (peak 88*; decline 60*) reported by Hilborn (1974). My estimate that 86* of the young are l o s t between b i r t h and recruitment into the trappable population i s very s i m i l a r to that of Krebs et a l . (1969) of 80* l n M. oohrogaster and 85* i n M. pennsylvanlcus. 57 Dispersal Since mortality i s equated with disappearance from the trappable population, It i s of considerable i n t e r e s t to know how much of the mortality i s due to di s p e r s a l rather than death. Dispersal has been shown to play an important part i n population regulation of voles by Clarke (1955) and by Krebs et a l . (1969). Myers and Krebs (1971b) measured d i s p e r s a l from a control area onto a trapped out area and found d i s p e r s a l the highest during the Increase phase of the population cycle (56-69*), reduced by almost half In the peak phase (25-33*), and reduced to less than a quarter the highest l e v e l l n the decline phase (12-15*). The dis p e r s a l measured i n thi s study followed the above trend i n that more dis p e r s a l was observed i n the peak (19*) rather than the decline phase (11*), however these rates were lower than those of the above study. 1 58 Home Ranges The adjusted range lengths of males i n t h i s population (44-70 m) were longer than those reported by Krebs (1970) of 40-59 m for male Mlcrotus ochrogaster and 40-43 m for male Mlcrotus pennsylvanlous. However, i n terms of area, the home ranges l n the l i n e a r habitat were considerably smaller (male ave., .036 acre; female, .024 acre) than those usually found i n continuous habitats. Van Vleck (I969) recorded home ranges in three separate studies averaging .38-.49 acre for males l n both high and low density populations of Mlcrotus pennsylvanlous. and .30-35 acre f o r females. In other studies reviewed by Van Vleck (I969), home ranges of Mlcrotus pennsylvanlous were found to vary from .10-.86 acre for males and .09-.77 for females. No home range sizes are availa b l e f o r other populations of M.  townsendll, but a cursory look at the data collected by LeDuc and Krebs (1975)t suggests that range sizes are more t y p i c a l l y l i k e those of M. pennsylvanlous than the very small sizes I observed l n t h i s population. It would be in t e r e s t i n g to see i f other populations of voles l n l i n e a r habitats would exhibit s i m i l a r c h a r a c t e r i s t i c s of small home ranges and small body weights. In conclusion, although t h i s study was mainly descriptive i n nature, there are several other Interesting aspects of microtine biology which might be studied l n a l i n e a r habitat. If the Individuals could be typed according to behavioral char-a c t e r i s t i c s i n r e l a t i o n to nearest neighbors (l/e. dominant, (5 59 /A subordlnant/), then Introduction and removal experiments using j fa animals of known behavioral types could be conducted. It would be Interesting to know the behavioral types of I n d i v i -duals which disperse onto a removal area and those which d i s -appear from the population. Home range sizes could a l s o be correlated with behavior and the s u r v i v a l of juveniles studied i n terms of the behavior of the adults In the v i c i n i t y of th e i r nest. 60 LITERATURE CITED Brody, s. 1945. Bloenergetlcs and growth. Reinhold, New York. l,023p. Chitty, D. 1952. Mortality among voles (Mlcrotus agrestls) at Lake Vyrnwy, Montgomeryshire i n 1936-9. P h i l . Trans. Roy. Soc. London, Ser. B, 236: 505-552. Chitty, D. I960. Population processes i n the vole and th e i r relevance to general theory. Can. J . Zool. 38: 99-113. Chitty, D. 196?. The natural s e l e c t i o n of self-regulatory behaviour i n animal populations. Proc. Ecol. Soc. A u s t r a l i a 2: 51-78. Chitty, D. 1969. Regulatory effects of a random v a r i a b l e . Am. Zool. 9: 400. Chitty, H., and D. Chitty. 1962. Body weight i n r e l a t i o n to population phase i n Mlcrotus a g r e s t l s . Symposium Theriologicum, Brno, I960, pp. 77-86. Clarke, J. R. 1955. influence of numbers on reproduction and su r v i v a l in two experimental vole populations. Proc. R. Soc. Ser. B, 144: 68-85. Getz, L. L. 1961. Home ranges, t e r r i t o r i a l i t y , and movement of the meadow vole. J . Mammal. 42: 24-36. Healey, M. C. 1967. Aggression and s e l f - r e g u l a t i o n of population s i z e In deermlce. Ecology 48: 377-392. Hilborn, R. 1974. Ph.D. Thesis. The Univ. of B.C. Fates of disappearing individuals i n fl u c t u a t i n g populations of M. townsendll. Iverson, F. L., and B. N. Turner. 1974. Winter weight dynamics i n Mlcrotus pennsylvanlcus. Ecol. 55s 1030-1041. K e l l e r , B. L., and C. J . Krebs. 1970. Mlcrotus population biology I I I . Reproductive changes i n fl u c t u a t i n g populations of M. ochrogaster and M. pennsylvanlcus In southern Indiana, 1965-67. Ecol. Monogr. 40: 263-294. Krebs, C. J . 1964a. The lemming cycle at Baker Lake, Northwest T e r r i t o r i e s , during 1959-62. A r c t i c Inst. N. Amer. Tech. Pap. 15. 104 p. Krebs, C. J . 1964b. C y c l i c v a r i a t i o n i n skull-body regressions of lemmings. Can. J . Zool. 42: 631-643. 61 Krebs, C. J . 1966. Demographic changes l n fl u c t u a t i n g popula-tions of Mlcrotus c a l i f o r n l o u s . E c o l. Monogr. 36: 239-273. Krebs, C. J . , B. L. K e l l e r , and R. H. Tamarln. 1969. Mlcrotus population biology: Demographic changes l n f l u c t u a t i n g popu-l a t i o n s of M. ochrogaster and M. pennsylvanlous l n southern Indlana. Ecology 50: 587-607. Krebs, C. J . , and J . H. Myers. 1974. Population cycles l n small mammals. Ad/. Iff Ecol. Res. 8: 267-399. LeDuc, J . 1975. Masters Thesis. The University of B.C. Demographic consequences of a r t i f i c i a l s e l e c t i o n at the lap locus i n voles (Mlcrotus townsendll). LeDuc, J . , and C. J . Krebs. 1975. Demographic consequences of a r t i f i c i a l s e l e c t i o n at the LAP locus i n voles (Mlcrotus  townsendll). Can. J . Zool. 53: i n press, Dec. L e s l i e , P. H., J . S. Perry, and J . S. Watson. 1945. The determination of the median body-weight at which female rats reach maturity. Proc. Zool. Soc. Lond. 115: 473-488. Myers, J . H., and C. J . Krebs. 1971a. Sex r a t i o s i n open and enclosed vole populations: demographic implications. Am. Nat. 105: 325-344. Myers, J . H., and C. J. Krebs. 1971b. Genetic, behavioural, and reproductive attributes of dispersing f i e l d voles Mlcrotus pennsylvanlous and Mlcrotus ochrogaster. Sad l e l r , R. M. F. S. 1965. The re l a t i o n s h i p between agonistic behaviour and population changes i n the deermouse. Peromysous  manlculatus (Wagner). J . Anim. Ecol. 34: 331-352. Schaffer, W. M., and R. H. Tamarin. 1973. Changing reproduc-t i v e rates and population cycles i n lemmings and voles. Evolution 27: 111-124. Van Vleck, D. B. 1969. Standardization of Mlcrotus home range c a l c u l a t i o n . J . Mammal. 50: 69-8O. 

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