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Demographic consequences of invasion by a native, controphic competitor to an insular bird population Johnson, Kate Marie; Germain, Ryan Ross; Tarwarter, C. E.; Arcese, Peter 2017-08

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                                                                                                                         Johnson et al.    1 Running head: Demographic consequences of invasion by a native competitor 1  2 Demographic consequences of invasion by a native, controphic competitor to an insular 3 bird population 4 Kate M. Johnson1,2, R.R. Germain1,3, C.E. Tarwarter1,4, and Peter Arcese1* 5  6 1Department of Forest and Conservation Sciences, University of British Columbia, 2424 Main 7 Mall, Vancouver, BC, Canada V6T 1Z4 8 2Faculty of Pharmaceutical Sciences, University of British Columbia, 2405 Wesbrook Mall, 9 Vancouver, BC, Canada, V6T 1Z3 10 3Institute of Biological and Environmental Sciences, School of Biological Sciences, Zoology 11 Building, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK 12 4Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071 13 *Corresponding author:, Phone: 604-822-1886  14 Word count: 4,331 (excluding abstract, acknowledgement, references, and figure legends)  15                                                                                                                          Johnson et al.    2 Abstract. Species invasions and range shifts can lead to novel competitive interactions between 16 historically resident and colonizing species, but the demographic consequences of such 17 interactions remain controversial. We present results from field experiments and 45 yrs of 18 demographic monitoring to test the hypothesis that the colonization of Mandarte Is., BC, Canada, 19 by fox sparrows (Passerella iliaca) caused the long-term decline of the song sparrow (Melospiza 20 melodia) population resident there. Several lines of evidence indicate that competition with fox 21 sparrows for winter food reduced over-winter survival in juvenile song sparrows, enforcing 22 population decline despite an increase in annual reproductive rate in song sparrows over the 23 same period. In contrast, we found no evidence of interspecific competition for resources during 24 the breeding season. Our results indicate that in the absence of a sufficient ecological or 25 evolutionary shift in niche dimension, range expansions by dominant competitors have the 26 potential to cause the extirpation of historically resident species when competitive interactions 27 between them are strong and resources not equitably partitioned. 28 Word count: 165 29 Keywords: interspecific competition; competitive exclusion; invasive species; range shifts; 30 Melospiza melodia; Passerella iliaca 31 32                                                                                                                          Johnson et al.    3 INTRODUCTION 33 Exotic species are well-known to affect community composition via competitive, predatory and 34 pathogenic interactions, especially on islands (Reaser et al. 2007; Dhondt 2012; Doherty et al. 35 2016). However, despite many native species undergoing range shifts linked to climate and land 36 use change (e.g., Parmesan 2006; Early and Sax 2014; Krosby et al. 2015; Elmhagen et al. 2015), 37 relatively little is known about the demographic impacts of these colonists on historically extant 38 species (Davis and Shaw 2001; Loarie et al. 2008; Sorte et al. 2010; Rodewald and Arcese 39 2016). Theory suggests that the response of native species to controphic colonists will depend on 40 their overlap in resource use, the demographic effects of resource limitation, the time-frame over 41 which competitive exclusion might occur relative to the rate at which native species can adapt 42 via ecological or evolutionary shifts in niche dimension, and the spatial scale examined (Shea 43 and Chesson 2002; Davis 2003; Gurevitch and Padilla 2004; Reaser et al. 2007; Sax et al. 2007; 44 Bennett et al. 2012; Dhondt 2012; Stuart et al. 2014). Although examples of competitive 45 exclusion by colonist species remain rare, they are thought to be most likely to occur where 46 environmental heterogeneity is low, such as small islands or isolated water bodies (e.g., Chesson 47 2000; Davies et al. 2005; Melbourne et al. 2007; MacDougall et al. 2009). Given the potential for 48 novel competitive interactions to shape species demography and community composition 49 (Simberloff 2005; Reaser et al. 2007), we used a 45 yr study of an island song sparrow 50 (Melospiza melodia) population to ask how the colonisation and expansion of a colonist, 51 controphic competitor, the fox sparrow (Paserella illiaca) has affected its demography.  52 Although described as migratory throughout its range in North America (Weckstein et al. 53 2002), fox sparrows established resident populations in the San Juan and Gulf Islands of western 54 North America in the latter half of the 20th century, where they now survive and reproduce at 55                                                                                                                          Johnson et al.    4 rates consistent with rapid population growth (Visty et al. 2017). Because song and fox sparrows 56 are territorial, very similar in life history, but differ in size, we used field experiments and 57 demographic analyses to test for evidence of interspecific competition between song sparrows 58 and this colonist following Dhondt (2012) and Jankowski et al. (2010).  59 Specifically, we first used a life-table response experiment and 45 years of demographic 60 data to identify vital rates contributing most to song sparrow population growth over time, and to 61 test if those rates varied with fox sparrow abundance. We next tested for interspecific 62 competition for breeding habitat by conducting simulated territorial intrusions to quantify 63 interference competition. Because exploitative competition might reduce breeding habitat quality 64 even in the absence of interspecific territoriality, we also tested for long-term declines in site 65 quality following Germain and Arcese (2014). Last, we tested for evidence of interspecific 66 competition for access to winter food by measuring diet overlap and inter-specific dominance.  67  68 METHODS 69 Study system 70 Mandarte Is. is a c. 6 ha islet in southwestern BC, Canada, where a resident, individually-banded 71 song sparrow population was monitored from 1960–63 and 1975–2016 (Tompa 1963; Germain 72 et al. 2015). Song sparrows are a c. 24 g passerine that occur over much of North America at 73 densities of ~1–9 pairs/ha (Arcese et al. 2002). On Mandarte Is. song sparrows lay 2–5 eggs in 1-74 7 open-cup nests annually (Arcese et al. 1992) and defend 200–5000 m2 territories year-round 75 (Arcese 1989). From April – July 1960–63, 1975-79, and 1981-2016, the territorial status and 76 breeding activity of all song sparrows was monitored at least weekly to locate all nests annually. 77 All nestlings were colour-banded, followed to independence from parental care (~24 days post-78                                                                                                                          Johnson et al.    5 hatch) and their recruitment or disappearance from the population was recorded, which provided 79 precise estimates of annual population size, age structure, and reproductive and survival rates 80 (annual re-sighting probability >99%; Wilson et al. 2007). About one immigrant song sparrow 81 settles on Mandarte Is. annually, but because that rate is low and about constant it is not 82 considered here.  83 Fox sparrows are ~19% larger than song sparrows in mass and linear traits and, like song 84 sparrows are territorial, multi-brooded, open-cup nesters that feed mainly on seeds (winter) and 85 insects (breeding; Weckstein et al. 2002, Visty et al. 2017). Fox sparrows are native to BC, but 86 were absent from Mandarte Is. prior to 1975 (Tompa 1963; Drent et al. 1964), when they first 87 bred there (J.N.M. Smith, pers. com.). Fox sparrows were counted systematically in 13 yrs from 88 1960-2016 by spot-mapping singing males, their mates and nest locations, and by mapping their 89 territories in detail in 2010 and 2013–16 when up to 70% of adults were individually identified.   90 Rate of annual change in the number of female sparrows on the island in late April was 91 estimated using a generalized linear model with year as a fixed effect (fox sparrow: Poisson 92 distribution, log link; song sparrow, Gaussian distribution). We tested for temporal 93 autocorrelation in the time series (finding none) and employed R 3.1.3 (R Core Team 2015) for 94 all statistical analyses. 95 Demographic rates 96 We identified the demographic vital rate contributing most to song sparrow population growth 97 using a stage-structured life table response experiment (LTRE) to estimate the contribution of 98 each vital rate to growth from 1975–2014. We used juvenile and adult age-classes for both sexes 99 because adults differ modestly in survival and reproductive rate after reaching adulthood. Local 100 juvenile survival was estimated as the proportion of offspring surviving from independence (day 101                                                                                                                          Johnson et al.    6 24 post-hatch) to April 30 the next year. Adult survival was the proportion of individuals alive 102 on April 30 in year t that survived to year t + 1. Reproductive rate was the mean number of 103 independent young produced per female annually, excluding birds used in experiments (1979, n 104 = 70; 1985, n = 87; 1988, n = 114; Arcese and Smith 1988; Smith et al. 2006). Juvenile survival 105 was unknown in 1979 and 1980, reproductive rate unknown in 1980, and adult survival unknown 106 in 1975.  107 The LTRE included a treatment matrix parameterized with juvenile and adult survival 108 and adult reproductive rate in each of 37 yrs. Vital rates were arranged in 2 x 2 treatment 109 matrices wherein the 1st and 2nd columns included juvenile and adult vital rates, respectively, and 110 the first and second rows specified reproduction and survival rates, respectively. Because 111 juveniles do not breed, the 1st row of column one always equalled zero. Treatment matrices were 112 compared to a 2 x 2 reference matrix of mean vital rate over all years to determine the 113 contribution (cij) of each vital rate to annual population growth (cf Caswell 1996) such that: 114  115 𝑐!" = 𝑎!"  !"# − 𝑎!"  !"! ∗   𝑠!"  !"#                                            eqn. 1 116  117 where aij is the (i,j) element of a, the reference (ref) or treatment (trt) matrix, and sij is the 118 sensitivity of the reference matrix, indicating the impact of an absolute change in a vital rate on 119 population growth (de Kroon et al. 1986; Caswell 1996). Analyses were implemented in the 120 popbio package (Stubben and Milligan 2007) and trends in vital rate contribution estimated by 121 linear model (Gaussian). Because the LTRE indicated that juvenile survival was a key vital rate, 122 we tested for an effect (α ≤ 0.05) of fox sparrow abundance on juvenile song sparrow survival 123 using a linear model (Gaussian distribution), with fox and song sparrow numbers as predictors. 124                                                                                                                          Johnson et al.    7 Song sparrow numbers was included because it was shown previously to predict juvenile 125 survival in that species (Arcese et al. 1992). Estimates of fox sparrow population size and song 126 sparrow juvenile survival were available for 11 years from 1960 to 2016. 127  128 Competition for winter food 129 Dhondt (2012) identified space, nesting habitat and food as common limiting factors in bird 130 communities. We tested for interspecific competition for winter food between fox and song 131 sparrows using a seed preference experiment to estimate diet overlap, and two arena experiments 132 to assess behavioral dominance in contests over food. We assessed the breadth of winter food 133 available by characterising the type and abundance of seeds in soil, given that both species feed 134 mainly on seeds in winter (Tompa 1963; Willson 1971; Arcese et al. 1992; Weckstein et al. 135 2002). To do so we excavated 250 ml of soil (10 x 15cm, 2cm deep) at 15 sites across the island 136 in December 2013, extracting all seeds by sieve to estimate abundance by identity. Seeds 137 recovered in soil included 64% blackberry (Rubus armeniacus), 17% Oregon grape (Mahonia 138 aquifolium), 8% Nootka rose (Rosa nootkensis), 8% red elderberry (Sambucus racemosa), and 139 3% other (bitter cherry, Prunus emarginata; choke cherry, P. virginiana; snowberry, 140 Symphoricarpos albus; grape, Vitis sp.) by volume. 141 We estimated fox and song sparrow preference for seeds in March 2015 by cleaning, then 142 freezing blackberry, Nootka rose, elderberry, and snowberry seeds collected from fruits in 143 summer 2014. We chose the seed types that were most abundant in soil samples with the 144 exception of Oregon grape and cherry, which are ~1.5x larger than all other seed types and are 145 likely inedible to song and fox sparrows. Seeds were arranged by species in one of four 98cm3 146 circular depressions (‘cups’) in 60 x 12 x 3 cm plywood feeders, and rotated among cups in each 147                                                                                                                          Johnson et al.    8 trial to avoid location effects. Feeders were placed on the ground at 6 locations on the island used 148 regularly by foraging fox and song sparrows. In each trial we recorded by video the fraction of 149 time a visiting fox or song sparrow fed on each seed type (N=14 trials, including 50 visits by 6 150 different song sparrows and 9 different fox sparrows). A ‘visit’ comprised the time elapsed from 151 when a focal bird picked up its first seed, to the time the focal bird’s lower mandible stopped 152 moving after last seed was eaten, prior to leaving. Seed preference was estimated by recording 153 the total time from when a focal bird picked up a first seed in cup x, to the point its lower 154 mandible stopped moving before selecting a seed from a different cup or leaving. The proportion 155 of total time spent feeding on each seed type during a visit was then used as the dependent 156 variable in a generalized linear mixed model (GLMM, quasibinomial distribution, logit link). 157 Each visit was numbered and included in the model as a random effect, as was fox or song 158 sparrow identity. We used the glht function in the multcomp package (Hothorn et al. 2008) to 159 assess statistical significance of all pairwise comparisons of fox or song sparrow with each seed 160 species using Tukey contrasts for unequal groups (Tukey 1949; Kramer 1956). 161 We conducted arena experiments in October 2013 to assess interspecific dominance at 162 winter food sources by piling 250ml of commercial bird seed at 5 sheltered locations across the 163 island and using video cameras to record interactions. We recorded 19 fox and 19 song sparrows 164 in 68 aggressive interactions. The wining individual stayed in the arena (datum = 1); the loser 165 was chased (datum = 0; Arcese and Smith 1985). We tested the null expectation of equality 166 among species using two GLMMs (binomial distributions, logit links), each including fox and 167 song sparrow identity as random effects. In the first model, we estimated the displacement rate 168 using maximum likelihood; in the second model, we tested the hypothesis of displacement rate 169 equal to 0.5 for both species. Finding a significant effect of ‘species’ on displacement is 170                                                                                                                          Johnson et al.    9 consistent with a hypothesis of interspecific dominance.  We replicated the arena experiment in 171 March 2015 using 20 feeders (15 x 20cm plastic tray on ~20cm stake; 250ml of commercial 172 birdseed), distributed in sheltered sites across the island. We monitored, scored and analysed 31 173 interactions between 20 song and 16 fox sparrows. 174  175 Competition for space 176 To test whether fox and song sparrows compete for breeding space, we first calculated the spatial 177 overlap of song and fox sparrow territories in 2010, 2013, 2014 using ArcMap (ESRI 2011). We 178 also conducted playback experiments prior to breeding in April 2014 to quantify the response of 179 song sparrows to fox sparrow, song sparrow, and Swainson’s thrush (Catharus ustulatus) (a 180 control species) following Jankowski et al. (2010). Playback experiments involved placing a 181 taxidermic mount on an artificial perch at the center of 27 song sparrow territories, and playing 182 species-appropriate song from a speaker placed below the mount. In each 12 min trial we 183 recorded the closest approach of the territorial male and female song sparrow to the mounts, 184 during: 2 min of pre-trial observation, 5 min of playback, and 5 min follow-up observation. All 185 mounts were presented in random order and prepared in a neutral, perched position. Swainson’s 186 thrushes are similar to fox sparrows in size but do not breed on Mandarte Is. Trials were 187 conducted throughout the day but separated by > 1 hour on focal and neighboring territories. 188 Closest approach to the mount was used as a dependent variable to indicate aggression (cf  189 Jankowski et al. 2010) and compared among mounts using a GLMM (negative binomial 190 distribution, log link), and including the identity of focal male song sparrows as a random effect. 191 Time of day (categorical effect: morning, <10am, n = 22; midday, 10–1pm, n = 27; afternoon, 1–192 4pm, n = 6; evening, >4pm, n = 22), whether or not the focal male was singing prior to playback 193                                                                                                                          Johnson et al.    10 (1/0 fixed effect), and whether one or more neighbor males sang in response to playbacks were 194 also recorded and included as covariates (1/0 fixed effect).  195  196 Competition for nesting habitat 197 We followed Germain and Arcese (2014) to estimate site quality as the mean number of 198 independent song sparrow young produced annually in each of 146 20 x 20 m grid cells 199 distributed continuously over the island. Doing so allowed us to test the prediction that site 200 quality for song sparrows declined as fox sparrow abundance increased over time, as expected 201 given competition by song and fox sparrows for breeding resources. Specifically, we regressed 202 site quality on year using a linear mixed model (year of study included as a fixed effect, grid cell 203 identity as a random effect). We also tested for a decline in the mean number of independent 204 young produced by female song sparrows over the study using a GLMM (negative binomial 205 distribution, log link) with year as a fixed effect and female identity as a random effect to 206 account for repeat observations across years.  207  208 RESULTS 209 Population size and demography 210 Female song sparrow population size varied widely over 45 years (range = 4–71, mean = 35.0 ± 211 17.1 SD), but declined on average (β = -0.60 ± 0.14 SE, t(43) = -4.19, p < 0.001, R2 = 0.29; 212 figure 1).  In contrast, fox sparrow abundance increased after 1975 (range = 1–30 females, β = 213 0.06 ± 0.01 SE, z(11) = 9.13, p < 0.001; figure 1).   214 Juvenile song sparrow survival also varied widely over the 37 yrs it was recorded from 215 1960 to 2016 (range = 0.04–0.88, mean = 0.37 ± 0.18 SD, nyrs = 37; figure 2), as did adult annual 216                                                                                                                          Johnson et al.    11 survival (range = 0.07–0.88, mean = 0.59 ± 0.17 SD, nyrs = 38), and annual reproductive rate 217 (range = 1.10–6.90, mean = 3.25 ± 1.32 SD, nyrs = 38). However, despite wide variation in song 218 sparrow vital rates over time, when contribution (as determined from the LTRE) was regressed 219 on year, juvenile survival was the only vital rate to increase in influence over time (β = -0.01 ± 220 0.004 SE, t(35)  = -3.37, R2 = 0.25, p = 0.002; figure 3a). Variation in annual reproductive rate 221 had no detectable effect on long-term change in song sparrow population size (t(35)  = -1.29, p = 222 0.21, R2 = 0.05; figure 3b). Similarly, the contributions of annual adult survival to change in 223 population size were smaller and unrelated to long-term decline in song sparrow abundance 224 (t(35)  = 1.29, p = 0.20, R2 = 0.05; figure 3c).  225 The fraction of juvenile song sparrows surviving overwinter also declined as fox sparrow 226 abundance increased (figure 4), amounting to a 40% decline in the expected value of survival 227 from 1960 to 2015 (0.39 ± 0.06 SE and 0.23 ± 0.07 SE, respectively). Juvenile song sparrow 228 survival was also inversely related to fox sparrow population size (β = -0.009, ± 0.004 SE, t(8) = 229 -2.46, p = 0.04, R2 = 0.44), but unrelated to song sparrow population size (t(8) = -0.63, p = 0.55).  230  231 Competition for winter food 232 Fox and song sparrows exhibited a strong preference for elderberry (mean proportion of time 233 spent feeding 0.34 ± 0.05 SE and 0.34 ± 0.12 SE, respectively), and against blackberry (mean 234 0.05 ± 0.02 SE and <0.01 ± 0.003 SE, respectively). But we found no differences in the time 235 spent feeding by each species on blackberry, Nootka rose, red elderberry or snowberry seeds 236 (blackberry: z(59) = -0.81, p = 1.0, elderberry: z(59)  = -0.19, p = 1.0, rose: z(59)  = -0.37, p = 237 1.0, snowberry: z(59)  = -0.18, p = 1.0), implying complete overlap in preference for these seed 238 species (figure 5).  239                                                                                                                          Johnson et al.    12 Our observations of song and fox sparrows at experimental arenas baited with 240 commercial bird seed revealed that song sparrows were displaced by fox sparrows in 91% of 68 241 contests (Χ2 = 25.6, df = 1, p < 0.001) in October. In a replicate experiment in early March fox 242 sparrows displaced song sparrows in 100% of 31 interactions, obviating further analysis. 243  244 Competition for space and nest sites 245 We observed no evidence of competition for space or nest sites during the breeding period. Fox 246 and song sparrow breeding territories overlapped 100% in 2010, 2013 and 2014, and no 247 aggressive interactions between them were observed despite regularly perching or singing within 248 1 m of each other. Territorial song sparrows approached conspecific mounts in simulated 249 intrusions much more closely than fox sparrow or Swainson’s thrush mounts (t(41) = -7.83, p < 250 0.001 and t(41) = -8.28, p < 0.001; respectively). Song sparrows also responded similarly to fox 251 sparrows and Swainson’s thrush mounts (t(41) = -0.82, p = 0.42; figure 6), indicating that song 252 sparrows did not respond to territorial intrusions by fox sparrows as expected if these species 253 compete for breeding space. 254 We observed no evidence of a long-term decline in nest site quality (expected number of 255 song sparrow young produced per nest; t(2671) = -1.29, p = 0.20) evaluated in 147 grid squares 256 distributed continuously over the island and monitored annually over the study (see Methods).  257 However, we observed a statistically significant increase in the mean annual reproductive 258 success of female song sparrows from 1975 to 2014 (β = 0.014, SE = 0.002, t(643) = 6.63, p < 259 0.001). These results are opposite to the hypothesis that fox and song sparrows compete during 260 the breeding period. 261  262                                                                                                                          Johnson et al.    13 DISCUSSION 263 We tested whether the colonization of Mandarte Is. by fox sparrows in 1975 led to the decline of 264 the song sparrow population resident there (figure 1). Song sparrows have, on average, declined 265 over the past 46 years whilst fox sparrows increased from 0 to 30 breeding pairs. Demographic 266 analyses indicate that juvenile survival was the most influential of three vital rates affecting 267 population growth in song sparrows and that it declined as fox sparrows increased (figure 4). In 268 comparison, adult survival and reproductive rate had no detectable effect on the long-term 269 decline in song sparrow abundance (figure 3). These findings mirror analyses conducted at much 270 larger scales which indicate a long-term, regional decline in song sparrow abundance (Jewell and 271 Arcese 2008), but increases in fox sparrow abundance, particularly in winter (Visty et al. 2017). 272 Because these results are consistent with the hypothesis that interspecific competition may be 273 contributing to song sparrow declines on Mandarte Is. and regionally, we conducted additional 274 tests to discover potential mechanisms, focusing on competition during breeding and overwinter 275 periods.  276 Contrary to the idea that fox and song sparrows compete for breeding space, we observed 277 complete overlap in song and fox sparrow territories. Moreover, territorial song sparrows largely 278 ignored simulated intrusions by fox sparrows (and the control), despite responding strongly to 279 simulated intrusions by song sparrows (figure 6). These results are opposite to expectation if fox 280 and song sparrows compete by interference for breeding space (Jankowski et al. 2010; Dhondt 281 2012). 282 We also found no evidence of exploitative competition between fox and song sparrows in 283 the breeding period. First, annual reproductive rate in female song sparrows increased as song 284 sparrow population size declined and fox sparrows increased (figure 3c), opposite to 285                                                                                                                          Johnson et al.    14 expectations under exploitative competition (Dhondt 2012), but consistent with earlier reports of 286 density-dependent reproductive success in song sparrows (Arcese and Smith 1988; Arcese et al. 287 1992). Second, we detected no change in habitat quality for song sparrows (cf Germain and 288 Arcese 2014; Crombie et al. 2017), contrary to the expectation that sharing habitat with fox 289 sparrows might reduce food or nest site availability during breeding. 290 Intraspecific competition for winter food and space are well-known to affect juvenile 291 survival and population growth in song sparrows (Nice 1943; Arcese 1989; Arcese et al. 1992; 292 Wilson and Arcese 2008), raising the possibility that interspecific competition with fox sparrows 293 may also reduce survival in juvenile song sparrows sufficient to cause population decline. 294 Specifically, Dhondt (2012) notes that competitive exclusion becomes more likely when, in the 295 presence of intraspecific competition for a limiting resource, the addition of an interspecific 296 competitor further reduces the fitness of subordinate competitors by further reducing access to 297 those resources. Consistent with these expectations, we observed a strong overlap in preference 298 for native seeds in fox and song sparrows, mirroring the results of Willson (1971) who reported 299 strong overlap in preference for commercial seed in Illinois, USA, and also found fox sparrows 300 to be significantly more efficient at handling seeds on average. Moreover, on Mandarte Is., fox 301 sparrows excluded song sparrows from access to supplemental food in 91 and 100% of contests 302 in October and March, respectively. Because these periods correspond with annual peaks in 303 intraspecific aggression and dispersal in song sparrow (Arcese 1989; Wilson and Arcese 2008), 304 these findings suggest that fox sparrows limit song sparrow abundance on Mandate Is. via 305 aggressive competition for winter fool. Overall, therefore, our findings are consistent with the 306 hypothesis that range shifts in colonizing species have the potential to drive community 307 composition via interspecific competition. 308                                                                                                                          Johnson et al.    15 The coexistence of interspecific competitors has been shown to depend in part on the 309 ability of species to partition resources in ways that allow each to maintain positive growth rates. 310 In ground finches (Geospiza spp) on the Galapagos Islands, a drought-mediated decline in seed 311 abundance intensified competition between a resident and colonist species, but also facilitated 312 their rapid morphological divergence and coexistence (Grant and Grant 2006). Stuart et al. 313 (2014) also reported the rapid evolution of feeding behavior and morphology in a native lizard 314 following the invasion of its habitat by a competitively dominant congener. Similarly, Jankowski 315 et al. (2010) demonstrated a strong role for interspecific competition in maintaining elevational 316 range boundaries in congeneric Andean forest birds, but noted that as ranges shift upwards, the 317 species at the top may be limited in their response. These studies illustrate a potential for 318 ‘evolutionary rescue’ to facilitate co-existence via rapid adaptation (e.g., Carlson et al. 2014), 319 and imply that song and fox sparrows are most like to co-exist where sufficient heterogeneity in 320 habitat type or resource use allows each species to diverge sufficiently in its use of limiting 321 resources to maintain positive growth rates.  322 In contrast, low habitat heterogeneity on Mandarte Is. (Lameris et al. 2016; Crombie et al. 323 2017), small population size and low juvenile survival rates in song sparrows all increase the 324 likelihood of their local extirpation on Mandarte Is. (Arcese et al. 1992; Arcese and Marr 2006). 325 Although divergence in quantitative traits potentially affecting co-existence is possible (Schluter 326 and Smith 1986), small population size, random genetic drift and gene swamping are likely to 327 limit rapid local adaptation in the Mandarte Is. song sparrow population (Keller et al. 2001; Marr 328 et al. 2002). Given regional increases in fox sparrow abundance (Visty et al. 2017) and declines 329 in song sparrow abundance (Jewell and Arcese 2008; National Audubon Society 2010; Sauer et 330                                                                                                                          Johnson et al.    16 al. 2017), it remains an open question as to how competition between these species may be 331 affecting abundance at larger spatial scales. 332 Alternative explanations for the decline of our focal song sparrow population seem 333 unlikely. Jewell and Arcese (2008) showed that brown-headed cowbirds (Molothrus ater) can 334 limit song sparrow population growth by reducing reproductive success, but cowbirds were 335 absent from Mandarte Is. in 16 of 17 yrs since 2000, and female reproductive success increased 336 over the course of our study. Severe weather can also decimate sparrow populations (Arcese et 337 al. 1992; Keller et al. 1994; Smith et al. 2006) but has been ameliorated by climate warming (P. 338 Arcese and R. Norris, unpubl. res). Despite changes in vegetation cover, we detected no 339 reduction in the cover of fruiting shrubs (Lameris et al. 2016) or increases in nest failure 340 (Crombie et al. 2017). Overall, therefore, our results strongly support the hypothesis that fox 341 sparrows caused the Mandarte Is. song sparrow population to decline, but more work is needed 342 to understand how fox sparrows may limit song sparrow abundance and distribution regionally. 343 Shifts in species distribution could have far-reaching effects on plant and animal 344 communities via their effects on predation, pathogens and competition (e.g., Simberloff 2005; 345 Parmesan 2006; Early and Sax 2014; Elmhagen et al. 2015; Rodewald and Arcese 2016). 346 Although the threat of competitive exclusion by colonizing species is sometimes downplayed 347 (Davis 2003; Gurevitch and Padilla 2004; Krosby et al. 2015), novel competitive interactions can 348 be expected to increase as climate and habitat change promote shifts in species ranges further. 349 Because interspecific competition can act subtly in communities (Dhondt 2012), long-term and 350 experimental studies of the competitive exclusion of native species by colonists undergoing 351 range expansion will be needed to predict community dynamics in the future. Our results 352 indicate that in the absence of ecological or evolutionary shifts in niche dimension, range 353                                                                                                                          Johnson et al.    17 expansions by dominant competitors have the potential to cause the extirpation of historically 354 resident species when competitive interactions between them are strong and resources not 355 equitably partitioned. 356  357 ACKNOWLEDGEMENTS 358 We thank many people that have contributed to monitoring on Mandarte Is. and the Tsawout and 359 Tseycum Bands who kindly provide us permission to work there. We are grateful to J.M. Reid, 360 L. Keller, R. Schuster, M. Crombie, J. Krippel, N. Morrell, E. Hampshire and N. Chen for help 361 with data, experimental design, analysis, or preparation of the manuscript. Our work was 362 supported by the University of British Columbia, W. and H. 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Number of breeding female song sparrows in 45 years from 1960–63 and 1975–2016 507 (black circles) and fox sparrow breeding females in 13 years from 1960–2016 (grey diamonds). 508 Song sparrow population size has declined significantly over the study period while fox sparrow 509 population size has increased. Shaded areas represent predicted values ± 1 SE. 510   511 FIGURE 2: Percent of yearling male and female song sparrows surviving from the end of 512 parental care (day 24 after hatching) to April 30th of the following year (juvenile survival) from 513 1975-2016 (excluding 1979 and 1980 when juvenile survival was unknown). 514  515 FIGURE 3. Contributions of (a) juvenile survival, (b) adult reproductive rate and (c) adult 516 survival to song sparrow population growth from 1975–2014, derived from a stage-structured 517 life table response experiment (see Methods). The contribution of juvenile survival changed 518 significantly over time, but reproductive rate and adult survival remained approximately zero, 519 indicating that the observed decline in song sparrows is best explained by the decrease in 520 juvenile survival (Figure 3). The shaded areas around the line indicate predicted values ± 1 SE.  521  522 FIGURE 4. Juvenile song sparrow survival declined as the number of fox sparrow breeding 523 females increased. The shaded areas around the line indicate predicted values ± 1 SE. The black 524 circles are observed juvenile song sparrow survival in each study year for which fox sparrow 525 population size was known (Nyrs = 11). 526  527                                                                                                                          Johnson et al.    26 FIGURE 5. Proportion of time song (dark) and fox (light) sparrows fed on each seed type during 528 feeder visits (see Methods). Seeds were presented by type in identical circular depressions in 529 plywood feeders dispersed across Mandarte Island. Fox and song sparrow seed preference 530 overlapped completely. Whiskers represent approximate 95% confidence intervals around the 531 median (solid line), and the box spans the lower and upper quartiles (25%–75%). 532  533 FIGURE 6. Closest approach by territorial male and female song sparrows to taxidermic mounts 534 presented at the center of song sparrow territories during playback trials. Song sparrows (SOSP) 535 came closer to the conspecific mount than to the fox sparrow (FOSP) or control (Swainson’s 536 thrush) mounts, and there was no difference in song sparrow response to the fox sparrow and 537 control mounts, indicating that song sparrows do not respond to simulated territorial intrusions 538 by fox sparrows. Whiskers represent approximate 95% confidence intervals around the median 539 (solid line), and the box spans the lower and upper quartiles (25%–75%). 540                                                                                                                            Johnson et al.    27  541 FIGURE 1542                                                                                                                          Johnson et al.    28  543 FIGURE 2 544                                                                                                                          Johnson et al.    29     545 FIGURE 3 546                                                                                                                          Johnson et al.    30  547 FIGURE 4 548                                                                                                                          Johnson et al.    31            FIGURE 5                                                                                                                                      Johnson et al.   32                FIGURE 6 


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