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Dissertation Reproductive Ecology and Population DISSERTATION REPRODUCTIVE ECOLOGY AND POPULATION VIABILITY OF ALPINE-ENDEMIC PTARMIGAN POPULATIONS IN COLORADO Submitted by Gregory T. Wann Graduate Degree Program in Ecology In partial fulfillment of the requirements For the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Fall 2017 Doctoral committee: Advisor: Cameron L. Aldridge Cameron K. Ghalambor N. Thompson Hobbs Barry R. Noon Copyright by Gregory T. Wann 2017 All Rights Reserved ABSTRACT REPRODUCTIVE ECOLOGY AND POPULATION VIABILITY OF ALPINE-ENDEMIC PTARMIGAN POPULATIONS IN COLORADO Understanding factors regulating populations is a fundamental goal of population ecology. Life-history traits such as survival and fecundity are key vital rates responsible for population change and may vary across elevational gradients. At the upper end of this gradient, the alpine zone, populations are faced with extremely short growing seasons, unpredictable winter conditions dictated by snowpack, and the continued threat of habitat loss due to temperatures increasing beyond the range that defines these cold systems. To date, few studies have addressed population regulation of alpine-endemic species in the context of the aforementioned factors. I used long-term demographic data collected over a 51-year period at two study sites (Mt. Evans and Trail Ridge) together with a contemporary field study (2013- 2015) at three sites (Mt. Evans, Trail Ridge, and Mesa Seco) to examine factors regulating alpine-endemic white-tailed ptarmigan (Lagopus leucura) in Colorado. My first research question addressed fitness consequences of reproductive timing. Past research examining data at the long-term study sites indicated populations responded to warming springs by breeding earlier, and the population that advanced its breeding phenology the most also had a concomitant decreasing trend in observed fecundity. The phenology mismatch hypothesis states that individuals in a population failing to time reproductive events during highest availability of resources will have lowered fitness. I used data from a three-year telemetry study on radio-marked hens to examine fitness consequences of timing of reproduction ii relative to abundance of food resources. I measured temporal changes in plant productivity using NDVI measurements recorded at weekly intervals in brood-rearing habitats. At a subset of sites and years I measured temporal changes in arthropod abundance to assess the usefulness of NDVI as a proxy for insect availability. NDVI and arthropod abundance were strongly correlated for a subset of arthropod taxa known to be consumed by chicks. A covariate for the degree of synchrony between breeding events and NDVI (termed “mismatch”) was then used as a covariate in nest and chick survival models. There was a weak negative relationship between daily nest survival and mismatch, indicating birds nesting out of synchrony suffered reduced daily nest survival. The relationship was much stronger for chicks, and the predicted percent decline in daily survival ranged from 4% to 47% for chicks hatched with the largest calculated mismatch values compared to chicks hatching at the peak (dependent on site and year). My second research question tested for the presence of density dependence using the long-term population datasets. Density dependence is widely recognized as being prevalent in wildlife populations, but in the absence of strong demographic signals, it can be difficult to detect. More contemporary studies have focused on age- and sex-specific responses to density dependence, which may vary dramatically, and interactions with abiotic factors. Such studies generally require individual-based data on multiple age classes monitored over multiple decades, and datasets meeting these criteria have been extremely limited - particularly for avian species - until only recently. I constructed an integrated population model (IPM) fit in a Bayesian statistical framework to combine spring survey, fecundity, and capture-recapture data in a single model using a joint likelihood. The IPM provided a way to estimate juvenile survival from the combined data and incorporated uncertainty from multiple demographic processes. The Mt. Evans population increased over the 51-year period (average annual increase of 2%) while the iii TR population sharply declined (average annual decrease of 3%). An interaction between minimum winter temperature and fall population density indicated both populations suffered lower survival rates when densities were high. However, density interacted with temperature positively at Mt. Evans (at highest densities, survival was higher when minimum winter temperatures were high) and negatively at Trail Ridge (at highest densities, survival was higher when temperatures were low). This finding may have been due to differences in temperature regimes at wintering sites due to elevational differences between populations. Juvenile survival was negatively related to fall population density at both sites for males and females. My final research question provided a viability assessment for the Mt. Evans and Trail Ridge populations. In this analysis, I used reproductive data collected from radio-marked hens during the 2013-2016 field seasons at Mt. Evans and Trail Ridge to estimate components of age- specific fecundity. Fecundity estimates were based on a previous breeding study using Bayesian models and informative priors, and age-specific estimates of survival rates were obtained from the IPM and included estimates of environmental variance. I used a life-stage simulation analysis (LSA) to assess the influence of vital rates on population growth (휆). LSA is based on simulating many population projection matrices, each of which is populated with vital rate values drawn from probability distributions with associated means and variances based on empirical data, regressing the resulting dominant eigenvalues (휆) on these rates, and using the coefficient of determination (푅2) as a measure of explanatory power. Next, I used cumulative distribution functions (CDFs) to calculate quasi-extinction probabilities given the underlying vital rates, age structure, and within-year and between-year correlations in vital rates. Population growth for both Mt. Evans and Trail Ridge was most sensitive to survival of hens 2 years or older (푅2 = 69% at Mt. Evans and 61% at Trail Ridge), followed by juvenile survival (푅2 = 14% at Mt. iv Evans and 23% at Trail Ridge). Fecundity rates explained only minor amounts of variation (all 푅2 ≤ 6%). Overall, stochastic growth rates for both populations indicated the internal demographics were insufficient to maintain populations (Mt. Evans: 휆푆 = 0.78, Trail Ridge: 휆푆 = 0.72), and both populations were at risk of surpassing a 50% probability of quasi extinction within 5 (Trail Ridge) and 7 (Mt. Evans) years. However, updating the population model to include only fecundity and survival estimates post-1994 (a time after hunting restrictions went into effect at Mt. Evans) produced a much more optimistic estimate for the Mt. Evans population (<3% probability of becoming quasi-extinct within 50 years, 휆푆 = 1.04), and a slightly more optimistic estimate for Trail Ridge (50% probability of becoming quasi-extinct within 11 years, 휆푆 = 0.88). Discrepancies between stochastic growth rates and those obtained from the IPM were likely due to effects of immigration into the breeding populations which I did not directly model, but this highlights the importance of maintaining population connectivity. v ACKNOWLEDGEMENTS I would like to first thank my advisor Cameron Aldridge. I could not have asked for a more supportive advisor throughout my time as a graduate student. Cam was always positive and enthusiastic about ecology and nature and it was difficult to not feel the same when in his company. I received a great deal of help and support throughout my studies from Professors Tom Hobbs, Cameron Ghalambor, and Barry Noon. My committee members were generous with their time and ideas; any of the good stuff that may exist in this dissertation I owe to them, and the rest is on my shoulders. I would particularly like to thank Cameron Ghalambor for giving me the opportunity to teach ornithology lab for two semesters. I was also fortunate to be part of a lab group full of very bright and talented students and post-docs. To everyone in the Aldridge Lab, thank you for your friendship and support. I would particularly like to thank Shelley Spear for sticking out three field seasons of ptarmigan work with me while completing her thesis work, Katie Langin for exposing me to a completely different perspective of ptarmigan (genomics!), and Adrian Monroe for his endless willingness to help debug JAGS code and think through Bayesian modeling. This project would not have been possible without the dedicated efforts of many field technicians. Billy Dooling was instrumental in the project; he was there to help me during the first field season of dissertation research and even came back to do it all again the following year. I thank Jeremy Austin, Justin Borcherding, Matthew Broadway, Jack Bushman, Chris Potter, Andy Valencia, Chelsea Weithman, and Chad Young for their dedicated efforts collecting data. The pay was low, the hours were long, the weather was cold, and my extended company was a poor reward, but they all worked very hard and did so with tremendous energy and humor. vi I owe a special thanks to Amy Seglund (Colorado Parks and Wildlife) for her support as a collaborator and colleague. I learned
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