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Impacts of non-native ungulate grazing on reproduction and physiology in Greater Sage-grouse
AdvisorSedinger, James S
Ecology, Evolution and Conservation Biology
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Understanding the mechanisms underlying dynamics of animal populations is critical to planning appropriate management actions for species of concern. Greater Sage-grouse (Centrocercus urophasianus; hereafter “sage-grouse”) face many threats across their range in North America. My dissertation examines the effects of two of these threats, livestock and feral horse grazing, on sage-grouse reproductive strategies and physiology. In Chapter 1, I investigated influences on the start of the sage-grouse breeding process. Sage-grouse exhibit an intermediate pace on the life-history spectrum, which leads us to ask what kind of trade-offs they may experience and what costs might incur. I used data on the breeding status and survival of marked females to estimate breeding propensity and the potential cost of breeding under climatic variation and a range of non-native grazing pressure. I found both past and current weather influenced the decision to breed and increasing livestock numbers led to reduced breeding in drought years. Breeding rates increased slightly with feral horse presence, likely due to overlap in habitat use during drier conditions. Breeding birds had lower daily survival rates than birds what did not breed at all, revealing an immediate cost of reproduction. In Chapter 2, I evaluated the physiological response of grouse to variation in climate and non-native grazers as a possible mechanism for population decline. I used an assay of feathers collected from wild sage-grouse to measure levels of the avian stress hormone, corticosterone. I also examined short-term demographic consequences of increased hormone levels. Both horses and livestock increased corticosterone, primarily under dry climatic conditions. Nest survival was lower with increased hormone levels, but other demographic rates did not correlate with this measure. Lastly, in Chapter 3 I addressed aspects of reproduction and the maintenance of genetic diversity in sage-grouse populations. I measured the occurrence of multiple paternity and conspecific nest parasitism using genetic samples from monitored nests. I found several nests that were the result of mating with multiple males, and one nest that had an egg laid by another female. These phenomena are likely important to female fitness, particularly for persisting through environmental variation. The project goal was to better understand the complex interactions of sage-grouse, their habitat, and non-native grazers. I showed the importance of including precipitation in assessing the impacts of grazers, and that genetic analyses can reveal even more about these populations. This work adds to other recent evidence supporting the need for better management of livestock and feral horses across sage-grouse habitat.