Use and Survival of Greater Sage-Grouse in Meadows Managed for Non-Native Ungulate Grazing in Nevada
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Authors
Golden, James E.
Issue Date
2023
Type
Thesis
Language
Keywords
Abundance estimation , Bayesian analysis , Brood rearing habitat , Greater sage-grouse , Hierarchical distance sampling , Survival analysis
Alternative Title
Abstract
Population declines of greater sage-grouse (Centrocercus urophasianus, hereafter sage-grouse) have largely been attributed to extensive loss of sagebrush habitat. Consequently, investigating how disturbance to seasonal sagebrush habitat, such as winter and nesting habitat, affects population dynamics of sage-grouse has been a major focus of research. However, low juvenile survival and recruitment has also been implicated as a potential driver in sage-grouse population declines.Nutritional requirements of juvenile sage-grouse combined with the desiccation of key food plants in shrub communities push sage-grouse in the Great Basin to seek mesic environments such as higher elevation shrub communities or riparian meadows during the drier summer months. This use of riparian meadows typically occurs during what is known as the late brood-rearing period. Late brood-rearing habitat comprises only about 2% of the landscape, yet is essential for fledging young sage-grouse. Consequently, late brood-rearing habitat could have disproportionate effects on sage-grouse population dynamics. Non- native ungulates also rely heavily on these mesic ecosystems during the summer. With high densities of non-native ungulates grazing these relatively small areas during a large time span, severe degradation to these habitats may occur. This type of high-intensity grazing of late brood habitat may negatively affect sage-grouse populations, although moderate grazing may benefit sage-grouse. However, the effects of grazing in late brood-rearing habitats are understudied. I investigate sage-grouse use and survival at five meadow sites with differing grazing management strategies in northwestern Nevada. I employed a trapping protocol, traditionally performed at leks, but instead captured brooding and non-brooding hens at late brood-rearing habitat. I integrated distance sampling methods to simultaneously obtain data for estimating local population abundance and survival.In chapter 1, I used single-variate Cox proportional hazards models to investigate the effects of vegetative cover within the meadows on brood and hen survival. However, low encounters of broods and low hen mortality occurring at certain sites resulted in large uncertainty, making comparison between grazing management treatments implausible. In response to this finding, I shifted focus to obtaining estimates of local abundance using data collected via point counts, rather than survival at each meadow to understand better variation in population sizes. I integrated my point count data with a hierarchical integrated distance sampling model to derive habitat-informed local population estimates of sage-grouse. I found that sage-grouse density and abundance at heavily grazed meadows, which have since been protected through the exclusion of non-native ungulates, were lowest compared to late brood-rearing habitats that have experienced restricted and unrestricted grazing. I speculate the reason sage-grouse abundance is lower at sites that have been recently protected is that although the meadows now have protection from grazing, their condition was deteriorated such that sage-grouse stopped using the sites and have not yet returned. Meanwhile, sites that have a history of responsible grazing management continue to support sage-grouse use. These results indicate that improper grazing of late brood habitat may lead to persistence of low abundances or extirpation, despite implementation of protection and restoration efforts. It is unclear if and when a degraded riparian meadow will be used again by sage-grouse for late-season brood-rearing habitat after it has been protected. Conversely, the results indicate continued use and presence of sage-grouse using late brood-rearing habitat undergoing restricted grazing.In chapter 2, I fully describe development of the hierarchical integrated distance sampling model used to estimate density and abundance of sage-grouse at each meadow site. I used simulated data generated using known parameters to prove its ability to recover known parameters and identify estimation biases. Results of the model fitted to simulated data mirroring the survey protocol indicated that the model could accurately recover estimates with little bias. The validation of this model suggests the opportunity for these cost-effective methods to be applied to future late brood-rearing studies, as well as adaptation into traditional trapping around leks.