Greater Sage-grouse habitat and demographic response to grazing by non-native ungulates
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Authors
Street, Phillip
Issue Date
2020
Type
Dissertation
Language
Keywords
Centrocercus urophasianus , Grazing , Greater Sage-grouse , Habitat selection , Population biology , Survival
Alternative Title
Abstract
Within the Great Basin of the Western United States, management discussions regarding the impacts of grazing by livestock and feral horses on Greater Sage-grouse (\textit{Centrocercus urophasianus}, hereafter sage-grouse) often focus on the negative impacts to habitat, and speculate how the sage-grouse populations will respond in turn. While the linkage between sage-grouse demographics and habitat is well documented, quantifying the direct impacts of non-native grazing on sage-grouse has been fraught with difficulties. These struggles include the logistical constraints and cost associated with monitoring multiple sage-grouse populations across large landscapes, an adequate temporal span to detect responses, grazing manipulations at a large enough spatial scale to affect sage-grouse populations, and the ability to account for the spatiotemporal dynamics of non-native grazers as well as sage-grouse. In my dissertation, I investigated the response of sage-grouse demographics, movements, and habitat to grazing by non-native ungulates in Northern Nevada and Southern Oregon over the course of 5 years. The study area was comprised of three large subunits, with substantially different grazing regimes. I used Bayesian Hierarchal modeling to account for uncertainty in each component of our data due to the observation process and the spatiotemporal dynamics of sage-grouse and non-native grazers. In chapter 1 of my dissertation, I used hierarchical distance sampling of feces to investigate the distribution of horses and livestock on the landscape and whether they were impacting the vegetation components at sites available to sage grouse for breeding, sites chosen by sage grouse to nest, and sites chosen by sage-grouse to brood their chicks. When grazing was not present, sage-grouse selected nest and brood sites with higher percentages of perennial grasses, key forbs, and other forbs. When livestock were present at high rates, we detected a decline in the herbaceous understory and increases in the amount of bare ground, regardless of whether the site was a random site, or if it was a site chosen to nest or brood their chicks. We observed similar trends at available sites when feral horses were present at high rates. Conversely, at sites chosen by sage-grouse to nest and brood their chicks with high rates of feral horse activity, we found evidence for increased amounts of cheatgrass. We hypothesize that increasing annual grasses at nest and brood sites as horse activity increased reflected sage-grouse selecting sites containing more cover to compensate for reduced cover of other vegetation at higher levels of grazing by horses. One explanation for why we observed lower amounts of annual grass at sites chosen by sage-grouse to nest and brood the chicks where livestock were present, is that livestock densities were higher than feral horses. Whether the presence of grazing resulted in increases in bare ground or increases in annual grass, the end results was less than optimal vegetation for breeding sage-grouse. In chapter 2 of my dissertation, I develop an innovative data collection method to count chicks with a uniquely identifiable adult, as well as a novel analytical method to estimate chick survival when detection is imperfect and adoption between broods occurs. One assumption of all studies is that the methods do not influence survival or behavior of individuals. In many species, marking young is impractical, and may negatively affect individual health and mobility. When young are dependent on an adult, being able to follow a marked adult and count the offspring is an alternative to marking offspring. In precocial avian species, counts are obtained by flushing the adult while it is brooding chicks. This approach temporarily separates the brood from their mother, and may negatively impact survival or induce adoptions by another parent. Further, existing chick survival models based on counts make various assumptions that are often difficult to satisfy, such as perfect detection or no brood mixing. We developed a method to obtain counts of chicks using remote video cameras that minimizes disturbance. We fit our model to sage-grouse data, and found evidence that weather patterns were regulating sage-grouse reproduction in two ways. First, at nests that received higher amounts of winter precipitation we found that females were hatching smaller clutches. Second, if it rained or snowed on the brood when it was young, the chicks survived at a lower rate. The observation method I present has the potential to be applied to other species with dependent offspring. In chapter 3 of my dissertation, I model early and late brood rearing habitat for sage-grouse. Sage-grouse have declined throughout much of their range, and substantial conservation efforts have been devoted to the persistence of the species. Chick survival has been identified as a critical life history stage, yet conservation planning tools that address this stage do not exist for many populations. I develop a Hierarchical Bayesian Model that integrates data on the species' ability to travel from leks (breeding sites) with data on habitat selection at the landscape scale. I monitored 279 individual females with broods captured from 24 leks in the Northern Great Basin. For these 24 leks, I classified the habitat into unsuitable, low, moderate, and high quality habitat. We found that only 5.35\% of the habitat was classified as high quality early brood-rearing habitat, and 4.64\% was classified as high quality late brood-rearing habitat. These results suggest that both early and late brood-rearing habitat are limited for sage-grouse in the Great-Basin. When taken together, the results from my dissertation provide evidence that livestock and feral horses are altering the vegetation structure of the Northwestern Great Basin to less than optimal conditions for sage-grouse reproduction, sage-grouse are not, or unable to, choose nesting or brooding sites to avoid these impacts, and sage-grouse chicks are most vulnerable at young ages. Lastly we provide a flexible conservation planning tool that directs managers to areas most critical for early and late brood rearing sage-grouse when making decisions about grazing practices.