Graduate Thesis Or Dissertation
 

The Role of Trees and Thermal Refugia in Greater Sage-Grouse Brood Habitat Selection

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  • It is well documented that greater sage-grouse (Centrocercus urophasianus; hereafter sage-grouse) populations have been in decline throughout their range in the western United States. Currently, sage-grouse occupy approximately 56% of their historic distribution prior to European settlement. Numerous factors have been cited as potential causes for declining productivity and contraction of the species historic distribution including: loss, fragmentation and degradation of habitat, anthropogenically subsidized predation, conifer encroachment and certain land use practices. Sage-grouse resource selection has been widely studied as it is an important factor driving land management and sage-grouse conservation decisions. However, in my study, I specifically examined the brood-rearing life history stage and provided further insights into how broods were using habitats at daily and seasonal scales in the Northern Great Basin. Obtaining a better understanding of how broods are using the landscape is of critical importance because chick survival is the second most influential vital rate driving population dynamics of the species. During crepuscular morning hours, broods forage for forbs and invertebrates to increase body growth which benefits their thermoregulatory capacity and ability to survive in inclement and extreme weather conditions. During the afternoon, when not foraging, grouse are exhibiting loafing behavior to reduce thermal stress, and increase their concealment from predators. There is a need to further describe sage-grouse habitat selection within these specific hours of the day to attain a better understanding of the resources being utilized for foraging, thermoregulation, or concealment from predators. My study first looked beyond vegetation components which typically describe sage-grouse habitats, and was the first to investigate how microscale thermal environments on the landscape influence sage-grouse brood habitat selection. I then examined brood resource selection as it related to vegetation cover attempting to evaluate the relative importance of tree cover, which is generally thought to reduce available sage-grouse habitat and indirectly increase their risk of predation. Researchers have attempted to describe mechanisms driving sage-grouse habitat selection, but have generally omitted the thermal component of their space use. In Chapter 2, I monitored 37 sage-grouse broods in 2018 and 2019, and collected >113,000 black bulb temperature (Tbb) measurements from 367 unique thermal arrays that each collected Tbb measurements for 24 hours. I attempted to describe thermal variation across the landscape using thermal arrays, and as expected, my findings demonstrated that at any given ambient temperature, there were numerous microscale thermal options available to sage-grouse broods throughout the brood-rearing period. For example, when ambient temperature was 20°C, Tbb ranged from 5.8 to 58.8°C. I compared brood morning (i.e., foraging), afternoon (i.e., loafing) and associated random locations between morning and afternoon hours and this was necessary to disentangle the importance of microclimate and other potential resource needs. By comparing microclimates when broods were present at a given location type, and when they were not, I provided insights into the thermal characteristics they may have been selecting. Additionally, I found that microclimates at brood foraging locations heated more rapidly than either their loafing or proximate, random locations. Alternatively, brood loafing locations moderated ambient temperature more effectively than brood foraging locations but were similar to random locations at extreme ambient temperatures. My results indicated that broods were using warmer areas to forage in the morning, cooler areas to loaf in the afternoon, and these patterns suggested that microclimate was influencing brood habitat selection. I measured vegetation composition and structure and this provided insights into possible mechanisms for the observed microclimates as well as sage-grouse brood space use. As expected, shrub cover was a primary driver of microscale thermal environments for both the morning and afternoon locations, and with greater shrub cover, microclimates were cooler in the afternoon. My findings also supported the importance of increasing and maintaining shrub structure on the landscape, as it is known to moderate extreme conditions and provide thermal stability. In Chapter 3, I used resource selection functions to determine the relative probability of brood use of tree cover and whether their use of trees differed between their morning (i.e., foraging) and afternoon (i.e., loafing) times of day. Previously published literature is inconclusive as to whether sage-grouse broods avoid trees during the summer as they do throughout the rest of the year. Previous work in my study area in the South Warner Mountains found that female sage-grouse selected for low conifer cover, and therefore, I sought to further examine this phenomenon and focused solely on the brood-rearing period. Between 2016 and 2019, I monitored 49 broods and obtained a total of 4,585 morning and afternoon brood locations. Each location was confirmed as a brood location by ensuring that at least one chick was present with the marked female after each location was recorded. I found that broods used space with greater shrub cover and did not select for low tree cover which was contrary to previous work in the South Warner Mountains. In my study, I included only brood locations while other previous studies that showed that sage-grouse are selecting for tree cover during brood-rearing included all female sage-grouse during the brood-rearing period. Therefore, it is possible differences in use of trees during brood-rearing observed in my study relative to previous research was because previous studies included females without broods. Females without broods may have been selecting for areas with sparse tree cover during the summer, while females with broods generally chose not to use trees to that extent due to potential brood survival risks associated with trees (i.e., predator perching structures). My data indicated that sage-grouse broods were using the landscape in various ways throughout the day. Brood avoidance of tree cover was twice as high in the morning (100-m buffer; β = -0.160, 95% CI: -0.189 – -0.134) relative to the afternoon hours of day (200-m buffer; β = -0.080, 95% CI: -0.095 – -0.065), but overall, my findings demonstrated that sage-grouse broods avoided trees throughout brood-rearing. There are likely mechanisms driving a reduction in conifer avoidance in the afternoon during the summer (e.g., thermal refugia), and these mechanisms may present indirect demographic consequences for sage-grouse broods. Temperature is an important driver of ecological processes, and how organisms respond to changes in temperature can impact growth, reproduction and survival. Collectively, microclimates during foraging and loafing times of day provided conditions suitable for each behavioral need throughout brood-rearing, and these habitats will be imperative to maintain microscale thermal heterogeneity across the landscape as climatic conditions continue to change. My study provided evidence that sage-grouse broods were using habitats during specific times of day, in part, due to microclimates and tree cover on the landscape.
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