Thesis (M.S.)--University of Nevada, Reno, 2006. / "December 2006." Includes bibliographical references (leaves 23-27). Online version available on the World Wide Web. Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2006]. 1 microfilm reel ; 35 mm.

Human activities and unpredictable changes to environmental habitats impose a suite of stress and challenges to animal homeostatic function. Stress responses are often controlled by the release of glucocorticoids to mobilize energy, primarily corticosterone in small mammals, to help regulate homeostatic function such as heterothermy and changes to body condition, the latter of which serves as a proxy for energy reserves. Adaptive heterothermy is influenced by daily and seasonal patterns, heat produced from daily activity, and has been shown to increase in small mammals during times of environmental stress to conserve energy budgets. Body condition also changes in response to environmental perturbations, mobilization of energy by corticosterone, seasonal changes, and activity. My study aimed to disentangle the effects of environment and activity on homeostatic responses by pharmacologically manipulating corticosterone in kangaroo rats. Kangaroo rats are ecosystem engineers, heterothermic, and their activity periods are functions of their thermoregulatory patterns as well as environmental conditions thus making the species a great candidate for this form of study. I conducted two in-situ field experiments to assess for the effects of stress responses on thermoregulation, activity, and body condition. In my first experiment, I used body temperature (Tb) as a proxy for activity time and examined how pharmacologically increased corticosterone influenced kangaroo rat heterothermy responses to the moon phases and ambient temperatures. I also examined their fat, lean mass, and water content at the end of the study. Moon phase was a significant predictor of activity period as animals typically waited longer during the waxing moon phase to become active and cooled down below activity earlier in the night during the waning moon phase. As nights shortened, activity decreased despite environmental conditions becoming warmer. Corticosterone also significantly decreased total activity time and thus steadily increased heterothermy across the length of my experiment. These results indicate activity, not environment, are stronger drivers of heterothermy patterns. Total fat content (energy content) at the end of the study was not affected by corticosterone. The lack of change in fat content was presumably because kangaroo rat body condition was measured once at the end of the study and likely not a reflection of changed body condition over time. In my second experiment, I examined body condition across a longer period by measuring lean mass, fat content, and total body water across a 2-month period in the summer by pharmacologically increasing corticosterone to test the relationship between stress and body condition and to dissociate the two from environmental factors. Body condition indices generally increased across the summer, but corticosterone implantation did not significantly affect body any of the indices. The loss of heterothermic control and decrease in activity time across the summer suggests that animals are likely conserving energy budgets leading to preservation of condition. Further, banner-tailed kangaroo rats generally breed in the spring where body condition falls due to stress induced by increased competition and then increases across the summer as late summer monsoons promote the growth of primary resources utilized by animals thereby preserving body condition. This study suggests that body condition is driven more by life-history traits, activity time, and environment rather than stress responses.

A majority of the arid grasslands in the western U.S. have been dramatically altered by anthropogenic influences resulting in degradation and desertification. Within the arid grasslands of North America a guild of burrowing herbivorous rodents that includes kangaroo rats (Dipodomys spp.) and prairie dogs (Cynomys spp.) is often considered integral to arid grassland maintenance. As part of the larger guild of burrowing herbivorous rodents, kangaroo rats are considered to be an important keystone guild whose role as ecosystem engineers and habitat modifiers complements that of prairie dogs. Together these species organize and structure arid grassland ecosystems and the biodiversity therein, by providing a mosaic of microhabitat patches, thus increasing overall heterogeneity. In an area where black-tailed prairie dogs (C. ludovicianus) were reintroduced, I used Giving-up Density (GUD) to assess the indirect effects black-tailed prairie dogs might have on the foraging patterns of resident kangaroo rats (D. spectabilis and D. merriamii). My objective was to compare and contrast kangaroo rat foraging GUD within and along the boundary of a on a recently established black-tailed prairie dog colony with that in the surrounding unmodified native habitat. This enabled assessment of whether black-tailed prairie dogs had an influence on the perceived quality of the habitat by kangaroo rats. Kangaroo rats visited off-colony feeding trays more frequently, and collected a greater mean mass of seed per tray as well. This indicates that the kangaroo rats perceived the area off the prairie dog colony as having a lower foraging cost than on the colony or along the colony edge. I conclude that from the perspective of the seed-eating kangaroo rat, the colony is not viewed as high quality habitat. What impact the reintroduction and management of one keystone species might have on another keystone species deserves additional consideration as we attempt to restore arid grassland ecosystems.

Climate change and exotic plant invasions are significant anthropogenic threats to desert community structure and resilience . In the Mojave Desert, the invasive grass red brome (Bromusrubens L) is increasing fire frequency and extent in response to climatic factors. The resilience of this ecosystem will be affected by how plant and animal communities respond to fire. To better understand these dynamics, we studied the environmental factors underlying changes in invasive grass fire regimes in the Mojave Desert and its structural and functional effects on plant and animal communities. Following fire, reestablishment of native vegetation can be preempted by repeated burning associated with the abundant exotic grass red brome. Red brome density is correlated with various climate and landscape variables, but to establish causality, we experimentally assessed germination and growth of red brome. Red brome responded positively to fall precipitation, finer-textured soils, fertile-islands soils, and soils from burned landscapes. Red brome germination is maximized in wet fall periods when adequate water and optimal temperatures overlap . To evaluate landscape responses of pre- and post-fire plant communities and the potential for repeated burning we analyzed vegetation greenness (NDVI) data from 1985-2011 in response to temperature and precipitation. Landscape analysis indicated that the dominance of exotic grasses increases on post-fire landscapes. Following wet fall and winter seasons, high red brome productivity increases fire potential. Without mitigation, the establishment of an invasive-plant-driven fire regime is likely and may drive state transitions from arid shrublands to arid annual grasslands. Potential revegetation of post-fire landscapes will depend at least in part upon the physiological response of surviving vegetation to post-fire landscapes. Plant physiological responses to post-fire landscapes were generally neutral or positive, suggesting that revegetation of post-fire landscapes is not precluded by resource loss associated with fire and may even be enhanced by post-fire conditions. This will likely translate to increased reproductive potential of surviving plants. Alterations to small mammal populations will likely play a role in the reestablishment of vegetation (both native and exotics) as small mammals have strong top-down effects in arid ecosystems. Diversity and species richness responded negatively to burned landscapes as Merriam's kangaroo rat (Dipodomys merriami) increased in abundance while other species practically disappeared from burned landscapes. Merriam's kangaroo rat affects propagule sources through direct consumption, and seed dispersal. Increases in abundance and dominance of Merriam's kangaroo rat will likely alter plant recruitment.

Geographic Information Systems (GIS) and remote sensing technologies were used to identify and describe potential habitat for three species endemic to the Southwestern United States; the Golden-cheeked Warbler (Dendroica chrysoparia), the Black-capped Vireo (Vireo atricapillus), and the Texas kangaroo rat (Dipodomys elator). For each species, the computerized classification of digital satellite imagery was integrated with ancillary spatial information (e.g. soils, geology, and land use) to construct a data base to be used for ecological evaluation as well as habitat protection and management measures.