Investigating individual-based habitat settlement and space use decisions (i.e., how individuals distribute themselves across a landscape) is a central theme in ecology, with potential consequences for fitness (e.g., survival, reproduction). Among mammals, habitat selection and spatial organization (home range size and overlap) may be influenced by inter-annual variation in population density, and space use decisions are often related to behavior and social structure. In this dissertation, I combined the results of field work, experimental assays, mapping, and modeling techniques to examine long-term behavioral and spatial dynamics in a high elevation population of golden-mantled ground squirrels (Callospermophilus lateralis), an asocial and presumably territorial species.
In chapter one, I described and quantified litter relocation behavior –a poorly documented phenomenon, in which a female moves her litter to a new location –during long-term studies of two species of ground-dwelling squirrels: golden-mantled ground squirrels and yellow-bellied marmots (Marmota flaviventer). In this natural history study, I identified several possible costs (e.g., energy expenditure and predation risk while transporting young) and benefits (e.g., increased habitat quality and social benefits) of litter relocation, and highlighted the need to better understand habitat preferences and the role of kinship on space use in golden-mantled ground squirrels. Accordingly, chapters two through four focused on patterns of habitat selection, behavior, and the social and spatial organization of this species, with particular emphasis on the effects of density, behavior, and kinship on space use.
Habitat selection dictates the distribution of individuals across space and time, which influences access to key resources such as preferred food items and cover from predators. As population density increases in high quality habitat and competition for limited resources becomes intense, animals are believed to preferentially settle in lower quality habitat, where they are equally likely to survive or reproduce due to lack of competition; however, studies that quantify density-dependent habitat selection or tie fitness measures to resource selection decisions in free-ranging animal populations remain rare. In chapter two, I quantified habitat selection in adult female golden-mantled ground squirrels across two spatial scales (home range placement, and occurrence within the home range) by using 11 consecutive years of data on individual space use, and I used compositional analysis, resource selection functions, and multilevel modeling to address how habitat preferences may be influenced by density or linked to fitness outcomes. Squirrels preferred dry meadow over all other habitat types (aspen, spruce, wet meadow, and willow) at both spatial scales, and were more likely to use dry meadow that contained shorter vegetation and vision-enhancing prominences such as rocks (“perches”). Use of dry meadow at each spatial scale was not influenced by changes in density, and use of dry meadow did not influence fitness (as measured by litter size, pre-hibernation mass, or survival). However, squirrels that experienced a greater number of perches or smaller local densities had higher survival rates, suggesting that a lack of visual obstruction, probably to promote detection of predators, drives habitat selection in this system. Surprisingly, squirrels maintained their preference for dry meadow as density increased, and they experienced reduced survival as a result, perhaps because marginal habitat of sufficient quality was not available.
Although spatial patterns are typically studied at the population or species level, individual differences in space use dynamics often account for a substantial amount of the variation exhibited at higher levels of biological organization, and thus may act as a link between process and pattern in ecology. A growing body of research suggests that variation in patterns of space- and resource use in wild animals may in part be explained by consistent individual differences in behavior across time or contexts, a concept known as animal personality. In chapter three, I used repeated standardized assays to perform the first characterization of personality in golden-mantled ground squirrels, and I used multilevel modeling to determine if personality influenced 95% home range size, 50% core area size, movement speed, or use of perches in nature. Data collected over three years showed that squirrels consistently differed in activity, sociability, boldness, and aggressiveness, and that individual squirrels differed in space- and perch use in a non-random manner that was dependent on their personality type. I found that bolder individuals maintained larger core areas, more active and bold individuals moved faster, and more active, bold, and aggressive individuals had greater access to fitness-enhancing perches in their home ranges. I also found that sociability was associated with access to perches in the home range, suggesting there is a potential benefit within an asocial species of being relatively more social.
Spatial organization may be influenced by kinship, and tolerance among kin is believed to play a role in the evolution of sociality. Ground-dwelling squirrels exhibit a wide range of sociality, from solitary to highly gregarious. Golden-mantled ground squirrels are considered to be asocial and presumably territorial, with the expectation that adults, including close kin, have home ranges that are distinct and non-overlapping; however, this classification was supported by sparse data. In chapter four, I characterized the spatial organization, including the role of kinship, in a population of golden-mantled ground squirrels, and I capitalized on changes in population size over 25 years to quantify the influence of density on that spatial organization. I found extensive overlap between female home ranges (30%), and between male and female home ranges (28%) after the breeding season. However, the core areas of adult female home ranges overlapped minimally (7%), suggesting squirrels were territorial, but only for the inner part of their home range. We did not find an effect of population density on home range or core area size, but local-level intruder pressure led to an increase in home range size, perhaps because females ranged farther to either gain familiarity with neighbors or shift into less crowded areas. Contrary to expectation, females shared more space with kin than non-kin, but only at high densities. This density-mediated increase in space-sharing among kin suggests that familiarity and inclusive fitness benefits may offset costs of competition and promote philopatry under certain conditions. Overall, I provide evidence of a facultative transition in this species from asociality to the formation of single-family kin clusters, the first step towards developing sociality.
Together, these chapters further our understanding of the causes and consequences of changes in habitat use, providing insight to the fields of wildlife management and conservation. This work highlights how the personality of free-ranging animals may interact with environmental variables to differentially affect space- and resource use, with potentially important consequences for population-level processes. In addition, this dissertation emphasizes the importance of considering familiarity among neighbors in asocial species, as well as the interactive effects of environmental conditions and kinship when assessing the spatial and social organization of mammals. I hope this work serves as a valuable contribution to our understanding of ground squirrel ecology, as well as inspires future research that links individual processes to spatial patterns of wildlife populations in natural ecosystems.
|Advisor:||Van Vuren, Dirk H.|
|Commitee:||Sih, Andrew, Hijmans, Robert J.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 82/8(E), Dissertation Abstracts International|
|Subjects:||Ecology, Wildlife Management, Behavioral Sciences|
|Keywords:||Animal personality, Habitat selection, Population dynamics, Animal sociality, Spatial ecology, Callospermophilus lateralis, Golden-mantled ground squirrels|
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