Density-dependent habitat selection and consequences for fitness in a fluctuating ground squirrel population
A habitat map of our study site at the Rocky Mountain Biological Laboratory in Gothic, Colorado. Squirrels have access to five microhabitat types: Aspen forest, Dry Meadow, Spruce Forest, Wet meadow, and Willow community.
Standardized Manly resource selection ratios for golden-mantled ground squirrels at two spatial scales: where an individual chooses to place its home range, or the area needed for day-to-day activities (Second Order), and where within its home range it spends most of its time (Third Order). Everything above the dotted red line indicates positive selection.
Habitat comprises a combination of resources (for example, food, water, space) and conditions (for example, temperature, absence of enemies) that are critical to an animal's performance, or 'fitness' (the ability to survive or reproduce). However, selection for certain habitat types or features may vary with changing population density. While wildlife management requires understanding these habitat-fitness relationships, access to these data is rare in wild mammal populations.
I explore habitat selection decisions, including the effect of population density on selection and the fitness consequences of non-random resource use, in a population of golden-mantled ground squirrels (Callospermophilus laterals). My research takes place at the Rocky Mountain Biological Laboratory in Gothic, Colorado, where I trap, mark, and observe individuals over 3-month periods of time as part of a long-term study.
After assessing habitat use on two different spatial scales, results show that golden-mantled ground squirrels preferentially select for dry meadow habitat, and they select against other habitat types, such as wet meadow (Veratrum plant communities) and willow communities. A series of statistical models suggest that access to perches, such as rocks or other vantage points from which squirrels could look out for predators, is the key variable that makes dry meadow habitat desirable. Local population density, or the density realized by each individual, has a negative effect on access to preferred habitat. Preliminary results suggest that this matters because access to dry meadow habitat significantly increases body mass upon emergence from hibernation, which may be important for survival.
Predictive models show that access to perches, from which golden-mantled ground squirrels can gain predator visibility, influences habitat selection. As the number of perches within a given space increases, preferred use of that space increases.
Golden-mantled ground squirrels hibernate over winter and, because our study site is at high elevation (2900 m), the summer active season is very short. Squirrels only have about 4 months to emerge from hibernation, find a mate, care for young, and then gain enough weight to have sufficient fat stores to be hibernation-ready again by the end of the season. Drawings courtesy of Elsa Cousins.