Rodent Responses To Drought In The Southwestern United States: Resource Utilization Strategies And Effects To Keystone Resources

The role of abiotic and biotic factors in regulating community and population dynamics is a central question of ecological inquiry. In the southwestern United States, the North American monsoon supplies vital pulses of moisture to Sonoran desert, arid grassland, and montane communities. I evaluated abiotic limitation in prairie and montane populations of Gunnison’s prairie dog (Cynomys gunnisoni) using stable isotope analysis to quantify foraging niche widths. Niche widths declined under periods of drought stress. Prairie dogs in montane habitats exhibited seasonal shifts in dietary niche width during favorable growth periods for more nutritious plants using the C3 photosynthetic pathway. Production of offspring was positively correlated with C3 plant use. Body condition improved after emergence from hibernation, except in montane females, who exhibited evidence of early-season reproductive investment. Despite similar body condition and initial population densities, montane C. gunnisoni reached densities up to 20x those of the prairie site. The link between plant nutritional quality and demographic parameters suggests bottom-up regulation within this reportedly disease-limited species. In the Sonoran Desert, white-throated woodrats (Neotoma albigula) supplement herbaceous diets with succulents such as the saguaro cactus (Carnegiea gigantea). Massive saguaros store water for annual production of flowers, fruits, and stem growth that feeds desert consumers during droughts. Saguaros with high levels of herbivory (>20% of the surface) produced fewer flowers and fruits than similar plants with no herbivory. These findings suggest that periodic use of saguaros by N. albigula, such as during extended droughts, reduces long-term reproductive capacities of this keystone desert resource.