Abstract:
I investigated effects of migration and population density on habitat and diet selection in a population of mule deer (Odocoileus hemionus) in southern California from 1989 to 1991. All male deer were migratory, whereas females exhibited a mixed strategy with both migrant and resident individuals. No difference occurred in sizes of home ranges for migratory or resident deer. Home-range size of deer was smaller in summer than in winter, however. Size of home range was positively associated with proximity to human disturbance and the amount of avoided habitat (use $<$ available) in the home range. Deer avoided human disturbance in all seasons. Clear tradeoffs existed for deer in montane southern California with respect to whether they migrated. Migratory females were farther from human disturbance and used high-quality habitats more often than did their nonmigratory conspecifics. Nonetheless, during migration deer were at increased risk of predation, and in years of low precipitation (low snow) had higher rates of mortality than did resident deer. Thus, in areas with extremely variable precipitation and snow cover, a mixed strategy for migration can be maintained. Migration patterns of deer resulted in drastic shifts of population density between seasons as deer migrated into and out of ranges. Quality of diet (as indexed by fecal crude protein) for deer in a low-density area was higher than that of a high-density area in winter, when deer densities were most different. Diet quality was similar in summer when both areas had similar densities of deer. Contrary to predictions of the ideal-free distribution, diet quality was different between the two areas in autumn when population densities were similar; this may have been due to an elevated availability of graminoids on the high-density area. Niche breadth, as measured by diet diversity, differed in a manner opposite to the predictions of the ideal-free distribution. During winter, when differences in density between the two study areas were most evident, niche breadth along the dietary axis in the low-density group was twice the size of this measure for the high-density area. Theoretical models for changes in niche dimension need to consider such empirical outcomes.
