Comparative population ecology of Peromyscus leucopus and Peromyscus maniculatus

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Abstract
The reproductive biology and population structure of Peromyscus leucopus and P. maniculatus were studied from 1981 to 1983 to determine if differences in demographic features were sufficient to explain the relative frequencies of the two species. A total of 444 P. leucopus and 244 P. maniculatus were caught 3912 times. The population density of P. leucopus declined from a high of 38 animals/ha to a low of 6 animals/ha from 1981 to 1983, and P. maniculatus declined from 19 to 2 animals/ha over the same time period. The reproductive season averaged 197 days for P. leucopus and 204 days for P. maniculatus and was bimodal, with peak activity in April-June and September-October. Litter sizes aceraged 3.5 for P. leucopus and 3.4 for P. maniculatus. Two-week survival rates varied between ages and sexes, but ranged from 0.75 to 0.91 for both species. Immigration occurred throughout the year in both species but was most pronounced during the breeidng seasons. Sixty-three percent of adult male and 47% of adult female immigrants were reproductively active. Fifty-six percent of P. leucopus and 55% of P. maniculatus attained residency. Mean length of residency varied among sexes, ages, and years for P. leucopus, but not for P. maniculatus. For P. leucopus, adults had longer residencies than juveniles at high densities, but juvenile females had longer residencies than adults or juvenile males at low densities. The lack of significant differences in the demographic features between the two species suggests that other factors are responsible for mediating their coexistence and maintaining a P. leucopus/P. maniculatus ratio of approximately 2:1.