Spatial Segregation in Three Sympatric Species of Peromyscus from the North Coast Range of California

Purpose of study: Three sympatric species of white-footed mice, Peromyscus boylii, P. maniculatus and P. truei occur in the northern Coast Range of California. Closely related, coexisting species present useful opportunities to investigate niche resource partitioning. Previous research has indicated that a variety of factors can influence the niche separation in Peromyscus. The purpose of this study is to determine the role of spatial segregation, and habitat association in the niche partitioning in northern California Peromyscus. Procedure: Prior to field work, museum specimens from the California Academy of Sciences in San Francisco, and the Museum of Vertebrate Zoology, U.C. Berkeley, were examined to: 1) develop a key to the identification of Peromyscus on the basis of external morphology 2) obtain historical information on the distribution and sympatry of the Peromyscus in the study area. New collections were vouchered and a key to the live identification of P. boylii, P. maniculatus and P. truei was developed. Live-trapping and vegetation-habitat sampling were used in combination to determine the spatial segregation and habitat association of the three Peromyscus spp. on 10 habitat sampling grids in Lake, Mendocino, and Sonoma Counties. Trapping grids consisted of 24 trap stations set at 20-meter intervals with the grids being 8 trap stations long and three wide. A total collection effort of 5,544 trap nights was conducted from June 1982 to Aug. 1983. Vegetation was sampled with line intercept methods on four 10 meter line transects at each trap station. Physical features, slope, slope exposure, substrate and non-plant cover were also recorded. Findings: The three Peromyscus species were found to be 'macro-sympatric' and 'micro-allotopic', spatially partitioning the available habitats. P. truei was the most abundant species captured in the study area and occurred in the greatest variety of habitats. P. truei was most strongly associated with short chamise chaparral slopes where low ground cover was available. P. boylii was found to associate strongly with taller vegetation on north-facing slopes. P. maniculatus was found to 1 inhabit chaparral edge locations where large amounts of grass were present. f:.. maniculatus also associated with rocks and logs on the ground. Conclusions: The prominence of a patchy 'mosaic' vegetation pattern in California has resulted in a spatial segregation of the macro-sympatric Peromyscus species. This segregation has not been previously described for the California Coast Range because the vegetation patterns and the Peromyscus species are closely intermingled. P. truei inhabits the widest range of habitats and occurs in all habitats that are not occupied by P. boylii or P. maniculatus. P. maniculatus inhabits ecotone areas where vegetation disturbance is present and where grass seeds are available. The most likely hypothesis to account for P. boylii's preference for taller chaparral is that this species exhibits more arboreal activity, perhaps giving it resource advantages over P. truei. The relative needs for water, and the different temperatures of the different chaparral stands cannot be excluded as potential modes of habitat partitioning.