Habitat Selection and Species Interactions: An Experimental Analysis with Small Mammal Populations

Abstract

Habitat associations and habitat utilization patterns of small mammals (Sorex cinereus, Blarina brevicauda, Peromyscus leucopus, Microtus pennsylvanicus, and Zapus hudsonius) were experimentally investigated in three communities (Broadmoor, Great Island, and Nantucket) in eastern Massachusetts (USA). Structure of the habitat was ordinated using principal components analysis. Habitat associations of small mammals colonizing experimental trapping grids where all individuals were regularly removed were compared with control grids using multiple linear regression. Intergrid differences in habitat selection were observed in one population of Blarina, two populations of Peromyscus, three populations of Microtus, and one population of Zapus. Differences in habitat associations in four populations could be attributed at least partly to differences in habitat structure. An index of habitat utilization (discrimination) was developed using discriminant function analysis. Peromyscus showed large increases in discrimination on the experimental grids, whereas one population of Microtus showed a decrease in discrimination on the experimental grid. Seasonal discrimination values were regressed on seasonal density estimates of small mammals on the control grids to quantify the relationship between population density and habitat utilization in the Peromyscus and Microtus populations. The Peromyscus populations and the Broadmoor Microtus population demonstrated the predicted negative relationship between intraspecific population density and discrimination. The Great Island Microtus population showed a positive relationship between density and discrimination, which could be explained by the structure of the habitat. There was no linear relationship between Microtus density and discrimination on Nantucket, but discrimination followed a pattern similar to that of density. No significant relationships were found between habitat discrimination and densites of other small mammal species. These results suggest that intraspecific competition was more important than interspecific interactions in determining habitat use patterns, and that habitat selection was an important determinant of structure in these small mammal communities.