Habitat Differentiation Among Three Species of Sorex and Neurotrichus gibbsi in Washington

Abstract

Environmental parameters in serai stages of the Tsuga heterophylla vegetation zone were measured and correlated with the presence of four insectivores: Neurotrichus gibbsi, Sorex trowbridgii, S. vagrans and S. monticolus. Correlation coefficients were used to determine the specific habitat occupied by each of the species. Neurotrichus occupied forested areas where it burrowed deeply in the soil. Sorex trowbridgii also burrowed, yet was commonly found on the surface of the soil foraging among the litter and moss. Sorex monticolus was more restricted to the layer of debris on the forest floor and perhaps was more successful at foraging there than S. trowbridgii. Sorex vagrans did not burrow and was found in patchy open areas, alder stands and in areas with high water tables where the numbers of S. trowbridgii were low. Introduction During a survey of small mammals in the spring of 1972 on the Cedar River watershed, 32 km E of Issaquah, King Co., Washington, in the Tsuga heterophylla zone (Franklin and Dyrness, 1973), I frequently collected Sorex trowbridgii, S. monticolus { = S. obscurus ?Hennings and Hoffmann, 1977), S. vagrans and Neurotrichus gibbsi at the same and adjacent trapline stations. Because these species are very similar in size and food habitats (Dalquest, 1948; Ingles, 1965; Criddle, 1973; Terry, 1978; Whitaker and Maser, 1976), competition should be expected if they coexist in the same habitats and critical resources are limited (Hutchinson, 1957). The occurrence of these species in close spatial association indicates that: (1) They are coexisting in the same habitat and their environmental requisites are less similar than the literature suggests; (2) critical resources are not limiting, or (3) they occupy distinct subdivisions of the habitat. Although the data reported in this paper are based on unequal and sometimes small sample sizes, the results are useful in indicating which of these three hypotheses is most likely to be correct and to suggest more detailed, testable hypotheses regarding critical features of the ecology of these small mammals. Methods To determine precise habitat characteristics for the above species, I planned to correlate numbers of each insectivore species in an area with measured environmental parameters. Traplines were set in 10 serai forest stages with internally homogeneous vegetation (Table 1), and 30 vegetational and soil variables were measured in each trapping area. Two parallel traplines, 12 m apart, were placed in each vegetation type. Each trapline consisted of 10 stations at 6-m intervals, each containing two Victor mouse traps baited with a mixture of peanut butter, bacon grease and rolled oats. Because of low trapping success (due to rain and other interfering factors), traplines were run for 58 62 days. Environmental parameters were measured at 20 random points (10 for vegetation, 10 for soil) on each trapline pair in April and early May, a time when spring growth was just beginning and vegetational cover was minimal. Because cover is probably of major importance to shrews in avoiding pr?dation, estimates of various types of cover were made on a m2 plot at each sampling point. Estimates of cover provided by various levels of tree canopy, different types of shrubs, dominant ferns, small herbs and ground cover at 2-10 cm were taken. The cover of major moss species, dead leaves, and dead and decaying wood also was estimated. A constancy value {sensu Mueller-Dombois and Ellenberg, 1974) was calculated for each trapping area. Present address: Department of Systematics and Ecology and the Museum of Natural History, University of Kansas, Lawrence 66045.