Factors Influencing the Local Distribution of Shrews

Abstract

A field study was made of the factors influencing the local distribution of Sorex cinereus and Blarina brevicauda in south- ern Michigan. The environmental factors considered were vegetation type, cover (fallen trees, brush, leaf litter), temperature, food, moisture and interspecific competition. Blarina brevicauda was found only in moist situations but avoided standing watcr. It was scarce or absent in those habitats in which the available food supply (consisting of the larger forms of invertebrates) was low. Vegetation, type of cover, temperature, and interspecific competition were not important factors. Sorex cinereus occurred in all habitats except upland hardwoods. The reason for the avoidance of this habitat was not determined, but it apparently was not a response to the type of vegetation. Sorex was relatively less abundant in the drier situations than was B. brevicauda and did not avoid standing water. S. cinereus may be able to utilize smaller food items (collembolans, ants, spiders) than B. brevicauda, so that its local distribution is not influenced by the availability of the larger invertebrates. Type of cover, temperature, and interspecific competition were not important factors in its local distribution. Shrews have been found to normally inhabit moist situations (Pruitt, 1953, 1959; Manville, 1949; Wetzel, 1958). Such a restric- tion to moist areas most likely results from a large turnover of water by shrews (Chew, 1951). Pruitt (op. cit.) has further analyzed the physical factors influencing the local distribution of shrews and found both temperature and moisture to be important factors. The relative importance of the various biotic factors (i.e., food availability, vege- tation type, and interspecific competition) have not been thoroughly investigated. Pearson (1947) has shown that shrews have high metabolic rates and as a result require large intakes of food. As a result one might expect the availability of food to play an important role in the local distribution of shrews. The present paper contains data pertaining to the masked shrew, Sorex cinereus, and the shorttailed shrew, Blarina brevicauda, ob- tained during a study of the factors influencing the local distribution of small mammals in southern Michigan. Data concerning other species will be presented elsewhere (Microtus pennsylvanicus and Synaptomys cooperi, in press; Peromyscus leucopus and Zapus hud- sonius, in preparation). Most of the habitats studied had conditions moist enough to be considered favorable in regard to this factor. Shrews did not occur in all study areas, however; it was desired, therefore, to determine the factors influencing distribution within these moist areas.