Masked Shrew (Sorex Cinereus) Abundance, Diet and Prey Selection in an Irrigated Forest
McCay, Timothy S., Storm, Gerald L., The American Midland Naturalist
ABSTRACT.-Moisture has been proposed as the primary factor affecting local abundance of shrews. We studied invertebrate availability and masked shrew (Sorex cinereus) diet in wastewater-irrigated and nonirrigated forests in central Pennsylvania to better understand the relationships among moisture, invertebrate abundance and shrew diet. Earthworms, gastropods, isopods, millipedes, larval beetles and larval flies were more abundant in irrigated than nonirrigated plots during both spring and autumn, indicating greater availability of certain foods. Larval beetles and larval flies composed a greater portion of the masked shrew diet in irrigated plots during autumn (P < 0.05). Spiders, which were less abundant in irrigated than in nonirrigated plots during spring and autumn, made up a smaller portion of the masked shrew diet in irrigated plots during autumn (P < 0.01). Masked shrews rarely ate millipedes and isopods, although these invertebrates were very abundant in irrigated plots. Shrews did not feed on invertebrates in proportion to their abundance (P < 0.05), but selected some taxa (e.g., insect larvae) and avoided others (e.g., millipedes). Our study suggests that increased populations of invertebrates in moist forests, especially those selected by shrews, affected the diet of shrews and may be a mechanism for the increased abundance of shrews in moist environments.
TIMOTHY S. McCAY1AND GERALD L. STORM
Pennsylvania Cooperative Fish and Wildlife Research Unit, The Pennsylvania State University, University Park 16802
Moisture has been proposed as the primary factor affecting local abundance of shrews (Getz, 1961; Wrigley et al., 1979; Kirkland, 1991). Because shrews have relatively high metabolic rates (Pearson, 1947), they consume large amounts of food relative to their mass (e.g., Hawkins and Jewell, 1962) and have high water requirements (Chew, 1951). Environmental moisture may benefit shrews directly by increasing relative humidity at the forest floor, which may help shrews to maintain proper water balance, or indirectly by increasing the activity and availability of invertebrate prey (Kirkland, 1991). Evidence for the importance of moisture, however, has been largely inferential, based on capture rates in natural environments differing in moisture (e.g., Wrigley et al., 1979). Moreover, the mechanisms by which moisture may indirectly affect shrew abundance, such as increased invertebrate productivity, have received little attention.
Holling (1959) attributed functional and numerical responses of the masked shrew (Sorex cinereus) to changes in the abundance of the European sawfly (Neodiprion sertifer), the major prey item at his study area. Increased density of sawflies was accompanied by increased consumption of sawflies by masked shrews (functional response) and increased density of masked shrew populations (numerical response). Stewart et al. (1989) suggested that abundant littoral foods were related to the high density and extended breeding season of an insular population of the masked shrew on Bon Portage Island, Nova Scotia.
Application of treated sewage effluent (wastewater) in forests of central Pennsylvania provided us an opportunity to examine invertebrate abundance and diversity and masked shrew abundance and diet in an artificially moist environment. Water and nutrient enrichment by wastewater irrigation may affect shrews directly, or indirectly through changes in the abundance and distribution of plants (Sopper and Kardos, 1973) and animals (Mastrota et aL, 1989; Rollfinke et aL, 1990). For example, wastewater irrigation in a mixed-oak forest in Pennsylvania was associated with increased biomass of earthworms (Dindal et al., 1979; Mastrota et al., 1989) and gastropods (Mastrota et al., 1989), indicating an increase in the quantity of certain foods available to shrews.
We collected invertebrates and shrews in wastewater-irrigated and nonirrigated forest plots in Pennsylvania during spring and autumn 1992. …