Impact of Precipitation and Grazing on the Water Vole in the Beartooth Mountains of Montana and Wyoming, U.S.A.

Abstract

This paper examines the influence of increased precipitation levels and grazing on the demographics of Microtus richardsoni. Water voles were trapped and marked during the summers of 1990, 1991, and 1992 along four headwater watersheds of the Clark's Fork of the Yellowstone River in Wyoming and Montana. The summer of 1992 had more than double the precipitation of either 1990 or 1991. During the wet summer of 1992, capture success was significantly greater, as was the proportion of young voles captured. In 1992, several factors contributed to increased water vole populations. There were significantly more indications of male reproductive activity. Class I water voles (13–49 g) of both sexes showed signs of reproductive activity indicating they were reaching sexual maturity at smaller body mass. Significantly more embryos/trap-killed female were found. In 1995, the water vole was listed as a sensitive species because it is rare and requires specific alpine riparian habitat that is declining and may be damaged by poor grazing practices. Capture success was significantly greater, and there were significantly more young water voles in ungrazed drainages. Measured indicators of reproductive activity did not vary significantly between grazed and ungrazed drainages. Grazing might affect survival of young water voles and should be studied further.