PARENT SESSION

Population Ecology 3 -- Session Chair: Winston Smith-- Van Duzer Theater

FACTORS INFLUENCING THE ABUNDANCE OF TWO SYMPATRIC VOLES IN THE SACRAMENTO MOUNTAINS, NEW MEXICO. James P. Ward¹ and William M. Block². ¹ Rocky Mountain Research Station, c/o 1101 New York Ave., Alamogordo, NM; ² Rocky Mountain Research Station, SW Forestry Sciences Complex, 2500 South Pine Knoll Dr., Flagstaff, AZ.

ABSTRACT- Long-tailed voles (Microtus longicaudus) and Mexican voles (M. mexicanus) occupy canyon meadows, forest openings, and associated ecotones throughout portions of southwestern mountain ranges. The population ecology of these species in the southwestern United States is not well documented. This paper reports the primary factors influencing abundance of these voles where they are sympatric in the Sacramento Mountains, New Mexico. During 1992–1996 in montane meadows, summer density of Mexican voles fluctuated from 0 to 5,634 individuals/ha while long-tailed voles fluctuated from 0 to 1,871 individuals/ha. Peak years for Mexican voles occurred in 1992 and 1995, where as long-tailed voles peaked only during 1992. Environmental correlates with abundance of these two voles were identified by (1) constructing envirograms to develop a series of a priori multiple regression models, then (2) using the information theoretic approach to model selection to estimate model parameters and rank relative importance of factors correlated with vole abundance. According to Akaike's Information Criterion (corrected for small samples), the top rated model included 7 ecological variables and explained 95% of variation in Mexican vole abundance where as the top rated model for long-tailed voles included 3 ecological variables and explained 80% of variation in long-tailed vole abundance. The top three factors potentially influencing Mexican vole abundance were identified as positive associations with grass-forb height, number of reproductively active females in the population, and number of interference competitors (predominantly long-tailed voles). The top three factors influencing long-tailed vole populations were identified as positive associations with number of reproductively active females in the population, and number of interference competitors (Mexican voles) and negative association with grass-forb heights.

KEY WORDS: Microtus longicaudus, population ecology, abundance factors, M. mexicanus