Pitfall versus Live-Trap Enumeration of Fluctuating Populations of Microtus townsendii

Abstract

Four populations of Microtus townsendii were trapped concurrently with live-traps and pitfall traps from May 1976 until June 1978. The number of voles enumerated by pitfall traps in both increasing and peak populations was up to two times larger than the number enumerated by live-traps. Pitfall traps generally caught voles under 40 g, live-traps over 40 g. About 45% of 3,677 voles captured first by pitfall traps were never caught by live-traps. Among those voles subsequently entering livetraps, over 50% stayed away for more than 5 weeks, and 10% stayed away for more than 20 weeks. Pitfall traps previously caught more animals that dispersed than did livetraps. An index of juvenile survival based on data derived from live-traps may not be indicative of actual trends. Voles caught only in pitfall traps have faster growth rates than those first caught in pitfall traps and later in live-traps. Live-traps and pitfall traps sample different segments of the population with respect to size, wounding levels, and parasite loads.