Pitfall Trapping of Microtus townsendii
A field population of Microtus townsendii was trapped concurrently with live-traps and pitfall traps throughout the spring and summer of 1975. The population was at peak densities, with a maximum of 519 voles per acre. The number enumerated by pitfalls was up to two times larger than the number enumerated by live-traps. Capture of a large number of young in pitfalls indicated that mortality was higher among post-weanlings than among any other age group. Recruitment to live-traps was largely restricted to animals over 40 g, whereas in pitfalls animals were generally lighter. Over half of the 1,100 animals captured first by pitfalls were never caught by live-traps. Among those subsequently entering live-traps, many stayed away for 6 to 10 weeks, and some of the smaller ones stayed away even longer. Few animals dispersed to other nearby trapping grids. Pitfall trapping and live-trapping sample different subpopulations, young animals and transient adults being a large fraction of the individuals caught in pitfalls.
Microhabitat heterogeneity and population ecology of an herbivorous rodent, Microtus californicus
SummaryExisting models of microtine population regulation and life history evolution rely on data derived from empirical studies which assume spatial homogeneity of the sampling unit, and infer that population dynamics can be described adequately by aggregation of individuals differing in sex, age and social status. These assumptions were not supported in the present study, and their application would have obscured relationships of ecological and evolutionary importance. From 1975 to 1981 a population of Microtus californicus exhibited seasonal fluctuations in numbers along with two extreme peaks in abundance. Floristic richness changed under the impact of heavy grazing by M. californicus, but floristic spatial pattern showed little interannual variation. All parameters of female survival and reproduction were correlated with floristic micropattern. Voles performed better in vegetation dominated by the native perennial grass Elymus triticoides. Female dispersion was more restricted than that of males. There are no reasons to believe the microhabitat preferred by females affords superior nutrition or protection from predation during peaks of abundance. However, it should provide higher probability of survival during summer, the time of greatest stringency in central California. The implications of these results for current theory are discussed, and a hypothesis developed which emphasises the importance of improved conditions in the most unfavourable season for peaks in abundance.
Differences in Numbers, Reproduction and Survival Between Two Neighboring populations of Bank Voles (Clethrionomys glareolus)
This research is supported by grant G-14533 from the National Science Foundation and grant M-5001 from the National Institute of Mental Health, U. S. Public Health Service. I am indebted to Hermine Kavanau for the illustrations and to Drs. Daniel H. Brant, Robert Glaser, Kenneth S. Norris and Frederick B. Turner for valuable suggestions concerning the manuscript. Thanks also are due to Marcyn Brown, Bill Landy, Tony Dymond and Greg Sarna for aid in data reduction.