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.
A Field Experiment on Intraspecific Interactions in the Red-Backed Vole, Clethrionomys Gapperi
Acting on the hypothesis that dominant male red-backed voles, Clethrionomys gapperi, killed or drove out juveniles during the breeding season, large males were selectively removed from four areas of woodland during the spring and summer of 1967. the results obtained were compared with those from four untouched areas. Experimental and control areas alternated along a narrow strip of forest atop a stabilized sand dune bordered by uncolonized habitat (marsh) in southern Manitoba. Each area measured 300 by 50 to 80 meters. There was little movement between areas. Numbers known to be present increased more rapidly on three out of the four experimental areas than on their respective control areas due mainly to a greater number of juveniles in the catches on these areas. An examination of size distributions indicated that juveniles on the control areas entered the "trapable" populations about 3 weeks later than those on the areas where large males had been removed. This caused the numbers known to be present on the control areas to lag behind those on the experimental areas during the early part of the breeding season. This analysis led to the suggestion that the presence of large males in some way inhibits the first approach to a trap by juveniles. the evidence gathered provided no support for the initial hypothesis.
Population Density, Life Span, and Mortality Rates of Small Mammals in the Blue-Grass Meadow and Blue-Grass Field Associations of Southern Michigan
Introduction ----.... 395 Acknowledgments -------395 Methods . 396 Populations of small mammals -----------397 Perom_scus maniculatus bairdii 397 Fluctuations in population density -397 Estimated life span and mortality rates 399 Peromyscus leucopus noveboracensis -401 Fluctuations in population density A401 Estimated life span and mortality rates -402 Microtus penns_lvanicus pennsylvanicus -. 404 Fluctuations in population density .404 Estimated life span and mortality rates 405 Blarina brevicauda talpoides 405 Zapus hudsonius hudsonius 406 S_naptom_s cooperi cooperi 407 Other species ---------------408 Fluctuations in density of small mammal populations 408 Life span and mortality rates of small mammals 410 3ummary414 References 415 Tables . 417