Interaction of Natality, Mortality and Movement during One Annual Cycle in a Microtus Population

Abstract

A population of meadow mice, Microtus pennsylvanicus, was studied through one annual cycle, near East Lansing, Michigan. In May the mice were at a very low density and living under good conditions of food and cover and experiencing little competition. Because of high natality the population increased rapidly throughout the summer and fall. Breeding ceased in January, and the population continued to increase in February as a result of immigration. Mortality was highest in the summer and least in the winter. This was the result of (1) change in the age composition of the population, and (2) probable reduction of predation due to a persistent snow cover. A life table showed that mortality was highest in the postnestling juveniles and young adults. In other age classes mortality continued at approximately 50 per cent. The natural rate of increase of the population was -0.0315 daughters per female per day. Although the rate of increase was negative, immigration was sufficient to cause the population to increase in size. Although the population dynamics of meadow mice of the genus Microtus has been the subject of considerable research both in North America and Europe, our knowledge of the interaction of mortality, natality, and movement in a single population of this genus is still far from complete. In this paper I intend to present a detailed analysis of these factors in a population of Microtus pennsylvanicus. The data for this analysis were obtained during an intensive one and one-half year investigation of the energy flow through an old-field vegetation Microtus pennsylvanicus -Mustela rixosa food chain. (Golley, 1960). My studies were made in 1956 and 1957 in an abandoned field near the Michigan State University campus, East Lansing, Michigan. Blue grass (Poa compressa), wild carrot (Dauca carota) and Canada thistle (Cirsium arvense) were the chief dominants in the vegetation. Since occasional burning was the only disturbance to the vegetation for about 15 years prior to the study, a dense sod of grasses and perennial herbs had developed and provided an abundance of cover and food for the meadow mice. During the study period the weather was unusually mild. The temperature was above average in every month except September, 1956, and January, 1957, and precipitation was below average throughout 'the fall and winter. The setudy area was surrounded on four sides by orchards, alfalfa fields, and old-field vegetation, all of which furnished habitat for Microtus. Acknowledgments.-I am grateful to Dr. Don W. Hayne, Fish and Wildlife Service, for his advice on methodolgy and analysis of the data on density, natality and mortality. I am also grateful to Dr. Helmut K. Buechner, Dr.