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Effect of Exploitation on Birth, Mortality, and Movement Rates in a Woodchuck Population

1964; Wiley; Volume: 28; Issue: 1 Linguagem: Inglês

10.2307/3797927

1937-2817

David E. Davis, John J. Christian, F. H. Bronson,

Census and Population Estimation

To assist in understanding the compensatory features of population changes, a detailed examination of the changes in numbers and age composition of a population of woodchucks (Marmota monax) was conducted from 1957 to 1960 inclusive at the Letterkenny Ordnance Depot near Chambersburg, Pennsylvania. The land is divided into areas of about 600 acres each. Changes in birthrate, mortality rate, and movement in an area from which large numbers of woodchucks were regularly removed were compared with the same kinds of changes in a reference area from which few woodchucks were removed. The woodchucks were captured in box traps that produced an unbiased sample on an age and sex basis but were more efficient in the spring (May). The population was estimated by an efficiency-of-capture procedure. The populations in the two areas remained numerically indistinguishable even though 1,040 woodchucks were removed from Area C and only 299 from Area D. The percentage of adults in Area C declined from 70 to 30 but in Area D remained at 70. Survival of young in C increased substantially. The birthrate increased in Area C from 1.29 to 1.63 and then declined to 0.93; in Area D it increased and remained high. Emigration from Area D was much greater than from Area C. Thus, in response to the removal of woodchucks, all three rates changed in directions that compensated for the losses. It is axiomatic that changes in the numbers of animals reflect changes in birthrates, death rates, and movement rates that result in turn from changes in the environmental factors. A detailed examination of the changes in numbers and age composition of a population of woodchucks was an integral part of a continuing study of the role of social behavior in limiting population growth. Changes in age composition are recognized as an important manifestation of changes in the population itself. The experiment to be described was designed to determine the quantitative effects of removal of animals on population parameters. Therefore, changes in birth, mortality, and movement rates in an area from which large numbers of woodchucks were regularly removed were compared with the same kinds of changes in a reference area from which few woodchucks were removed. High precision in estimating numbers has not been attained; hence the changes are indicated in magnitude and direction rather than in detailed counts. The woodchucks were studied on the Letterkenny Army Ordnance Depot near Chambersburg, Pennsylvania, beginning in 1955. The research was conducted under 1Authorized for publication as paper No. 2728 in the Journal series of the Pennsylvania Agricultural Experiment Station.