Dynamics of Enclosed Small Mammal Populations in Relation to an Experimental Pesticide Application

Abstract

Long-term effects of a pesticide (Sevin) application on populations of cotton rats (Sigmodon hispidus), house mice (Mus musculus) and old-field mice (Peromyscus polionotus) were investigated over a 1-year period in two enclosed, 1-acre experimental areas, one of which served as a control. Mammals were live-trapped weekly and the following characteristics compiled: natality rate, populatio'n change, mortality rate, sex ratios, trapping efficiency, recapture frequency, survival rate and Sigmodon weight changes. The pesticide apparently inhibited Sigmodon reproduction, resulting in a lower peak population density than in the nontreated area. Mus population density was higher in the sprayed area, a possible result of fewer interactions with the larger Sigmodon. Peromyscus did not survive well in either area. Sigmodon reproduction ceased in winter, whereas Mus continued breeding at a reduced rate. All mammals exhibited a 1: 1 sex ratio except for Mus in the treated grid. However, females of all species, except Sigmodon in the treated area, were recaptured more frequently than males. 'Cotton rats were recaptured more frequently than either of the other two species. Turnover rate for 50% of the population was approximately 1 month for Mus and 2 months for Sigmodon. Cotton rats lost weight during the winter in the pesticide-treated area while those in the nontreated area maintained or gained weight. Although ecosystem resources were similar in both areas, small mammal population composition remained altered for several months following pesticide treatment. INTRODUCTION This investigation is one in a series of coordinated studies of plantanimal relationships in two 1-acre, fenced areas which were planted to brown-top, millet (Panicum ramosum) and stocked with three species of rodents. One was treated in midsummer 1966 with a single application of Sevin, a carbamate insecticide. T'he overall effects of the pesticide treatment on the whole ecosystem were described by Barrett (1969). This paper deals with the rodent population dynamics in the two enclosures from 30 July 1966 to 31 May 1967. There have been few intensive studies of mixed populations of small mammals confined under field conditions. By combining systematic live trapping with measurements of important ecosystem resources (i.e., primary production, food, etc.), an attempt was made to evaluate the effects of an acute pesticide stress upon population changes and species interactions. The study was carried out at the Institute of Ecology Field Station, adjacent to. the University of Georgia campus, Clarke Co., Ga. The 1 Present address: Battelle Columbus Laboratories, 505 King Ave., Columbus, Ohio 43201.