Meadow Vole Population Response to Vegetational Changes Resulting from 2,4-D Application

Abstract

Abstragt: The effects of a herbicide (2,4-D) on the small mammal component of an old-field community were investigated. The study area consisted of two 0.4-ha plots surrounded by 20-gauge galvanized steel walls, into which seven pairs of meadow voles (Microtus pennsylvanicus) were introduced. An amount equal to 567.5 g/ha of the N-Oleyl 1, 3 Propylendiamine salt of 2,4-Dichlorophenoxyacetic acid (2,4-D), mixed in 151.4 liters of water, was applied to one of the experimental plots on 8 July 1976; the other plot was left untreated. Meadow vole population dynamics were monitored by trapping twice weekly, both before and after herbicide treatment, from 26 June through 11 December 1976. Vegetation was analyzed by the harvest method. In the control plot, 250 animals were captured a total of 1203 times, whereas in the herbicide-treated plot 134 animals were captured a total of 1042 times. The ratio of animals captured between control and treatment plots was 1.87:1.0. The control plot reached a peak density of 116 animals/0.4 ha on 6 November 1976; the treated plot reached a peak density of 68 voles/0.4 ha on 9 October 1976 (a ratio of 1.71:1.0). Male:female sex ratios were skewed in favor of males in the treated grid, mainly because of reduced female survival rates. Plant community structure (e.g., standing crop biomass) and function (e.g., net primary productivity) were significantly altered by herbicide application. It appears that voles in the treated grid, which became essentially a monoculture type giant foxtail (Setaria faberii) habitat, were subjected to food quality differences (e.g., protein deficiencies) as compared to voles in the more diverse untreated habitat.