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

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Abstract
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 7 pairs of meadow voles (M. 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 l of water, was applied to one of the experimental plots on July 8, 1976; the other plot was left untreated. Meadow vole population dynamics were monitored by trapping twice weekly, both before and after herbicide treatment, from June 26-Dec. 11, 1976. Vegetation was analyzed by the harvest method. In the control plot, 250 animals were captured 1203 times, whereas in the herbicide-treated plot 134 animals were captured 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 Nov. 6, 1976; the treated plot reached a peak density of 68 voles/0.4 ha on Oct. 9, 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. Voles in the treated grid, which became essentially a monoculture type giant foxtail (Setaria faberii) habitat, were probably subjected to food quality differences (e.g., protein deficiencies) as compared to voles in the more diverse untreated habitat.