Effects of Meadow Vole Population Density on Tree Seedling Survival in Oil Fields

Abstract

Voles (Mammalia: Arvicolidae) are reported to be key consumers of tree seedlings in some habitats, but their importance in old—field succession is not known. Natural populations of voles fluctuate widely in density, and we sought to determine the effects of vole density on seedling predation. We used a randomized block design, with meadow voles maintained at three density levels (low, medium, high; three replicates of each) in field enclosures of 17 mo. We introduced young seedlings of five tree species into the enclosures. Three cohorts of seedlings were planted under intact vegetation, and one was planted in small areas from which we had cleared all vegetation. Overall predation rates were high for all cohorts planted under intact vegetation. There was a significant effect of vole density on seedling predation rate for two of these three cohorts; nearly all seedlings were destroyed in enclosures with high vole density. Predation rates by voles were highest for white ash and red maple, intermediate for white pine, and lowest for eastern hemlock and red oak. For the cohort planted in small clearings, mean predation rates were low and there was no density effect. Spatial variation in seedling predation rates was higher in enclosures with low and medium densities of voles than in high—density enclosures. In three of the cohorts there was a significant block effect, with the same block consistently having the highest predation rates. Evidence suggests that the block effect was caused by genetic differences among voles. Seedling predation by voles can: (1) delay old—field succession by reducing the number of tree invaders; (2) create patchiness by concentrating surviving seedling in certain microsites; and (3) alter relative abundances of invading species via differences in palatability or apparency. We suggest that the population dynamics of voles be viewed as a keystone process in old—field succession.