The effects of tree seed and seedling density on predation rates by rodents in old fields

Abstract

Seed and seedling predation by rodents along forest-field edges can influence the dynamics of tree invasion into old fields. As predicted by the Janzen-Connell escape hypothesis and the predator satiation hypothesis, the effects of rodents on tree propagules may be modified by variation in the distribution and abundance of tree propagules. We tested these hypotheses in old-fields using a series of U-shaped enclosures in southeastern New York State. We maintained two densities (high versus low) of meadow voles (Microtus pennsylvanicus), which are the principal seedling predators and dominant rodent competitors at our sites. High density of voles significantly reduced density of white-footed mice (Peromyscus leucopus), which are the primary seed predators. Within enclosures we manipulated the density and spacing of red maple (Acer rubrum) seeds and black birch (Betula lenta) seedlings. We used three densities of red maple seeds (1500, 750, and 150 seeds/2 m2) arranged to mimic natural leptokurtic declines in seed density with distance from a hypothetical seed source. The density of black birch seedlings was also manipulated at three levels (50, 20, and 10 seedlings/ 2 m2). Seed predation was uniformly high overall (approx. 99%) in three adjacent plots irrespective of seed density. However, predation was significantly lower in isolated, low-density plots. These results suggest that mice were not satiated by the range of seed densities typical for old fields and instead foraged in a distance-responsive manner consistent with predictions of the escape hypothesis. In contrast, seedlings in high density plots experienced significantly lower predation than those in medium- and low-density plots, suggesting that voles were satiated by plots of high seedling density. These patterns were clearest within enclosures having high vole density, highlighting the importance of competitive interactions in modifying the foraging decisions of rodents in old fields.