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Effects of Rodent Consumers on Tree Invasion of Old Fields

Richard S. Ostfeld
Institute of Ecosystem Studies

PI: R. Ostfeld; coPIs: C. Canham (IES); and R. Manson (IES and Rutgers University).

In abandoned agricultural fields of the northeastern U. S., meadow voles (Microtus pennsylvanicus) and white-footed mice (Peromyscus leucopus) are the most common small vertebrates. Meadow voles are herbivorous, and recent studies have shown that they are voracious consumers of tree seedlings. White-footed mice are granivorous and omnivorous, and are efficient predators of tree seeds. Because the ability of trees to invade open fields, and the speed with which they can do so, depends critically on the survival of seeds and young seedlings, the population dynamics, behavior, and interspecific interactions of these rodent consumers may play a strong role in the tree invasion process.

We use an experimental approach to determine how rodents affect the dynamics of tree invasion. Inside partially fenced enclosures in natural old fields, we maintain distinctly different densities of meadow voles by transferring targeted numbers of animals between plots designated as high-vole-density and low-vole-density treatments. We allow white-footed mice to adjust their densities and space use to the abundance and distribution of voles. Because voles are competitively superior to mice, the enclosures with high vole density maintain low mouse density, and vice versa. We introduce cohorts of native tree seeds and seedlings and monitor their fates as a function of tree species, rodent density, distance from the forest edge, microhabitat (e.g., herbaceous or shrubby). As a check on our experimental approach, we also monitor the abundance and distribution of naturally occurring seedlings in our study plots.

We have found that mice typically remove >80% of the seeds within 1-2 months, and that seed predation increases with increasing mouse density. Mice prefer some seeds, such as white pine and red oak, over others, such as tree-of-heaven. In contrast, voles tend to kill 40-60% of seedlings within 1-2 months, but when vole density is high, depredation rates on seedlings may approach 100%. Voles tend to prefer species avoided by mice, including tree of heaven, red maple, and white ash, and tend to avoid those preferred by mice, including pine and oak. Mice tend to concentrate their activities near the forest edge and in shrubby microhabitats, whereas voles are most active >10m from the forest edge and in grassy microhabitats. The net effect is that mice and voles have very different impacts on the species of trees invading, the speed of invasion, and the spatial pattern of invasion.

By monitoring the abundance and distribution of naturally occurring seedlings at our sites, we have found that native seedlings are almost three times as abundant in plots with low vole density as in those sustaining high vole density. We conclude from this that predation on experimentally introduced seeds and seedlings well represents natural invasion dynamics, and that the net effect of voles is greater than that of mice in old fields.

Supported by NSF

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