LONG-TERM EFFECTS OF RODENT HERBIVORES ON TREE INVASION DYNAMICS ALONG FOREST–FIELD EDGES

Abstract

Despite many observations of herbivores and granivores influencing the survival of tree seeds and seedlings, the net effects of these consumers on plant succession remain unclear. We used both experimental herbivore manipulations and observations of changes in vegetation cover to evaluate the longer-term impacts of small-mammal herbivores and granivores on woody plant invasion into old fields in New York State. Over a 5-yr period, we manipulated the density of meadow voles (Microtus pennsylvanicus) within partial enclosures replicated at five study sites. Average vole activity in enclosures with high vole density was significantly greater than that in enclosures with low vole density. In contrast, activity of competitively inferior white-footed mice (Peromyscus leucopus) was significantly greater within enclosures designated as low vole density. Colonization by tree seedlings, as well as shrubs and vines, was significantly greater in enclosures under the low vole density treatment. Further, tree seedling abundance was higher in shrub-dominated enclosures with low vole, but high mouse, activity. Five years of contrasting mouse and vole activity within adjacent enclosures revealed that seedling predation by voles had a greater net effect on changes in woody vegetation cover in old fields than did seed predation by mice, despite studies reporting higher rates of seed vs. seedling predation in these areas. We conclude that short-term measures of predation intensity may not accurately predict the longer-term effects of small-mammal herbivores on vegetation dynamics and suggest that voles may be a keystone herbivore in old-field succession.