ANTHROPOGENIC EDGES, TREEFALL GAPS, AND FRUIT–FRUGIVORE INTERACTIONS IN A NEOTROPICAL MONTANE FOREST

Abstract

In a montane tropical forest in southwestern Colombia, we investigated how anthropogenic edges may alter bird-mediated seed dispersal from edge to forest interior as a function of edge age and presence of treefall gaps. We estimated fruit abundance and mist-netted birds at four distances from edge to forest interior (0–10, 30–40, 60–70, and 190–200 m) in three young ( 40 yr) edges. Fruit-sampling plots (50-m2 plots) at each of the four distances were classified into gap and intact forest. Fruit abundance and frugivore capture rates varied from edge to forest interior, but such changes depended on edge age. At new edges, the total number of fruits was higher at the forest edge than at the forest interior, whereas bird captures showed the opposite trend. At old edges, the total number of fruits and bird capture rates did not vary among the four distances. In a first group of 12 plant and four bird species, the distribution of individuals in fruit (7 species) and captures (3 species) from edge to forest interior differed between old and new edges. In a second group of 18 plant and five bird species, which included those that were not amenable for a comparison between old and new edges and those that were not influenced by edge age, the distribution of individuals in fruit (12 species) and captures (3 species) was not uniform from forest edge to forest interior. Lastly, 124 plant and 19 bird species with <20 individuals in fruit and captures, respectively, were classified into very sparse and sparse species. We found that all but the sparse frugivores were more abundant at the forest edge than in the forest interior. Because very sparse and sparse plant species showed such a clear trend, we used seeds retrieved from mist-netted birds to assess potential seed movement of these species from edge to forest interior. Seeds of very sparse and sparse plant species were found both at forest “edge” (0–10 m) and at forest “interior” (the three other distances combined). Our results suggest that birds are not responding to changes in fruit abundance (resource-base-driven mechanism). Instead, they indicate that frugivore capture rates reflect either a direct edge effect or a non-edge induced effect on birds. The apparent uncoupling of processes generating the observed patterns in fruit and frugivore abundance may affect seed dispersal in important ways. Furthermore, our results indicate that, as edges age, “edge effects” (i.e., maximum distance at which changes induced by edge creation are apparent within forest stands) change.