Increased herbivory in forest isolates:: Implications for plant community structure and composition

Understanding processes driving population declines and, ultimately, species loss in forest isolates has significant implication for the long-term maintenance of species diversity. We investigated a potential mechanism driving loss of plant species in small, medium, and large land-bridge islands in Lago Guri, a 4300-km(2) hydroelectric impoundment in the State of Bolivar, Venezuela. Our hypothesis was that elevated Atta (leaf-cutter ants) herbivory on small Guri islands, attributable to release from predation, could result in reduced sapling densities, with potentially negative effects on preferred plant species. We experimentally established Atta preferences among 43 common tree species. Vegetation plots on small, medium, and large land masses were examined for variation in density, size-class distribution, and representation of preferred and less preferred species of small stems. Densities of adult trees of preferred and less preferred species were compared between Atta colony foraging zones and control plots without active Atta colonies, and between foraging zones on the small islands and the large island. Small stem densities were significantly reduced on the small islands, especially in the sapling category (greater than or equal to 1m tall, <1cm dbh) and juvenile (<greater than or equal to>1cm dbh, <10 cm dbh) size classes. On small islands, however, both preferred and less preferred species showed reduced representation in the sapling size class. Adults of preferred species were significantly reduced with Atta foraging zones compared with control plots without Atta colonies. Small stem densities were independent of adult tree densities for preferred species on small islands and for both preferred and less preferred species on large land masses. High levels of Atta herbivory could be potentially important cause of reduced plant recruitment on small Guri islands. Our findings contribute to understanding of the role of altered ecological interactions as deterministic mechanisms driving a process of change in the structure and composition of fragmented communities.