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

Understanding processes driving population declines and, ultimately, specie s loss in forest isolates has significant implication for the long-term mai ntenance of species diversity. We investigated a potential mechanism drivin g 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) herbivo ry on small Guri islands, attributable to release from predation, could res ult in reduced sapling densities, with potentially negative effects on pref erred plant species. We experimentally established Atta preferences among 4 3 common tree species. Vegetation plots on small, medium, and large land ma sses were examined for variation in density, size-class distribution, and r epresentation of preferred and less preferred species of small stems. Densi ties of adult trees of preferred and less preferred species were compared b etween Atta colony foraging zones and control plots without active Atta col onies, and between foraging zones on the small islands and the large island . Small stem densities were significantly reduced on the small islands, esp ecially 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 show ed reduced representation in the sapling size class. Adults of preferred sp ecies were significantly reduced with Atta foraging zones compared with con trol plots without Atta colonies. Small stem densities were independent of adult tree densities for preferred species on small islands and for both pr eferred and less preferred species on large land masses. High levels of Att a herbivory could be potentially important cause of reduced plant recruitme nt on small Guri islands. Our findings contribute to understanding of the r ole of altered ecological interactions as deterministic mechanisms driving a process of change in the structure and composition of fragmented communit ies.