TERBORGH 9707281 A number of recent studies document dramatic losses of plant species in isolated forest fragments, but the mechanisms of species loss remain largely unknown. The investigators will examine ongoing compositional change in the tree communities of 10-year old land-bridge islands in Lago Guri, a 4,300 km2 hydroelectric impoundment in the State of Bolivar, Venezuela. Lago Guri islands range in size from < 1 ha to > 650 ha and are covered by tropical dry forest. Preliminary work has shown that the animal communities of small (1 ha) and medium (10-15 ha) Lago Guri islands are strikingly imbalanced from a functional standpoint: they are deficient in vertebrate predators, seed dispersers, large seed predators, and pollinators, and contain abnormally high densities of small seed predators (rodents) and arboreal folivores. The proposed research will test the overall hypothesis that imbalanced animal communities are generating forces that will result in transformation of the tree communities of Lago Guri islands as the existing canopy is replaced by trees that established subsequent to isolation of the land-bridge islands. A better understanding of the mechanisms that drive extinctions in habitat fragments will facilitate conservation planning in managed and fragmented landscapes. The investigators' results suggest that the absence of top predators in forest ecosystems leads to runaway growth of prey populations, especially herbivores, which then severely alter the pattern of tree recruitment. Forests affected by herbivores are destined to change inexorably in composition with a resulting loss of diversity of both plants and animals.
