The manner in which terrestrial ecosystems are regulated is controversial. The "top-down" school holds that predators limit herbivores and thereby prevent them from overexploiting vegetation. "Bottom-up" proponents stress the role of plant chemical defenses in limiting plant depredation by herbivores. A set of predator-free islands created by a hydroelectric impoundment in Venezuela allows a test of these competing world views. Limited area restricts the fauna of small (0.25 to 0.9 hectare) islands to predators of invertebrates (birds, lizards, anurans, and spiders), seed predators (rodents), and herbivores (howler monkeys, iguanas, and leaf-cutter ants). Predators of vertebrates are absent, and densities of rodents, howler monkeys, iguanas, and leaf-cutter ants are 10 to 100 times greater than on the nearby mainland, suggesting that predators normally limit their populations. The densities of seedlings and saplings of canopy trees are severely reduced on herbivore-affected islands, providing evidence of a trophic cascade unleashed in the absence of top-down regulation.

We report on the bird communities of a set of 12 7‐yr‐old forested land‐bridge islands in Lago Guri, a 4300 km2 hydroelectric impoundment in the State of Bolivar, Venezuela. Birds were censused on all islands and at mainland control sites by spot mapping in 1993, and via point counts in 1995. Instead of orderly “nested sets” of species on landmasses of graded size, the species composition of small (≈1 ha) and medium (11–12 ha) islands was highly variable. Spot mapping substantiated the occurrence of 58 species of forest‐nesting birds, collectively, on the 11 small and medium islands, which supported means of only 9 and 12 resident species, respectively. No species was found on all islands, and only five species were found on as many as 7 of the 11 small and medium islands. The mean number of islands per resident species was 3.0 for the 11 islands. Transients of many species were detected on all islands, but were more frequent on near (≤0.5 km from a larger landmass) than on far islands (≥1.0 km from a larger landmass). A large majority of the populations on the 1‐ha islands consisted of a single pair, implying that few such populations could have survived since isolation without new colonizations to replenish individuals lost to dispersal and mortality. Notwithstanding low species numbers, both census methods indicated that avian densities were approximately twice as high on the 1‐ha islands as on the mainland. In contrast, two of three medium‐sized islands supported anomalously low densities of approximately one‐fifth the mainland level. Both islands retain relict populations of capuchin monkeys (Cebus olivaceus). One hundred percent of artificial nests set out on one of these islands were raided, whereas no more than 30% of nests were lost on any of the other islands or on the mainland. For small and medium islands, we conclude that the founding communities present when the waters of Lago Guri reached their final level have already collapsed and been reconstituted. Species loss on some islands may have been accelerated by relict populations of predators, such as capuchins. Changes in composition thus appear to have been driven by a combination of biological (nest predation) and stochastic processes (high turnover).

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The results are reported of a survey of rodents on 10 forested land-bridge islands ranging in size from 0.2 to 350 ha in the state of Bolívar, Venezuela. The islands were contained within a lake formed c.12 years before the study by the damming of the Caroni River for hydroelectric power. Rodents were sampled on each island by live-trapping along transects that sampled all available habitat types on each island, and microhabitat structure was measured at each trap station. A total of 674 captures of 359 individuals of six species of rodents was recorded. Species composition changed from the largest to the smallest islands, and small and medium islands (0.2-11 ha) displayed the typical effects of insularity, with fewer species and increased abundances and biomass. The largest island (350 ha) seemed to function more like a mainland. Most species were associated with a suite of microhabitat variables. It is suggested that release from top-down control by predators was responsible for higher abundances and biomass on the smaller islands and that predators moving between large islands and other nearby landmasses help maintain a mainland community structure on large islands. However, changes in species composition on smaller islands may be the result of patchy occurrences of some species before isolation, changes in microhabitat structure following isolation, and species-specific microhabitat requirements.

Sapling mortality exceeds recruitment for many species of trees in the dry forest of 16-y-old islets in Lago Guri, Venezuela. Failure of sapling recruitment is potentially attributable to the aberrant animal communities of these islands. Predators of vertebrates are absent and densities of pollinators and seed dispersers are substantially reduced in comparison to the nearby mainland. In contrast, predators of invertebrates, rodents and generalist herbivores (leaf-cutter ants, howler monkeys, common iguanas) are present at greatly elevated densities. Given these distortions in the animal community, recruitment failure of saplings could potentially be attributable to several causes: reduced pollination or seed dispersal, excessive seed predation or seedling herbivory. Two of these hypotheses are tested herein. The seed predation hypothesis predicts greater seed removal in the presence of hyperabundant rodents, and the seedling herbivory hypothesis predicts reduced seedling survival in the presence of hyperabundant herbivores. Seed removal trials were conducted with 18 species of forest trees, using both exposed and lightly buried seeds. Seed removal was not generally higher on islands supporting hyperabundant rodents, contrary to the seed predation hypothesis. Seedlings exposed to herbivores for 4 mo suffered the highest mortality on small islands supporting hyperabundant rodents and leaf-cutter ants. Saplings survived equally well in cages open to arthropods + rodents as in cages open only to arthropods, suggesting that there was no additive effect of rodents on seedling mortality. Current evidence points to excessive seedling herbivory by arthropods, especially by leaf-cutter ants, as the principal cause of recruitment failure on predator-free Lago Guri islets.