The Green World Hypothesis, which argues that predators keep the world green by limiting consumer densities, is well-supported in systems with arthropod consumers. However, it predicts that plants are resource-limited, and thus unaffected by reduced consumer densities. Yet studies of overhunted tropical forests show that seedling density increases and plant community composition shifts when mammalian consumer abundance is reduced, suggesting that both reduced and increased consumer densities affect plant communities. Collared peccaries (Pecari tajacu) are a dominant, but intensively-hunted, Neotropical consumer that have trophic (consuming seeds, leaves, and roots) and non-trophic (trampling, rooting) effects on plants. We tested four predictions consistent with the hypothesis of top-down consumer limitation of plants at six Central American forests along a peccary density gradient of 3 to >14 / sq. km., as well as within three sets of experimental mammal exclosures: (1) Collared peccaries reduce understory vegetation density and cover, and leaf litter depth, mass, and arthropod abundance; (2) Effect size of mammalian exclosure is negatively correlated with peccary density; (3) Vegetation density and cover is negatively correlated with peccary density across sites; and (4) Peccaries have stronger impacts on lianas than on free-standing plants.
Results/Conclusions
Relative to mammal exclosures, collared peccaries significantly reduced understory vegetation, woody plant, and vine density, and liana cover at La Selva Biological Station (highest peccary density), and vine density at Barro Colorado Island and Gigante Peninsula (intermediate peccary density). Peccaries reduced leaf litter depth and biomass at La Selva, but increased densities of predatory and herbivorous litter arthropods. Exclosure effect sizes were largest at La Selva, despite the younger exclosures, higher soil fertility, and presence within the exclosures of small terrestrial consumers, e.g., agoutis. Across six study sites, palm, liana, and forest cover declined with peccary density. Our research demonstrates that collared peccaries exert strong top-down effects on vegetation at a range of densities, independent of predator limitation. This study adds to the growing literature showing that both decreased and increased consumer densities have strong effects on the plant community, with cascading consequences for arthropods, birds, and other components of these lowland tropical forest ecosystems. In order to preserve plant communities and tropical forest biodiversity, large consumers such as peccaries should be maintained at intermediate densities.