Distance-dependence in two Amazonian palms: effects of spatial and temporal variation in seed predator communities.

Abstract

Animals aid population growth and fitness in tropical forest communities through dispersal and negatively impact populations through seed predation. The interaction between dispersal and seed predation can produce distance- or density-dependence; powerful mechanisms for maintaining species diversity incorporated in the Janzen-Connell model. Large mammals, the highest biomass seed predators of intact Amazonian communities and at risk due to human disturbance, are potentially central to these interactions. This study tests the Janzen-Connell model and investigates the impact of mammalian seed predators on seedling recruitment and maintenance of tree diversity. Patterns of both vertebrate and invertebrate seed predation and seedling recruitment were studied in the two most abundant canopy tree species in western Amazonia (Arecaceae: Astrocaryum murumuru and Iriartea deltoidea). We specifically examined effects of both spatial and temporal variation of the highest biomass seed predator in southwest Amazonian forests, the white-lipped peccary ( Tayassu pecari), on recruitment through disturbed and undisturbed sites and through a fortuitous 12 year natural extinction and recolonization event of T. pecari. Distance-dependent seedling recruitment was found in Astrocaryum and Iriartea at both sites. However, the median distance of seedlings was approximately 1.5x farther from reproductive adults in both palms at the undisturbed site. The number of Iriartea seeds escaping predation increased 6,000% in both space and time due to the decline of T. pecari abundance. The results demonstrate that Janzen-Connell effects are stronger in intact ecosystems and tie these mechanistically to changes in seed predator abundance. This study shows that anthropogenic changes in mammal communities decrease the magnitude of Janzen-Connell effects in Amazonian forests and may result in decreases in tree diversity.