Niche relationships in an assemblage of neotropical granivorous ants.

Abstract

Ants are key players in ecosystem function, especially in the tropics, yet little is known about the diversity and behavior of tropical ant communities. In a lowland wet forest of La Selva, Costa Rica ants are the primary predators of over a third of the sizes of bird-dispersed seeds. Dominated by the tribes Attini, Pheidolini and Solenopsidini, 35-38 species of ant preyed on seeds or seed baits. This is the most diverse granivorous ant community yet recorded, with the high diversity associated with higher population densities, smaller colony sizes and smaller body sizes than North American granivorous ant communities. The size of a frugivore dropping is isometric with the size of the bird producing it, and decreases with rain. Ant predation on these droppings was highly variable in time and space. Discovery and recruitment to droppings increased with dropping size as predicted by simple models. However, partial predation of large droppings produced the highest seed mortality at intermediate-size droppings. Seeds were found in 29% of meter-square samples of ant nests, suggesting seed rain was not highly localized. Small droppings were used by the greatest variety of species--this corresponded to observations of ant aggression at the largest droppings. Niche breadth increased with body size for both seed size and microclimate. Large ants foraged in a greater range of Vapor Pressure Deficits than small ants, as predicted by the law of surface area to volume. Large ants also took a greater variety of seed sizes than small ants, aided through intraspecific size matching in the large species (mostly attines). The tendency for small ant species to have niches nested within those of large ant species highlights the need to understand how body size and colony size influence interactions in ant communities.