Manna from heaven: Refuse from an arboreal ant links aboveground and belowground processes in a low land tropical forest

Abstract

Aboveground consumers can shape below ground processes by serving as conduits for resources. Social insects dominate in terms of biomass in tropical forests, but compared to studies on large mammals, or aggregate solitary insects, we know relatively little about the role of social insects as nutrient conduits particularly in complex environments like tropical forests. Social insects like ants in the tropical forest canopy can connect aboveground and below ground food webs by producing a nutrient stream (excreta) from large, long-lived and stationary nests. The excreta, in turn, would create enduring spatial heterogeneity in the forest floor. Here we evaluate this scenario in a low land Neotropical forest using Azteca trigona, a dominant canopy ant that feeds on honeydew and insects and rains refuse out of its hanging nests onto the leaf litter below. We investigate decomposition rates and detrital communities associated with areas near nests versus 10 m away. Further, we directly test refuse's impact on decomposition and detrital communities in a common garden experiment. Relative to leaf litter, refuse is enriched 7-fold in P, 23-fold in K, and 3-fold in N, all elements shown to limit decomposition in this forest. Accordingly, both artificial substrates and natural leaf litter substrates decomposed over 1.5- and 1.2-fold faster respectively below A. trigona nests and areas under nests supported more invertebrate detritivores and predators compared to controls 10 m away. These decomposition results were replicated in a 6-wk common garden experiment, but the changes in detrital invertebrate composition were not. Canopy ants like A. trigona act as dependable nutritional conduits to patches of the forest floor, transferring significant quantities of aboveground exudates and necromass. The general capacity for such social insect colonies to generate ecosystem heterogeneity remains an open question. © 2013 Clay et al.