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

Abstract

Abstract. Aboveground consumers can shape belowground 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 belowground 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 lowland 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.

The microbiome of the ant‐built home: the microbial communities of a tropical arboreal ant and its nest

“…Nutrient‐rich refuse below A. trigona nests can accelerate decomposition and alter the composition of the invertebrate community in the soil (Clay et al. ). While previous studies of refuse dumps have emphasized an enrichment in nutrients and higher fine root density (Farji‐Brener and Werenkraut ), our results suggest that the microbial community structure of refuse can also contribute to accelerated decomposition rates and provide a favorable environment for root growth.…”

“…Refuse collection buckets were placed below each nest to collect refuse before it could be inoculated with soil microbial communities, as described in Clay et al. (). Due to the close proximity to the forest floor, collection buckets capture >90% of the refuse fall.…”

“…For refuse samples, we collected 0.5 g of refuse from collection buckets (Clay et al. ). Finally, we took 0.5 g soil samples from locations 0.5 m away from directly below the center of nests.…”

“…The Neotropical ant, Azteca trigona , forms high‐density populations in Panama's seasonal forests (1–5 nests every 40 m) with colonies inhabited by >200,000 ants (Adams , Clay et al. ). Azteca trigona societies build and maintain large papery carton nests (0.5–4 m in length) by chewing, regurgitating, and gluing together plant fibers (Fig.…”

“…The constant refuse input generates a long‐term interaction between canopy and forest floor microbial communities (Clay et al. ).…”