Inferring ant (Hymenoptera: Formicidae) dietary responses to experimental drought in a tropical rainforest using stable isotopes

Abstract

With higher frequency and intensity of droughts predicted for many tropical forests, understanding the responses of plants and animals to changed climatic conditions will be increasingly important. Ants are considered ecosystem engineers in tropical rainforests due to their abundance and the diversity of functional roles they perform. Function in animals is often dictated by diet, with diets of rainforest ants ranging from purely carnivorous to mostly plant based. Stable isotopes of nitrogen and carbon have been utilised to infer information on diets, and more broadly on changes in ecosystem dynamics and water-use in plants. We used stable isotopes to investigate the potential effects of drought on the diets of two dominant ant species (Anonychomyrma gilberti and Oecophylla smaragdina) in an in-situ through-fall experiment in lowland rainforest. We observed a significant and sustained increase of δ¹⁵N in both ant species in the drought experiment and during the dry season in the control treatment. We also detected elevated although much smaller levels of δ¹⁵N values in leaves of some drought-stressed plant species. There was no detectable difference of the experiment for ant or plant δ¹³C values. We conclude that the observed elevation in ants' δ¹⁵N in response to drought may be partly caused by a drought-induced shift in δ¹⁵N in plants. Although, with average nitrogen enrichment values of 2.7‰ and 3.2‰, for A. gilberti and O. smaragdina, respectively, it is consistent with these ants deriving nitrogen largely from prey. Because ants are such dominant species in rainforests, even small trophic shifts in response to climate change if widespread could have broad implications for species interactions and food webs. Importantly, it is possible that such a change could potentially mediate the effects of drought on tropical forests if herbivory on drought-stressed plants is diminished by ant predation.