The big-headed ant (Pheidole megacephala) is a recent invader in East African arid savannas of the Laikipia Plateau. In many tropical and sub-tropical ecosystems, big-headed ants strongly reduce native insect diversity, but we know less about their effects on ecosystem functions (e.g., decomposition). Our study focused on the effects of P. megacephala invasion on the invertebrate detritivore community at Mpala Research Centre, and the decomposition function that those invertebrates support. We monitored insect diversity and abundance, as well as insect-mediated dung decomposition, at 4 sites experiencing ant invasion. Each site straddled the edge of the big-headed ant “invasion front” (spreading from a nearby human establishment), and thus contained matched plots that were invaded and non-invaded. Over 1 month, we repeatedly sampled native insect diversity and abundance in dung piles and in dung-baited pitfall traps. We identified insects as detritivores and non-detritivores, and then calculated the diversity and abundance of: 1) detritivores in pitfall traps, 2) detritivores in dung piles, and 3) the overall insect community sampled in pitfall traps. We also measured loss of dung mass at each site, to evaluate the effects of P. megacephala invasion on insect-mediated decomposition function.
Results/Conclusions
Overall diversity and abundance of insects in pitfall traps was significantly lower in invaded sites. Big-headed ants were 10x more abundant than native insects in invaded plots. Despite these strong changes in the arthropod community, detritivores (i.e., dung beetles and termites) in dung piles and pitfall traps did not change in abundance or diversity. Further, rates of dung decomposition did not change in invaded sites, though exclusion experiments showed that detritivores significantly increase dung mass loss rates. While P. megacephala often preys on termites, studies in other systems have identified termites that can effectively defend subterranean colonies against P. megacephala attacks. The high mobility of dung beetles, necessitated by foraging for widely dispersed resources, may allow them to avoid predation by P. megacephala. Resistance of detritivores to P. megacephala predation may explain why decomposition was unaffected by invasion. Our findings provide evidence that ecosystems may absorb large changes to community composition due to biological invasion, while retaining ecosystem functions that are provided by a specialized functional group of species.