Inquiline predator increases nutrient-cycling efficiency of Nepenthes rafflesiana pitchers

The modified-leaf pitchers of Nepenthes rafflesiana pitcher plants are aquatic, allochthonous ecosystems that are inhabited by specialist inquilines and sustained by the input of invertebrate prey. Detritivorous inquilines are known to increase the nutrient-cycling efficiency (NCE) of pitchers but it is unclear whether predatory inquilines that prey on these detritivores decrease the NCE of pitchers by reducing detritivore populations or increase the NCE of pitchers by processing nutrients that may otherwise be locked up in detritivore biomass. Nepenthosyrphus is a small and poorly studied genus of hoverflies and the larvae of one such species is a facultatively detritivorous predator that inhabits the pitchers of N. rafflesiana. We fitted a consumer-resource model to experimental data collected from this system. Simulations showed that systems containing the predator at equilibrium almost always had higher NCEs than those containing only prey (detritivore) species. We showed using a combination of simulated predator/prey exclusions that the processing of the resource through multiple pathways and trophic levels in this system is more efficient than that accomplished through fewer pathways and trophic levels. Our results thus support the vertical diversity hypothesis, which predicts that greater diversity across trophic levels results in greater ecosystem functioning.