The biotic and abiotic drivers of ‘living’ diversity in the deadly traps of Nepenthes pitcher plants

Nepenthes pitcher plants are carnivorous plants that paradoxically harbor a living infauna (the inquilines) in their pitchers, which withstands the hostile conditions of the digestive fluid and plays a role in prey digestion. Because most Nepenthes species are threatened by human activity, we aimed to assess how their inquiline communities are likewise endangered. This involved testing whether arthropod infaunal composition is Nepenthes-specific or even species-specific, as well as determining the ecological drivers of its diversity. In a field experiment in Brunei (Borneo), prey items were introduced into the fluid of newly open pitchers in four sympatric Nepenthes species, and into water control reservoirs. Abundance, species richness and Shannon diversity of metazoans in all reservoirs were analyzed 1 month later. Reservoir dimensions and fluid pH were measured, and the natural prey and vegetal detritus were identified and quantified. The inquiline diversities of the Nepenthes pitchers were much greater than those of the water controls. Dissimilarity indices showed that the inquiline composition was specific to each Nepenthes species. The fate of the inquiline community is thus intrinsically linked to that of its host plant, underlining its threatened status. Inquiline abundance was determined by pitcher aperture diameter, pitcher volume, fluid pH and the prey number. Inquiline species richness increased solely with abiotic factors, such as fluid pH and pitcher aperture diameter, and thereby with habitat area, reflecting the well-known species–area relationship, but it did not vary with species richness of prey. Nepenthes pitcher plants thus control, to some extent, the establishment of their inquilines via fluid physico-chemistry and pitcher design. From a conservation perspective, priority protection should be given to Nepenthes species with pitchers of large aperture, keystone for a broader biodiversity.