Carnivorous pitcher plant facilitates its ant prey

Positive species interactions are ubiquitous in natural communities. However, positive species interactions are often overlooked or undetected because they tend to be more context dependent—that is, outcomes of interactions may change from positive to neutral/negative, depending on the ecological contexts in which these take place. It was recently shown that the pitchers of the carnivorous plant, Nepenthes gracilis, trapped more ant prey when found in close proximity to a congener, N. rafflesiana. As pitchers usually succeed in trapping only a fraction of all pitcher visitors, it was proposed that N. rafflesiana feeds ant colonies in its immediate vicinity with its copious nectar secretions. We tested this hypothesis using ex situ experiments. Philidris sp., Crematogaster treubi, and C. ferrarii colonies were reared in the presence or absence of N. rafflesiana plants in enclosed terraria. As such an interaction has the potential of being context dependent, the interaction was tested under high and low feeding conditions. Low feeding conditions resulted in strong declines in ant colony health, but this was mitigated by the presence of N. rafflesiana, although N. rafflesiana had no significant effect on ant colony health under high feeding conditions. This was observed even though pitchers of N. rafflesiana trapped large numbers of ant workers. Our results show that prey facilitation by a predator is possible, at least in eusocial insects like ants, and that prey facilitation conforms to the central prediction of the stress gradient hypothesis in being more positive at higher stress (lower resource) levels.