Positive species interactions are ubiquitous and crucial components of communities, but they are still not well incorporated into established ecological theories. The definitions of facilitation and mutualism overlap, and both are often context dependent. Many interactions that are facilitative under stressful conditions become competitive under more benign ones. This is known as the stress-gradient hypothesis, which is a specific case of context dependency. Stress can be further divided into resource and non-resource categories, but a better mechanistic understanding is necessary to improve the theory's predictions. We examined if two pitcher-dwelling crab spiders (Thomisidae), Thomisus nepenthiphilus and Misumenops nepenthicola, can facilitate nitrogen sequestration in their pitcher plant host, Nepenthes gracilis, by ambushing pitcher-visiting flies and dropping their carcasses into pitchers after consumption. This relationship is, by definition, both mutualistic and facilitative. Laboratory experiments found that both crab spiders increased prey-capture rates of N. gracilis. Nutrient analyses showed that both crab spiders also decreased per unit nitrogen yield of prey. Using experiment duration as a proxy of prey-resource availability, we constructed a mechanistic conceptual model of nutritional benefit. The nutritional benefit received by N. gracilis from T. nepenthiphilus decreases with increasing levels of the limiting resource in the environment (i.e., decreasing levels of resource stress). Our findings suggest that any nutritional mutualism that increases the quantity of resource capture (e.g. number of prey individuals) but decreases the quality of the captured resource (e.g. nitrogen content of individual prey) will necessarily conform to the resource-based predictions of the stress gradient hypothesis.

The fluids of pitcher plants are habitats to many specialized animals known as inquilines, which facilitate the conversion of prey protein into pitcher-absorbable nitrogen forms such as ammonium. (Diptera: Mycetophilidae) is a predatory dipteran inquiline that inhabits the pitchers of Larvae of construct sticky webs over the fluid surface of to ensnare emerging adult dipteran inquilines. However, the interaction between and its host has never been examined before, and it is not known if can contribute to nutrient sequestration in individuals were reared in artificial pitchers in the laboratory on a diet of emergent mosquitoes, and the ammonium concentration of the pitcher fluids was measured over time. Fluid ammonium concentration in tubes containing was significantly greater than those of the controls. Furthermore, fluid ammonium concentrations increased greatly after larvae metamorphosed, although the cause of this increase could not be identified. Our results show that a terrestrial, inquiline predator can contribute significantly to nutrient sequestration in the phytotelma it inhabits, and suggest that this interaction has a net mutualistic outcome for both species.

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