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The stress-gradient hypothesis (SGH) predicts that the strength and frequency of facilitative interactions increase monotonically with increasing environmental stress, but some empirical studies have found this decrease at extreme stress levels, suggesting a hump-shaped SGH instead. However, empirical studies of the SGH are often hindered by confounding resource and non-resource stress gradients. Nepenthes pitcher plants trap animal prey using modified-leaf pitfall traps which are also inhabited by organisms known as inquilines. Inquilines may assist pitchers in the digestion of trapped prey. This interaction is known as a digestive mutualism and is both mutualistic and facilitative by definition. Inquiline species may also facilitate each other via processing chain commensalisms. We used in vitro experiments to examine the isolated effect of resource stress on the outcomes of two facilitative interactions: (i) digestive mutualism—facilitation of pitcher nutrient sequestration by two inquiline dipteran larvae, culicids and phorids; (ii) processing chain commensalism—facilitation between these two inquiline taxa. The net nutritional benefit of phorids on N. gracilis was found to conform more to a monotonic rather than hump-shaped SGH model. However, the effect of culicids on N. gracilis and the effects of culicids and phorids on each other were weak. These findings provide compelling evidence that changes in facilitation along an isolated resource stress gradient conform to the predictions of the monotonic SGH model rather than that of the revised hump-shaped model, and highlight the importance of isolating stress gradients in empirical tests of the SGH. [ABSTRACT FROM AUTHOR]

Nutritional mutualisms are one of the three major categories of mutualisms and involve the provision of limiting nutrients (resources) to one species by another. It was recently shown in laboratory experiments that two species of pitcher‐dwelling crab spiders (Thomisidae), Thomisus nepenthiphilus and Misumenops nepenthicola, increased capture rates of flesh flies (Sarcophagidae) for their host, Nepenthes gracilis. The spiders ambushed pitcher‐visiting flesh flies and dropped their carcasses into pitchers after consuming them. The consumption of shared prey‐resources by crab spiders and pitcher plants presents the possibility of parasitism between them. However, ecologically generalizable mechanisms that predict the context‐dependent outcomes of such mutualisms are not known.The effectiveness framework (mutualism effectiveness = quality × quantity) is useful for examining the total effect of mutualisms, but its quality component can be difficult to define. We identify the crab spider–pitcher plant interaction as a type of resource conversion mutualism and propose that the quality component in such interactions is the amount of the underlying resource contained in each unit of resource processed. We then used the crab spider–pitcher plant interaction to test the hypothesis that resource conversion mutualisms are more beneficial to the nutrient recipient when operating through high‐quality resources (i.e., large prey, in this interaction).We sampled the prey and inquilines of 107 N. gracilis upper pitches in situ and analysed the differences between pitchers that were inhabited or uninhabited by crab spiders, and the differences between nutritional contents of prey that were consumed by crab spiders or not.Pitchers inhabited by T. nepenthiphilus contained higher numbers of several prey taxa, many of which were flying insects. Consumption by T. nepenthiphilus reduced the nutrient contents in all prey examined. Overall, T. nepenthiphilus‐assisted prey capture is likely to result in a net nutrient gain for N. gracilis that is proportional to the size of prey consumed by T. nepenthiphilus.Our results suggest that resource conversion mutualisms are more likely to operate through high‐quality resources, since the nutrient‐processing species necessarily reduces the quality of the resource it processes while increasing its availability to the nutrient recipient species. The study confirms the existence of mutualism between a pitcher plant and a crab spider species living in it. It also reveals a generalizable trend between resource quality and total mutualistic effect in resource conversion mutualisms, a subcategory of nutritional mutualisms which is described in this study. [ABSTRACT FROM AUTHOR]