Novel pitcher plant-spider mutualism is dependent upon environmental resource abundance

Host Plant Specificity in Web-Building Spiders

Spiders are ubiquitous generalist predators playing an important role in regulating insect populations in many ecosystems. Traditionally they have not been thought to have strong influences on, or interactions with plants. However, this is slowly changing as several species of cursorial spiders have been reported engaging in either herbivory or inhabiting only one, or a handful of related plant species. In this review paper, we focus on web-building spiders on which very little information is available. We only find well-documented evidence from studies of host plant specificity in orb spiders in the genus Eustala, which are associated with specific species of swollen thorn acacias. We review what little is known of this group in the context of spider-plant interactions generally, and focus on how these interactions are established and maintained while providing suggestions on how spiders may locate and identify specific species of plants. Finally, we suggest ideas for future fruitful research aimed at understanding how web-building spiders find and utilise specific plant hosts.

Plant-animal interactions between carnivorous plants, sheet-web spiders, and ground-running spiders as guild predators in a wet meadow community

Plant-animal interactions are diverse and widespread shaping ecology, evolution, and biodiversity of most ecological communities. Carnivorous plants are unusual in that they can be simultaneously engaged with animals in multiple mutualistic and antagonistic interactions including reversed plant-animal interactions where they are the predator. Competition with animals is a potential antagonistic plant-animal interaction unique to carnivorous plants when they and animal predators consume the same prey.The goal of this field study was to test the hypothesis that under natural conditions, sundews and spiders are predators consuming the same prey thus creating an environment where interkingdom competition can occur.Over 12 months, we collected data on 15 dates in the only protected Highland Rim Wet Meadow Ecosystem in Kentucky where sundews, sheet-web spiders, and ground-running spiders co-exist. One each sampling day, we attempted to locate fifteen sites with: (a) both sheet-web spiders and sundews; (b) sundews only; and (c) where neither occurred. Sticky traps were set at each of these sites to determine prey (springtails) activity-density. Ground-running spiders were collected on sampling days. DNA extraction was performed on all spiders to determine which individuals had eaten springtails and comparing this to the density of sundews where the spiders were captured.Sundews and spiders consumed springtails. Springtail activity-densities were lower, the higher the density of sundews. Both sheet-web and ground-running spiders were found less often where sundew densities were high. Sheet-web size was smaller where sundew densities were high.The results of this study suggest that asymmetrical exploitative competition occurs between sundews and spiders. Sundews appear to have a greater negative impact on spiders, where spiders probably have little impact on sundews. In this example of interkingdom competition where the asymmetry should be most extreme, amensalism where one competitor experiences no cost of interaction may be occurring.

Digestive mutualism in a pitcher plant supports the monotonic rather than hump-shaped stress-gradient hypothesis model

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.

The crab spider-pitcher plant relationship is a nutritional mutualism that is dependent on prey-resource quality

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.