Abstract
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.
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Acknowledgements
We thank Sherry Hung for advice with statistical analysis, as well as Antonia Monteiro and Liew Chye Fong for the provision of various items of laboratory equipment that were essential to the conduct of the study.
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FWSL, WNL, and HTWT composed the study design. FWSL and WNL collected specimens. FWSL conducted the experiments and collected the data. WNL performed the analysis. FWSL and WNL wrote the manuscript together, and HTWT contributed to revisions.
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Data underlying this study are available at https://figshare.com/s/47eb9840e8d40b424c18.
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Communicated by Ian Kaplan.
This work uses a novel system, the pitcher plant phytotelma, to answer a question that has not been satisfactorily dealt with using more conventional systems.
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Leong, F.W.S., Lam, W.N. & Tan, H.T.W. Digestive mutualism in a pitcher plant supports the monotonic rather than hump-shaped stress-gradient hypothesis model. Oecologia 190, 523–534 (2019). https://doi.org/10.1007/s00442-019-04404-1
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DOI: https://doi.org/10.1007/s00442-019-04404-1