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Marine Ecology Progress Series

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The urchin Holopneustes purpurascens and the gastropod Phasianotrochus eximius enmeshed within the common kelp Ecklonia radiate. Image: J E Bell

Bell JE, Bishop MJ, Taylor RB, Williamson JE


Facilitation cascade maintains a kelp community


Facilitation cascades - nested positive indirect interactions involving three or more species - are recognised as common mechanisms that maintain the structure of communities. Here, the positive relationship between the kelp Ecklonia radiata and the gastropod Phasianotrochus eximius is mediated by a third species, the sea urchin Holopneustes purpurascens via a facilitation cascade. Both the urchin and gastropod are found in greater abundance when wrapped within the lamina of E. radiate, than on other algae. The gastropod was more abundant on kelp with urchins than without, and recolonised more frequently to plants with urchins. Our study indicates that the association between the kelp, gastropod and urchin is maintained via a unidirectional facilitation cascade. Further studies are now needed to elucidate the mechanism by which H. purpurascens facilitated P. eximius.


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