Facilitation cascade maintains a kelp community

Abstract

There is increasing interest in how facilitation cascades — nested positive indirect inter- actions involving at least 3 species — maintain com- munity structure. Here we investigated whether 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 cas- cade. Both the urchin and the gastropod are found enmeshed within the lamina of the common kelp E. radiata, which the urchin is known to consume. Sampling of urchin and gastropod abundances at 4 sites in Sydney, Australia, over 2 yr revealed that both H. purpurascens and P. eximius were more abundant on kelp than on other algal substrates and that the gastropod was more abundant on kelp with than without the urchin. Large P. eximius were found only on plants inhabited by H. pur- purascens, suggesting that either P. eximius is able to survive longer when it is part of this association or that adults actively move onto this substrate. When the presence/absence of H. purpurascens on kelp plants was manipulated experimentally in the field, greater recolonisation by P. eximius of kelp with than without urchins was observed. Urchins, by contrast, did not differentially respond to kelp with and without the gastropod, indicating that the association between the kelp and the urchin is maintained by a unidirectional facilitation cascade. Our study adds to growing evidence that facilita- tion cascades are a common mechanism by which community structure is maintained. Studies are now needed to assess their sensitivity to environmental change.