Positive effects of damselfish override negative effects of urchins to prevent an algal habitat switch

1. Understanding the factors that influence habitat persistence is a central theme in ecology, particularly for habitats created by terrestrial and aquatic primary producers that are some of the world's most extensive and ecologically important. 2. Many species have positive (e. g. farming) or negative effects (e. g. herbivory) on the abundance of primary producers, potentially causing wholesale switches in habitat structure if the net outcome of effects moves toward one extreme (e. g. over-grazing). Predicting the conditions under which such switches occur remains a key challenge for ecologists. 3. The purpose of this study was to understand how co-habiting species of opposing effect (damselfish as habitat facilitators vs. sea urchins as habitat consumers) can directly and indirectly influence the persistence of algal habitats on a tropical coast, including their potential to initiate switches among habitat types (productive 'turfs' of filamentous algae vs. 'barrens' of encrusting algae). 4. Using a series of five independent experiments, we observed that damselfish facilitated the production of algal turfs both directly, through active farming of selected species, and indirectly, by vigorously attacking and expelling invading urchins from the local area (i.e. preventing herbivory). In contrast, urchins consumed algal turf to directly maintain barrens. 5. The negative effects of urchins on algal turf were strong enough to initiate a habitat switch from turf to barrens, but this was conditional upon the absence of damselfish and the presence of a particular species of urchin. 6. Synthesis. These results build upon our understanding of the dynamics of habitat persistence by demonstrating the conditions where biological interactions of opposing direction (positive vs. negative) maintain or switch habitat types. Such knowledge is central to addressing global concerns about habitat loss and predicting the occurrence of switches to less-productive states.