Light, shade and predation: who wins and who loses in sessile fouling communities?

Sessile communities are regulated by biotic and abiotic forces that influence species composition and relative abundance. The top-down control exerted through predation has direct impacts on prey and indirect implications for other biota through altered competitive interactions. Concurrently, light is recognised as an important driver, especially in early successional stages, due to its influence on larval settlement. This study investigated the combined influence of light and predation by nektonic predators on sessile communities. At two locations, settlement panels were allocated to three treatments: open panels (exposed to light and accessible to predators); semi-caged panels (shaded but accessible to predators); and caged panels (shaded and inaccessible to predators). Regardless of location, community composition differed among all treatments, suggesting that communities are regulated by an interplay between light and predation. This pattern was driven by ascidians of different life forms, with colonial species dominating open panels while solitary taxa were most abundant on caged panels. Notably, there was no difference in abundance between solitary and colonial ascidians on semi-caged panels. These results suggest that together, light and predation regulate the abundance of colonial and solitary ascidians. These findings highlight the importance of considering the conjoint effect of multiple ecological factors in regulating communities.