How the timing of predation affects composition and diversity of species in a marine sessile community?

Predation during the early life history stages, when organisms are susceptible to both biotic and abiotic factors, is likely to have pervasive effects on community development. However, few studies have examined the relative importance of predation during early life-history stages on long-term community composition. Using a sessile marine community, we conducted a manipulative experiment to measure the effect of timing and extent of predator exposure. Every month over a 5 month period we assessed taxa composition, dominance and taxa richness in each of the following five treatments: 1) never predated, 2) always predated, 3) early predated (during the first month of community development), 4) late predated and 5) uncaged panels. Predation was very important in determining taxa identity and dominance, reducing spatial monopolization by ascidians and increasing the area occupied by bryozoans, but overall predation did not result in a more diverse community. The localized extinction of ascidians in predation treatments was compensated by an increase in the diversity of bryozoans and barnacles. Interestingly, predation during the early stages of community development had only short-term effects on the taxa richness and composition. After 5 months, predator exclusion during the early life-history stages had no effect on community composition and only current levels of predation determined differences in composition. Our results show that predation can cause changes in community composition, directly and indirectly, without necessary changing the total number of co-occurring taxa. More important, we demonstrate to our study system that although predation during the early life-history stages can be strong and have important short-term effects, predation during the adult stages determines long-term community composition.

► We manipulated the timing of predation during sessile community development.
► Predation determines community identity and dominancy but not species diversity.
► Predation during early stages of the community development had no pervasive effects.
► Ascidians dominated protected areas, while bryozoans were abundant in predated areas.