Predation and competition interact to determine space monopolization by non-indigenous species in a sessile community from the southwestern Atlantic Ocean.
Non-indigenous fouling species (NIS) often dominate coastal facilities, such as harbors and marinas. Along the subtropical Coast of Brazil, sessile communities from artificial habitats are mostly dominated by the NIS ascidian Didemnum perlucidum and the bryozoan Schizoporella errata, which show contrasting life-history traits. To understand the dynamics between these species and better predict the identity of the dominant NIS in the sessile community, we conducted an experiment where colonies of S. errata, D. perlucidum and the cryptogenic ascidian Botrylloides niger were submitted to the following three interaction scenarios: colonies growing without competition, under intraspecific competition and under interspecific competition. All competition treatments were crossed with the following two predation treatments: exposed or protected to predators. The experiment was repeated in two different seasons (Winter and Summer). When released from predation and competition, the three species grew at least 10 times faster in the Summer than in the Winter, and S. errata always grew slower than the ascidian species. Predation reduced D. perlucidum survival in the Winter but not during the Summer, when the fast colony growth seemed to buffer partial colony removal by predation. Colonial growth of B. niger was affected by competition only in treatments without predators, growing almost 50% more without competition than when competing, regardless of the competitors' identity. When exposed to predation, D. perlucidum was more limited by interspecific than intraspecific competition. S. errata growth was not affected by biotic interactions and was linked to intrinsic seasonal variation. Mineralized clonal organisms, such as bryozoans, were less predated than most of the ascidians species but they grew slower. Ascidians were more often predated but were also strong competitors that were able to overgrow several taxa. The contrasting growth strategies resulted in different successes of NIS and cryptogenic species at the studied locality. Ascidians were favored under low predation pressure while bryozoans were survivors when predation was intense.