Calcareous defence structures of prey mediate the effects of predation and biotic resistance towards the tropics

Aims: The importance of biotic interactions in creating and maintaining diversity is expected to increase towards low latitudes. However, the way in which predation affects diversity can depend on how predators mediate competitive interactions and also on defensive traits of prey. Here, we assessed the role of physical defences of prey to escape predation and how the importance of predation on community structure and diversity changes across latitude. Location: Six sites, in three regions distributed across 45 degrees of latitude in the Atlantic Ocean: a tropical region in Angola, a subtropical region in Brazil and a temperate region in Wales, UK. Methods: We manipulated predation on marine sessile communities, using exclusion cages and assessed community parameters, including their susceptibility to biological invasion during early and advanced succession. Results: Predation was more intense in the tropics and in advanced communities suggesting that predation effects increase through time. In the tropical region, predators reduced the number of co-occurring species and beta diversity, limited the occurrence of exotic species and promoted a change in the identity of the dominant organisms, replacing soft-bodied organisms with calcified animals. In the subtropical region, predation promoted a similar trait-mediated change in the identity of dominant prey, although it was not strong enough to affect diversity and did not prevent bioinvasion. In the temperate region, other processes than predation seem to drive the community organization and resistance to invasion. Main conclusions: Our results support both Biotic Interaction and Biotic Resistance Hypotheses, showing that the importance of predation to biodiversity increases towards the tropics. In addition, where predation is intense, morphological traits of prey drive the final structure and dominance in the community. Our results suggest that physical defences are the main traits preventing predation, perhaps explaining why calcified organisms are among the most common invasive species in coastal habitats.