Habitat Isolation Influences Population Structure of Intertidal Crustaceans and Molluscs on Boulder‐Dominated Rocky Shores in SE Brazil

ABSTRACT

On sheltered rocky shores in SE Brazil, variation in habitat configuration (non‐isolated vs. isolated boulders) acts as a natural exclusion experiment on a key isopod consumer, leading to potential interspecific competitive release. The semiterrestrial grazing isopod Ligia exotica forages almost exclusively on non‐isolated boulders, resulting in decreased biofilm biomass. Based on these divergent patterns, we tested whether the population structure of grazing gastropods and barnacles differs between non‐isolated and isolated boulders. We hypothesised that (i) grazing gastropods exhibit greater size, biomass, and abundance on isolated boulders, due to higher biofilm availability or competitive release; and (ii) barnacle density differs between boulder types, potentially due to the contrasting influence of biofilm on larval settlement or the accidental removal of newly settled larvae. Confirming previous observations, foraging of the isopod L. exotica was intense on non‐isolated boulders (1.58 isopods · min ^{− 1} · m ^{− 2} ) and almost absent on isolated boulders (0.04 isopods · min ^{− 1} · m ^{− 2} ). The density of grazing gastropods and barnacles varied between boulder types, with site‐specific effects. As predicted, grazing gastropods were 19%–37% larger, and their total biomass was 88% greater on isolated boulders. These findings underscore the ecological importance of habitat configuration at small scales, suggesting that even subtle changes in connectivity can trigger bottom‐up and top‐down processes that propagate through food webs.