Variation in a simple trait of mangrove roots governs predator access to, and assemblage composition of, epibiotic sponges

Many species create biogenic habitat that may vary in quality depending on its attributes. This variation may in turn affect species interactions among members of the attendant community. We describe a habitat-provisioning species that, with variation in a simple trait, produces dichotomous classes of habitat: one that serves as a predation refuge and the other that does not. Subtidal roots of the Caribbean red mangrove Rhizophora mangle are colonized by a diverse epibiont assemblage that includes many species of sponge. We experimentally demonstrated that roots touching the seafloor give benthic sea star predators access to their sponge prey living on the roots. After 6 wk, half of sponges on grounded roots were eaten, whereas sponges on suspended roots were uneaten. Correspondingly, in concomitant field surveys of mangrove root epibiont as semblages, we found very different sponge ensembles on the 2 root types. Even after standardizing for root area, suspended roots harbored 7 sponge species that covered an average of 91.3% of subtidal root length, while grounded roots, where sponges were exposed to sea star predation, had only 4 sponge species that covered 63.2% of root length. There was little overlap in species composition and a single sponge species Chondrilla caribensis, that was never eaten in our experiment, dominated grounded roots. This study suggests that a simple, dynamic trait of mangrove roots — groundedness — controls predator access, with consequences for assemblage composition.