Trade-offs in resistance to competitors and predators, and their effects on the diversity of tropical marine sponges

1. Consistently very high regional diversity of tropical marine sponges reflects a combination of high within-habitat diversity and distinctness of species composition among habitats. Distinctive sponge faunas of Caribbean coral reefs and mangroves seem to support the assumption that abiotic factors determine sponge diversity within habitats and faunal differences between habitats because these habitats differ greatly in abiotic characteristics and because lower species diversity on mangroves appears to reflect their inferiority as sponge habitat. 2. A way to test this assumption is provided by unusual mangrove cays in Belize that are inhabited by the typical Caribbean reef sponge fauna. Reciprocal transplant experiments, combined with caging (predator-free space) and artificial substrata (competitor-free space), demonstrated control of community membership by biological interactions for 12 common species: spongivorous predators excluded typical mangrove sponges from reef sponge assemblages, and reef sponges were excluded from mangrove sponge assemblages by competition. 3. Variation in growth rate was related inversely to variation in defences against predators in the species studied, suggesting a trade-off between resistance to competitors and to predators. 4. This trade-off influences community structure, as the key importance of competition for space among mangrove species results in lower within-habitat diversity, while multiple challenges, including predation, may maintain high diversity of reef sponges. Differences in species composition between habitats are maintained, as this trade-off precludes success of individual species as members of both faunas. 5. Most surprising is that typical faunas of mangroves and reefs are not tied to these habitats by abiotic factors. Greater sponge species diversity on the reef does not necessarily indicate superior conditions for sponges. Instead reefs may be a refuge for species that grow too slowly to coexist with typical mangrove species.