title: Sponge communities on Caribbean coral reefs are structured by factors that are top-down, not bottom-up.
abstract: Caribbean coral reefs have been transformed in the past few decades with the demise of reef-building corals, and sponges are now the dominant habitat-forming organisms on most reefs. Competing hypotheses propose that sponge communities are controlled primarily by predatory fishes (top-down) or by the availability of picoplankton to suspension-feeding sponges (bottom-up). We tested these hypotheses on Conch Reef, off Key Largo, Florida, by placing sponges inside and outside predator-excluding cages at sites with less and more planktonic food availability (15 m vs. 30 m depth). There was no evidence of a bottom-up effect on the growth of any of 5 sponge species, and 2 of 5 species grew more when caged at the shallow site with lower food abundance. There was, however, a strong effect of predation by fishes on sponge species that lacked chemical defenses. Sponges with chemical defenses grew slower than undefended species, demonstrating a resource trade-off between growth and the production of secondary metabolites. Surveys of the benthic community on Conch Reef similarly did not support a bottom-up effect, with higher sponge cover at the shallower depth. We conclude that the structure of sponge communities on Caribbean coral reefs is primarily top-down, and predict that removal of sponge predators by overfishing will shift communities toward faster-growing, undefended species that better compete for space with threatened reef-building corals.
authors.affiliation: Department of Biology and Marine Biology, Center for Marine Science, University of North Carolina Wilmington, Wilmington, North Carolina, United States of America. pawlikj@uncw.edu
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Pawlik
authors.forename: Tse-Lynn
authors.initials: TL
authors.lastname: Loh
authors.forename: Steven E
authors.initials: SE
authors.lastname: McMurray
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Finelli
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D057894:Coral Reefs; D005431:Florida; D020387:Food Chain; D011157:Population Dynamics; D011161:Porifera / Q000502:physiology*; D013045:Species Specificity; D013696:Temperature; D014872:Water Movements