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.
Highlights
Food chain dynamics is considered a central theory in ecology [1], and proposes that populations of organisms that make up communities are controlled by processes that are bottom-up or top-down
Greater growth was observed for Callyspongia armigera at 15 m vs. 30 m (F = 4.97, df = 1, p = 0.0294), and growth was similar between depths for the other 3 branching sponge species
The absence of a bottom-up effect on sponge growth at Conch Reef was surprising, given that most ecosystems that have been studied show evidence of both top-down and bottom-up processes in structuring communities [5], [7], and because the two previous studies of sponges at the same research location concluded that bottom-up effects were dominant [19], [20]
Summary
Food chain dynamics is considered a central theory in ecology [1], and proposes that populations of organisms that make up communities are controlled by processes that are bottom-up (nutrients, food) or top-down (predation). Macroalgae cover the greatest surface area on many reefs [14], and while arborescent gorgonian corals are often visually dominant [15], sponges have become a primary component of Caribbean coral reef ecosystems [16]. In addition to their large biomass on shallowwater reefs, sponges dominate light-limited reef interstices, caves and mesophotic reefs [17], but are found in grassbed, hardbottom, and mangrove habitats [18]. Recent evidence indicates that sponge populations on Caribbean reefs are increasing [16]
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