Food web interactions along seagrass–coral reef boundaries: effects of piscivore reductions on cross-habitat energy exchange

Abstract

Although early studies qualitatively documented the importance of cross-habitat energy transfers from seagrasses to coral reefs, such exchanges have yet to be quantified. Empirical evidence suggests that grazing by reef-associated herbivores along the coral reef -seagrass interface can be intense (e.g. conspicuous presence of bare-sand 'halos' surrounding coral reefs). This evidence must be interpreted with caution, however, as most of it comes from areas that have experienced sustained, intense overfishing. To quantify the impacts of piscivore removal on cross-habitat energy exchange at the coral reef -seagrass interface, we compared grazing intensity along fished and notake reefs in the upper and lower Florida Keys. Using visual census techniques and direct measures of seagrass grazing, we documented the impacts of piscivore density on herbivory along the seagrass-coral reef interface. Grazing rates were greater than observed seagrass (Thalassia testudinum) production near reefs in the upper Keys, but less than 48% of production in the lower Keys. Analyses showed that these large differences were not related to regional differences in either herbivore density or species composition. Seagrass biomass was also lower near reefs in the upper Keys, where estimates of grazing were highest. Piscivores were dominated by transient predators (jacks and barracudas) whose densities varied with region and distance from reef, but not with protection from fishing. A nonsignificant negative correlation between great barracuda density and leaf losses from tethered shoots, coupled with the greater abundances of larger grazers near reefs, suggests that predation risk, rather than direct reductions in density, may limit grazers to intense feeding on seagrasses adjacent to reefs in the upper Keys. The large-scale variation in grazing intensity illustrates the need for more detailed quantifications of energy exchanges along the seagrass-coral reef boundary.

Proximity to reef influences density of small predatory fishes, while type of seagrass influences intensity of their predation on crabs

“…In the Florida Keys (USA), the density of fish was significantly higher immediately adjacent to the reef than 30 m away (Valentine et al 2007). Several small predatory fishes, as well as some larger piscivorous fishes, followed this same trend (Valentine et al 2007).…”

“…Experimental studies have often shown that the intensity of predation and herbivory matches these trends in consumer density, with intensity declining with increasing distance from reefs (e.g. Shulman 1985, Valentine et al 2007). Prey densities often show the opposite trend, increasing with distance from reefs, a pattern consistent with a 'top-down' influence by predators (e.g.…”

“…Dorenbosch et al 2005, Wernberg et al 2006, Valentine et al 2007). Many reef-associated consumers show distinct patterns of movement to seek prey in adjacent habitats (e.g.…”

Piscivore assemblages and predation pressure affect relative safety of some back-reef habitats for juvenile fish in a Caribbean bay

“…Additionally, a video observation showed that the reef-associated Aulostomus maculatus consumed a tethered fish on the seagrass bed in the entrance of the bay (Table 2). This indicates that visiting piscivores enter the entrance area and join the local piscivore assemblage (Dorenbosch et al 2007, Valentine et al 2007). Larger piscivores are assumed to be more efficient predators on small juvenile fish than smaller predators (Scharf et al 2000, Kulbicki et al 2005.…”

“…Back-reef habitats may border coral reefs, which facilitates entry of reef-associated piscivores to these non-reef habitats for feeding (Baker & Sheaves 2007, Dorenbosch et al 2007, Valentine et al 2007). Aside from this, back-reef habitats located farther away from the coral reef may harbour resident populations of piscivores (Baker & Sheaves 2005), including larger juvenile piscivorous fish (Eggleston et al 2004).…”