Indirect Effects of an Exploited Predator on Recruitment of Coral-Reef Fishes

Abstract

The more ecologists examine the role of trait-mediated indirect interactions (TMIIs), especially in regulating predator-prey interactions, the more we recognize their fundamental role in structuring food webs. However, most empirical evidence for TMIIs comes from studies that are either conducted in laboratory or mesocosm venues or are restricted to simple food webs involving lower trophic-level animals. Here, I quantified the direct and indirect effects of interactions between high-level vertebrate predators on their vertebrate prey using a field experiment. Specifically, I tested how varying densities of a large-bodied, top predator (Nassau grouper; Epinephelus striatus) affected persistence, growth, and behavior of two smaller-bodied, intermediate predators (coney and graysby groupers; Cephalopholis fulva and C. cruentata) on 20 isolated patch reefs in the Bahamas. Large-bodied groupers are capable of consuming their smaller-bodied counterparts, and previous observational studies have indicated that local abundances of these groupers are negatively correlated. I measured the effects of interactions among groupers on lower trophic-level prey by quantifying recruitment of coral-reef fishes to the reefs. The field experiment demonstrated a strong trophic cascade that was entirely mediated by modified behavior of the intermediate predators. These results indicate that indirect, nonlethal interactions in natural systems can have strong cascading effects even at high trophic levels and in high-diversity food webs. Incorporating the complexity of such indirect effects into fisheries management may improve the sustainability of fished populations and strengthen marine conservation efforts; however these results also indicate that the effects of fishing are complex and difficult to predict.

Trophic restructuring of coral reef fish communities in a large marine reserve

“…One interpretation of these patterns of species' responses to the reserve is that large predators such as sharks, barracuda, and large grouper inside the marine reserve likely reduced the biomass of smaller predatory fishes such as snapper and smaller grouper. Although we did not study trophic interactions directly, such interactions have been empirically demonstrated in the Bahamas (Eggleston et al 1998, Stallings 2008. The lowered biomass of midsize predators in turn appeared to reduce the amount of predation on their prey, resulting in an increase in relative biomass in some of the smaller planktivorous and carnivorous species.…”

“…For example, in addition to direct consumption, large predatory species can also have non-lethal, negative effects on the biomass of smaller fishes. In the Bahamas, large groupers may out-compete smaller groupers for food and/or territory, causing a reduction in growth rates of small groupers (Stallings 2008). Growth rates can also be reduced when prey species choose inferior habitats in a tradeoff with predator avoidance (Werner et al 1983).…”

“…Even though, on average, herbivores did not exhibit a strong response to reserve protection, our results nonetheless suggest that the community of fishes is structured, at least in part, by strong, topdown processes. Such effects may be detected even without strong responses from herbivores and primary producers because of direct (Carpenter et al 1985) and indirect (Stallings 2008) interactions among largebodied and small-bodied carnivores/omnivores.…”

“…Additionally, body size may be a large determinant of trophic level and ecological role because the outcome of nonlethal interactions (e.g. interspecific competition) may be based on a body size hierarchy (Werner & Gilliam 1984, Stallings 2008. Combined with a more traditional approach using trophic categories based on trophic scores (i.e.…”

“…In the Caribbean, heavily affected species include sharks (mostly of the family Charcharhinidae), jacks (Carangidae), and grouper (Serranidae), including the Nassau grouper Epinephelus striatus (Parrish 1987). As large piscivorous fish are removed, their prey populations can increase in abundance and size due to release from predation (Frank et al 2005) and/or competition (Stallings 2008). Fishing intensity often does not decline with decreasing yields (Gordon 1991), which only exacerbates this effect.…”