Indirect effects of overfishing on Caribbean reefs: sponges overgrow reef-building corals

Tse-Lynn Loh; Steven E McMurray; Timothy P Henkel; Jan Vicente; Joseph R Pawlik

doi:10.7717/peerj.901

Indirect effects of overfishing on Caribbean reefs: sponges overgrow reef-building corals

PeerJ. 2015 Apr 28:3:e901. doi: 10.7717/peerj.901. eCollection 2015.

Authors

Tse-Lynn Loh¹, Steven E McMurray¹, Timothy P Henkel², Jan Vicente³, Joseph R Pawlik¹

Affiliations

¹ Department of Biology and Marine Biology and Center for Marine Science, University of North Carolina Wilmington , Wilmington, NC , USA.
² Department of Biology, Valdosta State University , Valdosta, GA , USA.
³ Institute of Marine and Environmental Technology, University of Maryland Center for Environmental Science , Baltimore, MD , USA.

Abstract

Consumer-mediated indirect effects at the community level are difficult to demonstrate empirically. Here, we show an explicit indirect effect of overfishing on competition between sponges and reef-building corals from surveys of 69 sites across the Caribbean. Leveraging the large-scale, long-term removal of sponge predators, we selected overfished sites where intensive methods, primarily fish-trapping, have been employed for decades or more, and compared them to sites in remote or marine protected areas (MPAs) with variable levels of enforcement. Sponge-eating fishes (angelfishes and parrotfishes) were counted at each site, and the benthos surveyed, with coral colonies scored for interaction with sponges. Overfished sites had >3 fold more overgrowth of corals by sponges, and mean coral contact with sponges was 25.6%, compared with 12.0% at less-fished sites. Greater contact with corals by sponges at overfished sites was mostly by sponge species palatable to sponge predators. Palatable species have faster rates of growth or reproduction than defended sponge species, which instead make metabolically expensive chemical defenses. These results validate the top-down conceptual model of sponge community ecology for Caribbean reefs, as well as provide an unambiguous justification for MPAs to protect threatened reef-building corals. An unanticipated outcome of the benthic survey component of this study was that overfished sites had lower mean macroalgal cover (23.1% vs. 38.1% for less-fished sites), a result that is contrary to prevailing assumptions about seaweed control by herbivorous fishes. Because we did not quantify herbivores for this study, we interpret this result with caution, but suggest that additional large-scale studies comparing intensively overfished and MPA sites are warranted to examine the relative impacts of herbivorous fishes and urchins on Caribbean reefs.

Keywords: Chemical defense; Coral reefs; Food webs; Indirect effects; MPAs; Marine protected areas; Resource trade-offs; Spatial competition; Top-down control; Trophic cascades.

Grants and funding

Funding for this study was provided by the AMNH Lerner Gray Fund for Marine Research, UNCW Brauer Fellowship and Graduate Student Association Travel Award, NOAA-NURC (NA96RU-0260), NOAA’s Coral Reef Conservation Program and the National Science Foundation (OCE-0550468, 1029515). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.