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Abstract Consumers can play critical roles in ecosystem resilience by modifying community resistance and recovery rates. In coral reefs, grazers can increase reef resilience by controlling algae and maintaining open space for coral recruitment, but can also erode the reef framework critical for coral recovery. Here we examine the context‐dependent effects of herbivores on reef persistence in Caribbean Panamá. Using a series of lab and field experiments, we found that the erosional effects of the herbivorous reef urchin (Echinometra viridis) were 2 orders of magnitude greater on dead corals than live corals, and surveys across multiple similarly overfished reefs revealed a positive relationship between urchin densities and percent cover of bare dead coral with urchin densities exceeding 150 m−2in some reefs. However, we observed that a mat‐forming zoanthid (Zoanthus pulchellus), found exclusively on dead corals, had an inverse spatial relationship with urchins. Through a series of field experiments, we found that zoanthid overgrowth repelled urchins, increased dead coral persistence, and decreased erosion of dead corals making up the reef framework by more than 50% over a 22‐month period. Our findings reveal that zoanthids can provide associational refuge to dead corals by enhancing their persistence under high urchin grazing pressure. We suggest that secondary space‐holders, such as zoanthids, may play increasingly important functional roles in degraded reef systems by shielding coral skeletons from external bioeroders. Moreover, the Stress Gradient Hypothesis, which predicts that the importance of positive interactions such as associational refuges increases with consumer pressure, extends to dead foundation species such as coral skeletons crucial for ecosystem recovery.
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Biodiversity has a positive but saturating effect on imperiled coral reefs
Species loss threatens ecosystems worldwide, but the ecological processes and thresholds that underpin positive biodiversity effects among critically important foundation species, such as corals on tropical reefs, remain inadequately understood. In field experiments, we manipulated coral species richness and intraspecific density to test whether, and how, biodiversity affects coral productivity and survival. Corals performed better in mixed species assemblages. Improved performance was unexplained by competition theory alone, suggesting that positive effects exceeded agonistic interactions during our experiments. Peak coral performance occurred at intermediate species richness and declined thereafter. Positive effects of coral diversity suggest that species’ losses on degraded reefs make recovery more difficult and further decline more likely. Harnessing these positive interactions may improve ecosystem conservation and restoration in a changing ocean.
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- PAR ID:
- 10330308
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 7
- Issue:
- 42
- ISSN:
- 2375-2548
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.abi8592
