Abstract
Marine reserves can effectively restore harvested populations, and ‘mega-reserves’ increasingly protect large tracts of ocean. However, no method exists of monitoring ecological responses at this large scale. Herbivory is a key mechanism structuring ecosystems, and this consumer–resource interaction's strength on coral reefs can indicate ecosystem health. We screened 1372, and measured features of 214, reefs throughout Australia's Great Barrier Reef using high-resolution satellite imagery, combined with remote underwater videography and assays on a subset, to quantify the prevalence, size and potential causes of ‘grazing halos’. Halos are known to be seascape-scale footprints of herbivory and other ecological interactions. Here we show that these halo-like footprints are more prevalent in reserves, particularly older ones (approx. 40 years old), resulting in predictable changes to reef habitat at scales visible from space. While the direct mechanisms for this pattern are relatively clear, the indirect mechanisms remain untested. By combining remote sensing and behavioural ecology, our findings demonstrate that reserves can shape large-scale habitat structure by altering herbivores' functional importance, suggesting that reserves may have greater value in restoring ecosystems than previously appreciated. Additionally, our results show that we can now detect macro-patterns in reef species interactions using freely available satellite imagery. Low-cost, ecosystem-level observation tools will be critical as reserves increase in number and scope; further investigation into whether halos may help seems warranted. Significance statement: Marine reserves are a widely used tool to mitigate fishing impacts on marine ecosystems. Predicting reserves' large-scale effects on habitat structure and ecosystem functioning is a major challenge, however, because these effects unfold over longer and larger scales than most ecological studies. We use a unique approach merging remote sensing and behavioural ecology to detect ecosystem change within reserves in Australia's vast Great Barrier Reef. We find evidence of changes in reefs' algal habitat structure occurring over large spatial (thousands of kilometres) and temporal (40+ years) scales, demonstrating that reserves can alter herbivory and habitat structure in predictable ways. This approach demonstrates that we can now detect aspects of reefs' ecological responses to protection even in remote and inaccessible reefs globally.
Highlights
As a globally pervasive conservation and ecosystem-based management tool, no-take marine reserves are increasing in both number and area, with 19 record-breaking ‘mega-reserves’ established since 2009 [1]
The well-known phenomenon of ‘grazing halos’ [4,5], which occur on coral reefs worldwide in a variety of primary producer habitats, are landscape-scale vegetation patterns consisting of a halo of heavily grazed, vegetation-free substrate surrounding spatially isolated coral patch reefs
Contrary to our initial hypothesis, halo size across the Great Barrier Reef (GBR) was not significantly related to marine reserve status nor reserve age, but rather was best explained by two environmental factors: patch reef area and sea surface temperature (SST)
Summary
As a globally pervasive conservation and ecosystem-based management tool, no-take marine reserves (hereafter ‘reserves’) are increasing in both number and area, with 19 record-breaking ‘mega-reserves’ (more than 100 000 km2) established since 2009 [1]. In these reserves, as well as in smaller and less remote reserves, no method currently exists for remotely observing and monitoring effects that reserves have on species interactions over scales of entire seascapes, despite the need for such methods due to the increasing number and scope or reserves worldwide [2]. Halos can affect a key ecosystem service: carbon sequestration [6]
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More From: Proceedings of the Royal Society B: Biological Sciences
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