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Marine Ecology Progress Series

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MEPS 627:71-81 (2019)  -  DOI:

Grazing halos on coral reefs: predation risk, herbivore density, and habitat size influence grazing patterns that are visible from space

Bartholomew P. DiFiore1,2,3,*, Simon A. Queenborough2, Elizabeth M. P. Madin4,5, Valerie J. Paul3, Mary Beth Decker6, Adrian C. Stier1

1Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106, USA
2Yale School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511, USA
3Smithsonian Marine Station at Fort Pierce, Fort Pierce, Florida 34949, USA
4Hawai’i Institute of Marine Biology, University of Hawai’i, Kane’ohe, Hawai’i 96744, USA
5Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia
6Department of Ecology and Evolution, Yale University, New Haven, Connecticut 06511, USA
*Corresponding author:

ABSTRACT: The behavior of organisms at local scales can have dramatic effects on the distribution of habitat and ecosystem processes at large spatial scales. Grazing halos—barren areas of sand surrounding coral reefs—are readily visible in remotely sensed imagery, and are formed by herbivores preferentially grazing close to the safety of the reef. Grazing halo size can vary by more than an order of magnitude, yet we lack an understanding of what underlies this variation. Changes in grazing intensity and the distance herbivores travel may be influenced by predation risk, resource availability, and structural attributes of the reef. Here, we tested how predator density, herbivore density, and patch reef area are correlated with halo size and grazing patterns by linking satellite imagery with mensurative experiments on patch reefs in the Caribbean. Chronic predation risk suppressed the overall foraging intensity of herbivorous fishes; a doubling in predator density led to a 29% decrease in grazing. Variation in herbivore density, which was tightly correlated with patch reef area, was linked to the distance herbivores foraged from the reef. Furthermore, herbivore density and/or reef area was the best predictor of halo width at field sites, and reef area explained a significant proportion of the variation in halo size across the landscape. Our results contribute to a growing body of research suggesting that halos may function as footprints of ecological processes, such as herbivory, that can be quantified using remote sensing, and offer a potential avenue to improve monitoring of coral reef management.

KEY WORDS: Predator-prey interaction · Foraging ecology · Herbivory · Predation risk · Remote sensing · Seagrass · Landscape ecology

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Cite this article as: DiFiore BP, Queenborough SA, Madin EMP, Paul VJ, Decker MB, Stier AC (2019) Grazing halos on coral reefs: predation risk, herbivore density, and habitat size influence grazing patterns that are visible from space. Mar Ecol Prog Ser 627:71-81.

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