Habitat and fishing control grazing potential on coral reefs

Robinson, James P.W., McDevitt-Irwin, Jamie M., Dajka, Jan-Claas, Hadj-Hammou, Jeneen, Howlett, Samantha, Graba-Landry, Alexia, Hoey, Andrew S., Nash, Kirsty L., Wilson, Shaun K., and Graham, Nicholas A.J. (2020) Habitat and fishing control grazing potential on coral reefs. Functional Ecology, 34 (1). pp. 240-251.

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Abstract

Herbivory is a key process on coral reefs, which, through grazing of algae, can help sustain coral-dominated states on frequently disturbed reefs and reverse macroalgal regime shifts on degraded ones. Our understanding of herbivory on reefs is largely founded on feeding observations at small spatial scales, yet the biomass and structure of herbivore populations is more closely linked to processes which can be highly variable across large areas, such as benthic habitat turnover and fishing pressure. Though our understanding of spatiotemporal variation in grazer biomass is well developed, equivalent macroscale approaches to understanding bottom-up and top-down controls on herbivory are lacking. Here, we integrate underwater survey data of fish abundances from four Indo-Pacific island regions with herbivore feeding observations to estimate grazing rates for two herbivore functions, cropping (which controls turf algae) and scraping (which promotes coral settlement by clearing benthic substrate), for 72 coral reefs. By including a range of reef states, from coral to algal dominance and heavily fished to remote wilderness areas, we evaluate the influences of benthic habitat and fishing on the grazing rates of fish assemblages. Cropping rates were primarily influenced by benthic condition, with cropping maximized on structurally complex reefs with high substratum availability and low macroalgal cover. Fishing was the primary driver of scraping function, with scraping rates depleted at most reefs relative to remote, unfished reefs, though scraping did increase with substratum availability and structural complexity. Ultimately, benthic and fishing conditions influenced herbivore functioning through their effect on grazer biomass, which was tightly correlated to grazing rates. For a given level of biomass, we show that grazing rates are higher on reefs dominated by small-bodied fishes, suggesting that grazing pressure is greatest when grazer size structure is truncated. Stressors which cause coral declines and clear substrate for turf algae will likely stimulate increases in cropping rates, in both fished and protected areas. In contrast, scraping functions are already impaired at reefs inhabited by people, particularly where structural complexity has collapsed, indicating that restoration of these key processes will require scraper biomass to be rebuilt towards wilderness levels.

Item ID: 61616
Item Type: Article (Research - C1)
ISSN: 1365-2435
Keywords: benthic, body size, bottom-up, ecosystem function, fishing, herbivory, top-down
Copyright Information: © 2019 The Authors. © 2019 British Ecological Society.
Additional Information:

This article is available Open Access via the publisher's website.

Funders: Australian Research Council (ARC), Royal Society (RS)
Projects and Grants: ARC Grant/Award Number: DE130100688 and DE130101705, RS Grant/Award Number: CHGR1‐ 170087 and UF140691
Date Deposited: 29 Jan 2020 07:39
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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