Limited Functional Redundancy in a High Diversity System: Single Species Dominates Key Ecological Process on Coral Reefs

Ecosystems (2009) 12: 1316–1328 DOI: 10.1007/s10021-009-9291-z Ó 2009 Springer Science+Business Media, LLC Limited Functional Redundancy in a High Diversity System: Single Species Dominates Key Ecological Process on Coral Reefs Andrew S. Hoey* and David R. Bellwood Australian Research Council Centre of Excellence for Coral Reef Studies and School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia ABSTRACT Herbivory is a key process structuring plant com- browsing species, the video footage revealed that a munities in both terrestrial and aquatic ecosystems, single species, Naso unicornis, was almost solely with variation in herbivory often being related to responsible for the removal of Sargassum biomass shifts between alternate states. On coral reefs, re- across all habitats. Of the 42,246 bites taken from gional reductions in herbivores have underpinned the Sargassum across all habitats, N. unicornis ac- shifts from coral to dominance by leathery macro- counted for 89.8% (37,982) of the total bites, and algae. These shifts appear difficult to reverse as 94.6% of the total mass standardized bites. This these macroalgae are unpalatable to the majority of limited redundancy, both within and across local herbivores, and the macroalgae suppress the scales, underscores the need to assess the func- recruitment and growth of corals. The removal of tional roles of individual species. Management and macroalgae is, therefore, viewed as a key ecological conservation strategies may need to look beyond process on coral reefs. On the Great Barrier Reef, the preservation of species diversity and focus on Sargassum is a dominant macroalgal species fol- the maintenance of ecological processes and the lowing experimentally induced coral–macroalgal protection of key species in critical functional phase-shifts. We, therefore, used Sargassum assays groups. and remote video cameras to directly quantify the species responsible for removing macroalgae across Key words: Naso unicornis; functional redun- a range of coral reef habitats on Lizard Island, dancy; phase-shift; macroalgae; Sargassum; coral northern Great Barrier Reef. Despite supporting reef; herbivory. over 50 herbivorous fish species and six macroalgal INTRODUCTION Herbivory is widely acknowledged as a key process structuring plant communities in both terrestrial and Received 30 July 2009; accepted 27 September 2009; aquatic ecosystems (Scheffer and others 2001). published online 23 October 2009 Whilst there are fundamental differences among Electronic supplementary material: The online version of this article ecosystems in the nature of herbivory and its (doi:10.1007/s10021-009-9291-z) contains supplementary material, importance relative to other processes (Shurin and which is available to authorized users. *Corresponding author; e-mail: [email protected] others 2006; Gruner and others 2008), areas of 1316 Limited Functional Redundancy on Coral Reefs 1317 moderate to high grazing are often characterized by a others 2006). Once established these shifts appear low biomass of highly productive plants; a grazing difficult to reverse as these macroalgae are unpal- lawn (sensu Bell 1971; McNaughton 1984). Within atable to the majority of herbivores (Bellwood and these systems a marked reduction in herbivory has others 2006) and have been shown to suppress the often led to a shift to an alternate state dominated by survival, fecundity, and recruitment of corals (Jo- a high biomass of larger, less productive, and less mpa and McCook 2002; Hughes and others 2007; palatable plants. Shifts between herbaceous and Mumby and others 2007). Given the potential woody vegetation have been documented for a importance of macroalgae in coral reef phase-shifts, range of terrestrial systems, including tropical and our ability to successfully manage coral reefs into subtropical savannas (Walker and others 1981; Ar- the future requires a clearer, quantitative under- cher and others 1988; Dublin and others 1990), standing of the roles of individual herbivorous fish mesic grasslands (Dobson and Crawley 1994), and species and the locations in which these roles are salt marshes (Bazely and Jefferies 1986). In marine exhibited. systems, shifts to macroalgal, or seaweed, domi- Herbivorous fishes may be broadly classified into nance have been documented on coral reefs and four functional groups based on their roles in eco- temperate rocky shores following reductions in system processes: excavators, scrapers, grazers, and herbivore populations (Hughes 1994; Steneck and macroalgal browsers (Steneck 1988; Bellwood and others 2002). The persistence of these shifts long others 2004). Whilst excavating, scraping, and after herbivore populations have been restored grazing taxa generally consume algal turfs, they highlight the difficulty of reversing such shifts as the perform different and complimentary roles in dominant vegetation reaches a size refuge from helping reefs to resist shifts to alternate states. In herbivory. Seedlings of woody plants and macroalgal contrast, the removal of adult macroalgae by her- propagules are easily eliminated by grazing herbi- bivorous fishes (that is, macroalgal browsers) ap- vores (Holmgren and others 2006), however, as they pears to represent a separate but critical process in grow they become less susceptible to the same suite the reversal of phase-shifts (Bellwood and others of herbivores. Quantifying the impact of different 2006). Recent studies on the Great Barrier Reef herbivore groups is fundamental to our under- (GBR), one of the world’s most intact coral reef standing and management of these ecosystems. systems, have demonstrated that only a few species Coral reefs are one of the worlds’ most produc- are responsible for the removal of erect brown tive and biologically diverse ecosystems. On heal- macroalgae within this system (Bellwood and thy coral-dominated reefs with intact herbivore others 2006; Mantyka and Bellwood 2007; Fox and populations the algal community is dominated by Bellwood 2008; Cvitanovic and Bellwood 2009). highly productive algal turfs (primarily filamentous However, all of these studies were spatially re- algae, macroalgal propagules, and detritus) and stricted. All were conducted on the leeward side of grazing resistant crustose coralline algae. Within a single inshore island, with the majority restricted these reefs over 90% of the daily algal production is to a single bay and/or a single habitat. Identifying consumed by a diverse assemblage of grazing fishes the species contributing to this function across a and invertebrates (Polunin and Klumpp 1992; range of spatial scales is central to our under- Bellwood and others 2004). Following large scale standing of this process and the resilience of the coral mortality the dead coral skeletons are rapidly system as a whole (Peterson and others 1998; Ny- colonized by algal turfs (Diaz-Pulido and McCook stro¨ m and Folke 2001). 2002), subsequently increasing algal abundance Within the GBR, there is a marked separation of and production. On reefs with intact herbivore inshore reefs from mid- and outer-shelf reefs in communities these algal communities are main- benthic composition, herbivore community struc- tained in a cropped state (Arthur and others 2005), ture, environmental parameters, and ecosystem suggesting there is an innate capacity to compen- processes (Fabricius and De’ath 2001; Hoey and sate for the increased algal production. However, Bellwood 2008; Wismer and others 2009). Erect regional reductions in herbivorous fishes through brown macroalgae, in particular Sargassum spp. overfishing have limited the ability of many reefs to (Ochrophyta: Phaeophyceae), are a dominant fea- absorb the increased algal production. This dis- ture of shallow coastal reefs where they form dense equilibrium between algal production and con- stands up to 3 m in height which can cover over sumption may release macroalgal propagules from 50% of the substratum (Bellwood and others 2006; top–down control, and ultimately lead to a shift to Wismer and others 2009). In contrast, erect brown dominance by erect brown macroalgae (Hughes macroalgae are present in low densities on mid- 1994; McClanahan and others 2001; Graham and and outer-shelf reefs (McCook and others 2000). 1318 A. S. Hoey, D. R. Bellwood Whilst variation in grazing intensity has been and back reef (2–4 m). The remaining three habi- shown to be a primary determinant of Sargassum tats were located on the leeward or sheltered side of distributions on mid-shelf reefs of the GBR the island: a patch reef habitat (4–6 m depth), a (McCook 1996), the identities of the species sheltered reef flat (1–2 m), and sheltered reef base responsible for this process are not known. The aim (6–8 m) on a fringing reef on the north-western of this study, therefore, was to identify the species side of the island (Figure 1C). responsible for removing erect brown macroalgae across multiple mid-shelf reef habitats on the GBR, Benthic Surveys and in doing so, to quantify the extent of functional To quantify the variation in the algal community redundancy within and among habitats. The iden- and benthic community structure 12 replicate 10 m tification of these species is an essential step to transects were censused within each habitat. Tran- understand the resilience of these habitats, and the sects were haphazardly placed within each habitat reef as a whole. and, where possible, laid parallel to the reef crest. The type of substratum immediately under

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