MARINE ECOLOGY PROGRESS SERIES Vol. 332: 143–153, 2007 Published March 5 Mar Ecol Prog Ser Mediating effects of sea urchins on interactions among corals, algae and herbivorous fish in the Moorea lagoon, French Polynesia B. D. Mapstone1,*, N. L. Andrew2, Y. Chancerelle3, B. Salvat3, 4 1Antarctic Climate & Ecosystem CRC, University of Tasmania, Private Bag 80, Hobart, Tasmania 7001, Australia 2WorldFish Center, PO Box 500, GPO, 10670 Penang, Malaysia 3Centre de Recherches Insulaires et Observatoire de l’Environnement, EPHE, BP 1013, Moorea, French Polynesia 4Laboratoire de Biologie Marine, URA CNRS 8046, EPHE, Université de Perpignan, 66860 Perpignan Cedex, France ABSTRACT: The relative importance of grazing by sea urchins in influencing the composition and structure of coral reef habitats has only occasionally been explored experimentally, and never on the coral reefs of Oceania, where both herbivorous fishes and sea urchins are often common. In this paper we report the results of an experiment in French Polynesia, in which densities of an abundant sea urchin, Echinometra mathaei, were manipulated within the territories of an abundant omnivo- rous, ‘gardening’ pomacentrid fish (Stegastes lividus) in thickets of Acropora pulchra. Increasing the sea urchin density resulted in reductions in the standing crop of algae and over-grazing of the dead coral substratum on which the algae grew. After 2 yr of this treatment, the coral thickets began to col- lapse. Reducing sea urchin densities to very low levels also resulted in collapse of the coral thickets and reduced densities of the fish, although algal biomass was apparently unaffected. We posit that the fish–coral–sea urchin–algal assemblage is relatively robust to wide fluctuations in sea urchin densities, but when sea urchin densities are driven to extremes the coral–algal habitat becomes destabilised and the entire system collapses, possibly as a result of different bioerosion processes in play at high and low sea urchin densities. KEY WORDS: Herbivory · Bioerosion · Sea urchins · Coral · Algae · Urchin–coral interactions · Algal–coral interactions Resale or republication not permitted without written consent of the publisher INTRODUCTION vores, including sea urchins, have been invoked as important mediators of this coral–algae competition, Sea urchins can significantly affect the ecological with corals apparently dominating when algae are structure of coral reefs through bioerosion of substrata suppressed by abundant grazers (Morrison 1988, (Bak 1990, 1994, Conand et al. 1998) and by influenc- Coyer et al. 1993). Changes in abundance of herbi- ing competition for space between corals and algae vores have been associated with blooms in benthic (e.g. Ogden & Lobel 1978, Hay 1981, 1984, 1991, algae and overgrowth and, ultimately, limitation of Williams 1981, Lewis & Wainwright 1985, Carpenter coral recruitment and survival (Hughes et al. 1987, 1986, 1990, Foster 1987a,b, Morrison 1988, Coyer et al. McManus et al. 2000, Williams & Polunin 2001). Simi- 1993). Focus on coral–algae competition is increasing larly, when herbivorous fish are prevented from graz- as concern grows about long-term alteration of ecolog- ing by the territoriality of some pomacentrids, algae ical states of coral reefs (see review by McCook et al. dominate the substratum in the protected territories 2001), with algal-dominated or coral-dominated com- (Potts 1977, Klump et al. 1987, Hata et al. 2004). munities considered by some to be alternate, poten- Bio-erosion by sea urchins, either through specific tially stable ecological states (Done 1992, Hughes burrowing activities or through inadvertent scraping of 1994, Miller 1998, McClanahan et al. 2002a,b). Herbi- substrata during feeding, has been found to interact *Email: [email protected] © Inter-Research 2007 · www.int-res.com 144 Mar Ecol Prog Ser 332: 143–153, 2007 with the herbivore–algae–coral dynamics, both influ- Diadema savignyi, and these sea urchins have been encing and responding to the dynamics of coral and implicated in both herbivory and bioerosion (Bak 1990, algal occupation of space on coral reefs (Risk & Sam- Carreiro-Silva & McClanahan 2001). marco 1982, Sammarco et al. 1987, Bak 1990, 1994, Relatively low densities of sea urchins are found in Eakin 1996, Conand et al. 1998, Carreiro-Silva & the Stegastes nigricans and S. lividus territories on McClanahan 2001, Stromberg & Kvamemo 2005). Sea consolidated reefs in the Moorea lagoon, where the urchins, therefore, can play diverse roles in a complex sea urchins are found in holes or crevices within small of interactions influencing the structure of coral reef patches of bare substratum surrounded by the algal communities. turf that characterises the damselfish territories (B. D. Fisheries for predators have also been invoked in the Mapstone unpubl. data). Sea urchins displaced from coral–algae dynamics because of the top-down effects the holes or crevices during daylight are evicted from of removing the predators of the herbivores that medi- the turfs by the damselfish, indicating active interfer- ate the algal–coral interactions (Hughes et al. 1987, ence competition between the fish and sea urchins. McManus et al. 2000). For example, in heavily fished Sea urchins are more abundant in damselfish territo- areas of the Caribbean, Diadema setosum consumed ries in thickets of Acropora spp., however, particularly much of the algal primary production in areas affected near the bases of the corals, below a band of dense epi- by fishing, and their removal, or loss to disease, caused phytic filamentous algae characteristic of the poma- significant increases in the standing crop of algae centrids’ territories. Roaming schools of grazing scarids (Hughes et al. 1987, Lessios 1988). In unfished areas, and acanthurids are prevented from grazing within the herbivorous fish are thought to out-compete sea thickets by attacks by the pomacentrids (B. D. Map- urchins and to play a more important ecological role stone unpubl. data). Predatory fishes such as haemu- (Hay 1981, Robertson 1991). lids and labrids also are driven off by the pomacen- Although the diadematid sea urchins are perhaps trids. the most studied ecologically, sea urchins in the genera Anecdotes indicated that the Acropora spp. thickets Tripneustes, Lytechinus and Echinometra may also be in the Moorea lagoon have changed considerably in abundant on coral reefs and in seagrass habitats. location and extent over recent decades. The reasons Recent reviews suggest that Tripneustes spp. have rel- for these changes are unknown, but one hypothesis is atively little impact on community structure (Lawrence that over-grazing by sea urchins may have weakened & Agatsuma 2001), and the large impacts observed for the bases of the Acropora spp. and, ultimately, led to Lytechinus spp. are restricted to seagrass and associ- the collapse of the thickets. Such a process would indi- ated habitats (Watts et al. 2001). McClanahan and cate an important structuring role for the sea urchin coworkers (Carreiro-Silva & McClanahan 2001, see grazers besides the suppression of algal growth. review by McClanahan & Muthiga 2001) have demon- In this paper we report the results of a manipulative strated, however, that Echinometra mathaei is an experiment to explore the hypothesis that sea urchins important grazer on heavily fished reefs in Kenya. In may have direct (bio-erosion) as well as indirect (via both Kenya and the Caribbean, sea urchins may play grazing algae) density-dependent effects on the algal– important roles in both herbivory and bioerosion, but coral–damselfish assemblage in coral thickets in the their impact is density-dependent and mediated by Moorea lagoon. We tested the null hypothesis that sea other processes such as predation and fishing. urchin density had no significant effect by directly ma- By contrast, on the Great Barrier Reef sea urchins are nipulating the densities of the sea urchin Echinometra generally at very low abundance and herbivorous mathaei within territories of the pomacentrids Ste- fishes are considered the most important herbivores gastes nigricans and S. lividus in lagoonal thickets of (Russ 1987, 2003). Sea urchins also are considered to Acropora pulchra and monitoring the subsequent status be less important than herbivorous fishes in the east- of the algal turf, living coral, fish and thicket structure ern Pacific and in Oceania (Ogden 1987, Sammarco over 2 yr. This work is one of the few attempts to test 1987), even though relatively high densities of sea through manipulative experiments the models derived urchins have been recorded in some areas, such as from observation of patterns or natural disturbances in some islands in French Polynesia. For example, sea one or more of these components of coral reef systems. urchins are very abundant on the lagoonal and fring- ing reefs around Moorea (Bak 1990, Conand et al. 1998), where a diversity of herbivorous fishes is also MATERIALS AND METHODS found, including scarids, acanthurids and the territorial damselfishes Stegastes nigricans and S. lividus (Galzin Study site. The experiment was done in an exten- 1987a,b). The sea urchins at Moorea are principally sive area of Acropora pulchra in the lee of Motu Fare Echinometra mathaei, Echinothrix diadema and off the north west tip of Moorea Island (17° 29’ S, Mapstone et al.: Sea urchin grazing in damselfish territories 145 149° 55’ W). Stegastes lividus was abundant, and indi- viduals were observed beyond the thickets during 2