Herbivory, algal distribution, and the maintenance of between-habitat diversity on a tropical fringing reef

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Hay, Mark E.
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The bases of coral reefs in the Caribbean often abut sandy plains covered by sea grasses (Randall 1965; Ogden et al. 1973b) or algae (Earle 1972; Dahl 1973). Interactions occurring at the border of reefs and sea grass beds have been studied on several occasions (Randall 1965; Ogden et al. 1973b; Ogden 1976; Parrish and Zimmerman 1977; Ogden and Lobel 1978), but little is known about those which occur between reefs and sandy plains dominated by algae. Unlike sea grasses, which root into the sand, many of the algal species that occur on sand plains require hard substrates (Dahl 1973) such as shells and coral fragments. Suitable attachment sites are uncommon on the sand plain at Galeta Point, Panama, and many are periodically buried or turned over during heavy seas. Paradoxically algal species that predominate on the sand plain tend to be rare or absent from the shallower reef slope where stable, hard substrate is abundant. The maintenance of such distinct distributional boundaries is often attributed to differential competitive abilities (Connell 1961; Holmes 1961) or to restrictive specialization to particular physical regimes (Doty 1946; Terborgh 1971). These factors have been hypothesized to be especially important (Dobzhansky 1950; Janzen 1967; Ashton 1969; Diamond 1975) in diverse tropical communities where specialization to narrow niches is thought to promote resource partitioning and allow increased coexistence. While such explanations are often consistent with observed patterns, they are seldom tested using controlled field experiments. Without such field manipulations it is impossible to adequately assess the relative importance of competition, predation, and physical stress in determining the distribution and abundance of species or the intensity of interactions that occur between them. In this paper I examine experimentally the factors maintaining these separate algal assemblages and contend that the sand-plain species (algae that are almost never found on the reef) would competitively exclude other species from the reef slope if they were not selectively removed by reef-associated grazers. Competition and physiological specialization appear to have no effect on excluding sandplain species from the reef slope. In the remainder of the paper I test the following hypotheses. (1) Low light and scarcity of attachment sites severely limit the growth of algae on the sand plain. (2) Sand-plain algae are not physiologically restricted to deep waters; they are most fit in the physical regime typical of shallower reef-slope habitats. (3) Reef-associated grazers are responsible for excluding sand-plain species from the reef slope. (4) In the absence of herbivores, sand-plain species competitively dominate reef-slope species. (5) Because of differential competitive ability, sand-plain genera are better adapted for temperate areas than are reef-slope genera.
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