Title:
Herbivory, algal distribution, and the maintenance of between-habitat diversity on a tropical fringing reef
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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Abstract
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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1981-10
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