Herbivory, Algal Distribution, and the Maintenance of Between-Habitat Diversity on a Tropical Fringing Reef

An extensive sand plain containing scattered hard substrates dominated by algae adjoins the shallower Galeta reef seaward from a depth of 11-14 m. This sand plain represents a physically marginal habitat where low light and lack of adequate attachment sites limit algal growth and where seasonal physical disturbances (turbidity and sand movement) cause large decreases in algal densities and cover. Light levels are higher and substrate more abundant on the nearby reef slope but species from the sand-plain assemblage rarely occur there. Algae from both reef-slope and sand-plain habitats become light saturated at approximately the same level (75-100 μE/m2 per s) but those from the sand plain were more productive (i.e., more efficient) throughout the range of light levels tested (13-170 μE/m2 per s). Efficient utilization of low light levels allows sand-plain species to survive on the dimly lit bottom but does not prohibit them from occupying areas of high light intensity. During two experimental periods of approximately 6 mo each, plants that settled on monofilament lines at about 3 m deep were 4-20 times more numerous, 6-23 times larger, more than three times as fertile, accumulated 27-400 times as much two-dimensional surface area, and were represented by 3-5 times as many species as algae that recruited at about 9 m deep. Although resources are more plentiful and sand-plain species more fit in shallower reef-slope areas, they are excluded from these habitats by intensive herbivory. It is hypothesized that in the absence of grazing, sand-plain species would competitively exclude reef-slope species from much of the subtidal reef. If this is valid, then reef hervibores, by limiting competitive dominants to refuge areas, are of primary importance in maintaining the large between-habitat diversity typical of shallow Caribbean reef systems.