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Direct competition for space between coral colonies at Heron Island, Australia.
Greenberg, David*,1, Connell, Joseph1, Goldwasser, Lloyd1, 2, 1 Department of Ecology, Evolution, and Marine Biology, Santa Barbara, CA, 931062 Department of Demography, Berkeley, CA, 94720-2120
ABSTRACT- Competition for space on the sea floor, both within and between species, is an important factor governing the abundance and species diversity of reef corals. Corals may compete for space via overgrowth and overtopping, or via physical and chemical attacks between neighboring colonies. We have studied the extent to which competition via direct attack occurs between coral colonies at Heron Island, Great Barrier Reef, Australia, using census data collected from fixed square meter plots at intervals since 1963. Attacks between coral colonies occur along neighboring portions of their perimeters, where each colony's polyps can physically reach each other. Competition for space should then constrain growth along these closely-neighboring perimeters more than along other regions further from neighbors. We tested this hypothesis using maps drawn from color photographs which show the perimeters of all colonies within each plot for each census year. The maps allow us to track changes in size and position of individual colonies as a function of their proximity to other colonies. We regard parts of colony perimeters relatively far from any neighbors as controls for the neighbored portions, but also analyze the extent to which this "neighbor effect" varies continuously, rather than discretely, along a colony's border as a function of distance from its nearest neighbored region. For each colony in each census year, the analysis involves: a) viewing its perimeter as a series of evenly-spaced points, b) computing each point's distance from the nearest point on the perimeter of its nearest neighbor, and then c) measuring its growth at that point to the next census. Under competition, growth at points on a colony border should vary as a continuously increasing function of each point's distance (measured in the first census) from a neighbor.
Key words: space, Australia, competition, corals