MEPS 196:187-194 (2000)  -  doi:10.3354/meps196187

Mechanisms and individual consequences of intraspecific competition in a coral-reef fish

Michael S. Webster*, Mark A. Hixon

Oregon State University, Department of Zoology, Corvallis, Oregon 97331-2914, USA

ABSTRACT: Species of coral-reef fish that exhibit dominance hierarchies provide opportunities for experimental studies of intraspecific competition within discrete social groups. The fairy basslet Gramma loreto (Grammatidae) lives in dominance hierarchies that occupy the undersides of open reef ledges in the Caribbean region. Each hierarchy is maintained by aggression between size classes such that larger fish occupy prime plankton-feeding positions closer to the outer edge of the ledge. We conducted a removal experiment in the central Bahamas to determine the role of larger fish in maintaining the hierarchy and the resultant consequences to smaller fish. When larger fish were removed, smaller individuals quickly occupied these prime feeding positions and were the recipients of less aggression compared to controls. As a result, the average feeding rates of smaller fish were more than 60% higher in aggregations in which larger fish had been removed. Differences in feeding rates were the result of 2 types of competitive effects: between-position and within-position. The between-position effect was the result of differential feeding rates associated with different feeding positions within an aggregation. The within-position effect was due to interactions within a given feeding position. Although both effects strongly influenced basslet feeding rate, the magnitude of the within-position effect was roughly 3 times that of the between-position effect. Together, these 2 components of competition may affect demographic rates and ultimately contribute to local population regulation.

KEY WORDS: Coral-reef fish · Intraspecific competition · Dominance hierarchy

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