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Ellison Abstract- 1989 Ellison and Harvell

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Ellison, A. M. and C. D. Harvell. 1989. Size hierarchies in Membranipora membranacea: do colonial animals follow the same rules as plants? Oikos 55: 349-355.


The relative importance of mode of intraspecific competition, growth rate, and recruitment time on size structure of populations of the bryozoan Membranipora membranacea was examined experimentally. Size distributions of populations of solitary colonies (1/225 cm2), low density colonies (<15/225 cm2) , and high density colonies (>25/225 cm2) were compared to determine if mechanisms that generate size hierarchies in plant populations also operate on populations of sessile invertebrates. Degree of inequality increased from solitary to low density populations, indicating that colony growth rate alone does not determine inequality. However, no difference in inequality was observed between low and high density populations, and degree of inequality neither increased nor decreased through time, indicating that the dominant mode of competition in these populations was resource depletion. Late-recruiting colonies were smaller than early recruits, but did not suffer disproportionately from competition relative to their size or recruitment time. This latter result lends further support to the hypothesis that resource depletion is the major determinant of size distributions in these populations. We conclude that processes similar to those that produce hierarchical size distributions in plant populations also structure populations of colonial invertebrates.

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