Temporal Changes in Species Number in an Assemblage of Sessile Marine Invertebrates

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Abstract
The MacArthur-Wilson model is frequently used to study communities that live in patchy habitats. Recent evidence shows that the model should be regarded as not adequately tested. The model makes three predictions, none of which can be used to test the model. As stochastic definition of the species equilibrium is provided, so that one of the predictions becomes falsifiable. A statistical test for the existence of an equilibrium is provided, based on this definition. Another method is suggested that allows the presence of an equilibrium to be tested for a whole group of patches. The latter technique also allows the quantitative assessment of the effect of predators or disturbance on the fluctuations in the number of species. A consequence of these tests is that at least six censuses of a patch are necessary for it to be possible to reject the null hypothesis of a species equilibrium. These methods were used to investigate the changes in the number of species of sessile invertebrates epizoic on the shells of a bivalve, Pinna bicolor, in a subtidal environment. The fluctuations in the number of species on individual Pinna shells are too great for the concept of an equilibrium to be useful. Predation by monacanthid fish on newly metamorphosed colonial tunicates decreased the fluctuations in species number, since tunicates were capable of causing dramatic changes in the number of species, owing to their competitive superiority. The wide fluctuations in species number are most likely due to chance extinctions of colonies and to dramatic variation in the colonization rates between species and between places and times for individual species.

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