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CONVERGENCE OF ECOLOGICAL PROCESSES BETWEEN INDEPENDENTLY EVOLVED COMPETITIVE DOMINANTS: A TUNICATE‐MUSSEL COMPARISON

It has long been recognized that physically comparable environments often are inhabited by morphologically similar species. The phenomenon is termed convergence, especially where there is some but not extreme genetic distance between the species, and it has become a popular comparative tool. The basic hypothesis is well-stated by Orians and Solbrig (1977): " . that, given 2 regions with identical climate, geology and topography, ecosystems that are identical in structure and function will result . . . despite differences in the initial flora and fauna." Marine ecologists are familiar with at least two community level derivatives of this approach, long practiced by terrestrially-oriented biologists. Thorson (1957) expanded parallel level-bottom community comparisons to a worldwide description; Stephenson and Stephenson (1972) have fostered a universal or "widespread" scheme of intertidal zonation. When convergence is expressed on a strictly morphological basis marine invertebrate groups provide numerous examples, often discussed under "homeomorphy." Cloud (1948) assembled many striking examples which include the near morphological identity, when viewed externally, of a Devonian tetracoral, a Permian brachiopod and a Cretaceous bivalve. Most ecological comparisons invariably involve species which share some degree of taxonomic closeness: lizards (Schall and Pianka, 1978), plants (Leigh, 1975; Cody and Mooney, 1978) or birds (Cody, 1974), for instance. The comparative questions asked are purposely constrained by the common taxonomic heritage. But suppose the focus is on the role played by some particular species or life form, where role is defined in terms of the organism's behavior, the type and range of its ecological interactions, and especially its influence on other community members. In phyletically rich marine communities comparable functional roles can be occupied by sea urchins or herbivorous fish, by carnivorous crabs or predatory snails, or by bryozoa or sponges. Many examples are provided by Woodin and Jackson (1979). Thus the phenomenon of complementarity in which taxonomically distant but functionally similar groups either compete with or replace each other geographically (Brown and Davidson, 1977; Wright, 1979) is apt to be commonplace. If one is interested in how similarly organized the communities are, then the appropriate comparisons between different communities are often not taxonomic. Our purpose is to examine a suite of ecological traits common to species potentially dominating a comparable limiting resource, space. The main comparison is purposely biased towards the extreme condition: surely, if similarities can be identified between such taxonomically distant examples as a chordate and a mollusc, then the same influences should also characterize less extreme comparisons since at some point the interphyletic examinations converge on those within orders, families or those with still closer genetic ties, and in the process gain that strong bias produced by immediate common ancestry. We begin by describing those processes thought to determine the distribution and local abundance of the tunicate Pyura praeputialis (Heller). These new data are then compared with our standard, the