Biogeography of Corals, Seagrasses, and Mangroves: an Alternative to the Center of Origin Concept

McCoy, E. D. , and K. L. Heck, Jr. (Department of Biological Science, Florida State University, Tallahassee, Florida 32306) 1976. Biogeography of corals, seagrasses, and mangroves: an alternative to the center of origin concept. Syst. Zool. 25:201-210. -A "center of origin" explanation for the distribution and diversity patterns of the organisms comprising the principal shillow-water habitats in the tropics-hermatypic corals, mangroves, and seagrasses-is highly unlikely, based on fossil data and the dispersal capabilities of the organisms. Instead, these biogeographic patterns are better explained by the existence of a previously widely-distributed biota which has since been modified by tectonic events, speciation, and extinction, in accordance with modern geological and biogeographical theory. [Corals; seagrasses; mangroves; biogeography; vicariance. ] The "center of origin" concept-that a taxon's place of origin is the region in which it is most diverse-is nearly axiomatic (Rotramel, 1973). Briggs (1961, 1966, 1967), Darlington (1957), and Mayr (1965) have all been active proponents of this assumption, although Darlington has warned against its application when a thorough fossil record does not exist to verify it. Other biogeographers (e.g. Cain, 1944, Croizat, 1958; Croizat et al., 1974) also have strongly questioned the uncritical application of the center of origin concept. Despite these warnings and criticisms the concept has a facile appeal, and remains viable (for a recent example, see Muller, 1973). The center of origin concept was originally conceived on the assumption that the positions of the continents remained constant through time. This implied that closely-related organisms inhabiting widelyseparated land masses or bodies of water must have somehow dispersed over long distances. Of course, it can no longer be accepted that the continents have remained stationary, and as a result, it is necessary to critically reevaluate the validity of earlier conclusions involving the center of origin concept. In place of the notion that 1 Order of authorship was determined by the toss of a coin. centers of origin can be identified by centers of diversity, we advocate acceptance of the following ideas which take into account current biogeographical, ecological, and geological reasoning: (1) Longterm climatic and tectonic events cause environmental changes with important biotic consequences; hence, present centers of diversity may not have always been so (cf. Carson, 1970); and (2) localized ecological processes, such as extinction and species-area relationships, are significant determinants of species diversity independently of where the taxon originated; thus, present distributions and diversity patterns represent more than long-term, monotonous radiation of species from some point of origin. An important ancillary point is that the evidence for long-distance dispersal of widely-distributed organisms is often tenuous, though it is rarely presented as such. To illustrate how these newer ideas can be applied in explaining biographical patterns, we use three important groups of marine organisms and compare results obtained from them with those generated by the center of origin concept. We have chosen hermatypic corals, seagrasses, and mangroves for a number of reasons: (1) They are the principal shallow-water sessile assemblages throughout the world's tropics; (2) they presently co-occur throughout the tropics, and their diversity patterns and