Biogeography of the Tropical Pacific

Many previous biogeographic regionalizations of the islands and reefs of the tropical Pacific are unsatisfactory: the regions as defined are heterogeneous, localities with unlike biotas are grouped together, and those with similar characteristics are placed in separate categories. Often distinctions appear to be based on cultural or political rather than biogeographic consider~ ations. Criteria are defined for the establishment of biogeographic boundaries. Instead of the hierarchical schemes often utilized, it is proposed that the basis of biogeographic regionalization be typological. A distinction is made between the biogeographic characteristics of atolls and other reef islands, elevated limestone (makatea) islands, and high (often volcanic) islands. It is concluded that if the first two categories are filtered out, the treatment of the biogeography of the third group and hence the regionalization of the Pacific becomes relatively unproblematical. BIOGEOGRAPHIC REGIONAL SCHEMES discuss the difficulties associated with existing schemes and suggest solutions for them. The problems may be illustrated by reference to regional schemes proposed by the zoologist Gressitt (1961, 1963) and the botanist Thorne (1963). Gressitt's scheme involves a hierarchy of regions, subregions, divisions, and subdivisions, though how these are related is not immediately apparent from his map (Figure 1). Papua, the Philippines, and the Ryukyus are each separated at the subregional level from "Polynesia. " This unit includes most of the tropical Pacific islands; it includes the divisions of New Caledonia and Hawaii, and the subdivisions of Micronesia, central Polynesia and Southeast Polynesia. Leaving to one side the problem of nomenclature, this regionalization has problems that are also frequently characteristic of other such schemes. The "Micronesian" subdivision is heterogeneous: it extends from the Marianas and Palau to Kiribati and includes the Carolines and the Marshalls. There is little in common between Wake Island and Palau, for example, 276 1 Manuscript accepted for publication 2 May 199I. 2 Department of Geography, University of California at Berkeley, Berkeley, California 94720. BIOGEOGRAPHERS HAVE BEEN CONCERNED with the distribution of plants and animals in the tropical Pacific for over a century (Sclater 1858, Wallace 1876,1882), and the differences between Pacific insular biotas and those of East and Southeast Asia and Australia have long been recognized. Hedley (1899) was among the first to describe the profound differences within the Pacific basin, especially between the islands of the western Pacific and those of the north, central, and eastern Pacific. Indeed he located a profound discontinuity in diversity, moving eastward across the ocean, between Tonga and Samoa. In more recent years, in addition to the accumulation of many new data, biogeographic problems have been discussed in the symposia edited by Gressitt (1963) and Radovsky et al. (1984), and notably in a masterly review by Kay (1979). In spite of this, the biogeographic regionalization of the tropical Pacific remains in a most unsatisfactory state: it is perhaps indicative that although Kay (1979) discussed a number of regional schemes she herself refrained from adding to them. In this paper I Juan Fernandez,. Tropical Pacific Biogeography-SToDDART