The linear approach to biogeography: Should we erase Wallace's Line?

Various attempts have been made to subdivide the earth into biogeographic regions. Textbooks of modem biogeography treat the problem of identifying biogeographic units to different extents. Some biogeographers explore sophisticated systems of subdividing the earth into realms and provinces while others omit the issue altogether. Naturally, the basis of such subdivisions are patterns of geographic distribution of species. For large-scale subdivisions of the earth, however, in most cases supraspecific systematic categories such as orders, families or genera have been compared. The different systems of subdividing the earth into biogeographic units reflect our knowledge of the Earth’s biota. Accordingly, relatively showy species or, in terms of their geographic distribution or systematic relationships, well known groups such as many of the higher plant species, birds, and mammals, formed the basis of the earlier subdivisions. Although the subdivisions presented to date reflect real patterns, independent of the underlying causal processes, these subdivisions have repeatedly been criticized. Criticisms were based on exceptions which did not fit the patterns that defined realms, regions or provinces. Lines drawn, the most famous certainly being Wallace’s Line, were redrawn, replaced or even omitted altogether. A particular problem occurred in zones such as Wallacea where lines blurred and scientists working on different groups of organisms reached different conclusions about biogeographic boundaries. If scientists working on different groups of organisms do not agree on the delimitation of a particular biogeographic unit, it might seem futile to attempt to draw lines. In this paper, however, it is argued that lines, however ‘false’ or unrepresentative for large arrays of taxonomic groups these may be, should be used as heuristic tools to facilitate and stimulate further research into the biogeography of a particular area. Even lines based on the geographic distribution of only a few taxa may indicate that these share common dispersal characteristics or, more generally, a common biogeographic history. The delimitation of the respective line, and its discussion within the context of other evidence available for, or against, a particular biogeographic hypothesis, might shed further light on the evolution of certain elements of a fauna or flora. This paper focuses on the heuristic values and biogeographic implications of the various attempts to draw biogeographic boundaries in the Wallacean region. Recent developments, problems and needs for a better understanding of the biogeography of the Indo- Australian region are outlined and discussed. ‘The western and eastern islands of the [Malay] archipelago belong to regions more distinct and contrasted than any other of the great zoological divisions of the globe. South America and Africa, separated by the Atlantic, do not differ so widely as Asia and Australia. ’ Wallace (1860)