The influence of geometric constraints on the colonisation, speciation and range expansion of orchids

Variation in species richness along a gradient is a well-known phenomenon and many attempts have been made to correlate this with various environmental factors. Recently, however, the potential influence of non-biological factors has also been highlighted. Species distributions might be constrained by hard geographical boundaries, such as the ocean surrounding an island, since ranges cannot extend either below the sea or above the highest point on the island. This means a greater probability of overlapping evaluational ranges, and also therefore species richness, towards middle elevations. THis has been termed the 'mid-domain effect' (MDE). The colonization and speciation of island floras has fascinated biologists since the days of Wallace and Darwin, since their isolation restricts the number of possible hypotheses to explain evolutionary events. In this study, species richness of orchids (Orchidaceae) was studied along the elevational gradient both of the Mascarene Islands and of the islands of the Gulf of Guinea. Here we show how geometric constraints can effect speciation, future colonisation and range expansion of these insular orchids.