The latitudinal gradient of diversity: Synthesis of pattern and process

• Dawn Kaufman

Abstract
The latitudinal gradient of species diversity is a nearly universal pattern (Brown 1995) and latitude has been said to have the "boldest signature" with respect to the spatial distribution of biodiversity (Lewin 1989, Gaston 1996). In fact, the inverse relationship between species richness and latitude is known from a diverse set of taxa, including vertebrates (mammals--Simpson 1964, Kaufman and Willig in press; birds--Cook 1969, Blackburn and Gaston 1996; herptiles--Kiester 1971, Rosenzweig 1995; fish-- Macpherson and Duarte 1994, Oberdorff et al. 1995), non-vertebrate chordates (tunicates--Fischer 1960), invertebrates (ants--Brener and Ruggiero 1994, papilionid butterflies--Scriber 1973, termites-- Collins 1989, crayfish--France 1992, amphipods--France 1992, gastropods--Rex et al. 1993, coral--Fischer 1960), protozoans (fossil foraminiferans--Stehli et al. 1969) and plants (columnar cacti--Mourelle and Ezcurra 1997, fossil angiosperms--Crane and Lidgard 1989, trees--Currie and Paquin 1987, orchids--Dressler 1981). Although many studies appear each year on this topic, progress to ascertain a convincing general underlying mechanism has been stymied. The current ecological approach to research on the latitudinal gradient suffers from a lack of rigor and creativity in the examination of the pattern and in the quest for the process (detailed below). In addition, progress should be enhanced by capitalizing on new technologies to handle large datasets and perform comparative research across taxa to search for generalities.