Inverse latitudinal gradients in species diversity

Introduction No single pattern of biodiversity has attracted ecologists more than the observed increase in species richness from the poles to the tropics (Pianka, 1966; Rohde, 1992; Rosenzweig & Sandlin, 1997; Gaston & Blackburn, 2000; Willig, Kaufman & Stevens, 2003; Hillebrand, 2004). An obstacle in the search for the primary cause of this latitudinal gradient is the ever-increasing number of hypotheses (Pianka, 1966; Rohde, 1992; Clarke, this volume), their interdependence (Currie, 1991; Gaston & Blackburn, 2000) and lack of rigorous falsification (Currie, Francis & Kerr, 1999; Currie, this volume). However, a general decline in species richness with latitude is commonly observed (Pielou, 1977; Colwell & Hurtt, 1994; Willig & Lyons, 1998; Colwell & Lees, 2000; Zapata, Gaston & Chown, 2003; Colwell, Rahbek & Gotelli, 2004). Some groups of organisms, however, show an opposite trend: a strong latitudinal decline in species diversity towards the tropics. These trends have been almost neglected in the literature and little is known about their underlying ecological and evolutionary causes. Therefore, the ecological explanations proffered are usually specific to the group in question. Here an account of the most important cases of inverse latitudinal gradients is given. The existing hypotheses explaining this phenomenon are summarized and the evidence that tends to favor one of these is presented.