Introduction

Understanding the causes and consequences of variation in biodiversity has long been a central focus of research in ecology and biogeography (von Humboldt, 1808; Hutchinson, 1959; MacArthur, 1969; Brown, 1981; Tilman, 1999; Hubbell, 2001; Clarke, this volume). Ecologists have been particularly fascinated by the latitudinal gradient of increasing biodiversity from the poles to the equator since at least the time of Darwin (1859) and Wallace (1878). Contemporary data indicate that this gradient holds for nearly all major groups of terrestrial, aquatic, and marine ectotherms, both plant and animal, and for endothermic birds and mammals (Rohde, 1992; Gaston, 2000; Allen, Brown & Gillooly, 2002; Willig, Kaufman & Stevens, 2003; Currie et al., 2004; Pautasso & Gaston, 2005; Clarke, this volume; Currie, this volume). Furthermore, fossil data indicate that this gradient has been maintained for over 200 million years (Stehli, Douglas & Newell, 1969). Despite more than 150 years of inquiry, the mechanisms responsible for the gradient are still not well understood (Allen, Brown & Gillooly, 2003; Hawkins et al., 2003; Huston et al., 2003; Storch, 2003; Currie et al., 2004; Clarke, this volume; Currie, this volume), but a large and growing list of hypotheses has been proposed to explain it (Rohde, 1992; Gaston, 2000; Hawkins et al., 2003; Currie et al., 2004).

In recent years, particular attention has focused on what we will refer to as the species–energy hypothesis, which proposes that the latitudinal biodiversity gradient has somehow been generated and maintained as a direct consequence of greater energy availability towards the equator.