Introduction

It has been known for over two centuries that the diversity of land plants, in the familiar sense of species richness, is not distributed evenly over the surface of the earth (von Humboldt, 1808). Similar patterns were soon established for terrestrial animals (Wallace, 1876) but it was a long time before our knowledge of marine organisms was sufficient to determine large-scale biogeographic patterns in the sea. Although humans had long exploited the nearshore and continental shelf seas for food and other resources, it was not until the pioneering oceanographic voyages of the late nineteenth and early twentieth century that we began to determine similar global patterns of marine biogeography (Angel, 1994, 1997). We now recognize that, as a broad generalization, diversity on land and in the sea attains its highest values in the tropics and is lowest at the poles, with temperate regions often intermediate (Gaston, 2000; Chown & Gaston, 2000).

Although these global scale (macroecological) patterns in diversity are dramatic, we still lack an agreed explanation for their cause. We can correlate these patterns with a range of environmental variables but ecology based solely on correlations is incomplete. We might be able to use such correlations to make predictions (Peters, 1983, 1991) but predictions without an underlying mechanistic understanding have limited usefulness because we lack knowledge of the conditions under which those predictions might break down.