"Biodiversity, the variety of life, is distributed heterogeneously across the Earth. Some areas teem with biological variation (for example, some moist tropical forests and coral reefs), others are virtually devoid of life (for example, some deserts and polar regions), and most fall somewhere in between. Determining why these differences occur has long been a core objective for ecologists and biogeographers. It constitutes a continuing, an important, and to many an enthralling, challenge."