Historical influences on community ecology.

W hy does California, a large and topographically complex state that spans fogdrenched redwood groves to desert sand dunes, have 30 species of snakes, whereas roughly twice that number occur in five square miles of lowland Costa Rican rainforest? Almost 400 species of birds, a bit more than half as many as breed in the entire continental United States, occupy that same small Central American site, and indeed, all over the world, most groups of organisms increase in species richness as one moves from the poles toward the equator. Lifestyle diversity also varies globally, and increased numbers of species often reflect primarily ecological diversification; most temperate-zone bats feed on insects, for example, whereas tropical chiropteran faunas include specialists on fruit, nectar, fish, frogs, birds, and blood. Finally, biological diversity can vary extensively and exclusive of latitude, among grossly similar habitats: lizards reach a maximum of 15–20 species at North American desert sites, whereas several times that many species occur at some Australian desert localities. Such dramatic global variation has intrigued naturalists for centuries, and even now its causes are only partly understood. Most conceptual and empirical work in 20th century community ecology focused on contemporary processes, whereas an article by Vitt and Pianka (1) in a recent issue of PNAS demonstrates that dietary relationships in modern lizard assemblages may, in large measure, reflect the evolutionary origins of a few morphological and behavioral novelties during the Mesozoic, 100,000,000 years ago.