What species occur where, and why, and why some places harbor more species than others are basic questions for ecologists. Some species simply live in different places: fish live underwater; birds do not. Adaptations follow: most fish have gills; birds have lungs. But as reveals, not all patterns are so trivial. Travel from island to island and the species change. Travel along any gradient—up a mountain, from forest into desert, from low tide to high tide on a shoreline —and again the species change, sometimes abruptly. What explains the patterns of these distributions? Some patterns might be as random as a coin toss. But as with a coin toss, can ecologists differentiate associations caused by a multiplicity of complex, idiosyncratic factors from those structured by some unidentified but simple mechanisms? Can simple mechanisms that structure communities be inferred from observations of which species associations naturally occur? For decades, community ecologists have debated about whether the patterns are random or show the geographically pervasive effect of competition between species. Bringing this vigorous debate up to date, this book undertakes the identification and interpretation of nature’s large-scale patterns of species co-occurrence to offer insight into how nature truly works. explains the computing and conceptual advances that allow us to explore these issues. It forces us to reexamine assumptions about species distribution patterns and will be of vital importance to ecologists and conservationists alike.