INTRODUCTION

As many studies in this volume and elsewhere have noted, there are persistent differences in the ecological characteristics of the individual species assemblages found on different continents (Terborgh & van Schaik, 1987; Fleagle & Reed, 1996; Kappeler & Heymann, 1996). For example, the primates of the Neotropics tend to be smaller and less ecologically diverse than those of other continents, while Madagascar has a larger number of folivores than other biogeographical areas. It seems almost certain that the differences between the primate assemblages of different biogeographical regions are the result of many causal factors and their interactions, including differences in productivity, in the composition of the plant communities, in climate and soil, and in the potential for competition with other groups of vertebrates. These factors are examined in other chapters of this volume.

However, the primate assemblages we see today are not simply epiphenomena of present ecological conditions. Rather, they are also the product of evolutionary and ecological processes that have been ongoing for millions of years. Environments are not stable today, and they never have been; they are constantly changing. The temporal scale of environmental change ranges from decades and centuries for human-induced activities of habitat destruction such as logging, land clearing, hunting, or introduction of exotic species (Struhsaker, chapter 17, this volume) and also for natural epidemics. Global climatic cycles seem to have periodicities ranging from a few years, such as the El Niño phenomena to tens of thousands of years for the glacial cycles that have dominated the past two million years (Tutin & White, chapter 13, this volume; Potts, 1994).