In recent times, the science of ecology has been rejuvenated and has moved to a central position in biology. This volume contains eighteen original, major contributions by leaders in the field, all associates of the late Robert MacArthur, whose work has stimulated many of the recent developments in ecology. The intellectual ferment of the field is reflected in these papers, which offer new models for ecological processes, new applications of theoretical and quantitative techniques, and new methods for analyzing and interpreting a wide variety of empirical data. The first five chapters explore the evolution of species abundance and diversity (R. Levins, E. Leigh, J. MacArthur, R. May, and M. Rosenzweig). The theory of loop analysis is newly applied to understanding stability of species communities under both mendelian and group selection. Species abundance relations, population fluctuations, and continental patterns of species diversity are illustrated and interpreted theoretically. The next section examines the competitive strategies of optimal resource allocation variously employed in plant life histories (W. Schaffer and M. Gadgil), bird diets and foraging techniques (H. Hespenheide), butterfly seasonal flights (A. Shapiro), and forest succession examined by the theory of Markov processes (H. Horn). The seven chapters of the third section study the structure of species communities, by comparing different natural communities in similar habitats (M. Cody, J. Karr and F. James, E. Pianka, J. Brown, J. Diamond), or by manipulating field situations experimentally (R. Patrick, J. Connell). The analyses are of communities of species as diverse as freshwater stream organisms, desert lizards and rodents, birds, invertebrates, and plants. These studies yield insights into the assembly of continental and insular communities, convergent evolution of morphology and of ecological structure, and the relative roles of predation, competition, and harsh physical conditions in limiting species ranges. Finally, the two remaining chapters illustrate how ecological advances depend on interaction of theory with field and laboratory observations (G. E. Hutchinson), and how ecological studies such as those of this volume may find practical application to conservation problems posed by man's accelerating modification of the natural world (E. Wilson and E. Willis).