Ecological Assembly Rules: Perspectives, Advances, Retreats

Abstract

Edited by Evan Weiher and Paul Keddy. 1999. Cambridge University Press, Cambridge, United Kingdom. xii + 418 pp. ISBN 0-521-65235-9. Cloth, $90.00.—Community ecology has encountered several crossroads in the past and is currently at another. In the 1960s and 1970, several ecologists recognized that advancement of the field would require testing hypotheses about the relative importance of bottom-up and top-down effects on the structure of communities and food webs. This was evident in the pioneering experiments by Paine, Connell, Dayton, Menge, Lubchenco, and Sousa in intertidal communities (many of which are reviewed in Chapter 3 by D. A. Kelt and J. H. Brown in this volume). In the 1970s and 1980s, a contentious debate began over the existence of what Jared Diamond called “forbidden combinations” of bird assemblages in archipelagoes. This spurred the development of novel statistical approaches (e.g. null models; Connor and Simberloff 1983) to test these ideas. Community ecologists discovered that making inferences about the importance of competition from assemblage patterns was not as straightforward as one might initially think, and the statistical methodology that may have been used mattered. The current crossroad is different in that it is not about whether people are going to test new hypotheses or develop new statistical methods to advance the state of knowledge in community ecology. Rather, it is about whether the time is right for a synthesis of descriptive and experimental approaches in community ecology. Can the study of community assemblages be taken in a new direction? Fundamental questions such as “Why are there so many species?” and “What factors limit species diversity?” have been addressed primarily with either descriptive or experimental approaches. An important conclusion that I drew after reading Weiher and Keddy's Ecological Assembly Rules is that a synthesis of experimental and null-model approaches may be the best way to answer these fundamental but vexing questions. Inferring process from pattern has its advantages and disadvantages, as illustrated in Part I: “The Search for Meaningful Patterns in Species Assemblages.” An advantage of using inference in such an approach is that it aims at describing combinations of species/taxa and, thus, applies to communities as a whole. A disadvantage is the difficulty in ruling out the importance of factors other than competition in structuring assemblages. The difficulties are well illustrated in exchanges between Fox (Chapter 1); Simberloff, Dayan, and Stone (Chapter 2); Kelt and Brown (Chapter 3); and Wilson (Chapter 5). In these chapters, we learn about a dizzying array of type I and type II statistical errors that may occur when using inappropriate null models. These are given colorful names such as “The Narcissus Effect,” “The Icarus Effect,” “The J.P. Morgan Effect,” and “The Jack Horner Effect.” Although disputes over methodology are just as contentious today as they were in the 1970s and 1980s, a gentler tone characterizes the debate in the current volume. For example, we find Brown (Chapter 3) betting Simberloff (Chapter 2) a beer over the results of what both agree would be an “interesting study” on the assembly of desert rodent communities. There are also advantages and disadvantages to strictly experimental approaches in community ecology. One advantage is that factors other than competition can be ruled out unequivocally. One disadvantage, however, is that the results of most experimental studies relate to present-day, short-term processes, usually describing the responses of individual species to competition and other factors, rather than how communities as a whole are structured. Interpretations about the role of competition in structuring communities derived from such studies may depend on which species and which stages of the life cycle are examined and a consideration of the indirect interactions among three or more species. In Chapter 4, Lockwood, Moulton, and Balent use an experimental null-model approach to examine patterns of successful colonization of islands by introduced avifauna. Thus, statistical approaches and a natural experiment are combined in a powerful demonstration of the importance of assembly rules. Such an experimental approach is revisited by Lockwood and Pimm in Chapter 13, “When does Restoration Succeed?” Because restoration ecology is inherently experimental in nature (or at least it should be), restoration projects may provide the best opportunity to test hypotheses about community assembly. In their review, Lockwood and Pimm discover that full restoration is rarely achieved, but that restoration efforts that include secondary succession are more likely to succeed than those that rely on micromanagement of community structure. In Part II, the reader learns about other nontraditional perspectives on assembly rules. In the introductory chapter of the book, Keddy and Weiher provocatively state that “asking i