NATURE'S RULES OF ASSEMBLY.
The quest to understand patterns in the distribution and abundance of species is seen by some as the quest for the very legitimization of community ecology as a predictive science. Others, however, see it as a quixotic exercise, one that contributes little to the advancement of ecology or environmental problem solving. It is not the existence of patterns that is at issue. Every fisher, hunter, mushroom stalker, or nature walker knows that species have predictable patterns of distribution and abundance (D&A). This is why outbreaks, local extinctions, invasions, and other significant changes in D&A give us a sense that something is amiss. As P. Keddy and E. Weiher say in their introduction to Ecological assembly rules, "To ask if there is pattern in nature is to ask if bears shit in the woods." The more critical issue is whether patterns in D&A are the result of underlying rules. That is, when local communities assemble from a regional pool of species, are there rules that govern their assembly? If there are n o rules, then communities are meaningless jumbles of species. If the quest for assembly rules can be equated with the quest for understanding patterns in D&A, then such a quest would hardly be quixotic. The quest for assembly rules, however, is a narrow quest and only a small part of the business of understanding D&A. Ecological assembly rules surveys advances in the study of assembly rules over the last twenty-five years, but it also attempts to reorganize all of community ecology into the narrow framework of assembly rules. This undertaking makes what is otherwise a wealth of interesting information a sometimes-confusing assemblage of chapters.
Although the concept of assembly rules may be only twenty-five years old, the concept of community assembly is a much older and much broader concept. Assembly rules restrict the realm of permissible species combinations that are likely to assemble from a regional species pool. Jared Diamond (1975. The assembly of species communities. Pages 342-444 in M. L. Cody and J. M. Diamond, editors. Ecology and evolution of communities. Harvard University Press, Cambridge, Massachusetts) is cited throughout the volume as the originator of this concept. His classic work argued that he pattern of species compositions of birds found among the islands of New Guinea reflected constraints on permissible combinations that are generated by inter-specific competition within guilds. B. D. Booth and D. W. Larson (Chapter seven) and M. V. Lomolino (Chapter ten), however, correctly trace ideas of community assembly back to Clements in the 1920s. Lomolino's chapter in particular nicely demonstrates the relationship between assembly rules and concepts such as species area curves, Preston's canonical log-normal distribution, and MacArthur and Wilson's theory of island biogeography. To be sure, Diamond stimulated a novel way of looking for the signal of process-generated assembly in D&A data, but this contribution, important as it is, is only a variant in the general business of community ecology.
Perhaps the most significant contribution of assembly rules to community ecology was to formalize the quest for patterns in D&A. Ecologists learned to provide explicit hypotheses that could be rigorously analyzed by statistical or computationally intensive methods. These methods, however, especially the use of null models, have been fraught with controversy and debate since their inception. To no one's surprise, the devil frequently turned up in the data that often contained embedded patterns that either masked or erroneously supported the existence of assembly rules. The first three chapters serve as an excellent overview of these issues by examining the "guild assembly rule" developed by B. J. Fox. Briefly stated, the guild assembly rule stipulates that species have higher probabilities of entry into an assembling community if their guild is under-represented. The simplicity of this rule belies the extraordinary complexity of proving its existence. D. A. Kelt and J. H. Brown's chapter is especially thoroug h and provides strong support for the guild assembly rule as a force governing D&A in desert mammalian assemblages. D. Simberloff et al.'s chapter, however, counters that the data employed in these kinds of studies are predisposed towards providing support for the guild assembly rule and they believe that it is unlikely that this or any simple rule will adequately describe D&A in nature. The four remaining chapters in the first part of Ecological assembly rules surveys the application of rules to plants (J. B. Wilson, M. L. Cody, B. D. Booth, D. W. Larson), mammals (B. J. Fox, Simberloff et al., Kelt et al.) and birds (J. L. Lockwood et al., Cody). Collectively, these seven chapters of Part 1 form an excellent package.
The second part of the book, however, is a mix of chapters thinly linked by Weiher and Keddy's attempt to equate all of community ecology with assembly rules. Chapters by J. A. Drake et al. and by Weiher and Keddy offer thermodynamic-based principles of self organization as a promising alternative to the traditional framework for understanding ecological assembly. S. Diaz et al. argue persuasively that plant functional traits (rather than taxonomic identity) are critical for identifying how environmental filters, which reduce the set of species likely to be found in a region, affect D&A. J. L. Lockwood and S. L. Pimm review the relative success of restoration projects. E. M. Strange and T. C. Foin examine how biotic and abiotic factors combine to predict fish invasions in Northern California coldwater streams. A curious compilation of chapters.
Why is part two of Ecological assembly rules such a heterogeneous assemblage of chapters? The key can be found in Strange and Foin's claim that, "Community assembly now encompasses the ecology and short-term, observable evolution of the community from initial colonization to species saturation, and logically includes a number of topics that are referred to by other names." These "other names" include trophic cascades, top-down versus bottom-up regulation of trophic webs, and the regulation of biodiversity. Few ecologists would define assembly rules so broadly. Most define assembly rules as interaction-based rules that explain post-interactive patterns in D&A, a definition closer to the spirit of Diamond 1975 (even if he is, as Weiher and Keddy inform us, currently more flexible in his definition). A narrow definition retains the clarity and utility of assembly rules rather than losing it in the enormous discipline of community ecology.
As Weiher and Keddy suggest in their epilogue, this volume would indeed make for a lively graduate seminar (though few students could afford to buy it). It would serve not only as a useful compendium of current thoughts on assembly rules, but other important issues in community ecology such as restoration ecology and the use of functional groups. Overall, the book is well written, the majority of chapters blending theory, data, and rigorous analyses. Unlike some symposium volumes, there is much new and unpublished information here. The volume's balance of authorship is superb, providing a variety of distinct perspectives on the issues. There are a number of minor errors (references missing from the bibliographies, grammatical and spelling errors) which are sometimes distracting, but probably par for a volume with so many (25) contributors.
As central as the issues in this volume are, however, community ecology is currently concerned more with theory, experiment, and mechanism than it is in searching for assembly rules. Widespread outbreaks, extinctions, and invasions and the urgent need to understand, manage, and predict the ecological consequences of these many changes in D&A, preoccupy us. Diamond himself, the editors note, could not contribute to the volume due to his busy field schedule. Nevertheless, Ecological assembly rules is a valuable contribution to community ecology, drawing attention to the challenges and utility of searching for nature's rules of assembly.
|Printer friendly Cite/link Email Feedback|
|Article Type:||Book Review|
|Date:||Jun 1, 2000|
|Previous Article:||DIVERSITY OF RESEARCHERS.|
|Next Article:||OPTIMAL DEFENSE THEORY PREDICTS THE ONTOGENY OF AN INDUCED NICOTINE DEFENSE.|