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Discontinuities in Ecosystems and Other Complex Systems . Allen, C. R., and C. S.Holling , editors . 2008 . Columbia University Press , New York , NY . 288 pp. $84.50 (hardcover). ISBN 978-0-231-14444-5 . $34.50 (paperback). ISBN 978-0-231-14445-2 .

Discontinuities in Ecosystems and other Complex Systems is another interesting entry in the resilience literature. In fact, it is largely a book-length treatment of some ideas from a 1992 paper by one of the editors, C.S. (Buzz) Holling (Holling 1992), an enormous and influential review paper that seems to have spawned a review-paper industry. Unfortunately, it has not spawned a large body of empirical tests of its ideas. The general thrust of this creative original paper is that there are body size “clumps” or “gaps” or “discontinuities” in ecological communities and that these are caused by processes that entrain the space and time over which the organisms make a living. It is a very satisfying idea: ecological communities can be circumscribed into clumps of body sizes, and these clumps represent groups of organisms whose life histories are constrained by similar spatial and temporal processes in the landscape. Holling dubbed this the textural discontinuity hypothesis (TDH).

Two questions arise. What if these ideas are correct? If they are, we have a wonderful, synthetic version of how the world works and how it can be managed. In many situations, people have already assumed these ideas are correct. What, however, if they are wrong? Then this simple and elegant way of viewing nature is just a construct of our own desire to explain things and to be able to control them based on our explanations. If we are wrong about the repeatability of these patterns and processes, many of the techniques we hold dear as conservation biologists, like resilience, may be inappropriate tools. Understanding and supporting these ideas seems very important to me for conservation as a whole.

The potential utility and theoretical elegance of Holling's original ideas presents a challenge to the authors involved in developing them: demonstrate that these patterns and processes are true, formalize the ideas in a (testable) mathematical way, and engage experimental and empirical ecologists in their testing. But where are these tests? Why have only one or two papers critical of the lumpy-world hypothesis discredited (or at the very least, dramatically slowed) the continued analysis of these ideas for research ecologists (Manly 1996; Siemann & Brown 1999)? It seems to me that Holling's explanations of how the world works are among the most important to demonstrate and empirically support of all ecological questions. From a conservation biology point of view, the patterns and processes explained by the TDH are very useful simplifications in complex systems. If a number of very complicated processes across multiple scales can be reduced to repeatable and expected patterns, then very difficult problems in multiple systems may have similar scale-specific processes that can be managed with similar approaches. Simplifying management is of course a truly desirable outcome and is at the heart of much of Holling's best-known work in resilience, adaptive management, and cross-scale processes. As stated in the book, “[b]y understanding these cycles and their scales, it may be possible to evaluate their contribution to sustainability, to identify the points at which a system is capable of positive change, and to indicate the points where it is vulnerable. These leverage points may theoretically be used to foster resilience and sustainability within a system” (pp. 5–6). Desirable indeed!

I like to keep up on the “discontinuities and resilience” literature to see whether many of the things I am practicing or teaching as a conservation biologist are in fact strongly supported. I therefore believe that books like this one are of interest to a broad conservation biology audience. Discontinuities in Ecosystems and other Complex Systems contains 12 chapters, organized into three sections—“Background,”“Patterns,” and “Consequences.” The “Background” section focuses on a quick review of some past discontinuities research and describes results that suggest how complex systems may discontinuously organize (i.e., into what are variously referred to as discontinuities or gaps and their lumps, clumps, or aggregations). The “Patterns” section reviews some patterns of body size and other ecosystem discontinuities in nature and how these discontinuities might be detected. The third section, “Consequences,” focuses on the potential significance of these discontinuities, should they exist.

“Patterns” does not examine only the body sizes around which Holling's TDH was conceived. Patterns of landscape structure are described with analyses of what are referred to as “keystone” structural features, including tree island size, water stage, and temperature in the Everglades (“Biophysical Discontinuities in the Everglades Ecosystem” by L.H. Gunderson). Range sizes of North American birds and butterflies are examined with the TDH in mind, with the general finding that discontinuities in range size exist for both birds and butterflies, consisting of several major “zooregions” (i.e., clusters of bird or butterfly species that exist at the continental scale—“Discontinuities in the Geographical Range Size of North American Birds And Butterflies” by C. Restrepo and N. Arango). In general, some birds and butterflies have restricted ranges, and a smaller number have large ranges. In other chapters, these ideas are extended beyond strict ecological systems, and discontinuities in urban systems or city processes are discussed and examined.

The thoughtful final chapter in the “Patterns” section summarizes many of the ideas behind how well the TDH works. It is effectively a plea for more thoughtful research on the TDH. Here is a main message for the ideas in this book: more testing is necessary to refine and enhance the conservation utility of the TDH (“Evaluating the Textural Discontinuity Hypothesis: a Case for Adaptive Inference” by C.A. Stow, J.P. Sendzimir, and C.S. Holling).

The final section of the book, “Consequences,” evaluates what the earlier described body size and landscape patterns, processes, and hypotheses may mean for understanding and management of natural systems. These evaluations are ultimately what I think most readers will be interested in—what are some examples of how the existence of discontinuities can be used in applied settings? In particular, J.J. Skillen and B.A. Maurer, in their chapter “The Ecological Significance of Discontinuities in Body-Mass Distributions,” provide a synopsis of different techniques used to test for body-mass discontinuities. They present evidence of gaps in body sizes in the data set examined and provide a summary of a possible landscape mechanism that could generate these body size gaps. These gaps and landscape mechanisms are important results, and two consequences are predicted. First, when landscapes change at specific scales, the body-size distributions of organisms in that landscape change. Second, body-size distribution changes within a community may indicate changes in the landscape. These body-size and landscape change results may be used as assays to monitor and evaluate populations and landscapes, a very useful conservation tool.

The book concludes with an excellent and thoughtful synthesis, written by Donald Ludwig. If a reader is considering buying this book but has not yet made up his or her mind, I recommend looking at Ludwig's chapter. If a person is interested in what he has to say, then he or she will likely want to fill in the details by reading the other chapters. In particular, Ludwig describes communications between himself and some researchers who might be described as cautious or skeptical about some of Holling's ideas. The correspondence between Ludwig and others really addresses a fundamental argument in ecology and conservation biology (and one that is discussed frequently in my own lab). That is, do we stop with recognition of patterns or must we identify the processes behind them? If there are repeatable patterns, then conservation inputs can target these patterns. With understanding of the processes behind the patterns, however, conservation biologists can make predictions and target root causes—a much stronger and deeper response to crises. Ludwig describes the difficulty of creating and interpreting tests of complex patterns and ends with a call to “…avoid ‘capricious’ interpretations based on our built-in tendency to see pattern everywhere” (pp. 238–239) and continue the “never-ending attempts to winnow out structures and processes that underlie the patterns” (p. 239). It is great advice.

The book is an interesting read for two groups—those already working in this field (although I sometimes feel most of those people have chapters in this book) and research ecologists who would like to explore in detail the ideas behind discontinuities and resilience. People in the first group do not need this book because they already know what is in it. It would be a wonderful and important contribution to conservation biology and ecology if more researchers from outside the resilience group would start testing the ideas in this book. I really encourage more ecologists, especially beginning ones looking for thesis topics, to consider reading part or all of this work. The ideas are intriguing and need better empirical support. I am not convinced this is currently happening. I work in a school explicitly devoted to resource and environmental study and management, and terms like resilience, panarchy, and cross-scale processes are thrown around as if they have already been demonstrated to be true—true not just in a few examples or systems, or even as heuristic models, but true universally. This is not only a testament to the strength and seductiveness of Holling's ideas, but also a touchstone for their problems. Without an empirical grounding, they are, at best, (vague) theories to be tested, rather than blanket prescriptions for ecological and management problems. I hope this book encourages more researchers to examine and add to these ideas so that the next time a book of this nature is published the list of contributors will extend well beyond the usual suspects.

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