© 2003 by The Society for Integrative and Comparative Biology
The Importance of Species: Perspectives on Expendability and Triage
1 Department of Ecology, Evolution, & Natural Resources, 14 College Farm Road, Rutgers University, New Brunswick, New Jersey 08901 E-mail: pjmorin{at}rci.rutgers.edu
The Importance of Species: Perspectives on Expendability and Triage. P. KAREIVA AND S. A. LEVIN. (EDS.) Princeton University Press, Princeton NJ, 2003. 427 pages.
Robert T. Paine is synonymous with the experimental approach to community ecology. Throughout his long and eminent career Paine has advocated the power of elegantly simple field experiments in illuminating the complex interactions that produce striking patterns in ecological communities. His influence helped to transform the discipline from a largely comparative hypothetico-deductive approach to a rigorous and more objective science. The authors of the chapters in this edited volume, a diverse group of Paine's friends, students, and colleagues, have marked his retirement with a series of papers that explore a recurring theme in Paine's life work, the special importance of particular species interactions in creating community patterns.
The central questions considered in this book are when, how, or even if the impacts of species extinctions or introductions can be reliably predicted. These questions are motivated in turn by whether any species can ever be considered "expendable," in terms of its contribution to the myriad workings of natural systems. The question of expendability figures prominently in the recent body of controversial research that attempts to establish links between biodiversity and ecosystem functioning. Two other recent books by Kinzig et al. (2001)
and Loreau et al. (2002) provide useful summaries of these views, and they are essential companion reading. The approach taken in most biodiversity and ecosystem functioning studies is distinctly different from the research program advocated by Paine, in that it glosses over much of the detailed natural history of species interactions, in hopes of finding tractable short cuts to predict the consequences of species loss. The short cuts used as possible predictors of functioning are typically aggregate properties of communities, usually the diversity or richness of species. Such measures obviously ignore the identity of individual species, and focus instead on emergent properties of communities. The point of contention is whether such aggregate measures provide reliable insights into how complex natural communities will function, or whether functioning depends critically on the idiosyncratic effects of particular species. The chapters of this book provide a valuable overview of the controversy.
As is often the case, some contributions to edited volumes are more successful or novel than others, but most chapters in this book provide insights into the ways that critical species properties can be assessed. Approaches taken include surveys of historical accounts of the consequences of species extirpations (Simberloff), which in their incompleteness provide a compelling argument for the value of long term ecosystem monitoring. Other case studies of interactions between native and introduced species (Louda and Rand, Harley, Ruesink) highlight the complexity of predicting the positive and negative effects of species introductions. Modeling efforts address several issues, including how many subpopulations of salmon can be lost before the species goes regionally extinct (Ruckelshaus et al.). Three chapters address the species importance—species richness controversy head on, using different approaches. Wootton and Downing point out that hybrid experiments that combine manipulations of individual species and overall species richness can unambiguously show the extent to which effects of species are largely idiosyncratic or predictable from averages. Naeem expands on a previous approach using reliability theory to explore how the degree of interdependence of species extinctions can influence the predictability of ecosystem processes, while pointing out the important complementarity between community and ecosystem approaches. Doak and Marvier also extend a previous modeling effort to assess how purely statistical processes can confer stability on community properties as species richness increases.
Surprisingly, the message of a number of these chapters is that species can often be lost from systems without clear effect. Losses of mutualists (Morris), producers or consumers (Simberloff, Schindler et al.), and specialist herbivores (Root), either have no clearly documented keystone consequences or are predicted to be relatively inconsequential. This is tempered by the observation that with each loss, an irreversible change in natural diversity occurs, an event which has striking ethical and esthetic consequences regardless of its measurable impacts on ecosystem responses (Leigh). Other contributions review longstanding research programs to remind readers that traditional field removal experiments have much to say about the context-dependence and asymmetric consequences of species loss (Monge, Schoener and Spiller).
Certainly one of the best ways to evaluate a species' importance is to remove that species and observe what happens to the remaining community. Menge suggests that we need more large scale, long term research requiring "new funding, new partnerships, new approaches, cooperation across disciplines, new technologies and methodologies." Though this would indeed improve predictive capacity as he notes, the striking current and growing loss of species argues for a swifter attempt at predicting the outcome of species loss in order to influence the potential preservation of still intact whole ecosystems. It is this dilemma that suggests that aggregate measures of community responses to species loss, since they are faster to assess than measuring all possible combinations of effects of individual species removals, may indeed provide that predictive power for a discipline under pressure to produce results.
Collectively, the chapters in this book make clear that natural communities are marvelously complex, in ways that are devilishly difficult to predict from natural history alone and only become apparent from the kinds of experiments that Robert Paine has championed. While experiments have become essential to evaluate the importance of particular species in various contexts, it is equally clear that individual experimental studies of the 3–30 million extant and incompletely described species do not provide a realistic way to evaluate whether any species can be (regrettably) consigned to oblivion. This volume makes clear that the unfinished task of community ecologists is to find tractable ways to predict the impacts of species loss. The very real risk is that the extent of extinctions wrought by our own ignorance will make communities irreversibly simpler, before the full value of their original complexity can be understood.
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Kinzig, A. P., S. W. Pacala, and D. Tilman.(eds.) 2001. The functional consequences of biodiversity: Empirical progress and theoretical extensions. Princeton University Press, Princeton, New Jersey.
Loreau, M., S. Naeem, and P. Inchausti.(eds.) 2002. Biodiversity and ecosystem functioning: Synthesis and perspectives. Oxford University Press, Oxford.
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