Two May Be Company but Three Is Seldom a Crowd: Allee Effects in Ecology and Conservation

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Allee Effects in Ecology and Conservation. Courchamp, F., L. Berec, and J. Gascoigne . 2008 . Oxford University Press , New York , NY . 256 pp. $90.00 (hardcover) . ISBN 978-0-19-857030-1 .

In a characteristically perceptive cartoon, Gary Larson depicts a dinosaur with baseball hat, bat, and glove, looking dejectedly at a row of caves, to each of which has been affixed a notice reading “Sorry, extinct.” As with many Larson cartoons, the anthropomorphic humor is tinged with poignancy: to whom do the last surviving members of any group of animals turn for recreation or, more importantly, procreation?

In contrast to negative density dependence, the processes and implications of positive density dependence have, until recently, received relatively limited scrutiny. Over the past 15 years, however, that situation has changed dramatically. Such has been the explosion of interest in positive density-dependent processes, that it is increasingly difficult to keep track of developments. This new book aims to remedy that situation by bringing together in one place, the historical development of this field, together with the causes, consequences, breadth of influence, and new directions in the study of positive density dependence.

Negative density dependence has long been regarded by ecologists as a fundamental feature of population dynamics. This focus arose from the initial preoccupations of ecology including, critically, the search to understand the regulation of populations and the factors preventing their indefinite increase. In the later half of the last century, the attention of ecologists (and especially conservation biologists) turned increasingly toward the processes of population decline and the problems of small populations (Caughley 1994). In that context, focus shifted away from the factors that prevent large populations from getting larger, to factors that might push small populations to become smaller (the “extinction vortex” of Gilpin and Soulé[1986]). As Courchamp et al. document, this shift in focus led to a new appreciation of the long-neglected work of Warder Allee. In contrast to the prevailing focus, Allee had spent much of the first half of the 20th century investigating the possible benefits of animal aggregations and the disadvantages of being few (e.g., Allee 1931, 1938). This led Odum (1953) to refer to the benefits of aggregations or higher densities as the “Allee principle.” They are now more commonly referred to as Allee effects.

Much of the recent surge in interest in Allee effects can be traced to the thorough and comprehensive work of Brian Dennis. Dennis (1989) summarized the history of and evidence for Allee effects, illustrated why they are expected and how they can be modeled, analyzed their consequences for conservation (especially in relation to harvesting), and discussed their pervasive effects in “virtually all areas of population biology,” from community dynamics to evolution and population genetics. Nearly 20 years since the appearance of Dennis' publication, it is timely to consider how the field has grown, what we have learned, and what remains to be studied. These are the challenges of the new book by Courchamp, Berec, and Gascoigne, authors (over the last decade) of a score of papers on the Allee effect.

Pleasingly laid out and entertainingly written, Courchamp et al.'s new book is a highly comprehensive review of the subject. Despite tackling some thorny issues to do with terminology and definitions, the introductory chapter is engaging and clear. It is followed by chapters reviewing the wealth of mechanisms that could lead to Allee effects and the manifold ways in which modeling has been used to explore these potential causes and their consequences. The difficulty of demonstrating Allee effects (especially demographic Allee effects, manifested as positive density-dependent mean population growth) empirically means that much of what we know about their likely consequences is derived from population models. Unsurprisingly therefore, the chapter on modeling is perhaps the most comprehensive, accounting for almost one-third of the book's length. On balance, this chapter does a good job of synthesizing a huge amount of material. Nevertheless, given its length, complexity, and necessary importance, it is perhaps a shame that the authors chose to restrict it to a single chapter. Several distinctions could have been further emphasized to merit separate chapters; for example, the distinction between purely exploratory theoretical models and models based on real systems or between models that assume Allee effects at the outset and those from which Allee effects arise as emergent consequences of simpler assumptions.

The loss of genetic diversity in smaller populations is well established but, perhaps more importantly, in chapter 4 the authors also consider the evolutionary consequences of Allee effects. Here, the profound implications of Allee effects are brought home: many aspects of behavior, morphology, and endocrinology have evolved to deal with low-density situations and to facilitate aggregation, cooperation, and mate finding. In this respect it is hard to overemphasize the significance of Allee effects in driving evolution. Despite this importance, the backdrop to renewed interest in Allee effects is conservation and management, and in chapter 5 Courchamp et al. consider the implications of Allee effects for these fields. Conceding the difficulties of demonstrating Allee effects unequivocally, they nevertheless advocate a precautionary approach: the burden of proof should be on those claiming that Allee effects play no role in a given system. This is an intriguing point that may well prove contentious in the context of model parsimony. It also remains to be seen how modelers will parameterize a process that is often so hard to detect—a point in favor of mechanistic models from which Allee effects may or may not emerge. They conclude with a concise exposition of the characteristics that might indicate the vulnerability of a population to Allee effects, together with a set of specific management areas for which this knowledge might have implications.

Although the book's final chapter is entitled “Conclusions and Perspectives,” the emphasis is definitely on the latter. There is relatively little recapitulation (as, in any case, the vast bulk of the book is a review of existing material). Instead, the authors finish by tackling a number of issues to do with the broader implications of Allee effects (e.g., in other disciplines or larger-scale ecological processes such as ecosystem shifts) and define areas of the subject they feel are ripe for further exploration. This is the most individual aspect of the whole book, and, regardless of whether the reader agrees with the authors, is bound to stimulate thought and discussion.

Unquestionably, this book represents a thorough review of published material on Allee effects and will be of great use as a resource for anyone looking for examples within their system of interest, approaches to modeling a given phenomenon, or rapid digests of previously published papers. In this regard, however, it is disappointing that the contents and index are not more comprehensive and that there is no index for the cited publications. To consider whether the book has greater value than as a review alone, it is valuable to consider the advances it presents in the context of three areas that have long undermined the utility of the Allee effect construct.

First, a lack of clarity has weakened the term Allee effect (Stephens et al. 1999), proliferating its meanings and making some hesitant to use it. I was pleased to note the authors' efforts to tighten language describing the Allee effect, endorsing the use of Haldane's “positive density dependence” (Haldane 1953), in preference to the more confusing descriptor “inverse density dependence” (which, at best, serves only to exaggerate the overriding importance of negative density dependence). One area that did disappoint me, however, was that the authors did not do more to dispel the notion that Allee effects may only be important when the population is so low as to be already inviable. Although they state that, “The behaviors and ensuing dynamics that generate an Allee effect when there are few conspecifics … shape its life history traits all along the spectrum of population sizes or densities” (p. 15), they also note that Allee effects “by definition occur only over a small range of population densities or sizes” (p. 190). They may be talking about different phenomena in the two instances (component vs. demographic Allee effects, respectively), but, nonetheless, it is important to emphasize that—in theory at least—Allee effects can play an important role over a broad range of population sizes.

Second, a common criticism of any preoccupation with Allee effects is that their occurrence (at least at a demographic level) is poorly supported by empirical data (e.g., Liermann & Hilborn 1997; Gerber & Hilborn 2001). Unfortunately, a critical evaluation of the frequency of occurrence of demographic Allee effects in natural populations is unlikely to be achievable. Estimating mean per-capita growth rates requires many data points, especially when population size is low and growth is subject to high levels of demographic stochasticity. Unfortunately, if populations are subject to Allee effects, the type of long-term data required to demonstrate those effects are unlikely to be available, precisely because stochasticity dictates that the population is unlikely to remain for long in the zone of low or negative population growth rates. Moreover, any attempt to survey across studies of natural populations is likely to be subject to strong biases: on the one hand, researchers are only likely to mention Allee effects if they are observed; on the other, populations for which meaningful data can be collected at low population densities or sizes are probably those least susceptible to Allee effects. In the face of uncertainty regarding the ubiquity of demographic Allee effects, Courchamp et al. can do little but suggest methods by which those effects can be identified. Certainly, this remains an area in which much more work is required if Allee effects are to be taken seriously as a fundamental conservation problem.

Last, there are those who question whether knowing that a population is likely to suffer from an Allee effect makes any genuine difference to management. We know that populations should not be allowed to become too small (owing to the dangers of demographic stochasticity, natural or anthropogenic catastrophes, and genetic threats). What difference does it make if one labels some of these threats Allee effects or identifies one population as more likely to be vulnerable to Allee effects than another? In response, Courchamp et al. stress the importance of acknowledging the potential for Allee effects, especially in the modeling that underlies so much of modern conservation and management. They point to four areas in particular, in which an acknowledgment of Allee effects is likely to have significant implications for management, including target setting for minimum viable populations (MVPs) and strategies for exploitation, reintroduction (or, conversely, pest control), and metapopulation management. Nevertheless, without strong evidence that Allee effects become important at greater population sizes than other types of threats, have led to a significant number of extinctions, or significantly alter estimates of MVPs, it will be hard to convince skeptics that an appreciation of Allee effects is essential for practical conservation and management.

The last 15 years have seen a rapid increase in the number of studies that have considered positive density dependence, its causes and consequences. Since the work of Dennis (1989) there have been few significant conceptual advances but much fleshing out of the detail in our understanding. Bringing together the available material in one comprehensive and highly readable review is a major achievement and has resulted in a resource that should be of great value to students, researchers, and managers alike. In addition, Allee Effects in Ecology and Conservation should act as a springboard for a new approach to studying Allee effects and, in particular, a renewed focus on the essential questions: how common are Allee effects and what are their practical (rather than theoretical) implications for targets and strategies in conservation and management?

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