Plant–pollinator interactions: from specialization to generalization
Waser NM, Ollerton J. eds. 2006.
Chicago: Chicago: The University of Chicago Press. $45 (paperback). 488 pp.
Email Klinkhamer{at}RULSFB.LeidenUniv.nl
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Ten years later, Nick Waser and Jeff Ollerton have brought together a wide range of experts in the field to discuss patterns of specialization and generalization in a book that covers the fields of ecology and evolutionary biology and reports on studies at the level of single species, communities and geographic regions. The core of the book consists of three parts, one on the ecology and evolution of specialized and generalized pollination, one on community and biogeographic perspectives, and one on applications in agriculture and conservation. Each part consists of several chapters that are mostly written as reviews that report not only on recent findings but often also address methodological problems and exciting new questions that are waiting to be answered. The reader is guided through the 18 chapters of the book by an introductory part that puts the question of specialization versus generalization in a historical perspective, and by a closing chapter that puts specialization in the broader perspective of mutualistic interactions in general. Each of the three main parts of the book (2–4) begins with introductory comments by the editors in which they draw attention to the most important questions and findings that are addressed.
Part 2 on ecology and evolutionary biology of specialized and generalized pollination reveals among other things that most ecologists now believe that plant–pollinator interactions are generalistic while evolutionary studies report on selection towards stronger specialization. How can these different views be reconciled? A possible answer is given in the chapter by Aigner, who discusses trade-offs and explains that special adaptations need not come at the expense of general utility. He elegantly uses an example from a completely different field, the cichlid fish fauna of the great lakes of East Africa to illustrate this. To me another example came to mind. For a long time research on the evolution of plant defence systems focused on the impact of specialized herbivores, while we now know that in many cases generalist herbivores may be responsible for the evolutionary trends that we can observe in these systems. Although generalist herbivores may have a much smaller effect on plant fitness than specialists, they may present a stronger selective force. Likewise, it is not necessarily the most effective pollinator that exerts the strongest evolutionary force. The take-home message from this part of the book is that we have come to the realization that some of our old ideas, such as co-adaptation resulting from tight reciprocal specialization or lack of adaptation resulting from extreme generalization, may have to be replaced by more elaborate concepts including adaptive generalism, asymmetric specialization and floral adaptation to uncommon or relatively ineffective floral visitors.
Part 3 mainly deals with community-level studies whereby communities are represented as a network of interactions. The main finding perhaps is that specialization is asymmetric. Specialized pollinators interact with generalized plants and generalized pollinators interact with specialized plants. Although this pattern had been shown in many plant–pollinator communities by now, surprisingly little seems to be known about the ecological and evolutionary processes leading to this pattern. A valuable aspect for the reader of this part is the emphasis on new statistical techniques that can be used to analyse community data and on methodological problems, for instance in data collection. Because only a part of the actual interactions between individuals will be observed in any study, rare species will seem relatively specialized and common species relatively generalized. In addition, there is a higher chance of observing a rare pollinator only on a common plant even though it may visit rare plants as well. Such sampling bias may contribute to or even cause the observed asymmetry, as explained in the chapter by Vazquez and Aizen. The final chapters of this part discuss whether communities in the tropics are indeed more specialized than those in temperate and artic regions.
Part 4 asks what lessons we can learn from the study of pollination biology for conservation of biodiversity. While this is of course an extremely important question and, some might say, perhaps even the reason why we want to know it all, this was for me from a scientific point of view the least interesting part. One of the lessons to be learned is that a functional classification of flowers and pollinators is more useful from a conservationist point of view than a taxonomic one. In another chapter the vulnerability of plants is discussed in a broader perspective, including life-history characteristics and seed dispersal and longevity. Studies on the effect of habitat isolation and fragmentation discuss the extinction risks for specialist and generalist plants, a matter of debate in recent literature.
The editors have aimed at producing a book that would interest those established in pollination biology, those entering the field, and those in allied fields. I guess time will show that they have succeeded in doing so. The book will not only prove to be a handy reference but it will also prove to stimulate new research by showing how recently developed tools, such as monte carlo simulations using null-models, have enabled researchers to address questions that for a long time have been inaccessible. After reading this book, it once more became clear to me how little in fact we know about the ecology and evolution of plant–pollinator systems. This book provides good suggestions on how we should proceed from here on, and presents an optimistic view of the rapid progress we can potentially make.
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