© 2001 by The Society for Integrative and Comparative Biology
An Integrative Approach to the Study of Plant-Animal Interactions1
1 Department of Biology, University of Richmond, Richmond, Virginia 23173
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INTRODUCTION |
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I am tempted to give one more instance showing how plants and animals, remote in the scale of nature, are bound together by a web of complex relations.(Darwin, 1859
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Ecologists have been studying the interactions between plants and animals for as long as there have been ecologists. Charles Darwin studied the complex web of interactions even before he wrote his revolutionary book On the Origin of Species. Indeed, even his grandfather, Erasmus Darwin, was keenly aware of these interactions; he earned a small fortune for the family by writing poetry about the love lives of plants, accomplished through their relationships with animals (Gould, 1977).
The study of plant-animal interactions has played a key role in the development of ecological theory. I give two examples here. Hairston et al. (1960) began with a particular view of the relationship between plants and their herbivores. This work inspired a generation of theory and empirical work on the structure of ecological communities, which helped define ideas of "top-down" and "bottom-up" effects. The concept of coevolution was first developed to describe the results of the relationship between particular plant species and the herbivorous insects that feed on them (Erlich and Raven, 1964). It has since been widely applied to describe the evolutionary results of a wide variety of ecological relationships between two or more biological entities (Futuyma and Slatkin, 1983).
However, the study of plant-animal interactions has only recently begun to define itself as a distinct subdiscipline. I searched for publications from 1980 to the present listing plant-animal or plant-insect interactions as a keyword in Biological Abstracts (Fig. 1). In the early 1980s there were generally fewer than five such publications per year. That number has grown more or less steadily (with a few dramatically high years). The end of the 1990s generally saw more than 20 publications per year that explicitly listed plant-animal interactions as a keyword. There were 60 such publications in 2000, and for the first half of the year 2001, there are already 26 citations listed.
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While the field of plant-animal interactions has begun to define itself, it has also diversified, with specializations reaching into realms far beyond the earliest concerns of community ecology. Today, biologists studying plant-animal interactions may employ molecular techniques to estimate phylogenies of coevolved plant and animal lineages. They may be plant or animal physiologists, studying the physiological and biochemical mechanisms of adaptations and counteradaptations. Some are behavioral ecologists, and some are geneticists. And of course many are ecologists, struggling to understand how these interactions shape the living world we study. This symposium includes speakers representing each of these specialized subdisciplines.
The intent of this symposium is to reconnect these specializations and to foster an integrative approach to the study of plant-animal interactions. The effort to develop the methodogical and conceptual skills to pursue our questions leaves the field increasingly fragmented. The fragmentation tends to constrain our interactions along methodological or taxonomic lines (e.g., recent symposia on molecular techniques, mammal-plant interactions). This symposium brings together biologists with different conceptual approaches, methods, and taxa, helping all of us to see the relationships between our studies. By its name and nature, the Society for Integrative and Comparative Biology is the most appropriate place to realize our objectives of fostering interactions, integrations and collaborations across the subdisciplines.
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OVERVIEW OF THE SYMPOSIUM |
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This collection of papers from the symposium begins with three studies of community-level relationships. Clay considers a new angle on the control of community structure. In addition to top-down and bottom-up effects, he reports evidence that the "inside-out" effects of microbial symbionts of plants are also critical in determining community structure. Bronstein follows with a study of the costs of mutualism in the yucca-yucca moth interaction. While students of plant-animal interactions have long studied the benefits of mutualisms, the costs have been understudied, and may be as important in shaping the ecology and evolution of these relationships. Smallwood et al. present a study of another system that is usually regarded as mutualistic: squirrel dispersal agents for oaks. However, squirrels are far more effective as dispersal agents for one of the two main groups of oak species. Smallwood et al. test alternative hypotheses for the ultimate advantage to the animals for their habit of discriminating between different species of oaks. Steele et al. continue the study of squirrels and oaks. They determine the proximate cue used by squirrels to distinguish acorn species more suited for storing from more perishable acorns.
While there may be many mutualistic relationships between plants and animals, often plants are more the victims rather than the beneficiaries of the relationship. Plants have developed a number of defenses against their consumers, and these defenses have ecological and evolutionary consequences for both plants and animals. Becerra et al. begin with a study of the interactions between chemical and mechanical defenses in the plant genus Bursera. These interactions not only influence the way Bursera's herbivores feed on Bursera; they also influence how these herbivores defend themselves against their own predators. Folgeman and Danielson present an overview of the cactus-microorganism-Drosophila system. Drosophila breed in wounds in living cacti. The cacti produce a variety of alleochemicals which influence Drosophila spp. directly, and indirectly through the bacteria and yeast living in the same wounds.
As Fogleman and Danielson report, understanding the effects of plant chemicals on animals requires physiological study. Continuing in a physiological vein, Dearing et al. examine the diuretic effect of plant defensive chemicals from plants, which may be a particularly effective defense for plants living in arid environments. They find the effects are more pronounced for a generalist herbivore than for a closely related specialist herbivore. In contrast, Martinez et al. examine the physiology of nectar digestion by nectar feeding birds, where the problem is too much water rather than too little. They find that birds are better able to discriminate differences in nectar concentrations at relatively low concentrations than at high concentrations. They relate this behavior to the physiology of nectar digestion, and in turn to the evolution of nectar concentrations in flowers.
The paper by Martinez et al. provides a bridge to the consideration of coevolution. In developing the concept of coevolution, Erlich and Raven (1964) assumed that herbivores exert selection pressure for plant defenses, and that these defenses have fitness costs. In a detailed study of a single species, Mauricio confirms these assumptions for the mouse-ear cress, an annual plant. Finally, Abrahamson et al. present their study of goldenrod host plants, gall-forming flies, and stem and gall-boring beetles. They find two distinct races of the gallfly, due to the adoption of a new host species of goldenrod. This in turn created a novel resource for a stem-boring beetle species living on the newly-adopted host plant. Abrahamson et al. show that stem-boring beetles that use the galls are genetically distinct from those using the stems of the goldenrod. Their work suggests that sympatric speciation may be an important contributor to biodiversity.
I note that these papers could have easily been grouped very differently. Most of the papers would fit well in more than one session because of the nature of plant-animal interactions. It is that interdisciplinary nature that makes integrative symposia important. I hope the speakers and participants found the symposium as inspiring and enjoyable as I did. This is not the first collection of papers on plant-animal interactions, and I am sure it will not be the last. However, we can hope that this collection of papers will contribute to the development of an integrative approach to the study of plant-animal interactions.
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ACKNOWLEDGMENTS |
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I thank the speakers and their institutions for supporting their participation in this symposium, as well as the research they present here. I also thank the Society for Integrative and Comparative Biology for their generous support of this symposium and for choosing this topic as a Society-Wide symposium. I thank May Berenbaum for her insight and help in organizing the symposium, and regret that she was unable to participate. I thank Dianna Padilla and Kathy Van Alstyne for their advice and support during the symposium, and for joining their symposium on Marine Plant-Animal interactions with this one on terrestrial interactions, and for their generosity in sharing their financial support for the joint symposium.
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FOOTNOTES |
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1 From the Symposium An Integrative Approach to the Study of Terrestrial Plant-Animal Interactions presented at the Annual Meeting of the Society for Comparative and Integrative Biology, 5–8 January 2000, at Atlanta, Georgia.
2 E-mail: psmallwo{at}richmond.edu
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References |
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TOP
INTRODUCTIONOVERVIEW OF THE SYMPOSIUMReferences |
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Darwin, C. 1859. On the origin of species, 1st ed. John Murray, Publishers. London.
Erlich, P. R., and P. H. Raven. 1964. Butterflies and plants: A study in coevolution. Evolution, 18:586-608.[CrossRef][ISI]
Futuyma, D. J., and M. Slatkin. 1983. Introduction to coevolution. In D. J. Futuyma and M. Slatkin (eds.), Coevolution, pp. 1–13. Sinauer Associates, Sunderland, Massachusetts, U.S.A.
Gould, S. J. 1977. Ever Since Darwin. W. W. Norton and Company, New York.
Hairston, N. G., F. E. Smith, and L. B. Slobodkin. 1960. Community structure, population control, and competition. Am. Nat, 94:421-425.[CrossRef][ISI]
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