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Chemical warfare between plants and their herbivores and pathogens was first brought to our attention by the publication 25 years ago of the paper by Fraenkel in Science. There, he pointed out that most plants have similar nutritional characteristics so that the selection of plants by insect herbivores must depend on the relative toxicity of secondary compounds. This led, rather gradually, to a host of papers on plant-herbivore interactions. More or less at the same time, insect physiologists and ecologists were starting to realise the importance of chemical communi­ cation systems in determining sexual and other characteristics of insect behaviour. Nine years ago the Phytochemical Society of North America published their Symposium on'Biochemical Interaction Between Plants and Insects'in which the plant apparency theory was expounded by both Paul Feeny and Rex Cates and David Rhoades. This stated that plants which are apparent usually contain secondary components which reduce digestibility (tannins and lignins) while ephemeral plants have more toxic, and perhaps less costly, compounds such as alkaloids. These papers stimulated much research on biochemical ecology. The recognition of the importance of the biochemical factors in such interactions is not just of scientific interest. It is vitally important in programs for the production of new varieties of cultivated plants, especially in tropical countries where about one-third or more of the crops are lost to predation or disease.

Diversity within and among living organisms is both a biological impera­ tive and a biological conundrum. Phenotypic and genotypic diversity is the critical currency ofecological interactions and the evolution of life. Thus, it is not unexpected to find vast phytochemical diversity among plants. However, among the most compelling questions which arise among those interested in ecological phytochemistry is the extent, nature, and reasons for the diversity of chemieals in plants. The idea that natural products (secondary metabolites) are accidents of metabolism and have no biological function is an old one which has resurfaced recently under a new term'redundancy.'Redundancy in the broader sense can be viewed as duplication of effort. The co-occurrence of several classes of phytochemieals in a given plant may be redundancy. Is there unnecessary duplication of chemical defense systems and ifso, why? What selective forces have produced this result? On the other hand, why does the same compound often have multiple functions? At a symposium of the Phytochemical Society of North America held in August 1995, in Sault Ste. Marie, Ontario, Canada, the topic'Phytochernical Redundancy in Ecological Interactions'was discussed. The chapters in this volume are based on that symposium. They both stimulate thought and provide some working hypotheses for future research. It is being increasingly recognized that functional diversity and multiplicity of function of natural products is the norm rather than the exception.