Since Fraenkel (1959) recognised that plant secondary metabolites (PSMs) were not simply plant waste products but served to defend them against insect herbivores, numerous ecological roles for these intriguing compounds have been established, notably as defences against a broad range of herbivores and pathogens, but also as mediators of interactions with competitors and mutualists, and as a defence against abiotic stress. A single compound can influence multiple components within an ecological system, and can have effects that act across many different scales. Add to this the huge diversity of PSMs that have now been characterised, and the possible interactive effects among them, and it is clear that PSMs either individually or as groups can no longer be considered only in the context of interactions between the plant and a single other species. They are now recognised as major contributors to the bridge between genes and ecosystems, because (context-dependent) gene expression patterns influence the phenotype of a plant. The effects of PSMs are now known to affect community dynamics and to cascade through ecosystems, driving their composition and function and acting as agents of their evolution (e.g. Whitham et al., 2006). Here, we summarise the key points and emergent themes from the chapters in this book and provide a synthesis of the recent developments in the ecology and evolution of PSMs, illustrating how a range of approaches, including molecular, transgenic and metabolomic techniques, have brought us to the cusp of a new understanding of their integrative roles in ecosystems.
Distribution, allocation and evolutionary selection for PSMs
The chemical diversity of PSMs, combined with the number and complexity of potential biotic and abiotic interactions in which they are involved, has hitherto prevented these systems being predictable beyond the outcome of the strongest, pairwise and most well defined of these interactions. However, several recent developments are moving us towards a better understanding and predictability of the roles of PSMs in more complex systems.