A central question in the ecology and evolution of plant-animal interactions is why do plants produce so many different chemical compounds and how does that chemical diversity shape interactions between plants and animals? An abundance of high-profile hypotheses, such as Ehrlich and Raven’s coevolution hypothesis, have been proposed to explain patterns of phytochemistry, yielding decades of fruitful research. We argue that a limitation holding back future progress on our understanding of the causes and consequences of phytochemical diversity is a lack of precision surrounding the concept of phytochemical diversity. That lack of precision exists in part because of a lack of a coherent methodology for describing and quantifying phytochemical diversity across spatial and temporal scales. In this talk, we present a quantitative framework that fills that gap, borrowing techniques and perspectives from the species diversity literature and modifying them based on modern analytical chemistry.
Results/Conclusions
A first key recognition from our work is that there is no one universal measure of phytochemical diversity. Phytochemical diversity occurs at every biological scale, in space and time, and includes components of within-plant diversity and between-plant diversity. Each of these scales and components is biologically relevant, though potentially for different reasons. We present an explicit framework that indicates which scales and components of phytochemical diversity are relevant for which ecological and evolutionary questions. A second key recognition is that the level of phytochemical diversity relevant for an interacting animal depends nonlinearly on the amount of plant tissue with which each animal interacts. We present a quantitative method, based on species diversity sample rarefaction, for uncovering biologically meaningful relationships between scale and phytochemical diversity. We show how these nonlinear scaling relationships, though previously unappreciated, are integral to the central hypotheses about the ecology and evolution of phytochemical diversity and plant-animal interactions. Finally, we explain how the perspective we present and our quantitative methods lead directly to novel, testable predictions that have the potential to advance our understanding of the causes and consequences of phytochemical diversity.