The many dimensions of phytochemical diversity: linking theory to practice

Abstract

Research on the ecological and evolutionary roles of phytochemicals has recently progressed from studying single compounds to examining chemical diversity itself. A key conceptual advance enabling this progression is the use of species diversity metrics for quantifying phytochemical diversity. In this perspective, we extend the theory developed for species diversity to further our understanding of what exactly phytochemical diversity is and how its many dimensions impact ecological and evolutionary processes. First, we discuss the major dimensions of phytochemical diversity – richness, evenness, functional diversity, and alpha, gamma and beta diversity. We describe their potential independent roles in biotic interactions and the practical challenges associated with their analysis. Second, we re‐analyse the published and unpublished datasets to reveal that the phytochemical diversity experienced by an organism (or observed by a researcher) depends strongly on the scale of the interaction and the total amount of phytochemicals involved. We argue that we must account for these frames of reference to meaningfully understand diversity. Moving from a general notion of phytochemical diversity as a single measure to a precise definition of its multidimensional and multiscale nature yields overlooked testable predictions that will facilitate novel insights about the evolutionary ecology of plant biotic interactions.

Soil chemical legacies trigger species-specific and context-dependent root responses in later arriving plants

“…2c; r=0.65, P=0.0001), thus showing that samples with high DOC concentration also had a high metabolite richness. This is consistent with the abundance-dependence of chemical diversity that has been observed across multiple levels of biological organisation (Wetzel and Whitehead, 2020).…”

Disentangling dimensions of phytochemical diversity: alpha and beta have contrasting effects on an insect herbivore

“…The multidimensional nature of phytochemical diversity suggests it may have a variety of important ecological roles, but this complexity also challenges our ability to isolate each dimension and resolve their effects experimentally. Studies that have examined only one of these scales rather than comparing effects between scales may have overlooked testable predictions (Wetzel and Whitehead 2020). This work has led to the discovery of important patterns, but it has made it difficult to link specific scales and dimensions of phytochemical diversity to ecological outcomes.…”

“…Traditionally, chemical ecology examined the role of phytochemicals by isolating and studying small numbers of compounds at a time (Gershenzon et al 2012, Richards et al 2016). Advances in metabolomics and new collaborations with chemists, however, have recently enabled ecologists to measure large numbers of compounds simultaneously and efficiently (Dyer et al 2018, Kessler and Kalske 2018, Wetzel and Whitehead 2020). Indeed, recent work indicates that the diversity of compounds per se present in plants is strongly correlated with key ecological variables, including herbivory (Johnson et al 2009, Richards et al 2015, Glassmire et al 2019).…”

Ecological convergence of secondary phytochemicals along elevation gradients

“…For instance, we observed that groups of plants bearing practically identical chemical richness values (S) can have completely different GSLs compositions. This indicates that focusing on arbitrarily-selected indices of phytochemical diversity can be misleading in interpreting the metabolomics data and their effects (Wetzel & Whitehead 2020). Taking into account different factors determining such diversity, such as compound class, metabolites' molecular metrics, or biological activity, we were able to add a functional dimension to phytochemical diversity, as was for instance done for cardenolides in milkweeds using polarity values (Rasmann & Agrawal 2011).…”

“…3) How are different metrics of phytochemical diversity related to plant-herbivore interaction? Because each metric of phytochemical diversity can only capture a fraction of the chemical complexity, we predicted that not all metrics of phytochemical diversity similarly predict plant resistance against specialist and generalist herbivores (Wetzel & Whitehead 2020). With this work, we thus expand on the ecological and evolutionary processes that drive and maintain phytochemical diversity across space and time, and integrate the functional axis of phytochemical diversity with the functional axis of plant growth forms.…”