Plant species present a variety of antiherbivore strategies both in type and degree, but the underlying causes of this variation remain unclear. Several ecological and evolutionary theories propose to explain this diversity along gradients of resource availability in plants communities. Among them, the resource availability hypothesis states that plants should evolve toward tolerance, i.e. fast-growing and poorly-defended strategies, when resources are abundant.
We investigated this hypothesis theoretically using a three-trophic-level food web, where plants compete for a single nutrient and are grazed upon by a shared herbivore. Plants strategies are defined by their investments in competitive ability, maximal growth rate and susceptibility to herbivory. We studied selection and adaptation of those strategies using a recently developed eco-evolutionary graphical method based on an extension of Tilman’s resource competition theory to adaptive dynamics.
Results/Conclusions
We computed evolutionary bifurcation diagrams summarizing the best adapted strategies for the different conditions along the resource gradient. Situations leading to evolutionary diversification and stable coexistence were also identified. This was done for a various range of trade-offs parameters, accounting for different costs of the growth- and defense-related physiological functions.
We found that the best strategy to cope with herbivory in resource-abundant conditions can indeed be fast-growth when building defenses is costly. However, cheap defenses could also lead to the evolution of resistant if not completely inedible strategies. When an inedible strategy occurs, it must coexist with a less-defended one. We also identified a parameter range that leads to a hump-shaped defense investment peaking at intermediate resource availability. To conclude, the shape of the trade-off functions appears to be crucial for predicting plant evolutionary responses along a resource gradient and discriminate between those different scenarios.