Eco-evolutionary dynamics of competition for non-substitutable resources: Why and how stoichiometry matters

Background/Question/Methods

Intuition, theory, and data all suggest that resource competition should select for diversification of resource use traits (character displacement). Traits that allow individuals to avoid competition by granting access to underutilized resources should be favored by selection. However, this picture assumes that resources are perfectly substitutable: an individual can consume one resource instead of another and attain the same fitness. In nature, many resources are non-substitutable (e.g., there are no substitutes for N or water), or only imperfectly substitutable (e.g., plants with different C:N ratios). When resources are not perfectly substitutable, avoiding competition does not necessarily increase fitness, and so competition does not necessarily select for diversification of resource use traits. I will describe simple models of the eco-evolutionary dynamics of two consumers competing for two non-substitutable resources, or two imperfectly substitutable resources. Key questions include: When does competition select for character displacement? If competition does not select for character displacement, what are the effects on coexistence and stability? In particular, does selection for character convergence lead to a neutrally-stable system of equivalent competitors?

Results/Conclusions

Competition for non-substitutable resources, such as competition among plants for N and P, always selects for character convergence. If competition makes an essential resource scarce, you are selected to get better at consuming it, not to consume a (non-existent) substitute. However, competitors do not evolve identical resource use traits unless they have identical resource requirements (stoichiometry). Character convergence leads to stable coexistence, not neutral stability. The same adaptive evolution that drives character convergence also stabilizes the resulting eco-evolutionary equilibrium. Competition for complementary resources (such as competition among herbivores for plants which vary in their C:N ratios) can lead to character displacement, convergence, or parallel shifts in resource use traits. Which pattern occurs depends only on the match between consumer and resource stoichiometry, and not on resource use overlap. Incorporating costs of maintaining homeostatic stoichiometry (e.g., costly excretion of excess non-limiting nutrients) increases the likelihood of character convergence. Character displacement, convergence, and parallel shifts all lead to stable coexistence. The results suggest that, when resources are not substitutable, classical concepts such as character displacement and resource use overlap are epiphenomena unhelpful in understanding or explaining competitive outcomes. The take-home message is that primary producers and herbivores likely are diverse despite competition rather than because of it. This hypothesis requires empirical testing, ideally via manipulative experiments.