Is it important to think of populations as emergent properties of individuals?

Erica M. Holdridge, Biological Sciences, Boise State University, Boise, ID; Ecology & Evolutionary Biology, Yale University, New Haven, CT and David A. Vasseur, Department of Ecology & Evolutionary Biology, Yale University, New Haven, CT

Background/Question/Methods
Calls for more work on the role of intraspecific trait variation (ITV) in community ecology have emphasized the need to derive the properties of ecological systems from the properties of individuals. However, up to this point, models of ITV have not explored whether or not it is important to consider population dynamics as emergent properties of individuals. There are two reasons for this: ITV in all models to date follows a prescribed distribution and most models assume populations are infinite. Since the processes that shape trait distributions and their response to ecological interactions and selection occur at the individual-level, it is important to ask what additional insights can we gain by allowing ITV to emerge naturally as a result of these individual-level processes under a realistic set of ecological constraints.
We use a stochastic simulation algorithm to describe the competitive dynamics and coexistence outcomes between two populations in which individuals vary in their uptake rates of two essential resources. This approach differs from other kinds of individual-based models in that they can be simulated exactly using Monte Carlo methods and analytically analyzed. The stochastic nature of our approach allows trait distributions to naturally emerge as a product of ecological interactions and selection, providing insights into what ITV looks like under ecological constraints and, further, how this naturally emerging ITV maps onto fitness through a well-defined function.
Results/Conclusions
We found that ITV can promote coexistence between competitors through both ecological and evolutionary mechanisms. Mutation and selection on standing ITV traits produced similar overall trait distributions, but different patterns within distributions. The average magnitude of trait change as a result of mutation was relatively small, which is most effective at promoting coexistence along the coexistence boundaries. Selection on standing ITV in partially and fully heritable traits resulted in larger average trait change, and the latter was most likely to promote coexistence along the persistence boundaries.
Although most studies have focused on the evolutionary effects of ITV, recent work has shown that it can also affect competitive dynamics and outcomes through exclusively ecological mechanisms. Ecologically, ITV allows some individuals within the population to be functionally different than their competitors. Evolutionarily, selection on ITV can promote coexistence through both trait convergence and divergence. Not only does this emphasize that ITV is a critical component in ecological theory, but it highlights the importance of a deeper understanding of the mechanisms through which ITV affects ecological processes.