Christopher X J. Jensen
Associate Professor, Pratt Institute

Ecological Exclusion

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Does natural selection only operate at the level of the individual? Or, are there emergent scales of organization at which differential survival and reproduction can affect the evolutionary process?

These are the questions that frame my projects on ecological exclusion. I credit my Ph.D. advisor, Lev Ginzburg, with both introducing this idea to me and coining the term. Ecological exclusion is the process by which groups, species, or whole communities go extinct not just as a result of the aggregated failure of all individuals but due to emergent instabilities in the system of interacting individuals.

So what does this mean, and how might ecological exclusion work? The most simple way to imagine ecological exclusion is with a single obligate predator-prey system. In this system, there is continual selection for better predators, as predators who catch more prey survive and reproduce at a rate higher than their competitors. Prey also are selected to better evade predation, because it is hard to survive and reproduce after being eaten. This coevolutionary process might stabilize the interaction between predator and prey, but it is not guaranteed to do so. One potential outcome of  the interaction is that the predator evolves faster than the prey, and natural selection on individual predators produces population-wide predation rates so high that the predators consume all their prey and both species go extinct. Ecological exclusion is all about irony: because natural selection at the level of the individual pushed a predator species to be so successful, it in the end went extinct due to destabilization of the ecological interaction upon which it depends. Ecological exclusion is also all about emergence: extinction emerges as the unexpected result of aggregated individual success, a result that can only be predicted by looking at the fitness of the overall system.

Simple predator-prey systems are not the only units of ecological organization that might be susceptible to ecological exclusion. Mutualisms and parasitisms also have the potential to produce the destabilization that drives ecological exclusion, and it is also possible that whole communities might be subject to the kind of stability selection that drives ecological exclusion.

So far I have two ongoing projects that explore the possibility of ecological exclusion. The first explores the fundamental properties of predator-prey interactions, and seeks to understand the potential for ecological exclusion in different theoretical representations of predation. The second explores a specific potential example of ecological exclusion: the mass extinction of megafauna during the Late Pleistocene. Both of these projects are in their early stages, and I hope to continue work on ecological exclusion in these as well as additional areas.

All images on this page courtesy of Wikimedia Commons. Click on the images to see their exact source.