BEE: The Evolution of Fluctuation-Dependent Species Coexistence

Understanding how similar species coexist has been a central question in ecology since its inception. The question is important because species diversity is a defining characteristic of natural ecosystems. Traditional answers to this question focus on how species differ in the types of resources they use in non-fluctuating environments.

Under this paradigm, evolution can facilitate species coexistence when it makes species less similar in their resource use. However, natural ecosystems are not constant, and there has been recent interest in understanding how temporal variation in the environment, and differences in how species respond to this variation, can lead to conditions that favor coexistence.

While we know about these fluctuation-dependent mechanisms in theory, we know little about whether they can promote species coexistence in natural populations. We know even less about how evolution either facilitates or impedes such fluctuation-dependent mechanisms. This research attacks these questions by leveraging studies of experimental evolution in nature together with mathematical models and experiments in artificial streams.

The research will engage 38 young scientists from diverse backgrounds per year in the Young Scientist Training Program. The Young Scientists gain research experience that is complemented by training in ecology, evolution, and statistics. The researchers will also engage the public through an online outreach website and will develop training in ecology and evolution through mobile gaming, social media, and teacher training modules based on this system.

This work will provide a multipronged test of fluctuation-dependent coexistence mechanisms in natural communities and investigate how they evolve. In doing so, this work will synthesize ideas in ecology and evolutionary biology. This research takes advantage of the natural laboratory provided by the structure of stream communities in Trinidad, West Indies where fish communities consist of either Trinidadian guppies and Hart’s killifish or Hart’s killifish by themselves.

Killifish-guppy communities are formed when guppies invade communities with only killifish. Prior research has shown that following these invasions, guppies and killifish rapidly evolve differences in their life histories and resource use. These streams are also highly seasonal with pronounced dry and wet seasons.

Do killifish and guppies respond differently to these seasonal fluctuations? Are guppies adapted to dry season conditions and killifish to wet season conditions? If so, then this difference provides an avenue for the two species to coexist.

This research will also address how important these fluctuation-dependent mechanisms are relative to traditional fluctuation-independent mechanisms and identify how these contributions change through the formation of the community. This research will leverage replicated experimental evolution studies in natural ecosystems with structured models, factorial experiments in artificial streams, and stable isotope work.

At the same time, the mathematical models will provide a framework for linking theory on species coexistence with evolutionary theory.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.