Ecology, evolution and the maintenance of ecosystem resilience: Predictions about the nature of ecosystem responses to environmental change from a model of a microbial ecosystem

Owen L. Petchey and Marcel Suleiman, Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland

Background/Question/Methods
The nature of ecosystem responses to environmental change are the foundation of ecological resilience. When an environmental change has little effect on ecosystem properties, then the system is resilient. A system that is not resilient may change gradually, may exhibit a threshold-type response, or may display a discontinuous (step) response. Such discontinuities occur if some values of an environmental variable are associated with multiple stable ecosystem states. Understanding and predicting this variation in response-types requires understanding the influence of factors such as species interactions, functional composition and diversity, and evolution. Nevertheless, there are few models that include and connect these factors. Particularly little is known about the effect of evolution on types of ecosystem response. We explored how ecological interactions, functional composition and diversity, and evolution affect the nature of ecosystem responses to environmental change. We focused on a mathematical model of cyanobacteria-sulphur bacteria, and the oxic and anoxic regimes that it can predict. Key features of this system include the physiological oxygen-intolerance of the sulphur bacteria, and the sulphide-intolerance of the cyanobacteria.

Results/Conclusions
We found that the modelled system can display all the types of response mentioned above, depending on the functional composition of the system. Moreover, we found that variation in and selection for the physiological tolerance of the bacterial groups to each other can greatly change how the ecosystem responds to environmental change. Increased tolerance, which may be expected to evolve when the environmental changes, leads to increases in the extent of alternate stable states. This is because evolution enhances the resilience of the presiding stable state. Such effects of evolution were found to be as large or larger than effects of changes in functional composition. We view these findings as preliminary explorations of how recognizing and including vital connections among ecology, evolution, and physiology, can produce joined-up theory of ecosystem responses to environmental change.