New paper on ecological disequilibrium

Think of a community populated by saguaros, prickly pears, and agaves – do you imagine a warm place or a cold place? These species are warm-adapted, so a guess that the community is warm is a reasonable one. But what if the community were actually cold and snowy, like below? This would reflect ecological disequilibrium, a mismatch between the niches of species and the climate observed in a community.

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Disequilibrium seems unreasonable – we do not expect to find bananas growing in Siberia – but it is not impossible. And this possibility challenges several areas of ecology that we all rely on. Computer models projecting species’ geographic distributions in the future assume that species have an instantaneous response to environmental change; similarly, paleoclimate reconstructions based on fossils or pollen also assume that communities’ past compositions reflect past climates.

glacier

Rapid climate change or slow species change can result in disequilibrium. Species may persist in place even as the climate changes away from their physiological tolerances; alternatively, a rapidly changed site may remain un-colonized by physiologically appropriate species because they have not yet had the time to disperse there, e.g. after glacial retreat following the last Ice Age, as seen above. These are just a few of the possible scenarios that can result in more modest versions of the Siberian banana or snow-covered agave scenario. But how would we know if they were occurring?

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We just published a new paper in Ecology (PDF reprint here) that explores ecological disequilibrium. We build a mathematical framework that lets us measure the strength of disequilibrium in a community, then test the framework in hundreds of forest communities across North and South America (data from BIEN). It comes with an associated R package, comclim, that lets other investigators assess disequilibrium in their own datasets.

What did we find? That most communities are actually in equilibrium with present-day climate. This is re-assuring for the assumptions of many current ecological models. But we also found that about a quarter of the communities we examined show strong lags with respect to present-day climate. We were unable to find strong explanations for why this subset of communities were out of equilibrium, providing an important problem for future research to focus on. We also found that disequilibrium will only become stronger under future climate change scenarios, suggesting that some of our equilibrium models will require modification in order to make accurate predictions for the future. Overall, I think the paper shows that communities’ response to climate change can be lagged and idiosyncratic, a message that underscores the complexity of interpreting past ecological change or predicting future change.

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This paper did not come together in a simple way. It began as an idea of mine to better reconstruct paleoclimate from species occurrence data, an idea that was funded by the Danish National Research Foundation and that ultimately sent me to visit the Center for Macroecology, Evolution, and Climate in Copenhagen. I originally decided to go not because of the science but because I wanted to escape a difficult personal situation at home. Once I arrived, my hosts and I soon realized that our proposal was not viable, as it assumed a level of ecological equilibrium for which we had no evidence. We re-oriented the project towards measuring disequilibrium instead. It took two years of visits to Denmark to finish major work on the project, and countless rounds of revisions and peer review to get it published. Three years later we have a product that I am proud of, and that bookends several important chapters in my life. I don’t think that the me who wrote the original proposal would quite recognize the me or the final paper that came out of this adventure.

I hope you’ll give the paper a read and see if it challenges you to think about climate change in some new ways!

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