What (and When) is the Point of No Return?

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How-and when-do ecosystems change character? Are those shifts reversible? And what signs might precede them? Such questions are hard enough to answer in a single place. One might think that incorporating different kinds of ecosystems would only complicate the problem. But a group of scientists in the Long-Term Ecological Research Network is finding a remarkably consistent pattern by combining models and data across several long-term ecological experiments.

Their study, published in Ecological Monographs, asks how the intensity and duration of changes in external drivers interact to determine whether an ecosystem shift is temporary or more likely to be permanent.

Intense grazing driving a dry grassland toward a persistent shrubland is one of the classic examples of regime-shift. The researchers ran many simulations using a model that has been used to describe a number of grasslands. Their model suggested that grasslands could re-establish after even moderately-intense grazing—as long as the grazing didn’t continue for too long. But the higher the intensity of disturbance, the narrower was the window of time in which change would be temporary.

They also examined the role of spatial patchiness and temporal variability, finding that as a system moved toward a threshold, spatial patchiness intensified—potentially serving as a marker for impending regime-shift.

To determine how generally applicable the results might be, they ran models of other ecosystems and found similar outcomes. And they delved into long-running on-the-ground experiments in which a disturbance—such as excess nutrients, invasive species, grazing, or fire suppression—was removed after driving the system to the brink of irreversible change.

At the ecosystem scale, such experiments are rare, as they require decades of manipulation and observation, but the results for the three examples they found were consistent with model results. In the near term, they argue, the most robust test of these principles would be to undertake deliberate experiments in systems that respond rapidly—such as lakes, annual plant communities, and microbial ecosystems. The framework that emerges from the paper also suggests that it might be possible to mine long-ceased or altered experimental treatments for evidence of irreversible transitions.

 

Photo credit:Scott Bauer/USDA (CC BY-NC-ND 2.0)

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