All species on the road to extinction reach a critical moment when their overall chances of survival drop to zero. We might be able to identify when this moment approaches, allowing us to save species on the brink of extinction.
This tipping point between extinction and survival is known as "critical slowing down", a term previously used in physics and earth sciences and which is only now for the first time being applied to biology. Essentially, critical slow down refers to the point at which a system - or, in this case, a species - is overwhelmed by even small disturbances and requires increasingly long times to recover, placing its overall survival at risk. Figuring out how to identify when a species approaches such a point is crucial to preventing extinction. Teams at the University of Georgia and the University of South Carolina have used populations of water fleas in deteriorating environments to find just such a tipping point.
Georgia's John Drake explains:
"This is the first experimental demonstration of critical slowing down in a biological system. The theory was originally used to describe drastic changes in other kinds of systems-everything from epileptic seizures to regime shifts in the earth's climate system. But these attributions of CSD primarily have been after-the-fact explanations of anomalous observations without clear controls."
Drake and his colleagues turned CSD into an experimental predictor by taking multiple populations of tiny water fleas, and placing the experiment groups in an environment with dwindling food supplies. Over 416 days, all the stressed populations of water fleas felt the pressure of their deteriorating environment and went "extinct", reaching a population tipping point after approximately 300 days. All the control groups in healthy environments survived the experiment.
The team then examined the data, looking for a variety of different statistical indicators of critical slowing down. They then compared these indicators with when the food supplies began to dwindle - something that happened at different points in different groups - and when the species became no longer viable, looking for patterns. Although no single indicator was a perfect predictor, overall these signals offered a good idea of when the species would no longer be viable. In a complicated dynamical system like those nature provides, a general idea is about the best you can hope for.
In fact, as Drake explains, it might be better to have an approximate, easy-to-calculate sense of when the tipping point is reached via than an exact answer:
"You don't have to know the underlying equations to use the theory, and this is important in biology, where the dynamics are typically sufficiently complex that we often do not know which equations to use. In fact, we may never come to such a complete understanding, given the range of biodiversity out there and the fact that species are evolving all the time."
With better understanding of how critical slowing down works in species and how it can be predicted, we will have a chance to save species on the brink of extinction before they pass the point of no return. Still, as Drake points out, their work still has a long way to go:
"This is the first step in the fundamental research that would underlie such an application. We have shown that CSD can happen in populations-that is all. The real world is a lot 'noisier' than the lab. Using early warning signals to predict approaching tipping points could eventually be a powerful tool for conservation planning, though, and for better understanding a host of other kinds of systems as well."