Animal populations headed for extinction may give the same signals seen before crashes in coral reefs, the Sahara’s climate and even stock markets.
These systems are very different, but each exhibits what’s known as “critical slowing down,” in which a loss of resilience magnifies the effect of small perturbations, which become more and more difficult to recover from.
“Critical slowing down is an early warning signal. If you can detect it, you can know that you’re headed for a tipping point,” said University of Georgia ecologist John Drake. “This could lead to new ways of quantifying population viability.”
Critical slowing down in animals was theoretically predicted, and has now been demonstrated in laboratory populations of water fleas. If it can be detected in real-world populations of other animals, scientists could have a valuable new tool for telling when animals are literally on the brink of extinction.
On the surface, predicting extinction risk seems like it should be easy: It’s generally the result of habitat loss, exploitation, and factors like disease or invasive species. But with these conditions changing constantly, modeling is often difficult, and it can be hard to tell exactly when the threat of extinction has gone from predicted to imminent.
Drake, who specializes in population dynamics, wanted to see whether the emerging science of so-called critical transitions — the sudden flip of a system from one condition to another, originally identified in physics and recently applied to ecology — also applied to animals. At least theoretically, their populations should behave in similar fashion when approaching the critical transition that is extinction.
In a study published Sept. 9 in Nature, Drake and University of South Carolina biologist Blaine Griffen tracked population changes in 60 laboratory colonies of water fleas. Half were given a steady food supply, while the others received one-quarter less food every month. Lacking the nutrients needed to reproduce more frequently than they died, the latter colonies inevitably went extinct, long before the food ran out.
In any animal population, the number of individuals oscillates naturally. A few extra offspring are born, putting a strain on resources; that leads to a few extra deaths or a drop in births, which frees up resources that allow the population to grow again. These fluctuations converge on an equilibrium somewhere in the middle. Both the control and nutrient-deprived populations followed this pattern.
But when Drake and Griffen looked closely at the data from declining groups, they found that populations took much longer to return to equilibrium. That’s a telltale sign of critical slowing down. Under too much stress, a system loses its balance easily, and takes longer to recover that balance. It was evident up to eight generations before extinction.
“Critical slowing down refers to the resilience that a system has, its ability to recover from perturbations,” said Drake. “A tipping point is a condition when that ability to recover goes all the way to zero. As you’re getting closer, it takes longer and longer for each perturbation to die down.”
The equations generated by the cycles of water flea populations matched those seen in other systems as they approached their own critical transition points — the then-verdant Sahara before turning to desert 5,500 years ago, once-clear freshwater lakes turned by nutrient pollution into algae-choked soup, even the stock market prior to its 1987 crash.
Wageningen University ecologist Marten Scheffer, a pioneer in critical transition research, called the findings a “really important step” in learning how to detect impending tipping points in animal populations. But he warned that detecting signals in real-world conditions is harder than in a laboratory.
Such experiments are currently being run by University of Wisconsin ecologist Steve Carpenter, who has turned several lakes into field laboratories for detecting ecosystem shifts. “It’s too early to draw conclusions, but so far it seems promising,” said Carpenter.
Another useful experiment would be to measure the results of interventions at different points in the water fleas’ slide to extinction. That might hint at when an early warning sign turns into a foregone conclusion.
Drake’s resources have been stretched thin, but “that would be a fantastic experiment, and I would love for someone to do it,” he said.
Images: 1) Male and female water fleas./West Group, Oxford University. 2). Differences between control and food-deprived populations in two measures of critical slowing down./Nature.
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Citation: Early warning signals of extinction in deteriorating environments John M. Drake & Blaine D. Griffen. Nature, Vol. 467 No. 7313, September 9, 2010.
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