This also is the first time the theory has been applied to extinction.
The experiment featured populations of water fleas that were assigned to either deteriorating environments (in this case, declining levels of food) or stable environments (the control group). The experiment lasted for 416 days, when the last population in the deteriorating environment group became extinct. Depending upon the amount of food they received, populations in the deteriorating environment group reached the population viability tipping point after approximately 300 days. Populations in the control group never reached it; those populations persisted.
The researchers next looked at a variety of statistical indicators, early warning signals that could detect the onset of CSD and thereby predict the approach to a tipping point. They compared the indicators with the timing of the decrease in food and with the achievement of the tipping point, mathematically referred to as a "transcritical bifurcation." They found that each of the indicatorssome more strongly than othersshowed evidence of the approaching tipping point well before it was reached.
According to Drake, what is even more important is the generality such statistical indicators are expected to exhibit. That is, although precise quantitative models are required to predict most natural phenomenain any domain of sciencewith any degree of accuracy, the theory of critical slowing down applies qualitatively anytime a bifurcation is in the vicinity. "You don't have to know the underlying equations to use the theory," Drake said, "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."
|Contact: John M. Drake|
University of Georgia