From Derek Bolton
Peter Abrams lists a number of negative feedbacks that put the brakes on population decline: fewer predators, less disease, more resources.
But taken at face value these are ordinary negative feedbacks – they will reduce the decline caused by, say, culling, but provide no apparent reason for a paradoxical increase in numbers. If the hydra effect is real, the explanation must be more subtle.
One complication is that, both in models and reality, a “steady state” often consists of a periodic cycle. The size of a population is interpreted as the average over the cycle. At once the paradox appears less startling: culling might lower the population in one part of the cycle while raising it in another, leading to a misleading increase in the average.
We must remember another of Abrams’s points: that culling rates only a little higher than those that maximise average population tend to lead to extinction.
Sydney, Australia
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