Peter Abrams discusses the Hydra Effect in NewScientist (30 May 2015, paywall):
A decade ago, my collaborator Hiroyuki Matsuda and I coined the term hydra effect to describe all situations where a higher death rate in a particular species ultimately increases the size of its population. We named this phenomenon after the multi-headed serpent of Greek myth that grew two heads for each one that Hercules cut off. The hydra effect is just one example of a more general phenomenon where adverse changes in the environment that reduce the growth rate of a population may ultimately lead to greater numbers.
What could cause populations of animals or plants to bounce back so strongly in spite of a continued higher death rate? When a population experiences increased mortality – due to, say, harvesting – the initial effect is almost always diminishing abundance. However, after this initial decline, other organisms they interact with are likely to change. In most cases the organisms they eat go up in abundance. Diseases and predators that depend on the harvested population decline. Also, a higher risk of death often causes individuals to reduce their activity levels and to spend more time hiding, allowing their over-exploited food species to recover. Tadpoles in ponds with more predatory dragonfly larvae have been shown to end up getting more algae to eat, despite spending less time feeding. In a large food web many other complicated positive feedbacks exist.
The mathematics of populations must be fascinating, mostly in the modeling, so as to best characterize the relationships and how a change at one point in the web of life ripples through the rest of it.
But what about the political dangers of recognizing the hydra effect?
“I don’t know if it’s true, but I hope it isn’t.” This was the response of a fisheries biologist after I gave a talk about the hydra effect a decade ago. He was right to be concerned. If it were widely known that harvesting can potentially increase the size of fish stocks,fisheries quotas might have been raised too rapidly. Fishers may welcome the counter-intuitive idea that catching fish may bump up the population, but hydra effects are not yet predictable. And modelling shows that the harvest rate which maximises a fish population is surprisingly often only slightly smaller than that which results in a dramatic collapse or extinction.
Which leaves me wondering how many Fishing Ministers ponder how to determine that sweet spot, without ever considering how much illegal fishing actually goes on….
