How Network Theory Can Prevent Extinctions

Over the last 40 years, the population of great sharks in the northern Atlantic has dropped dramatically. These sharks prey on cownose rays and consequently, this population has exploded. The rays, in turn, eat scallop and these have become all but extinct in some areas.

There have been are numerous examples of these kinds of extinction cascades in recent years. They raise an awkward question: given the initial extinction, what is the best way to prevent or mitigate the secondary extinctions?

Today, Sagar Sahasrabudhe and Adilson Motter at Northwestern University in Illinois suggest a provocative solution. They say that “perturbing” the ecosystems in other ways can dramatically reduce the spread of additional extinctions.

Their idea comes from the study of networks, which is revolutionising areas as diverse as neuroscience, genetics and communications. The ideas that apply to one network can often be transplanted lock stock and barrel from one area to another. For example, there is great interest in controlling gene networks to prevent disease.

Sahasrabudhe and Motter take exactly this approach to ecosystems, which also form complex networks. They have studied the effects of perturbations both to theoretical networks and to models of real networks, such as the Chesapeake Bay food web, an aquatic network with 33 species.

They say they can use the models to work out how to prevent the spread of extinctions. They say that controlling certain key species can completely stop the spread of extinctions, just as fire breaks can stop the spread of forest fires.

That makes sense but it is likely to be highly controversial. For example, Sahasrabudhe and Motter say that in certain circumstances “the early removal of a species that would otherwise be eventually extinct by the cascade can prevent all other secondary extinctions.”

One important question is to what extent the model captures the real behaviour of the ecosystem. If other effects can come into play, it may be that the culling of a species will have unanticipated consequences.

And there could be social and legal problems too. Interfere in a complex system like this, you’re likely to be blamed whatever happens.

Despite the risks, a network approach to conservation is surely the right way to go. Indeed, this kind of thinking is already being applied to conservation efforts

In the mid-1990s, the population of island foxes on the Channel Islands off the coast of northern California came close to extinction. The foxes were being killed by golden eagles which had been lured to the islands by the growing population of feral pigs, an invasive species introduced by humans.

The obvious solution was to kill the pigs (eagles being a politically sensitive species in the US). But it soon became clear that this could have the opposite effect: forcing the eagles to focus on the foxes thereby driving them even closer to the edge.

Instead, local authorities began netting the eagles and removing them non-lethally. When the eagles had gone, they wiped out the pigs. The important point is that it was not just the removal of the species that was important but the order in which it was performed.

This is a simple example of network effects. The real test will come when it is applied to more complex systems with much bigger networks.

Ref: arxiv.org/abs/1103.1653: Rescuing Ecosystems From Extinction Cascades Through Compensatory Perturbations

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