Shrinking habitat may turn animals into easy targets

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As some animals decline in number, the geographic areas they inhabit can also shrink, causing remaining animals to be easy, affordable targets for hunting and fishing—and at even greater risk for extinction.

“We often think of species like elephants and bluefin tunas as being over-harvested because of their high market value,” says lead author Matt Burgess, a postdoctoral researcher in the Sustainable Fisheries Group at the University of Santa Barbara.

“Our results suggest that we should also be paying attention to their range contractions. In fact, range contraction can put a species at risk of over-harvesting regardless of how high its market value is.”

In order to determine which species this phenomenon affects, researchers used a mathematical model to derive conditions under which it would be possible to profitably harvest a species to extinction.

Reviewing relevant literature to identify ocean and land animals, investigators found several endangered species—Bengal tigers, Asian elephants, and Atlantic and Pacific bluefin tunas—whose geographic ranges have shrunk faster than their population sizes have declined. This kind of range contraction makes species especially susceptible to extinction. The findings appear in the Proceedings of the National Academy of Sciences.

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“To date, humans have destroyed a much higher fraction of terrestrial habitats than marine habitats, so it’s not surprising that we have seen more range contractions on land,” says coauthor Steve Gaines, dean of the Bren School of Environmental Science & Management.

Further, schooling behavior—the tendency for animals to aggregate in large groups—was found to be an important risk factor for range contraction in declining fish populations. “It’s ironic that the very behaviors, such as schooling, that protected species from predators now make them more susceptible to extinction by humans,” says coauthor David Tilman, a professor at the Bren School and a professor at the University of Minnesota.

Because climate change causes range contraction in many species, it could become an increasingly important risk factor for over-harvesting in the future, says coauthor Malin Pinsky, assistant professor at Rutgers University.

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“It’s precisely those over-harvested species that are least able to cope with climate change. We now know that one of the best ways to help animals survive climate change is to eliminate over-harvesting.”

The findings underscore the importance of well-managed harvests, Burgess says. Imminent collapse or extinction is not necessarily the only available outcome for vulnerable populations.

“For example, the International Commission for the Conservation of Atlantic Tunas recently enacted a rebuilding plan for severely depleted Atlantic bluefin tuna populations and already the eastern Atlantic population is showing signs of recovery,” he says.

“Our hope is that by drawing attention to the risk factors for over-harvesting, we can inspire similar conservation and management success stories in other species.”

Source: UC Santa Barbara