Range contraction puts species at risk of overharvesting

March 28, 2017
elephants

When certain species decline in number, the geographic areas they inhabit also shrink. With less space to occupy, these decreasing populations manage to remain locally abundant. As a result, in the places where these species can still be found, they remain easy, affordable targets for hunters and fishermen. A recent study found that this, in turn, can drive the animals to extinction.

“We often think of species like elephants and bluefin tunas as being overharvested because of their high market value,” said lead author Matt Burgess, a postdoctoral researcher in the University of California, Santa Barbara’s Sustainable Fisheries Group who received his doctorate from the University of Minnesota. “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 overharvesting regardless of how high its market value is.”

The study was co-authored by University of Minnesota Regents Professors David Tilman and Stephen Polasky, along with with researchers from UCSB and Rutgers University.

In order to determine which species are affected by this phenomenon, the team 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 harvested ocean and land animals so impacted, the investigators found several endangered species — including 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 study found that land animals have experienced range contraction as their population numbers declined more commonly than marine animals. The study also found schooling behavior — the tendency for animals to aggregate in large groups — 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,” said Tilman.

Though the objective of the study was to identify the risk factors that incentivize severe overharvesting, its results underscore the importance of well-managed harvests. Imminent collapse or extinction is not necessarily the only available outcome for vulnerable populations, Burgess noted.

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

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