Bighorn sheep re-wilding raises conservation questions

The value of a project to introduce bighorns to an island off the coast of Mexico is in doubt after evidence showed the animals died out there more than a thousand years ago. Erin Brodwin reports.

Two Rocky Mountain bighorn rams clash, but climate change has been the bigger enemy. – W. Perry Conway/CORBIS

A conservation project to transplant North America’s iconic bighorn sheep to the Mexican island of Tiburon has raised important questions for the nascent “re-wilding” movement, which seeks to repopulate wild landscapes with endangered species.

Are conservationists, in the name of biodiversity, dropping animals into ecosystems that can’t support them?

By most counts, the project has been a success with the meagre founding population of just 20 bighorn sheep in 1975 multiplying to a respectable 500 today.

But a report published last month in the journal PLOS One shows that the curly-horned beasts had been there before – and become extinct more than a thousand years ago.

Scientists might never have known that the bighorn once roamed the Mexican island had University of California-Riverside ecologist Ben Wilder not stumbled across the ancient sheep DNA by accident. To capture a snapshot of how island plants are responding to climate change, Wilder traveled to Tiburon last year to collect rat faeces. They preserve an excellent record of plant matter because rats urinate over the deposits, preserving everything inside.

While Wilder was busy collecting these samples, however, he came across something that didn’t look – or smell – like it had come from a rat. So he added the specimen to his collection bag, drove it back up to the United States and showed it to his colleague Julio Betancourt, a palaeoecologist with the US Geological Survey.

Climate projections suggest the new population could also
be living on borrowed time.

As soon as Wilder opened the bag, Betancourt knew what was inside. “I said this smells like sheep,” he recalls.

By studying the DNA in the remains, the researchers were able to determine that it belonged to a bighorn, moreover one that had lived on the island around 1,600 years ago, based on how much its DNA differed to that of modern-day counterparts. What happened to the bighorns of Tiburon? Scientists surmise that a decades-long drought wiped them out, a theory backed by the island’s tree ring patterns that show cycles of dry and wet periods over the past 2,000 years.

So perhaps an isolated, drought-prone island is not a great place to re-wild the bighorn. And climate projections suggest the new population may, like the older population, be living on borrowed time: severe dry spells are predicted to hammer Mexico and the US Southwest in the coming decades.

The bighorn sheep is found across North America. – Stephen Matera/Aurora Open/Corbis

This scenario raises troubling implications for conservation biologists eager to re-wild landscapes with endangered species. Betancourt and Wilder say that scientists need more long-term data that shows where and when the species lived and what caused them to perish. This way, conservationists will be better equipped to decide if the plants and animals they are seeking to introduce will be able to thrive in the long term.

“If you’re going to predict the world is going a certain direction," Betancourt says, “you’re going to need to know where the world has been.”

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