CORVALLIS, Ore. – A research team has found evidence that bighorn sheep inhabited Tiburón Island in the Gulf of California some 1,500 years ago – a surprising find that calls into question just how to manage the population of bighorns that were introduced to the island in 1975.
The experimental introduction almost 40 years ago of what was thought to be a non-native species was intended to create a large breeding population of bighorn sheep at a site safe from predators that could be used to restock bighorn populations on the mainland. The discovery that bighorn sheep previously had lived on the island raises philosophical questions, the researchers say.
They report on the dilemma, which they call “unintentional rewilding,” this week in the journal PLOS ONE.
“This is a microcosm for situations in which animals regarded as non-natives are introduced into an area where they actually lived in the past,” said Clinton Epps, a wildlife ecologist at Oregon State University and co-author on the PLOS ONE article. “There are some interesting implications.
“If, for example, one goal was to restore native habitat and it looked like the introduced animals were having an impact on the flora, the solution might be to remove the animals,” Epps pointed out. “But now you’d have to say, ‘not so fast.’ What is the right thing to do? Does it matter if the animals lived there 10, a hundred, or a thousand years ago?”
The development first began to unfold with the incidental discovery by lead author Ben Wilder of the University of California, Riverside, of a dung mat on the floor of a small cave in the Sierra Kumkaak, a rugged mountain range on the east side of Tiburón Island. Samples of the sheep pellets were sent for DNA sequencing to Oregon State.
“The first thing we had to do was eliminate the possibility that the material had come from deer, mountain goats, domestic sheep or cows, or some other animals,” said Rachel Crowhurst, a faculty research assistant in OSU’s Department of Fisheries and Wildlife. “It closely matched bighorn sheep. Then we used a second genetic marker to compare it to the modern population of bighorns on the island – and it was completely different.”
The OSU researchers determined that the sequences from the bighorn sheep that lived on the island some 1,500 years ago exactly matched sequences from desert bighorn sheep living today in Arizona.
In 1975, 16 female and four male bighorn sheep were introduced to Tiburón Island, which is a large, mostly uninhabited island just off the coast of Sonora Mexico. On the mainland, historical land use had decimated populations of wild bighorn sheep. By the mid-1990s, the Tiburon herd had grown to some 500 animals and was considered one of the most successful large mammal introductions in the world.
As it turns out, this supposed introduction was actually an “unintentional rewilding” – a phrase coined by the authors and a concept that has implications for future research, according to Julio Betancourt, a paleo-ecologist with the United State Geological Survey and co-author on the paper.
“Molecular studies will become more than an afterthought in paleo-ecological studies to address previously unanswerable questions about evolutionary responses to climate change,” Betancourt said.
The research by Epps and Crowhurst was supported by Oregon State University.