Bighorn Sheep Inhabited Tiburon Island Centuries Ago, But Went Extinct
March 20, 2014

Bighorn History On Tiburon Island Goes Beyond Reintroduction In 1975

Brett Smith for - Your Universe Online

In 1975, 16 female and 4 male bighorn sheep were deliberately introduced to Tiburón Island, a large and mostly uninhabited island in the Gulf of California.

However, these sheep are not necessarily an invasive species as a new report in the journal PLOS ONE has found that these animals actually lived on the island before and went extinct between the 6th and 9th centuries.

Bighorn sheep were reintroduced to the island as part of a breeding program that could be used to restock the California mainland, where land use has decimated native sheep populations.

"Introduction success was expected on Tiburón Island, given the suitable habitat, lack of predators, absence of domestic sheep and their diseases, and minimal human disturbance," said study author Benjamin Wilder, a plant and botany researcher at the University of California, Riverside. "Indeed, by the mid-1990s, the Tiburón herd had grown to a stable population of 500 animals, one of the most successful large mammal introductions in the world."

The study team actually discovered the sheep’s history on the island by accident, when Wilder and a colleague stumbled upon a 1500- to 1600-year-old, urine-caked dung mat on the floor of a small cave in the Sierra Kunkaak, a mountain range of the eastern side of Tiburón Island. Study author Jim Mead, a paleontologist at East Tennessee State University, was able to determine that bighorn sheep formed the dung mat based on his extensive knowledge on both living and extinct herbivore dung.

Conservation geneticists at Oregon State University, who extracted and sequenced mitochondrial DNA from the ancient sheep dung, were able to confirm the discovery, as sequences from the ancient sheep species differed considerably from other large herbivores that may have been present. Additionally, the DNA sequences were not comparable to the modern bighorn sheep living on Tiburón Island, giving assurance to the researchers' theory that the sequences did not originate from modern use of the cave by introduced bighorn sheep.

"This finding raises a host of fascinating questions,” Wilder said. "Are bighorn sheep on Tiburón Island a restoration or a biological invasion? This extended biological baseline confirms that the Tiburón bighorn sheep went extinct before. Given the cultural and conservation significance of the unintentionally rewilded population, actions can be taken to avoid the same fate."

Study author Julio Betancourt, a USGS paleoecologist, said the study supports the notion that, in the future, "molecular caving, the application of molecular genetics to cave sediments, will become more than an afterthought to answer such questions in aridland paleoecology and conservation."

The authors theorized that native desert bighorn sheep may have earlier colonized the island when lower sea levels connected Tiburón to the American mainland, probably during the Pleistocene. They said that the animals' local extinction was probably because of the dynamics of isolated populations, prolonged drought, or human overkill.

"With extended biological baselines, such as the knowledge that the Tiburón bighorn sheep went extinct before, it is possible to refine conservation targets," Wilder added. "Given the cultural and conservation significance of the Tiburón bighorn, actions can be taken to avoid their past fate."