Iguanas on Clarion Island, Mexico, found to predate human presence in the Americas

An international team of biologists, including those from the Museum für Naturkunde Berlin, have discovered that the spiny-tailed iguanas of Clarion Island (Mexico), previously thought to be introduced by humans, were likely there before humans colonized the Americas.

Clarion Island, the oldest island in an ocean archipelago off the west coast of Mexico, has experienced dramatic ecological changes caused by the human introduction of sheep, pigs and rabbits. Clarion Island was also colonized naturally by over-water dispersal by a unique assemblage of birds, snakes, and lizards.

Using DNA analysis, researchers showed that Clarion spiny-tailed iguanas diverged from their continental relatives around 425,000 years ago, long before humans colonized the Americas. The researchers posit that the dense vegetation that once covered the island allowed the iguanas to avoid detection during early expeditions (1890s to 1950s) and that vegetation clearing in the 1970s, brought about by the introduction of sheep and pigs, made the iguanas more visible.

This has important conservation implications, as current management plans included eradicating iguanas, which should now be considered part of the island’s native wildlife based on this new research. The study is published in the journal Ecology and evolution.

Clarion Island is the oldest island (Pliocene, ~5 million years old) in the Revillagigedo Archipelago, off the west coast of Mexico. These oceanic islands arose from volcanic activity along a fracture zone in the Pacific Ocean and therefore were never connected to the continental continent. This archipelago has been compared to others like the Galapagos and the Hawaiian Islands because it is also home to a unique assemblage of plants and animals, including endemic species of snakes and lizards, as well as many endemic species and subspecies of birds.

Human-introduced wildlife often has devastating effects on native species and ecosystems, particularly on islands. Examples include the brown tree snake in Guam and cats, rats and other invasive species in Hawaii.

Clarion Island was once so densely covered with prickly pears that researchers described it as impenetrable without a machete. Spiny-tailed iguanas are very shy and wary of humans and will often retreat into burrows or rock crevices when approached. In the 1970s, the Mexican military established a base on Clarion Island and introduced sheep, pigs and rabbits, which destroyed much of the native vegetation. Later, biologists working on the island noticed the iguanas and assumed that they had also been introduced because they were absent from previous reports of the island’s native wildlife.

Using DNA sequence data, the researchers compared a specimen from Clarion Island with spiny-tailed iguanas from the mainland and conducted Bayesian evolutionary analyzes to estimate the divergence date at approximately 425,000 years ago. Humans are believed to have colonized the Americas, via the Bering Land Bridge, around 16,000 years ago. However, recent studies suggest it could be as old as 26,000 years ago. However, this is still long after the estimated divergence between Clarion spiny-tailed iguanas and their mainland relatives.

This suggests that the iguanas naturally dispersed across the water to Clarion Island, via rafting on vegetative mats, similar to what has been hypothesized for other snakes and lizards in the archipelago. Wildlife management on Clarion Island has now eradicated sheep and pigs, and has actively eradicated rabbits and iguanas.

This new study overturns the hypothesis behind the eradication program, and management experts will now consider iguanas part of Clarion Island’s native wildlife. This research highlights the importance of natural history collections and highlights the need for evidence-based research in conservation strategies.

The addition of spiny-tailed iguanas to the already unique flora and fauna of this remarkable archipelago provides a new element for future studies of island ecosystems, island biogeography and biodiversity.