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An international team of biologists, including at the Museum für Naturkunde Berlin, have discovered that the Spiny-tailed Iguanas on Clarion Island (Mexico), previously thought to be introduced by humans, have likely been there since before humans colonized the Americas.
Clarion Island, the oldest island in an oceanic archipelago off the west coast of Mexico, has experience dramatic ecological changes caused by human introduction of sheep, pigs and rabbits. Clarion Island has also been colonized naturally via overwater dispersal by a unique assemblage of birds, snakes and lizards. Using DNA analyses, researchers show that Clarion Spiny-tailed Iguanas diverged from their mainland relatives approximately 425,000 years ago, long before humans colonized the Americas. The researchers postulate that dense vegetation that once covered the island allowed the iguanas to avoid detection in early expeditions (1890’s–1950’s), and that the clearing of vegetation in the 1970’s, caused by the introduced sheep and pigs, made the iguanas more conspicuous. This has important conservation implications because current management plans were to eradicate the iguanas, which should now be considered part of the island’s native fauna based on this new research.
Clarion Island is the oldest island (Pliocene, ~5 million years) in the Revillagigedo Archipelago, off the western coast of Mexico. These oceanic islands arose from volcanic activity along a fracture zone in the Pacific Ocean, thus were never connected to the continental mainland. This archipelago has been compared to others such as the Galapagos and the Hawaiian Islands, because it also hosts a unique assemblage of plants and animals, including endemic species of snakes and lizards, and many endemic species and subspecies of birds. Human-introduced wildlife often has devastating effects on native species and ecosystems, especially on islands. Examples include the Brown Tree Snake on Guam and cats, rats, and other invasive species in Hawaii. Clarion Island was once so densely covered in pricky-pear cactus that researchers described as impenetrable without a machete. Spiny-tailed iguanas are very shy and wary around humans, and often retreat into burrows or rock crevices when approached. In the 1970’s, the Mexican military established a base on Clarion Island, and introduced sheep, pigs, and rabbits, which cleared much of the native vegetation. Later, biologists working on the island noticed the iguanas and assumed they were also introduced because they were absent from previous reports of native wildlife on the island.
Using DNA sequence data, researchers compared a specimen from Clarion Island with mainland spiny-tailed iguanas, and conducted Bayesian evolutionary analyses to estimate a date of divergence to be approximately 425,000 years ago. Humans are thought to have colonized the Americas, via the Bering Land Bridge, approximately 16,000 years ago. Although, recent studies suggested this could be as old as 26,000 years ago. However, that is still long after the estimated divergence of the Clarion Spiny-tailed iguanas from their mainland relatives. This suggests that the iguanas naturally dispersed overwater to Clarion Island, via rafting on vegetative mats like what has been assumed for other snakes and lizards in the archipelago. Wildlife management on Clarion Island has now eradicated the sheep and pigs, and has been actively eradicating the rabbits and iguanas. This new study overturns the assumption behind the eradication program, and management experts will now consider the iguanas part of the native wildlife on Clarion Island. This research highlights the importance of natural history collections and emphasizes the need for evidence-based research in conservation strategies. The addition of spiny-tailed iguanas to the already unique fauna and flora of this remarkable archipelago provides a new component for future studies on island ecosystems, island biogeography, and biodiversity.
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