# Anthropogenic or Natural Dispersal: Case of the Spiny‐Tailed Iguanas ( Ctenosaura ) on Clarion Island, Mexico

## TL;DR

Spiny-tailed iguanas on Clarion Island, Mexico, likely arrived naturally millions of years ago, not through human introduction, changing conservation strategies.

## Contribution

Molecular data shows natural dispersal of Spiny-tailed Iguanas to Clarion Island predates human presence in the Americas.

## Key findings

- Phylogenetic analysis shows Clarion iguanas are closely related to mainland C. brachylopha populations.
- Divergence of the Clarion population is estimated at 425,600 years ago, predating human colonization.
- The iguanas likely dispersed naturally via rafting over 1100 km, similar to Galapagos dispersal distances.

## Abstract

Clarion Island, in the Revillagigedo Archipelago off the Pacific Coast of Mexico, hosts a unique assemblage of vertebrates. Introduced species have caused significant ecological damage, and Spiny‐tailed Iguanas (
Ctenosaura pectinata
) were assumed to have been introduced in recent times, prompting plans for eradication. To investigate the origin of the Ctenosaura population on Clarion Island, we conducted phylogenetic analyses of the Clarion Island and mainland populations using a portion of the mitochondrial DNA gene ND4. We estimated the date of divergence of the Clarion Island population from mainland Mexico populations using a relaxed‐clock method. Phylogenetic analyses reveal that Clarion iguanas are sister to mainland C. brachylopha populations in northwestern Mexico, a species recently resurrected out of
C. pectinata
. We estimated a divergence of approximately 425,600 years ago for the Clarion population—predating human colonization of the Americas. These findings support natural dispersal, likely through rafting on vegetation mats, as the mechanism of colonization. Iguanas are well known for their ability to colonize islands, and this represents their second‐longest overwater dispersal (> 1100 km); slightly greater than the distance of the Galapagos Islands from mainland Ecuador. Our findings demonstrate that Spiny‐tailed Iguanas are native to Clarion Island and should be considered an integral part of the island's native fauna. Conservation plans must prioritize the protection of this population, which we identify as an evolutionarily significant unit (ESU). Further genetic sampling and analyses are needed to determine the population's genetic variation and taxonomic status. Our findings challenge prior assumptions and emphasize the need for evidence‐based conservation strategies to preserve the integrity of oceanic island ecosystems.

Species can colonize oceanic islands by both natural and human‐assisted dispersal and human‐caused species introductions have had catastrophic effects on ecosystems around the world. The Spiny‐tailed Iguanas inhabiting Clarion Island were previously considered introduced. Molecular data and analyses support iguanas' arrival on Clarion Island by natural dispersal long before humans were in the Americas.