Invasive species represent a major threat for biodiversity. The numbers of independent introductions, introduced propagules, and introduction episodes are critical aspects for invasion success. Here, we traced the source(s) of introduction and determined the historical route of invasion in order to understand the main stages of the invasion process. However, we often must rely on indirect information when studying invasive species (i.e., not knowing where the invasive population originated), hence requiring robust analytical methods to solve those questions. The invasive population of the snake Boa constrictor in the Mexican Caribbean (Cozumel Island) has been studied ecologically and genetically, but, despite several lines of evidence suggesting its invasive nature, a full account of its invasive history is lacking. Here, we aimed to reconstruct the boa's invasion history by deciphering the original source(s) of the Cozumel population, routes of invasion, likely number of propagules, and estimation of historical genetic and demographic parameters, based on a comprehensive set of analytical tools including tree topology-based methods and Approximate Bayesian Computational algorithms. The phylogenetic relationship of the Cozumel boa within the Boa constrictor complex was unknown; hence, to identify the source populations, we first needed to clarify its genealogical relationships. We used mitochondrial and nuclear sequences and nuclear microsatellites, together with the widest geographic sampling along the species' entire continental distribution. With our genetic approach, we demonstrate that the Cozumel population was derived from an admixture of individuals from different geographic localities. Moreover, our demography results allowed us to successfully confirm both anecdotal and previous genetic information, concordant with a scenario in which a likely small number of propagules were released on the island about 50 years ago. Notably, national law hinders the possibility of performing any control protocols for the boa, hence our results highlight a rather unique conservation paradox, where the Cozumel boa has a novel endangered protected species status as B. imperator, but it is also an invasive exotic predator threatening the critically endangered endemic and native biota of Cozumel. Therefore, any conservation decisions should consider that boas in Cozumel are invasive, opening the possibility to legally allow implementing control or eradication programs.
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