Impact of invasive snakes in the balearic islandsan ecological and molecular approach

Laburpena

Biological invasions have strongly increased during the last decades as a direct consequence of globalization and the associated emergence of market routes, faster transport and the intensification of travel and tourism worldwide. The invasion process includes four stages: transport, introduction, establishment and spread. The success of the invasion process is affected by social, biological, ecological and climatic factors, so it strongly varies in function of life history traits of the species, the introduction effort, the characteristics of the receiving area and, finally, the introduction pathway. Although the consequences derived from biological invasions are difficult to predict and evaluate, it is known that they cause ecological, evolutionary and socioeconomic impacts on the receiving ecosystems, which are especially deleterious on islands. In recent decades, three snake species have been introduced to the Balearic Islands through nursery trade from the Iberian Peninsula, their native range. The horseshoe whip snake Hemorrhois hippocrepis was reported for the first time in 2003 in Eivissa, the ladder snake Zamenis scalaris in 2006 in Formentera and the Montpellier snake Malpolon monspessulanus in 2006 in Mallorca. Currently, any species of the family Colubridae inhabiting Eivissa and Formentera is considered legally invasive according to the Spanish legislation. Extensive control and eradication campaigns using novel traps baited with live mice are carried out in these two islands since 2016 and continue nowadays. In contrast, exotic snake species inhabiting the island of Mallorca are not considered legally invasive by the Spanish law. In this case, low-scale control and eradication campaigns started in Mallorca in 2017 and are still currently working. The scope of this plan is significantly lower than the one conducted in the Pityusic Islands, as snake species remain non-categorized as invasive. The results of this dissertation suggest that the invasion process has successfully completed the four steps for H. hippocrepis and Z. scalaris, as both species are well established in Eivissa and Formentera, respectively, and they have spread from their entry points. A complex combination of factors such as multiple introductions, traits of the species as elevated reproductive potential or trophic plasticity and characteristics of the invaded area as climate matching would have contributed to the invasion process. Regarding the Montpellier snake, M. monspessulanus, although its complete spread across the island of Mallorca has not yet been reported, its establishment has been proven. However, generalist traits of the species such as a large native area of distribution, euriphagia and the ability to adapt to altered ecosystems suggest a highly probable successful spread throughout the island in a near future. Among the ecological and evolutionary impacts detected on insular fauna, behavioral and physiological antipredatory responses of the moorish gecko Tarentola mauritanica individuals exposed to the scent of H. hippocrepis have been detected, a fact that evidenced that the moorish gecko recognizes the chemical signals of the horseshoe whip snake after sixteen years of its first report at the island of Mallorca. Costs derived from prevention and management, economic losses from predation of domestic animals and irreversible scientific loss would be some of the socioeconomic effects of these biological introductions. The current legislation has proven to be ineffective in terms of prevention of transport and introduction of these exotic snakes, given that any biocontrol measure has never been applied in the Balearic Islands to avoid the introduction of exotic snakes. Besides, despite the early detection of these species, standardized control programs lasted more than a decade to be implemented after the first reports of the snakes. However, the strong capture pressure exerted during the long-term control and eradication plans applied since 2016 is believed to have caused a depletion of population densities, especially Z. scalaris in Formentera. With all this background, there is enough information to foresee the potential implications of snake naturalization and spread in insular ecosystems and to properly prevent and manage the introductions of these species in the Balearic Archipelago. For all these reasons, this dissertation suggests a combination of research actions, prevention and control programs and, finally, financial and administrative support of the local and national governments as further proposals not only to manage the current snake populations established at the Balearic Islands but also to prevent future potential introductions of both these and other species.