Snakes on the Balearic islands: an invasion tale with implications for native biodiversity conservation.

Iolanda Silva-Rocha,Jose A Mateo,Neftalí Sillero,Daniele Salvi,Miguel A Carretero,Diego Fontaneto

doi:10.1371/journal.pone.0121026

Abstract

Biological invasions are a major conservation threat for biodiversity worldwide. Islands are particularly vulnerable to invasive species, especially Mediterranean islands which have suffered human pressure since ancient times. In the Balearic archipelago, reptiles represent an outstanding case with more alien than native species. Moreover, in the last decade a new wave of alien snakes landed in the main islands of the archipelago, some of which were originally snake-free. The identification of the origin and colonization pathways of alien species, as well as the prediction of their expansion, is crucial to develop effective conservation strategies. In this study, we used molecular markers to assess the allochthonous status and the putative origin of the four introduced snake species (Hemorrhois hippocrepis, Malpolon monspessulanus, Macroprotodon mauritanicus and Rhinechis scalaris) as well as ecological niche models to infer their patterns of invasion and expansion based on current and future habitat suitability. For most species, DNA sequence data suggested the Iberian Peninsula as the potential origin of the allochthonous populations, although the shallow phylogeographic structure of these species prevented the identification of a restricted source-area. For all of them, the ecological niche models showed a current low habitat suitability in the Balearic, which is however predicted to increase significantly in the next few decades under climate change scenarios. Evidence from direct observations and spatial distribution of the first-occurrence records of alien snakes (but also lizards and worm lizards) suggest the nursery trade, and in particular olive tree importation from Iberian Peninsula, as the main pathway of introduction of alien reptiles in the Balearic islands. This trend has been reported also for recent invasions in NE Spain, thus showing that olive trees transplantation may be an effective vector for bioinvasion across the Mediterranean. The combination of molecular and ecological tools used in this study reveals a promising approach for the understanding of the complex invasion process, hence guiding conservation management actions.

Highlights

Biological invasions are a major cause for current species extinctions, together with the destruction and fragmentation of habitats and climate change [1,2]
Islands ecosystems are especially vulnerable to biological invasions since many insular native species have been evolving long time in isolation often losing their ability for competing with other species, eluding predators or defending themselves from parasites, which may arrive from nearby continents [6,7,8,9]
The process is so intensive and widespread that even the distinction between allochthonous and autochthonous species may be problematic [10]. This is true for reptiles, one of the groups most widely introduced in Mediterranean islands, but at the same most threatened by introductions [9,11]

Summary

Introduction

Biological invasions are a major cause for current species extinctions, together with the destruction and fragmentation of habitats and climate change [1,2]. The rates of these processes, and in particular of biological invasion, have strongly increased during the last century due to human growth and globalization causing a parallel increase in the rate of biodiversity loss [3,4,5]. The process is so intensive and widespread that even the distinction between allochthonous and autochthonous species may be problematic [10] This is true for reptiles, one of the groups most widely introduced in Mediterranean islands, but at the same most threatened by introductions [9,11]

Methods

Results

Conclusion