The perils of an invasive snakethe California Kingsnake in the Canary Islands

Resumen

Invasive species—one of the most pressing challenges to the conservation of global biodiversity—show particularly damaging impacts on islands. Invasive snakes are especially concerning in this context, as they are currently present in various regions around the globe—including numerous islands—where they have devastating ecological consequences for native biota. Nonetheless, current information on these organisms mostly relates to a few notable examples, whereas the vast majority of introduced snakes remains to be studied. One of these cases is the invasive California kingsnake (Lampropeltis californiae), a colubrid snake native to the western coast of North America that was introduced as a pet to the island of Gran Canaria (Canary Islands, Spain). The species was detected in the wild in 1998, after a mass escape or relase event occurred, and it was declared naturalized in 2007, when the Government of the Canary Islands and the Cabildo of Gran Canaria launched a management program that continues today. Despite the substantial conservation efforts made to control L. californiae invasion, the species is still expanding its range all throughout the island, which currently holds four main populations and numerous isolated capture locations. Although some previous research has been performed on life-history traits, diet, population genetics, and parasitism of L. californiae on Gran Canaria, available biological and ecological information for the species was still scarce until 2017. Snake’s impact on native ecosystems was also totally unknown, except for the reported consumption of native and endemic species, especially the endemic reptiles—i.e., the Gran Canaria giant lizard (Gallotia stehlini), the Gran Canaria skink (Chalcides sexlineatus) and Boettger’s wall gecko (Tarentola boettgeri boettgeri). Given this situation, the present PhD thesis aimed to analyse direct and indirect impacts of L. californiae upon native ecosystems through four specific objectives that structure the different chapters of this document: (1) examine the direct impact of L. californiae on the abundance of the three endemic reptile species on Gran Canaria (Chapter 1); (2) analyze the phenotypic responses of these three endemic reptiles to the presence of L. californiae (Chapter 2); (3) test the possible occurrence of a trophic cascade affecting invertebrate abundance, mediated by L. californiae’s predation pressure upon reptiles (Chapter 3); and (4) determine the Canary Islands’ climatic suitability to potentially host L. californiae under current and predicted future climatic conditions as a way to predict the scalability of the detected impacts throughout the archipelago (Chapter 4). The abundance of the three endemic reptile species was first estimated and subsequently compared between sites invaded and uninvaded by L. californiae, using spatially explicit capture-recapture (SECR) and distance-sampling methods for G. stehlini and active searches for C. sexlineatus and T. boettgeri. Secondly, the phenotypic responses of the endemic herpetofauna to L. californiae predation pressure were evaluated by comparing reptiles’ morphologies and body conditions between invaded and uninvaded sites. Thirdly, trophic-cascade effects were examined by firstly identifyng those highly, moderately and less consumed invertebrate orders by endemic herpetofauna using metabarcoding techniques on reptile fecal samples, and subsequently comparing these invertebrate order abundances—obtained via pitfall, sheet-beating and foliage-sweeping sampling methods—between invaded and uninvaded sites. Finally, in light of detected impacts, the Canary Islands’ climatic suitability to host the invasive L. californiae was quantified using species-distribution modelling under current and predicted future climatic scenarios. The results from this PhD dissertation indicate that the abundance of all endemic reptiles was lower in invaded than uninvaded sites, with a reduction in the number of G. stehlini estimated through SECR and distance-sampling of 99.6% and 93.5 %, respectively, of 82.8% for C. sexlineatus, and of 52.1% for T. boettgeri. All reptile species also exhibited strong phenotypic responses that affected their morphology and body condition, although these responses varied among species. Additionally, depletion of reptile numbers triggered a trophic cascade that affected all highly consumed invertebrate orders, one moderatetly consumed order, and four less consumed orders, whose abundances increased in invaded sites either across the whole year or seasonally. Finally, the archipelago’s climatic suitability for L. californiae was determined to be extremely high under current climatic conditions and will increase in the near future with predicted climatic change. Overall, the present PhD is a clear indication of the severe ecological menace posed by L. californiae to native reptiles and ecosystems on Gran Canaria, and this could extend to all the Canary Islands, making this snake a serious conservation challenge to the archipelago’s endemic biodiversity. From a broader perspective, this PhD thesis underlines the necessity to increase general awareness of invasive snakes and gather further knowledge on these highly detrimental invaders worldwide.