At the edge of meridionalization: the case of alien lizards Podarcis siculus Rafinesque-Schmaltz, 1810 (Sauria: Lacertidae) around lake Garda

Alien species constitute a serious threat to biodiversity, also contributing to reduce the ecological resilience of ecosystems to global climate changes. The Italian wall lizard Podarcis siculus Rafinesque-Schmaltz, 1810 (Sauria: Lacertidae) represents at this regard a case point since it has established exotic populations on three different continents, seeming capable of adapting to a variety of habitats, temperature ranges and food sources, also competing with native lizards through species substitution and/or hybridization. Even though endemic and nearly ubiquitous to most of the Italian peninsula, P. s. is considered relatively rare north of the Po’ river and its occurrence around Lake Garda was enlightened only very recently (2016), when authors discovered “new” P. s. populations in the municipality of Arco (TN), in the touristic area of Limone s/G (BS), in three garden centres in Padenghe s/G (BS), Peschiera d/G and Castelnuovo d/G, in Valeggio sul Mincio inside the historical garden park “Sigurtà” and in the pre-alpine hilly area Colline Veronesi (VR). We have re-confirmed the presence of P. s. on Trimelone islet (VR) as well, while we failed verifying all the other historical data regarding the southern edges of Lake Garda and nearby areas; our intensive surveys in the localities of Calliano, Rovereto, Spino and Ala (TN), Castellaro Lagusello (MN) and Mt. Maddalena (BS) lead us to sadly assess, albeit with some obvious misgivings, the local extinction of the species. We did not find the species even in other nearby historical localities cited by literature on the Astico river (Breganze, VI), on the Oglio river (Orzinuovi, BS) and in Montorio, Parona and Marcellise (VR). With the aim to better understand the conservation status of P. siculus in the southern Alps around Lake Garda, we have inferred the geographical origin of these newly discovered populations. Preliminary analysis of chromatic dorsal patterns, ventral scales number and SVL evidenced an extremely high inter-, and, in some cases intra-, population variability, together with typical traits of southern P. siculus lineages, not naturally occurring in the Po Plain. In addition, lizards exclusively inhabit restricted man-made areas as olive orchards, parks, private gardens, factories or garden centres increasing the chance for human mediated introductions. We used phylogenetic analyses sequencing 935 bp of the mitochondrial cyt-b gene and comparing our results with those from Podnar et al. (2005) and Senzuck (2017). Lizard’s DNA was obtained with non-invasive methods extracting it from fecal pellets or pieces of dead skin collected in the field. As expected, our results suggest an exotic origin of most of the analyzed populations. The central-southern Adriatic coast, together with the Province of Agrigento (Sicily) represent the two main areas from where alien P. s. have been imported. The presence of different haplotypes ensure the occurrence of multiple separated colonization events even in the same locality. In at least two sites, this phenomenon has realistically brought to hybridization, probably offsetting the negative consequences of the low genetic diversity typically occurring in newly established lizard populations. Aged olive trees trading activities clearly represent the main way by which P. s. was imported from southern to northern Italy. These old plants perfectly host P. s. acting as a Trojan horse for the colonization of new hostile environments, representing an all-in-one microhabitat, where lizards can find food and basking sites, lay eggs and survive to cold climate conditions during winter. Aged olive trees are frequently sold to private citizens for ornamental purposes, so garden centres act as stepping stones for a rapid and unpredictable lizard invasion, which will affect a large portion of northern Italy, firstly south of the 46° parallel. The presence of P. siculus exotic lineages brings forward major considerations on their impact on ecosystems and specifically on native P. s. campestris and P. muralis populations occurring in the same area. Our episodical observations regarding the ecological interactions with P. m. appear at least contradictory, since in human inhabited areas colonized by alien P. s. we have found P. m. very rarely, while garden centres and olive orchards host both species with high densities and without any apparent habitat delimitation between them. The diet of alien P. s. around Lake Garda seems to vary, sometimes including exotic fruits (eg. Passiflora sp., Capparis spinosa, Opuntia sp. and Trachycarpus fortunei); eating fruits means spreading their seeds and this makes P. s. an invasive alien species able to favour other invasive alien species throughout a positive feedback which starts from aged olive trees importation and ends up in a faster habitat meridionalization of the area. On the contrary, autochthonous P. s. lineages can be considered extremely sensitive to habitat alteration and to the colonizing power of allochthonous entities as Ailanthus altissima, Robinia pseudoacacia and Buddleja davidii. Each of these plant species are able to rapidly cover open areas specifically on sandy substrates (riverbanks or arenaceous deposits in hilly areas), which both apparently seems to be essential to the survival of autochthonous P. s. at the northernmost limit of its distributional range. It is therefore important to emphasise that, as far as we know, to these days the only remaining autochthonous P. s. populations in southern Alps around Lake Garda are those located in the Colline Veronesi (VR), which are on its own restricted to very small ecologically suitable areas, and are at risk to be completely replaced by P. s. exotic lineages, clearly better adapted to the Po Plain climate. Moreover, male-male competition for mate choice will be a big deal for smaller autochthonous, since exotic males reach bigger sizes (up to 9 cm SVLm, compared to 6 cm) and exotic females can host more eggs in their body, probably resulting in a higher reproductive success. If no conservation strategies will be adopted, we can expect the same fate for the remaining autochthonous P. s. populations north of the Po’ river, resulting in the complete loss of P. s. lineages endemic of this area.