The common wall gecko, Tarentola mauritanica is a native and widespread Mediterranean lizard (Vogrin et al. 2017). Therefore, in its native range the species is subject to mild wet winters, and warm and dry summers (Lionello et al. 2006). These specific climatic conditions coincide with the obtained response curves for the several temperature and precipitation variables used in this study. Not surprisingly, the correlative models predicted California, central Chile, the Cape Region of South Africa, around the Caspian Sea, south-eastern Asia, and south-western and southern Australia, as some of the most suitable geographic areas for this gecko to occur, which also correspond to the areas of the world with Mediterranean-type ecosystems (Esler et al. 2018; Sayre et al. 2020). Nevertheless, the cool temperate climate we find in Northern Europe is also predicted as moderately suitable for T. mauritanica.
Although this gecko species has been successfully introduced across several sub-tropical and tropical regions of the world (e.g., Madeira, Azores, Yucatán, Florida, Uruguay and Argentina in Arredondo, Núñez 2014; Báez, Biscoito 1993; Baldo et al. 2008; Barreiros et al. 2010; Díaz-Fernández et al. 2019; Huerta-Vera 2016; Jesus et al. 2008; Rato et al. 2015b), our ENMs have identified only one (not yet colonised) highly suitable tropical region located in south-western Iran, facing the United Arab Emirates Peninsula.
Despite some known limitations (Araújo, Peterson 2012), ENMs can be a powerful tool to predict where invasive species will spread next. However, until the last stage of invasion, an introduced species is not yet at equilibrium with its environment, as demonstrated in previous studies focusing on other invasive species (e.g., Barbet-Massin et al. 2018; Gallien et al. 2012; Václavík, Meentemeyer 2012). The equilibrium hypothesis is an important assumption, and its violation has to be acknowledged when interpreting ENMs predictions (Garcia et al. 2012). Indeed, violating the equilibrium hypothesis has some consequences when modelling species distribution, such as the underestimation of the potential climatic niche of a species, which can in turn underestimate the geographical area the species can invade (Václavík, Meentemeyer 2012). Considering that the introduction of T. mauritanica into tropical ecosystems has taken place only in the last 20 to 30 years, it is reasonable to assume that these populations have not yet reached an equilibrium with the novel environmental conditions, and that the habitat suitability models might be underestimating the probabilities of species occurrences. Actually, over the last three decades, the occurrence area of T. mauritanica in Madeira Island has increased by more than 20 km (Silva-Rocha et al. 2022). Moreover, the same study demonstrates that despite preferring Mediterranean-like climate areas, more humid regimes seem also suitable for the introduced populations of the common wall gecko in sub-tropical Madeira Island.
Furthermore, our niche overlap results demonstrate that T. mauritanica’s realised niche has not been conserved over space, as the naturalised climatic niche of the introduced populations differs significantly from its native one (e.g., Early, Sax 2014; Medley 2010; Parravicini et al. 2015). These results highlight two important aspects: first, there has been no climatic niche conservatism during the several introductions of T. mauritanica; and second, this species seems to be able to cope with novel and more humid environments.
Undoubtedly, biological invasions offer a rare opportunity to investigate how species colonise new environments (Kueffer et al. 2013; Richardson, Pyšek 2008; Sax et al. 2007), and whether they preserve their climatic niche in a new range (Pearman et al. 2008). Addressing this question has proven important in recent years as a test for ecological niche models, which depend heavily on climatic niche conservatism between native and exotic ranges (Colwell, Rangel 2009; Pearman et al. 2008; Peterson 2011). Evidence exists both for (e.g., Peterson 2011; Petitpierre et al. 2012; Strubbe et al. 2013), and against (e.g., Broennimann et al. 2007; Fitzpatrick et al. 2007; Lauzeral et al. 2011; Li et al. 2014; Medley 2010; Rödder, Lötters 2009) climatic niche conservatism during invasions, which is most likely related to the different types of niche change, biological and/methodological study contexts, data types, species characteristics, or methods being used (see references in Guisan et al. 2014).
The introduction and expansion of the common wall gecko populations across different tropical ranges (Arredondo, Núñez 2014; Baldo et al. 2008; Silva-Rocha et al. 2022), suggests that the species can cope with more humid environments. However, as the response curves demonstrate, when humidity is too high, the habitat becomes unsuitable for T. mauritanica. This is most likely due to the limited favourable conditions viable for gekkotan reproduction, particularly concerning humid environments. Like most gecko lizards, T. mauritanica produces rigid-shelled eggs (a pre-adaptation to arid environments) that in conditions of high humidity can limit embryo development (Pike et al. 2012). Moreover, evidence from the gecko Chondrodactylus turneri suggests that under high moisture conditions, fungal infections can decrease the viability of hard-shelled gekkotan eggs (Andrews 2015). Therefore, unless the individuals inhabiting the tropics can select suitable micro-habitats for egg laying, the future of these populations might be compromised under current climatic conditions. Yet, the climate is changing at a global scale and at such a fast pace that species are more likely to change their distributions as a response than to adapt in situ (Bradshaw, Holzapfel 2006).
Indeed, the species distribution models projected to 2061–2080, forecast that the range of T. mauritanica is likely to shift towards northern latitudes but, surprisingly, not to expand. South-eastern Spain and parts of North Africa will become unsuitable, while the south of the UK and the Scandinavian Peninsula seem to offer more relevant environmental conditions for this gecko species. Furthermore, the climte in the tropics will become unsuitable, while parts of the north-eastern USA seem to favour the establishment of T. mauritanica. At least in the Iberian Peninsula and France, a northward shift in the range of the common wall gecko’s populations has already been documented as a response to global warming (Geniez, Cheylan 2012; Moreno-Rueda et al. 2011). According to Sumner et al. (2003), by the late twenty-first century, the far south of Spain will undergo a continued spread of aridity of the climate, extending westwards. This is already the hottest and one of the aridests parts of Spain, and the predicted environmental changes will be disastrous to several taxa of the region (e.g., Algyroides marchii in Rato et al. 2021b), including a thermophilus reptile such T. mauritanica.
Contrary to what we obtained with the species distribution models, the dispersal models forecast that colonization by the common wall gecko will still take place across southern Spain and North Africa. Indeed, the largest colonisation will take place in the European continent, with T. mauritanica expanding its territory towards the northern and eastern regions. In South America, the dispersal models also predicted a potential expansion to the south, and towards the north following the Atlantic coast of Brazil. The population from California is also expected to colonise more northern territories.
The common wall gecko occurs naturally in typical Mediterranean climate and has mostly a crepuscular activity. It thermoregulates during the first 2–3 h of the day near the refugium, to where it returns once their optimal temperature is achieved (Martínez-Rica 1974). In comparison to other sympatric similar-sized lacertid lizards, this gecko has a higher resistance to dehydration (García-Muñoz, Carretero 2013; Osojnik et al. 2013), with great plasticity among populations for this trait (Rato, Carretero 2015). Indeed, this feature conveys the common wall gecko with a great capacity to withstand long periods without water, and able to survive during transmarine trips. Moreover, future global warming would not be a limiting factor for this species, as already documented in previous studies (Geniez, Cheylan 2012; Moreno-Rueda et al. 2011). In fact, Moreno-Rueda et al. (2011) saw that the southern range of T. mauritanica has not been affected by global warming during the last 70 years. Therefore, both the predicted future range and colonization models are in line with the ecophysiological requirements and dispersal capacity of this species.
Overall, globalisation is increasing the frequency in translocating species from their native ranges to novel transmarine environments. Over the last 30 years, the native Mediterranean common wall gecko, Tarentola mauritanica, has been introduced across several tropical and subtropical regions of the globe, where it has found novel environmental conditions. Our results confirm that the realised niche occupied by these introduced populations is significantly different from their native one, highlighting the capacity of this species to cope with novel ecosystems, which is key to a successful colonisation. Over the next 60 years, the geographic range of this gecko is likely to shift, benefiting from global climate change. The lack of knowledge on the effects it might have on local species and ecosystems reinforces the need for strong and serious monitoring actions.