It is known that invasive species often do not present a static niche. This niche can expand, contract or change, reducing the predictive capacity of distribution models based on niche conservatism for species with a dynamic niche (Becerra López et al. 2017; Broennimann et al. 2007; Fitzpatrick et al. 2006; Pearman et al. 2008). For this reason, we integrally georeferenced the population of V. velutina from Mallorca (Balearic Islands) (Leza et al. 2021) and developed a predictive model for the entire island to obtain a distribution map of the habitat suitability with management purposes (Peterson, Papes, and Kluza 2003). Furthermore, the high population density and dispersion rate of this invasive species highlight the real necessity for an early detection and eradication measures. This need is even more pronounced in biodiversity hotspots like Mallorca, which account for high endemicity levels that may be threatened by alien invaders (Bessa et al. 2016).
To achieve it, we used a specific strategy for the species distribution model with few occurrences (Breiner et al. 2018) which has been low used in the management of invasive species (Beukema et al. 2018; Di Febbraro et al. 2019). These studies cited before showed this methodology had good results for assessing the invasiveness of an alien species in a territory, representing an important step forward to define their potential spread and provide a sound base for prioritize management actions.
Our case study represents the first time that V. velutina species reaches a Mediterranean island after its arrival to Europe in 2004 (Haxaire, Bouguet, and Tamisier 2006), and despite accounting for few occurrences it has been possible to determine its optimum niche in the first stages of invasion for an area that, a priori, seems to be low suitable (Villemant et al. 2011).
As a general view a great part of the island appears less suitable for V. velutina (Fig. 2), in agreement with previous studies reporting the low suitability of the Mediterranean environments (Barbet-Massin et al. 2013; Villemant et al. 2011). Nonetheless, our distributional model calibrated under Mediterranean island conditions proves that there are suitable areas for the establishment of V. velutina (most part of “Serra de Tramuntana”, 307.45 km2) (Fig. 2). The ecological niche model developed for the Iberian Peninsula and the Balearic Islands, based on Villemant et al. (2011), shows similar suitability values from Sóller (Mallorca) to the north of the Iberian Peninsula where this species first stablished and spread (MAGRAMA 2015). Likewise, the spread modelling developed by Robinet et al. (2019) concludes that V. velutina could effectively establish in Mallorca. The comparison among the above mentioned studies and our model reflects that analyzing at different scales results in different patterns, with regional scales providing more suitable areas than continental ones (as in Bessa et al., 2016). Furthermore, this hornet and other invasive terrestrial pests are mainly translocated as a by-product of shipping, where the characteristics at arrival ports could influence the invasive spread rates across territory (Hudgins, Liebhold, and Leung 2017; Sardain, Sardain, and Leung 2019). In the Mallorcan suitable area there is the port of Sóller with commercial connections with the Iberian Peninsula and France, both areas invaded by V. velutina (Laurino et al. 2020). Likewise, literature cites the invasion of the argentine ant (Linepithema humile) in the Balearic Islands across the same port (Bernard 1956).
Since November 2020, V. velutina is officially eradicated in the island after two years of active searching with no detections (CAIB 2019). However, according to our results and if an early detection and eradication plan would not have been implemented, the species probably would have colonized and stablished in the entire northwest part of the island (Fig. 2), as predicted by Robinet et al. (2019). In fact, we detected an increase in the number of nests, 1 in 2015, 9 in 2016 and 20 in 2017, as in other colonized areas (Bertolino et al. 2016; Monceau and Thiéry 2016) showing the potential spreading of the pest.
Future studies should include density data of hives because honeybees are a quite important food resource (Bessa et al. 2016), as well as freight traffic (Lester and Beggs 2018; Pusceddu et al. 2019).
Principal component analysis (2D-PCA) of the environmental data revealed one significant axis of environmental variation, defining the suitable ecological space for V. velutina establishment (Fig. 4). It is important to note that since 2015 (Fig. 4, green dot) until 2017 (Fig. 4, pink dots) the ecological niche has shift, moving away from the prevailing ecological niche offered by the rest of the island, despite the overlap between background and V. velutina niches. This niche shift occurs along the first axis, which is associated with topography slope and isothermality gradient across the island. Supporting this idea, V. velutina will be able to spread and stablish in specific areas under Mediterranean conditions (Robinet, Darrouzet, and Suppo 2019).
Another important point is the approach of using niche models to predict the spread of potential invaders into new areas (Broennimann et al. 2007). Our results show that other Mediterranean islands are potentially suitable for the establishment and spread of V. velutina (Fig. 5), zones where this species has not been recorded yet (Laurino et al. 2020), making it necessary to implement preventive actions to avoid a possible invasion. Robinet et al. (2019) explored the human-mediated dispersal of V. velutina concluding that the Mediterranean islands could not be naturally colonised, this invasive species just could reach the Mediterranean islands by an accidental introduction by humans. Although the probability calculated of introducing the hornet in the Mediterranean islands is relatively low in this study: to Sicilia was 0.925%, to Sardinia was 0.206%, to Mallorca was 0.095% and to Corsica was 0.032% (Robinet, Darrouzet, and Suppo 2019). V. velutina reached Mallorca in spite of the low probability of introduction, so these model extrapolations is a valuable information regarding the areas in the other Mediterranean islands that might be invaded next due to it could be used by scientists and managers for an early detection of the invasive species (Fig. 5) (Broennimann et al. 2007), because it is more cost-effective outcome for conservation and can be most efficiently controlled (Holden, Nyrop, and Ellner 2016; Monceau, Bonnard, and Thiéry 2014; Robinet, Suppo, and Darrouzet 2017). There are detection and control methods for V. velutina across Europe, however, little has been done to date to limit its progression (Turchi and Derijard 2018) and there is no clear coordination between countries neither uniform methods for eradication since the first detection (Leza et al. 2021). Poor coordination between managers and stakeholders in France was thought to be one of the important reasons for the species continues to spread (Monceau, Bonnard, and Thiéry 2014). Hence, long-term monitoring of ports and nearby areas should be implemented for an early detection of V. velutina to be in time to implement a control and eradication protocol, as in Leza et al. (2021). Moreover, many studies have confirmed that citizen collaboration in control programs is essential for the best management of invasive species (Clusa et al. 2018; Graham, Henderson, and Schloss 2011; Klemann-Junior et al. 2017). In the case of V. velutina, a specific trap for this species has not yet been developed and for this reason citizen collaboration is crucial for its detection and control.
Future research will focus on the genetic characterization of the Balearic population of V. velutina to determine the path of introduction and to assess the role of shipping networks in spreading V. velutina across the Mediterranean Sea, which could help in promoting preventative measures such as the establishment of stringent biosecurity policies at the national, regional, and global scales.