A golden cage for the European red squirrel in Italy? Proposal for a targeted control of the grey squirrel

The grey squirrel is regarded as an invasive non-native species for Europe. This species is included in the EU 1143/2014 Regulation and listed in the international list of 100 of the worst invasive non-native species (Lowe et al. 2000). Due to this legal proscription, it is illegal to release a grey squirrel into the wild, or allow one to escape, and it is mandatory for all EU Member states including Italy to eradicate the species if feasible, or effectively control it, particularly where there is a risk of exporting it to other Member states. However, the natural expansion of the nuclei introduced in Europe is favouring the colonisation of new areas, increasing the non-native distribution of the invasive species. In fact, introduced tree squirrels can establish self-sustaining populations from only a few founders and are likely to become invasive and cause problems to native species (e.g. Wauters et al. 2005; Bertolino et al. 2014; Mazzamuto et al. 2016, 2017a, b). Alien tree squirrels have a strong propensity to spread through the landscape despite wide roads and land-use changes that are predicted to cause a loss in suitable habitat and dispersal corridors, potentially limiting their range expansion (Chen & Koprowski 2019; Di Febbraro et al. 2019). Their ecological impact on habitats and native wildlife can be dramatic, particularly on native tree squirrels. In fact, grey squirrels have caused, and continue to cause, the extinction of red squirrel populations through interspecific competition for high-quality food resources. This reduces female reproductive success and juvenile recruitment (Wauters et al. 2002a, 2002b; Gurnell et al. 2014) and, on the British Isles, the transmission of a squirrel poxvirus (SQPV, micro-parasite spillover) from the alien reservoir host to native red squirrels for which, in most cases, the disease is fatal (Tompkins et al. 2002; Everest et al. 2019; McInnes et al. 2020). Moreover, the effects of the alien species’ presence on native red squirrels extend also to personality and physiology; in red/grey sites, red squirrels show higher sociability and higher concentrations of stress hormones than in red-only sites (Santicchia et al. 2018; Wauters et al. 2019). Unlike Great Britain and Ireland, there is no evidence of SQPV circulating in Italy (Romeo et al. 2019) but spillover of an alien endoparasite (Strongyloides robustus) introduced by grey squirrels has been reported (Romeo et al. 2015). Santicchia et al. (2020) showed that in red squirrels this infection reduces the host’s expression of activity an energetically costly personality trait. Range expansion of grey squirrels is occurring, at different rates, in some areas in Italy (e.g. Piedmont, Lombardy, Veneto) and also in the United Kingdom (Martinoli et al. 2010; Bertolino et al. 2014; Gurnel et al. 2014; Signorile et al. 2016; Loy et al. 2019). In Ireland, the trend seems to have stopped, or even been reversed, apparently associated with an increase in the pine marten population (Flaherty & Lawton 2019). This phenomenon of generalised expansion, with the exception of some specific areas (e.g. Parks of Genoa Nervi in Italy where the grey squirrel has been eradicated), is partly due to the low priority given to it by public administrators and/or politicians in charge of territorial management. In part, it may also be due to limited support from local social groups perhaps emanating from poor media coverage and biased media pressure (Lioy et al. 2019). Another inhibitor is the high costs envisaged for management actions over such a large-scale project and over such a long period of time. From this point of view and with the need to enhance and fully respect European Regulation management directives it is necessary to evaluate, from a cost/benefit perspective, which management strategy is the most appropriate to promote. In particular, it is necessary to evaluate whether a resolutive intervention to eradicate the various nuclei present is preferable taking account of the need for a large number of operators, reduced intervention times and coordination. Alternatively, a long-term control strategy aimed at limiting the expansion of grey squirrel populations, particularly in areas with higher biodiversity and