Methods
The locations of known established P. muralis populations were obtained from the Surrey Amphibian and Reptile Group website (Langham, 2019). We determined the current geographic extent of as many of these populations as logistically possible using a combination of visual surveys, canvassing of the local public at sites of interest, and press releases in local and regional media encouraging members of the public to report their wall lizard sightings (see Appendix 1). Of the 30 extant populations recorded on the UK mainland, we visited 21 between three field seasons (April-September) of 2016, 2017, and 2018 (Fig 1). We did not visit the remaining nine locations because the lizard population were either known to be very small, access was restricted, and/or site locations were otherwise logistically challenging (i.e. distance from other populations). Specific attention was given to assessing the extent of P. muralis presence along railway habitat at West Worthing, Sussex (50.818° N, 0.390° W) during a five-week period in June-July 2018. The railway acting as a linear transect, along which we could assess the utility of railway habitat as a corridor for dispersal (Fig S1 Appendix 1). We also collected wall lizard location data from populations at two additional sites, Eastbourne (50.768° N, 0.291° E) and Kingswear (50.349° N, 3.568° W). The former was confirmed to be an established colony through a site visit, and the latter was reported to RW by a member of the public responding to a citizen science campaign in regional media. Sightings were confirmed for a further eight new locations as a result of the citizen science campaign (Fig 1) (see Appendix 1 for detailed methods).