Conclusions
Extensive nationwide range expansion through secondary introduction is
likely to be restricted by currently unsuitable climate beyond southern
regions of the UK. However, exponential growth of local populations in
habitats providing transport pathways (i.e., movement of aggregates,
timber, plants, general public) is likely to increase opportunities for
regional expansion. A similar scenario has been described in the spread
dynamics of invasive Pallas’s squirrel (Callosciurus erythraeus ),
where a constant increase in the appearance of new populations occurred
after a two decade lag, and could be explained by increased vector
activity (intentional translocations) as the population size at the
initial introduction foci increased – causing a potentially exponential
increase of translocation events when translocated populations start
acting as a source themselves (Guichon, Benitez, Gozzi, Hertzriken, &
Borgnia, 2015).
Despite the fundamental physiological importance of sun exposure to
diurnal reptiles, to the best of our knowledge, our models are the first
to incorporate estimates of solar insolation into models of probability
of occurrence at this fine scale (but see Bennie, Huntley, Wiltshire,
Hill, and Baxter (2008) for a detailed discussion on use of solar influx
as a variable in modelling fine scale topographic microclimate). Our
models demonstrate the inclusion of the variable at this scale, and
indeed our entire approach to developing a fine scale SDM, could be very
useful in other applications relating to ectotherm ecology (e.g., in
developing habitat suitability indices, directing habitat management,
guiding survey effort for rare/cryptic species). Furthermore, whilst the
use of SDMs and IBMs have become a widely used to further understanding
of mechanisms driving invasion dynamics (Kadoya & Washitani, 2010;
Fraser et al., 2015; Suzuki-Ohno et al., 2017), the benefits of
incorporating spatially explicit individual based models
into management plans for the control of invasive species has only
recently been recognised (Day, Landguth, Bearlin, Holden, & Whiteley,
2018). In this regard, our models provide a best estimate for future
expansion of P. muralis at both the UK national and local scale,
providing essential information (i.e., dispersal patterns, key habitat,
current and projected population sizes) on which management decisions
could be made. As our models show UK populations may be approaching an
end to an inherent lag phase, there is argument to suggest that a timely
precautionary intervention may be justified to halt some populations
before an abrupt end to the lag phase occurs.
Data accessibility: the following data is in the process of being
deposited with EIDC
-MaxEnt (local and national scale) input files including presence and
background samples with climate/environment data
- Inputs for Rangeshifter models and subsequent population outputs