The role of niche boundaries and dispersal limitations in shaping
range limits
The accuracy of our prediction of range limits suggests that they are
revealing ecological niche boundaries to some extent. If other
environmental factors (e.g. lack of prey or presence of competitors,
predators or parasites) had been constraining range expansions, our
inferred range would have extended beyond real range boundaries, and
realized range edges would be explained by the existence of limitations
to expansion before fulfilling all cells within the spatial projection
of the species’ niche (Holt 2003, Soberón and Peterson 2005, Holt 2009).
In fact, this happened only in the northeastern border of the range,
where GEAM predicted the presence of P. algirus beyond the
barrier imposed by the Rhône River delta (niche boundaries would
actually allow the species to reach Italy).
Also, our results point to an apparent lack of demographical
center-border effects such as gene swamping or increased homozygosity
near the edge of the range (Herrera and Bazaga 2008; Polechová et al.
2009; Pironon et al. 2017). If these processes were acting, they would
be limiting the persistence of marginal populations near range
boundaries, and we would systematically infer false positives beyond
range limits (Case and Taper 2000; Bridle and Vines 2007; Lee-Yaw et al.
2018). However, this line of reasoning may be wrong, given the likely
polygenic nature of adaptations to temperature and humidity (e.g.
Rodríguez et al. 2017), if our approach had missed many of the
small-effect variants that contribute to such adaptations; these
variants should be relatively prone to gene swamping, showing lower
core-margin differentiation that the one we inferred (Lenormand 2002,
Yeaman 2015). In that case, both allele fixation at a small number of
major-effect loci (either the ones included in our model or others with
similar differentiation patterns), and gene swamping at a large number
of small-effect loci not included in our model, could explain the shape
of range limits. In fact, we did wrongly infer a relatively large inland
area of false positives on the northern side of the Pyrenees (Fig. 3 and
Fig. 4), which could thus be interpreted as consistent with the
hypothesis of gene swamping. However, this area was suitable for a small
number of genotypes (< 3 out of 46), so that another
plausible explanation for these false positives is the low probability
that the few genotypes that could be adapted to these unoccupied areas
were available in nearby marginal populations (Pujol et al. 2009; Dawson
et al. 2010; Barton and Etheridge 2018).
The dispersal ability of the genotypes arising at range margins could
play an important role in the colonization of new areas beyond range
limits (Simmons and Thomas 2004; Hardie and Hutchings 2010). In our
system, for example, the range of P. algirus would extend ca.
13% beyond its eastern European border if lizards were able to disperse
across the Rhône River and no other factors were constraining its
expansion. Furthermore, genetic diversity would be fostered by greater
dispersion abilities (Duckworth 2008), which should facilitate the
co-occurrence of adaptive allele combinations at range margins. For
instance, the expansion of these small terrestrial ectotherms towards
suitable areas north of the Pyrenees (false positives in our model)
could be constrained by the synergistic effects of low dispersal rates
and a low probability of finding adapted genotypes at nearby marginal
populations. To clarify the relative role of dispersal ability vs. other
possible constraints in the colonization of new areas beyond range
limits (see above), our genotype-based modelling approach could be
applied to species showing different dispersal abilities (Sanford et al.
2006; Dawson et al. 2010), or we could perform transplant experiments
beyond range limits (i.e. manipulating species’ dispersal abilities;
Hargreaves et al. 2014). By doing this, we should be able to discern
between the role of dispersal ability per se and the genetic
contribution of pre-adapted genotypes arising (or not) at marginal
populations (Bridle and Vines 2007; Sexton et al. 2009; Phillipsen et
al. 2015).