Discussion
As documented in Guisan et al. (2014) and Sexton et al. (2017), recent
applications of niche models (based on ENMs and PCA-env) analyze niche
shifts have provided new insights to the roles of abiotic climate and
geographical conditions in shaping range limits.
Moreover,
intraspecific
variation along allopatric genetic clades in the potential for realized
niche shifts has important implications not only for forecasting
biological invasion, but for studying the process of origin and
evolution across clades within species level.
Our results agree with previous
progressive
uplifts of Tibet (Mulch et al. 2006). Molecular datings of E. D, W. a
and W. b are almost identical to the fourth uplift of high mountain
ranges and aridification of Central Asia (ca. 20 to 10 Ma), and the
divergence times of clades E. A, E. B and E. C are coincident with the
final extension of the last uplift (ca. 10 Ma to present) predicted by
previous studies (Favre et al. 2015). However, future researches with
more detailed complementary analyses are needed to understand a link
among clades molecular dating, gene flow routes and past geological and
climatic changes across these regions.
Our results show PCA-env based approach well supports E. A, E. B and
partial of E.C clades with significant divergence. However, ENMs based
method just supports divergence of E. B in bi-directions, while the
divergence of E. A exhibits one-sided significance, the opposite
comparisons not deviated from null expectation. While inconsistent
conclusions drawn in divergence of E. C based two methods. In
literatures, inconsistent conclusions regarding niche conservatism or
divergence have been frequently reported based on different approaches.
For example, Guo et al. (2013) applied the ordination and ENMs for the
globally introduced Phragmites australis and found inconsistent
results. Our results based on two approaches draw almost the same
conclusion in one-sided tests, but PCA-env based approach is sensitive
to niche divergence, while ENMs is more inclined to niche conservatism
in bi-directions. We analyze reasons leading to these differences
further and find the native-range model, such as ENMs, always
under-predicts the species’ invaded range in exactly the types of
climates in which there was evidence of niche expansion as shown in
previous studies. Strubbe et al. (2013) showed that the predictive
performance of native range ENMs increased with increasing niche overlap
and decreased with increasing niche change. Similarly, Tingley et al.
(2014) found that a native-range ENMs under-predicted the extent of the
species’ Australian invasion.
In addition, we found E. A, E. B and E. C clades within S.
boulengeri have an obviously expansion of climatic niche (Figure 3h and
Supplementary material Appendix Figure A5). Intriguingly, they shift
beyond the realized niche with the new conditions but still overlap the
fundamental niche, which provides positive proof for the niche
conservatism hypothesis (Wiens 2004, Kozak and Wiens 2006) and niche
divergence hypothesis (Graham et al. 2004, Evans et al. 2009) are not
contradictory. We found a gradient of realized niche change in the
invaded ranges across clades within S. boulengeri : niche stasis
on E. D (96.3%) and W. a (96.6%), niche unfilling in W. b (75.2%),
and niche expansion (vs. E. D and W. a separately) in E. A (mean =
99.3%), E. B
(mean
= 97.1%) and E. C (mean = 90%). Our results seem to be inconsistent
with conclusion of previous studies in Petitpierre et al. (2012), in
which realized niche shifts between the native and invaded ranges were
largely due to niche unfilling. Our results are also different from the
results of cane toad in Tingley et al. (2014): the shift in the realized
niche of the cane toad Rhinella marina was solely due to niche
expansion. In our results, niche expansion into novel environments is
more popular than niche unfilling, suggesting that our niche divergence
due to niche expansion in the shifted range and thus represents true
niche changes. Why do E. D and W. a fail to fill its fundamental niche
in its native range? One possibility is that the presence of closely
related species (S. glandulatus and/or S. mammatus ) may
prevent S. boulengeri from colonizing suitable environments south
of its present range. Indeed, previous study found there were low rates
of interspecific hybridization (Chen et al. 2009). We cannot exclude
dispersal limitation in the native range as a possible contributing
factor, such as Jinsha River and Yalong River (Li et al. 2009), which
also can enforce stable parapatric range boundaries. Future studies will
be able to test this hypothesis using laboratory or field experiments.
Numerous examples of rapid adaptation in non-native niche suggest that
rapid evolution may be common during invasions in species level (Alström
et al. 2015, Bartels et al. 2012, Sherratt et al. 2017). The degree to
which species adapt to novel environments is important to a range of
topics in ecology and evolution (Wiens et al. 2008), but is of special
concern for the study of intraspecific niche evolution (Tingley et al.
2016). In our study, niche divergence caused by niche expansion indeed
accompanied key morphological innovations of preadaption in novel
climates vs. niche unfilling and stability. Our finding of significant
phylogenetic signal in SVL (Table 5) and Elevation (Table 4 and Figure
2c), indicates that these acquired data are not random and our results
are robust. Furthermore, our findings of significant phylogenetic signal
in these traits are consistent with previous studies (Freckleton et al.
2002, Blomberg et al. 2003,
Oufiero
et al. 2011). We found that Elevation (AIC = 21.3; P = 0.002),
Isothermality (AIC = 24.47; P = 0.007), Mean diurnal range (AIC = 29.33;
P = 0.037) and Max temperature of warmest month (AIC = 29.31; P = 0.037)
are significantly negative predictors of SVL under phylogenetic models,
which suggest S. boulengeri toads from warmer and more arid
environments tend to be larger, which is in concert with true records in
our field work.
Several factors may underlie the observed pattern of SVL variation inS. boulengeri clades. One possibility pertains to the expected
relationship between fasting endurance and SVL (Mautz 1982). The second
possibility is ecological release in novel shifted areas may allow for
larger SVL (Losos and Queiroz 1997, Yoder et al. 2010). The third is
likely that maintenance of preferred body temperature influences the
evolution of SVL (Oufiero et al. 2011). Our results highlight reduced
competitors (ecological release) in newly shifted niche may be the most
likely reason for enlarged SVL in E. A, E. B and E. C clades.
Moreover, we found species tolerance of newly shifted niche tends to
have morphological attributes important for locomotor performance. These
traits may be a key preadaptation in toads that helps overcoming the
challenge of insufficient precipitation or high temperature in novel
habitats, which are in accord with the evolutionary shifts mechanistic
model highlighted by prior studies
(Tingley
et al. 2014, Phillips et al. 2010, Kolbe et al. 2010). In our study,
LAHL (mean value: 27.19 mm in clade E. A, 26.27 mm in clade E. B and
23.95mm in E. C, 21.77–23.57 mm in the remained clades, Supplementary
material Appendix Table A1), HLL and FL have the same trend.
Interestingly, these character values, without size-correction, have a
high phylogenetic signal, best fitted BM model. However, once
size-corrected, these character values will have a completely different
scenario — with a low phylogenetic signal but trait correlations still
exist, there seems to be a trade-off strategy by
locomotor
performance combined enlarged SVL for speed and endurance in thermal
reaction norms (Angilletta et al. 2003). Collectively, we found
phenotypic plasticity
(i.e
LAHL, HLL and FL) and evolution changes (i.e. SVL) may together
contribute to niche expansion towards adapting novel niche. Indeed,
because the proximate mechanisms that underlie variation between body
length and locomotor performance can be complex, quantifying the fitness
consequences of the resulting trade-offs will be challenging, novel
analytical tools and optimization models are needed in further studies.