High gene flow across a heterogeneous landscape
Seasonally stable temperatures and steep ecological gradients on
tropical mountains are predicted to limit dispersal across elevations
(Janzen, 1967; Ghalambor et al., 2006). This prediction is supported by
several studies that show limited gene flow and significant genetic
differentiation across elevations in taxa with continuous elevational
distributions (Bertrand et al., 2014; Milá, Warren, Heeb & Thébaud,
2010; Gadek et al., 2018), sometimes with elevational parapatry between
sister species (Linck et al., 2019; Raxworthy et al. 2008a; DuBay &
Witt, 2014). For example, Gadek et al. (2018) found cryptic population
genetic structure differentiating high from low elevation populations in
3 of 4 Peruvian songbird species with continuous elevational
distributions.
Our results are not consistent with a hypothesis of restricted gene flow
across elevational gradients. We report evidence of high gene flow
between MK and MT as well as between low and high elevations on both
peaks, indicating that neither the lowland habitat connecting the two
peaks, nor the steep elevational gradient across which mountain
treeshrews occur on each peak, has significantly limited effective
dispersal.
The mountain treeshrew population in KNP is best described as comprising
two or three clusters. The primary population division does not
correspond to the two peaks; rather, the summit region of MT (≥ 2000
masl) is distinct (Figure 5a). When dividing the population into three
clusters, individuals at low elevation MK show mixed ancestry with low
elevation MT (Figure 5b). Additionally, the MIGRATE-N analysis supports
the division of individuals into three population clusters (high
elevation MT, low elevation MT, and all of MK), with high migration
rates between all pairs (Figure 2).
The temperature lapse rate in KNP is -0.55°C per 100 meters of elevation
gain (Kitayama, 1992) which means that mountain treeshrews experience a
12.65°C average range in temperature between 900 and 3200 masl on MK.
This temperature range is higher than the thermal neutral zone for most
mammals that weigh < 1 kg (Khaliq, Hof, Prinzinger,
Böhning-Gaese, & Pfenninger, 2014). On MT, mountain treeshrews
experience an 8.8°C range from 900 to 2500 masl (Camacho-Sanchez et al.,
2018). On MK, the species occupies four distinct vegetation zones, while
on MT it occupies three (Kitayama, 1992). If gene flow were restricted
due to limited elevational dispersal or selection against
cross-elevation migrants, we would expect to find greater genetic
differentiation on MK because of the broader elevational range and
associated diversity of environmental factors on this slope compared to
MT. However, the summit of MT was consistently recovered as the most
distinct population cluster while individuals caught along the entire
elevational gradient on MK form a single cluster (Figures 4 & 5). This
suggests that elevation is not the primary variable influencing genetic
structure in KNP.
Tropical montane species tend to have narrower ecological tolerance and
elevational ranges than those in the temperate zone. McCain (2009)
reviewed 170 montane gradients across vertebrate taxa to test this
hypothesis and found that most groups showed the predicted pattern of
decreasing range size with decreasing latitude. However, in rodents
there was either no relationship or range size increased with declining
latitude. This finding could be explained by the presence of cryptic
species pairs or genetic differentiation separating low elevation
populations from high (den Tex, Thorington, Maldonado, & Leonard, 2010;
Hinckley, Hawkins, Achmadi, Maldonado, & Leonard, in review). However,
McCain (2009) hypothesized that rodents may be able to cope with the
lower temperatures associated with increasing altitude through
behavioral adaptations including burrowing. Further studies are
necessary to determine whether mountain treeshrews show behavioral or
phenotypic plasticity, adaptive phenotypic (e.g. Branch et al., 2017) or
genetic differentiation despite gene flow, or a generalist phenotype
(e.g. Feijó et al., 2019) that allows them to persist across such broad
environmental conditions which is rare among small mammals in this
landscape (Nor, 2001; Camacho-Sanchez et al., 2019).