Implications for evolution in insular systems and tropical
mountains
Our results are interesting in the context of evolution within insular
systems. We found that dispersal limitations drive community structure
across multi-hierarchical levels within a single habitat in a
geographically limited sky-island. This is congruent with analyses in
oceanic islands, showing that when dispersal ability and climate
tolerance are restricted, strong geographic isolation within an island
can occur even in a few kms but extending back even millions of years
(Salces‐Castellano et al., 2019). Therefore, our results represent an
additional source of evidence of how topography and neutral processes
can promote biodiversity diversification even at short geographic
distances within insular systems. This pattern of dispersal constraints
is expected to be more pronounced in the tropics than temperate areas,
as tropical species have typically narrower thermal tolerances and lower
dispersal than temperate species, leading to higher
isolation-by-distance and isolation-by-elevation (e.g., Polato et al.,
2018).
Nevado de Toluca is part of a geographically extensive sky-islands
complex, and considering this broader spatial context has interesting
implications for why tropical mountains are biodiversity hotspots. It
has been hypothesized that the global pattern of hyperdiverse tropical
mountains likely reflects the differentiation of small, spatially
isolated populations combined with the long-term maintenance of these
populations, leading to speciation (Rahbek et al., 2019a). In this
context, spatial isolation normally refers to habitat fragments
distributed across different mountain peaks (Fjeldså et al., 2012;
Rahbek et al., 2019a, 2019b), however our results support that the
processes of differentiation and long-term persistence of small
populations may also hold at local scales. Firstly, our data shows that
a single sky-island harbour arthropod communities that are spatially
structured, at the haplotype and lineage levels, even within a single
type of forest with presumably similar environmental conditions at short
geographic distances. Secondly, a previous study on sky-islands of the
TMVB (including Nevado de Toluca) showed that montane ecosystems are
able to persist within the same mountain during climate fluctuations,
but shifting up and down slope (Mastretta-Yanes et al., 2018). Coupling
these results together, it is supported that a single sky-island can act
as a cradle for population differentiation and that this differentiation
can persist, and accumulate, relatively in situ over evolutionary
time scales. Previous case studies on beetle species have reached
similar conclusions (Bray & Bocak, 2016), but here we show that rather
than a particular case restricted to extremely poor dispersing taxa, the
phenomenon could be widespread among tropical arthropods.