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.