How solid is the analogy between mountains and islands to understand endemism?

In this new paper @GEB_macro we dive into the concept of ISOLATION THROUGH SPACE AND TIME and discover the links between true islands and islands in mountains. https://doi.org/10.1111/geb.13155. Thread!

There is a long-standing idea that high endemism in mountains parallels true islands due to similar island-like formations. Especially mountain tops can look very much like their oceanic relatives.

Names like alpine islands, sky islands, and habitat islands have arisen to describe these island-like formations in mountains around the world.

So does this apparent isolation in the present explain the common high endemism in mountains and islands?

Isolation is not as simple as it seems. A system is not simply in "a state of being isolated", but its isolation represents a continuum of different processes that interact with species traits and have varied in space and time. Endemism is the result of all this mix together.

For true islands, the main driver of changes in isolation has been sea level fluctuations (a). Though values vary globally, low sea stands (e.g. LGM) could have been over 120 m and high sea stands (e.g. LIG) at +6. But intermediate sea levels have been much more common (b).

For "mountain islands" (sky islands, alpine islands, habitat islands in mountains) changes have been equally substantial due to global temperature fluctuations.

Extreme temperature were super rare and short lasting. Many shifts at intermediate levels were the rule!

These sea level and temperature fluctuations caused changes in the spatial configuration of true islands and mountain islands, thus resulting in variable isolation through time.

We conceptualized isolation at any given moment in time as having two dimensions: (i) the environmental difference of a location from its surroundings (b) the effective distance from an equivalent environment.

Mountain islands and true islands differ already quite a lot here.

We know that isolation changes through time due to sea levels and temperature, but an island also starts with a certain level of isolation!

Did they arise "new"? Were they part of a continent or large mountain range? All this influences the degree of endemism found.

And then of course....how long has the island been isolated? Endemism is surely expected to be different on an island of a few million years old or a deep-time formation carrying loads of history along.

This all brings together that the isolated "state" of the mountain islands and true islands we see today is just a snapshot in time.

Current endemism carries the legacy of isolation history and to quantify that is one of the most exciting endeavours in #biogeography.

We hope that more people will embrace the manifold dimensions of isolation that may affect endemism, and that we inspire valuable comparisons between mountain islands vs true islands in the future.

And that's a wrap!

Hugh thanks to awesome co-riders on this trip: @Weigelt_P @mkborregaard @CarlatGCE @ManuSteinbauer @FranzEssl1 @Severin_Irl
@BioGeoKreft @Bernd_le_ @SietzeNorder @dodotalks
@r_field1 with support of @GMBA_IPO @ELD_IBED_UvA @BioUiB