Climate relicts

Carl Zimmer wrote an interesting piece recently on “hardy relicts”.  “Relict” species or populations are those that are left behind as all the other populations or individuals of a species die off.

What makes a relict?

I was going to construct a hypothetical example, but my girlfriend [bit of a Katatrepsis bump, there, my dear] just pointed out, the Pink Lake stickleback are a brilliant case study.  The Champlain Sea flooded southern Ontario and, as it receded during glacial rebound (the land rising back up after having been depressed by the weight of the glaciers) there were pockets of salt water left behind.  The stickleback in Pink Lake were caught in one of the pockets and represent a relict which has persisted outside of the current range.  More commonly these relicts occur as a result of climate change.  As the planet warms, regions closer to the equator become too warm for some species.  Populations of those species closer to the equator go extinct and new populations are formed in regions that were formerly too cool but are now suitable habitat.  These new regions may be further north, but they may also be at greater altitudes.  When species move up mountains and the populations on the lowlands die-off due to high temperatures, they are left as an isolated relict.

Why are they so interesting?

These relict populations exhibit a number of interesting features.  First, they are necessarily cut-off from the main part of their species which limits the opportunities for breeding between the two groups.  This means that the genetics of relict species can be quite distinct from that of the main species and has been proposed as a mechanism by which new species can develop.  Even if these relicts can still interbreed with the main group of populations, there are arguments for conserving them as unique “evolutionarily significant units” – they have unique combinations of genes which make them important to the overall genetic diversity of the species.

Relicts can also facilitate later range expansion.  It has been proposed that some of the range expansion that we are seeing now as a result of climate change is, at least partly, the result of unknown relict populations that have been hiding away in warmer pockets of habitat in the north.  Rather than poleward range shifts of species being a result of an inexorable poleward march be southern populations, it may be that these relict populations are finally having the opportunity to expand into areas that the species formerly occupied.

This combination of preservation of populations where they would not usually be able to persist and facilitation of range shifts is a positive attribute for the conservation of species, but there are limits.  “There ain’t no mountain high enough” to preserve these species indefinitely.  A 1 degree increase in temperature corresponds to around a 100m vertical movement in temperature.  Eventually, on most hills and some mountains, species will simply be “squeezed out” when they reach the summit.  This is exacerbated by the tapering of mountains towards the summit – there is a lot more space in the bottom 100m of a (roughly conical) mountain than at the top 100m.  The biology of these kinds of relict populations is rarely understood as they tend to be small, diffuse and rarely discovered.  However, they do represent important resources for conservation biology and really do need to be protected.

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