Heated Debates...

I’ve come across an interesting debate between 2 groups of scientists about the possible responses of high-latitude species to climatic change during the Pleistocene. I realise this post is a big long, but bear with me as I want to make sure I present both sides of the debate and explain everything clearly – here goes…

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Pruettand Winker (2005) conducted a phylogeographic study in which they used mitochondrial DNA sequencing to establish the effects of past global climate change on the establishment and differentiation of the rock sandpiper (the little bird in the picture) in Beringia (for more information on Beringia click here). Their investigation focused on climate change during the Wisconsin glaciation (10,000-117,000 years B.P.) where cycles of glaciation expanded and contracted ice free habitats, as well as periodically exposing a land bridge between Asia and North America (due to variations in sea level). They suggest that ice free areas served as refugium for many species, including the rock sandpiper.

They concluded that the genetic diversity in the rock sandpiper could be accounted for by the species’ existence in a ‘mosaic’ of refugia during several glacial cycles, highlighting the role of refugia in creating biodiversity. Climatic change and associated isolation of rock sandpipers in refugia seems to be the reason behind the ‘morphological and sub-specific differentiation’ observed in present day populations – the sub-species represent significantly different ‘ecological units’. They pointed out that although obtaining physical evidence for the geological reconstruction of these cryptic refugia is very difficult, the ‘genetic echoes of these refugia are unmistakable’.

Almost 3 years after this piece of research was published, Stewart and Dalén (2008) commented that the study would ‘benefit from an alternative interpretive perspective’. They propose that species that are found at high latitudes today, for example the rock sandpiper, were not restricted to refugia during glaciations of the late Pleistocene. Their explanation for this is quite interesting: They first pointed out that it was only at the time of the last glacial maxima that the ‘stereotypical ice age conditions’ were observed. During this time, species were isolated in areas free of ice, however, for the rest of the glacial period (when the ice was less extensive), they offer an opposing idea.

They acknowledge that a multitude of research has been published in support of the existence of cryptic Northern refugia, but they also highlight the fact that species adapted to much colder climates had a significantly larger range during periods of glaciation. It then follows that cold-adapted species are restricted to refugium during interglacials, including the present day interglacial – turns things on their head a bit, doesn’t it? But I guess it makes sense!

They point out that whilst glacial refugia were important in generating biodiversity for temperate species, the cold-adapted species are much more significantly restricted during interglacials (than temperate species during glacial periods). They suggest that different species will experience ‘variable isolation events as species-specific habitats’ shift in response to climatic change, concluding that temperate species respond differently to high-latitude species in the face of a fluctuating climate.

Pruett and Winker (2008) were quick to refute Stuart and Dalén’s (2008) ‘simple dichotomous treatment of northern versus temperate species’, presenting genetic evidence for both temperate and high-latitude species existing in similar glacial refugia.

Their argument is that if temperate and high latitude species were isolated in different areas by different factors – as Stewart and Dalén (2008) suggest – then the genetic analysis of the 2 types of species would possibly show different ‘patterns of isolation’. Pruett and Winker (2008) present evidence for the high-latitude rock ptarmigan, the high and temperate-latitude common raven and the temperate latitude common wren all showing ‘genetic echoes’ of similar refugia to the rock sandpiper – they showed that they had similar genetic signals and inferred from this that they were isolated in refugia at similar times.

Pruett and Winker (2008) do present some convincing evidence for temperate and high-latitude species existing in similar glacial refugium, but Stewart and Dalén (2008) must be basing their argument  on their own, equally convincing evidence in order to publicly comment – so who’s right? It could be that the landbird species in Pruett and Winker’s (2008) study responded in a different way to other temperate and high-latitude species. I don't think that either groups of scientist are wrong; elements of each body of work can be used to develop a better understanding of how different species responded to climate change in the past. I do think, however, that we have to be aware of the fact that generalised statements about the behaviour of temperate and high-latitude species as a whole may not be applicable to all species in all areas so case-by-case empirical studies would be useful to develop a greater understanding - although I know that time and resources can't always stretch that far!