A Cryptic Northern Ending

Sadly, my blogging time is coming to an end but I hope you have enjoyed following my updates! Over the past couple of months I have tried to provide a well-rounded account of a topic which, to be honest, I didn’t know hardily anything about when I first began! I have tried my best to review a variety of studies, from a range of disciplines, using a mixture of research methods, presenting different points of view where possible. Here is a summary of the development of the cryptic Northern refugia hypothesis:

Let me start by reminding you of a paper I discussed in one of my early posts by Davis et al (2005), which is apparently now becoming a ‘classic’ publication, focusing on species responses to changing climate. According to Davis et al (2005), Palaeoecologists have predicted that when species are faced with a significant change in climate they will respond in one of the following ways:

1. Migration/shift in range – Altering their distributions in order to remain within suitable climatic conditions
2. Persistence in refugia
3. Adaption
4. Extinction – If a species is not able to respond to significant changes in climate in any of the above ways then it will no longer be able to survive

Genetic evidence had already began to mount up in favour of the refugial hypothesis that stated that species responded to significant changes in climate by contracting to and then persisting for some time in refugia, that is ‘isolated areas of habitat that retain the environmental conditions that were once widespread’ (Stewart and Lister 2001). More recently, Quaternary refugia have been defined as ‘the geographical region or regions that a species inhabits during the period of a glacial/interglacial cycle that represents the species’ maximum contraction in geographical range. This is a flexible definition that accommodates species that are adapted to different climatic conditions, while at the same time highlighting the idea that species in general respond to climatic changes independently of each other’ (Stewart et al2010).

Traditionally, it was assumed that temperate (mid-latitude) species were driven southwards by the cold climatic conditions that characterise glacial periods. It was from these Southern refugia, where the conditions were warm enough for them to survive, that they re-colonised the more Northern latitudes when the climate once again became more favourable. Refugia studies of the 1990s generally provided evidence in support of the Southern peninsulas of Europe as major ice age refugia for temperate species (and in most cases demonstrate that genetically distinct taxa emerged from them). Here are links to a few of these earlier studies that support the Southern refugial hypothesis:

• Willis, K. J. (1996) ‘Where did all the flowers go? The fate of temperate European flora during glacial periods’ Click here
• Bennett, K. D., Tzedakis, P. C. and Willis, K. J. (1991) ‘Quaternary Refugia of North European Trees’ Click here 
• Hewitt, G. M. (1999) ‘Post-glacial re-colonization of European biota’ Click here 

 

Emergence of the Cryptic Northern Refugia Hypothesis

More recent studies have shown the Southern refugial hypothesis to be incomplete (again, I don’t want to say incorrect here because this hypothesis does apply to some species). Scientists such as Willis et al (2000) proposed that temperate species may not have been restricted just to the Southern European peninsulas, but that the role of ‘Quaternary refugia’ in Europe may have been more complex than previously thought (Stewart and Lister 2001). Studies in the late 1990s and early 2000s found evidence (in multiple forms) of certain temperate species surviving and re-colonising from more Northern refugia, backing up the cryptic Northern refugia hypothesis. The map in Figure 1 show the locations of the hypothesised cryptic Northern refugia mentioned in Stewart and Lister (2001).

Source: Stewart and Lister (2001)

Categories of Refugium

Evidence for the cryptic Northern refugium hypothesis has been put forward in relation to many temperate species, some examples of which have been discussed in this blog. Debate arose, however, when some scientists began to apply the refugial concept to cold-adapted species (remember the debate between Pruett and Winker (2005) and Stewart and Dalen(2008) over the rock sandpiper?). Two years on, Stewart et al (2010) stick by their argument, pointing out that these cold-adapted species will have had a much larger range during cold stages so they must, in fact, be in refugia during warm phases. As we’ve seen, however, evidence has been put forward against this and the debate over cold-adapted taxa and cryptic refugia continues. There has also been significant attention paid to emphasising the need for careful and precise terminology when using the word ‘refugia’ as it can mean different things depending on the environment it provides and the characteristics of the species that persist within it, as well as the spatial and temporal aspects of its existence.

Cryptic refugia remains one of the most interesting categories of refugia, defined precisely by Stewart et al (2010) as ‘refugia situated at different latitudes or longitudes than would normally be expected, and often resemble climatic islands in which conditions differ favourably from the surrounding areas’. The good schematic diagram in Figure 2 shows the locations of different types of refugia, with the location cryptic refugia represented by the smaller patches of red and yellow at higher latitudes (away from the large shaded areas to the South that represent Southern refugia). Refugia for cold-adapted species are shaded in blue (as mentioned above, scientists debate over whether these are considered cryptic refugia or not). Remember, not all the refugia will have been inhabited simultaneously.

Source: Stewart et al (2010)

Evolutionary Implications: Refugia as a Harbourer of Genetic Diversity and New Species

It seems that the unique microclimates of refugial areas expose the temperate species to slightly different selection pressures which they evolve in accordance with (if they are isolated long enough), as well as independently undergoing genetic drift and being subject to different genetic mutations. These refugial areas may be vital not only for the long-term survival of temperate species but also for the future development of new ones. Stewart et al (2010) point out that as glacials are longer than interglacials, temperate species will persist in refugia for longer amounts of time and, therefore, have more time to adapt and differentiate. The question is whether this time in refugial isolation can lead to the development of new species. In the case of cryptic Northern (and cryptic Southern) refugia, it seems that their unique characteristics mean that they have the potential to produce new species; they are generally much smaller in area, it is likely that the species within them are exposed to different environmental conditions (and, therefore, different selection pressures) than those in larger refugia, and predators often become extinct (due to the change in climate or the fact that the area of suitable climate isn’t large enough to sustain them) which alters selection pressures further (Stewart et al 2010). These processes could lead to rapid divergence and speciation if the reproductive isolation continued. 

The potential for speciation is higher within cryptic Northern refugia than it is in cryptic Southern refugia because the former exist during the longer glacial periods. This is why I chose to focus a few of my posts specifically on the role of cryptic Northern refugia in the creation of Arctic species, such as the Arctic fox and the Polar bear (see my posts on Dalén et al (2005) and Provan and Bennett (2008)’s work).