Cryptic Northern Refugia 101: Setting the Scene

Source: Stewart and Lister (2001)

‘Cryptic Northern Refugia’ – sounds a bit ominous, doesn’t it? So, let’s go back to basics and picture this scene: It’s the time of the Late Pleistocene ‘cold stages’ (see Fig. 1) and those unglaciated parts of Northern Europe are covered with the treeless, herbaceous vegetation of the Steppe-tundra biome. Steppe-tundra can be seen as a mixture of modern day steppe grasslands and northern tundra – quite different from Northern Europe’s current vegetation, climate and landscape and quite unfavourable for the temperate woodland species that thrive in these areas today. So the question is, how did these temperate species spread and re-colonise this part of the world so quickly (remember we’re talking geological time-scales here, so not exactly ‘quick’) when the warmer weather of the Holocene epoch kicked in?

In order to answer this question, I need to introduce a key term; ‘refugium’, or ‘an isolated area of habitat that retains the environmental conditions that were once widespread’ (Stewart and Lister 2001). The temperate woodland species, such as yew, Scott’s pine and red squirrel, were able to withstand these long periods of extremely cold climate in so called ‘refugia’ that were warmer than the surrounding landscape. When climatic conditions became favourable at the beginning of the Holocene these species were able to expand their ranges and re-colonise.

Source: Stewart and Lister (2001)

But what’s so ‘cryptic’ about that? Well, until recently it was thought that the temperate species that re-colonised Northern Europe at the beginning of the Holocene spread from refugia that were located as far away as Asia. However, relatively new empirical studies on specific taxa have found evidence for refugia much closer to home. The locations of these cryptic European refugia (‘cryptic’ because they were unknown until recently) are shown in Fig. 2.

I hope this short introduction to the topic has been helpful! Over the coming weeks I will be discussing some of the debates surrounding the idea of cryptic Northern refugia and selecting some case study examples to look at in more detail.