Glacial Refugia - A Refresher and Possible Explanation of Some of the Flaws

As previously stated, this blog will now focus on two main aspects; glacial refugia and species migration models. To kickstart this, a refresher to glacial refugia is required. During glaciations, temperatures plummet and aridity increases. Biologists often hypothesize the fate of temperate fauna and flora during these periods. Refugia describes the areas in which these species inhabit during the full glacial conditions, and often determine the current patterns of biodiversity we see in temperate species. The precise locations and impact on present day distribution is often under debate and new information is constantly being found that enhances our understanding.

Willis & Whittaker (2000) propose that the function of glacial refugia in temperate latitudes is to protect species diversity, while in tropical zones its primary function is to encourage speciation. It has also been proposed that glaciation can complete speciation events inaugurated in earlier geological time. An example of this can be seen in the formation of three new species of black-throated warblers in North America during the last glacial period. Studies of the Amazon basin have led to questioning of the glacial refugia hypotheses. Increasing aridity would lead to the wide scale change to savanna from lowland tropical rainforest. Under the hypothesis this would lead to small refugial belts in the mountains where the conditions would be wetter. However study of pollen records from the Amazon basin have shown that the area was not replaced with savanna and the forests have dominated throughout the glacial period. It has been suggested that the effectiveness of cold-stage aridity to roll back the rainforest has been greatly overestimated. Study of genetic diversity of the lowland canopy tree Poulsenai armata in Central America has shown a greater within-population diversity than would be consistent with postglacial expansion from a distant refugia.

Another twist in the Amazon-refugia debate is the distribution of endemic species within lowland Amazonia. It is hypothesized by Nores (1999) that this patterning of endemic species is the product of sea level rises of over 100 metres during the interglacials of the Quaternary and late Tertiary periods. This led to the formation of two large islands and several smaller archipelagos encouraging speciation through geographic isolation. It is important to always question the current paradigm as this is the way in which scientific discoveries are found. What is generally accepted as true is not necessarily so, only the best possible explanation of the current evidence, and I feel it is important to question areas that do not fit in with this paradigm, in this case the example of Central American forests and Amazonian species diversity.

The debate over glacial refugia is still in its infancy and will be delved deeper in subsequent posts of this blog. The next post will focus on how species diversity is assessed and what impact this has on determining glacial refugia.