Final Conclusions on Glacial Refugia

The post will attempt to give a well rounded review of the debate over cryptic glacial refugia in the Northern Hemisphere.

The consensus is that glacial refugia existed in the mountains of the mediterranean peninsulas in Iberia, Italy and the Balkans. This is where temperate tree species are thought to have persisted during full glaciations. However, there is another theory that cryptic glacial refugia existed in parts of northern Europe. For example the tree pollen records in northern Europe in the early postglacial period suggest huge migrations rates of trees of up to 1500m per year if they colonized from southern refugia. The theory of cryptic refugia involves some tree populations being able to survive in favourable microclimates during glaciations where the regional climate would not normally be suitable (Stewart & Lister 2001). How the population manage to survive are not fully understood, and many scientists (Tzedakis et al. 2003) put the presence of the early post glacial pollen to isolated long distance dispersal events.

Examples of cryptic populations can be seen today in certain areas in the current climate. In the central Sahara a total of 223 individuals of the conifer, Cupressus dupreziana, managed to persists in ravines, where rainfall averages at 30 mm per annum (subject to large variability) (Abdoun & Beddiaf 2002). This is similar in the way in which certain temperate tree species may have been able to survive in northern Europe during glaciations.

Techniques used to identify refugia are: pollen analysis, macrofossils and molecular techniques. Pollen analysis can be used to show the presence of species in an area. For species with low production and that are non wind pollinated, presence of the species can be concluded with confidence, however with wind pollinated species the pollen could have been produced outside the regional area. The records are quite vast and are an important tool in assessing species presence.

Macrofossils are the most precise way to identify species. Macrofossils have been recovered from paleolithic sites in northern Spain, dated between 17,000 and 15,000 years BP revealing a diverse assemblage of temperate and mediterranean species. This suggests a glacial refugium was present. Although this suggests the prescence of tree species, it does not make clear the type of vegetation (e.g. forest, woodland, steppe, steppe-tundra). Willis et al. (2000) studied macroscopic charcoal particles in glacial sediments in Hungry. They concluded that there were ‘oases’ in central Europe where temperate flora and fauna could survive the glacial conditions.

Molecular techniques involve the phylogeographic techniques discussed in a previous post. They involve studying, amongst others, species diversity as identifier of refugia. See ‘Glacial refugia - are they being too cryptic’ for a refresher on phylogeographic techniques. An example of how it is used is the prescence of Scot’s pine, Pinus sylvestris, in Scotland. High nuclear genetic diversity in scottish populations of P. sylvestris suggest a cryptic glacial refugium (Sinclair et al. 1999) during the last glaciation.

The evidence suggests that plant species persisted during glaciations in cryptic glacial refugia. While some academics still believe post glacial recolonization took place by long distance dispersal from southern refugia, I believe the cyptic glacial refugia hypothesis to be a better fit. 

The hypothesis manages to explain the huge migration rates had been inferred from colonization from southern refugia, which were highly unrealistic. This breakthrough has impact on future modelling of vegetation change due to climate change. After understanding how glacial refugia exists and the debates over post glacial colonization, the blog will move on to looking at the future and how vegetation modelling will be important for assessing climate impacts on species migration.

Diversity Estimators for Locating Glacial Refugia

This next post is going to focus on how species diversity is assessed in order to infer areas of glacial refugia. A common assumption is that glacial refugia harbor higher levels of genetic diversity than areas of subsequent colonization. This is the basis on which many glacial refugia are located. However Comps et al. (2001) believe this to be too simplistic, as the way in which genetic diversity is estimated can influence the results. To give a brief background of Comps et al. (2001), they studied European beech (Fagus sylvatica) across Europe. As F. sylvatica is wind pollinated a detailed pollen record is available in which the genetic data can be compared against. In addition it was chosen as its closest relative is Fagus orientalis so there are no problems of interspecies introgression.

The question is what pattern of diversity is expected from recolonization of a refugium? Colonizations of new areas normally consist of populations of a few individuals therefore showing a small sample of the genetic diversity of the source population. How much of a reduction is dependent of diversity estimators. For the new population, allelic richness (which is the number of alleles per population) would be expected to be lower than genetic diversity (which is the probability that two alleles sampled at random from a population are different). In F. sylvatica, allelic richness was lower in populations in Northern Europe, consistent with loss of rare alleles due to recolonization. 

However a reduction in genetic diversity was not found. Comps et al. (2001) found a negative correlation between allelic richness and genetic diversity. This meant areas with a decreased allelic richness displayed an increased genetic diversity. Since the majority of papers only calculate genetic diversity, academics may be misled in the identification of glacial refugia.

Personally I take the findings of Comps et al. (2001) with a pinch of salt. I have used this example not to show that a wrong estimator is being used, just that other estimators are available and might give slightly different results. I believe that the increase in genetic diversity when allelic richness decreased was a conclusion unique to the species in question and general conclusions should not be taken from this case study. It does bring an extra addition to the glacial refugia debate and I believe that diversity estimators should be taken into account when discussing methods of glacial refugia identification.

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.

A Shift in Emphasis

After having provided a historical context and examples of how and why species migrate due to climate change this blog has decided to focus on two main aspects. The first is to deal with the debate over glacial refugia and species migrationary capacity to disperse following glaciation. The second being to look in further detail at species migration models and critically assess how future migrations can be predicted. Both of these aspects are linked, inferring past migrations from paleoecological data in to help model future major species migrations in response to climate change.

Species Migrationary Response to Climate Change - A Poster

Below is the poster created from the work already put forward by this blog. An innovative design breathes fresh life into academic posters that can sometimes be very much alike.

Glacial Refugia - Are They Being too Cryptic?

As previously promised, todays post will be discussing glacial refugia. During the Last Glacial Maximum at around 23,000 to 18,000 BP most of the temperate biota existed in lower latitude refugia. Extensive analysis has revealed the existence of three main refugia in the Mediterranean peninsulas of Iberia, Italy and the Balkans. After the LGM, Europe was recolonized from these refugia by species migrating northwards. However using phylogeographical evidence, the possibility of cryptic altitudinal refugia has arisen (Provan & Bennett 2008). This may have led to gross overestimations of species dispersal capabilities, which could in turn cause problems for estimations of species migrations in the future.

Phylogeographic techniques involve exploring species diversity as an indicator for possible refugia. Species that have survived the glacial maxima in refugia will expect to have high levels of genetic diversity. Secondly long term isolation of populations will lead distinct genetic lineages and lead to possible recolonization routes which can then be used to locate refugia. Areas with high genetic diversity that have been recolonized by many separate refugia might lead to confusion and should therefore be taken into account. Figure 2 below shows phylogeographic evidence of red seaweed (Palmaria palmata) showing a refugia in the English channel. The different colours represent different genetic types, with the highest proportion being shown in the dotted area.

Figure 2. Distribution of Palmaria palmata in Northern Europe (Provan et al. 2005)

Although phylogeographic evidence has had success locating recolonization pathways, it does not explain population dynamics during phases of species contraction. It is unclear whether species migrate to refugia during changes in climate or simply species outside the refugia become extinct. Studies of the Artic fox (Alopex lagopus) using ancient DNA data techniques have established that during post glacial contraction species outside the refugia became extinct (Dalen et al. 2007). This might have a massive effect on cold-adapted species that contract during periods of warming.

Phylogeographic techniques has provided new insights into the locations of glacial refugia. They have challenged the theory that the southern refugia were the only source of recolonization for temperate species after the LGM. The identification of cyrptic refugia has important implications for future climate change due to estimations of species migration range.

The Bioclimate Envelope - Outdated and Simple?

Todays post will focus on the bioclimate envelope model and discuss whether they are useful in determining species migrationary response to climate change. The bioclimate envelope model has its roots in the ecological niche theory. The ecological niche is a conceptual space comprising all of the environmental variables in which a species can survive and grow. The bioclimate envelope can be defined as the climatic component of the ecological niche. Therefore it involves understanding the current species distribution, and how species physiologically will respond to climate change. It might enable us to make a redistribution of species over time using climate models.

The model is widely used in decision making on future climate change, however there are some criticisms. Pearson & Dawson (2003) express concern at how the model does not take into account how species interact with each other, rather how only they function alone. Since ecosystems are a complex web of interactions and feedbacks, taking into account only the species that is being studied could lead to erroneous model predictions. Another possible facet that is not included is evolutionary change. Studies by Thomas et al. (2001) and Woodward (1990) has studied rapid evolutionary change response to climate change of plant and insect species. This involves studying climate induced shifts in species, often excluding phenotypes that are poor dispersers or poorly adapted to local conditions.

Looking forward, the development of dynamic global species models would be a more accurate approach. There has been recent development of models on regional scales that break down ecosystems processes into key components and use that to spatially and temporally model species range shifts. However due to the complexity of these models, application to global scales have not yet been possible. This means that bioclimate envelope models are still perhaps the best available guide for policy making at the present time, though they must be viewed skeptically.