Safe Havens during the Ice Age: Refugia

The Quaternary Period is the period in geologic history ranging from 2.58 million years ago to the present. It is commonly known as the Ice Age. The cooling process started at the beginning of the Quaternary. We are still considered to be in 1 of 5 ice ages in Earth’s history because of the permanent ice sheets at our poles. 

Over the last 100 million years, Earth’s average temperature could reach up to 10°C more than the present, and ice was completely absent. Hence, we reside in a very rare state of the Earth. The Pleistocene Epoch during the Quaternary Period is characterised by repeated glacial and interglacial cycles, driven by the Milankovitch Cycles; this Ice Age refers to 11 major ice ages and many minor ones, interspersed by periods of higher temperature (interglacial). 

During the glacial period, the northern ice sheets extended several kilometres down south in North America and Eurasia; 30% of Earth surface was thought to be covered by ice. No macroorganisms can survive in a landscape fully covered in ice with no vegetation: devoid of food and shelter. When glaciation (icing up) happens, animals retreat to the south because of the inhabitable ice and the retreat of habitats to warmer temperatures nearer to the equator. Those on the mountains retreat downwards (as mountain glaciers form); those at the equator… wait, where do the equatorial living things go?

Surprisingly, we have not recorded much extinction at the onset of the Pleistocene (only some evidence of deep-sea Pleistocene extinction). In fact, mammals went through a significant evolutionary change, forming megafaunas such as the woolly mammoths; the genus Homo also appeared (humans are Pleistocene species!). Of course, there would be minor, localised extinction events attributed to sudden global warmings resulting in interglacial periods and re-glaciations. But overall, species were able to survive and evolve to adapt in these environments. How did they survive?

Refugia

In the 1930s, Swedish Arctic explorer, Eric Hultén, found endemic plant species in Alaska, which decreased in numbers further away. This suggests that the endemic plant species evolved in the Alaskan peninsula, and recently dispersed from the epicentre. What Hultén stumbled upon is the concept of ‘Refugia’ – areas of relatively stable environmental conditions that house species safe from harsh environments during the height of each glacial cycle. So, how did the flora and fauna survive? Because they were able to escape into refugia, of course!

Biomes: Map of Europe during the Ice Age reflecting Refugia
Figure 1: Map of Europe during the Last Glacial Maximum. source

At higher latitudes, nearer to the poles, a refugium is a place that remains ice-free and habitable at a glacial maxima. In Europe, at the height of glaciation, scientists have identified three refugia: the Iberian Peninsula, the Italian (Apennine) Peninsula, and the Balkan Peninsula. All of them remained ice-free for most of the Pleistocene. During the interglacial periods, species at these refugia recolonised the rest of Europe. With the onset of glaciation, they retreated towards the refugia again. 

What about in the tropics? Tropical rainforests have been viewed as places with a very stable climate; they have persisted for millions of years while the rest of the world fluctuates. Scientists attributed the stable climate as the reason for the high biodiversity in the tropics (and the low extinction rate). But, they have now uncovered evidence that suggests a much cooler climate in the tropics. 

In 1969, Jürgen Haffer studied bird species distributions in the Amazon rainforests, and proposed the Refugia Hypothesis. According to the hypothesis, the cooler climate divided the Amazon into smaller isolated patches, separated by non-forest vegetation. The patches acted as ‘refuge areas’ for forest species; this prolonged geographic isolation allowed speciation to occur (allopatric speciation). Haffer noticed that the patches he identified using rainfall patterns coincided with the observed centres of endemicity, much like that of Hultén! This hypothesis also answers the question of why tropical areas are higher in biodiversity. 

Functions of Refugia and Present Life Distribution

There are competing hypotheses and theories on how refugia in the Pleistocene help determine the present distribution of life. The two scenarios mentioned above can be summarised as follows:

  1. ‘Museum’ function – refugia as protection to old species that survived glaciation (relict species); and
  2. ‘Cradle’ function – refugia as a species pump, where repeated expansion and contraction of habitable lands drives speciation (the Refugia Hypothesis).

There are still many debates on which of these functions are the main drivers of high biodiversity. In fact, scientists have challenged the ‘museum’ role of temperate refugia and the ‘cradle’ role of tropical refugia. The post-glacial recolonisation seen in present day Europe shows significant differentiation of populations isolated at different refugia, suggesting a ‘cradle’ function rather than a ‘museum’ one. 

Continuous pollen history suggests that the Amazon rainforest never fragmented, but the present montane flora species replaced it instead. Some scientists also reject the Refugia Hypothesis. According to them, the areas with the highest rates of speciation are not areas with the most stable climate (refugia), but rather, areas with the greatest environmental changes (Intermediate Disturbance Hypothesis). 

Yet, in Southeast Asian Sundaland, during the Last Glacial Maximum, while a model predicted a suitable climate for rainforests, evidence from tooth fossils of animals suggested a savannah-like environment instead. 

In Other Context

As you can see, the ‘refugia’ concept developed from a simple idea of a safe haven preserving species. More complex ideas evolved delving into how refugia determine current biodiversity and patterns, or even undermining refugia as a safe haven! While the mechanisms and concepts of refugia are an active area of research today, the concept of refugia extends to many other places. 

During the glacial periods, montane species can extend down to the lowlands, but are now restricted to areas of higher elevation; this could function as refugia for cold-adapted species in the mountains. The Pleistocene also caused the surface water systems (rivers and lakes) to connect and disconnect; lowered rainfall and dried up rivers meant that surviving, disjointed freshwater features acted as refugia! 

Furthermore, during glacial periods, Indonesia and Borneo are all connected to the mainland via exposed seafloor in the South China Sea (Sundaland), so the current Indonesian islands and Borneo are in the refugial state! Current research also toyed with the idea of Anthropocene refugium, a place safe from human intervention and alteration during the present times!

By Tan Zhixian