Wednesday, 23 September 2015

The early Holocene of Europe - from 11,000 to 6000 BP

The Quaternary extinction is a much talked about subject today, as it is both fascinating in the context of prehistory, and simultaneously important in modern conservation. Typically defined as the rapid decline in global biodiversity during the late Pleistocene up until present, it is sometimes referred to as the sixth mass extinction in earth's history, the previous one wiping out the non-avian dinosaurs 66 million years ago. Personally, I dislike the term Quaternary extinction. The event mostly occurred within the last 50,000 years, making the term "Quaternary" odd, since the Pliocene and majority of the Pleistocene was not involved. Some have also referred to it as the Pleistocene extinction, which is equally incorrect, as it has continued up until the present. I prefer the term "Anthropogenic extinction", and have used it previously, as I believe it is more apt. It ignores the timeframe in which it has occurred, instead describing the primary causal factor, humans.

While it is true that the most dramatic of the Anthropogenic extinctions occurred either during the late Pleistocene or around the industrial revolution, in this post I will be focusing on a very different, but in some ways even more significant period, the early Holocene. The early Holocene is special in that it represents a relatively brief span of time, approximately 5000 years, in which environmental conditions were similar to today, yet direct human influence on the landscape was not. Instead, the most major human influences during the early Holocene were the same as those during the Pleistocene - hunting. While many of the most important megafauna were already gone by the time the Holocene began, several were not, and for just a few thousand years, we got a glimpse of something similar to what the world may have looked like had humans not existed.

Late Pleistocene Eurasia, by Mauricio Anton

The typical view of the faunal changes through the Holocene is as follows: In the beginning of the period, the climate was still quite cold, and open tundra-like conditions were dominant. During these times, Reindeer, Horses, and other such animals thrived. As time went on however, the temperatures increased, and the open landscapes soon became covered in forest. Over the course of the early Holocene, the forests grew increasingly denser and darker, thus causing the extinction of the species dependent on more open habitats. This is of course an extreme simplification, but roughly represents what has traditionally been said. After this, the story goes, the forests remained deep and dark for several thousand years, before finally being cut down by humans during the early Bronze age. In this version of history, the Aurochs and Wisents are also typically referred to as forest animals. As I am going to explain here, this view is false in several ways. The core problem with this model of the Holocene is that it simultaneously downplays the role of Megafauna in shaping their environments, while exaggerating the effects of the climate. I am not a scientist, much less a paleoecologist, but here is a more modern version of the early Holocene, based on what I have read from actual researchers:

In the earliest Holocene, over 11,000 years ago, northern Europe was indeed covered mostly in tundra. Willows and Dwarf birch were present, but did not form dense stands. At this point, the landmass known as Doggerland still covered much of what is now the North Sea, but was already decreasing in size. Outer Silver Pit Lake, now a valley at the bottom of the sea, was, as the name suggests, a lake at this point, and the rivers Rhine and Thames were both connected in a now submerged delta that flowed into the atlantic, and would have formed enormous wetlands. Mammoths were still clinging onto existence in some regions, while Saiga antelopes and Musk oxen continued to prosper. This was rapidly changing however. As the climate warmed, and trees began forming open woodlands, humans became more abundant, greatly increasing hunting pressure. By the end of the first part of the Holocene, the Preboreal, 10,000 years ago, Scots pine forests were common across most of western Europe, and animals such as Elk, Bison and Aurochs had returned from their southern refugees. 

Open Scots pine forests like this would have been common during the Preboreal

Following the Preboreal period came the Boreal. At this point the climate was almost as warm as today, and the signs of the previous glacial period were beginning to disappear. Over the course of a thousand years, Doggerland almost entirely disappeared, resulting in Britain being cut off from the rest of Europe. The only part of the once large landmass that remained was an island known as the Dogger bank, which lasted until around 8000 years ago. All of the fauna we know today arrived during this period, but many of the now extinct species began to decline. At the end of the ice age, the European horse, also known as the Tarpan, had replaced the Przewalski's horse, a steppe species, but now it too was in heavy decline in many parts of Europe. This has typically been assumed to have been due to natural causes, as, it was claimed, the warming climate allowed the forests to grow denser, crowding out species such as the Tarpan. As our understanding of megafauna and the effects they have on the landscapes they inhabit has improved, it has become increasingly clear that this view is not accurate. High populations of grazing megafauna were almost certainly capable of keeping forest growth at bay, resulting in the creation of savanna-like wood pastures instead of dense forest. Ecologist Frans Vera has long contested the traditional view of early Holocene Europe as a continent-spanning forest, claiming instead that it consisted mostly of wide open plains and the aforementioned wood pastures. Partly as an experiment to demonstrate his hypothesis, he helped create the Oostvaardersplassen in the Netherlands, where huge herds of thousands of Horses and deer, along with some couple hundred Cattle, have succeeded in creating the open landscape he predicted. 

There is a big problem with Vera's hypothesis however; It relies on the assumption that such huge herds existed during the early Holocene. Hunting pressure 10,000 years ago was not much lower than it was 6000 years ago, and it seems unlikely that herds of thousands of horses existed, at least not for long. During the early Boreal, it is probable that very large herds of Tarpans and other grazing animals did exist in some places at least, but they were already greatly reduced due to human hunting, and seem to have quickly been wiped out. By the late Boreal period, most of the grazing species had probably been reduced to such an extent that they could no longer prevent forest growth. As a result, they entered a sort of positive feedback loop, as the more their populations shrank, the more open habitat became overgrown, and the more open habitat became overgrown, the more their populations shrank. By the end of the Boreal, the Tarpan was extinct in most of western Europe, though it persisted for far longer in the regions where dry climate prevented forest growth.

A herd of wild horses, akin to those that inhabited the Boreal period

Thus we come to the Atlantic period, the last part of what I have termed the early Holocene. During the Atlantic period, temperatures were actually higher than they are today, and sea levels rose up to 3 meters, before falling again during the Subboreal. During the Atlantic, the extinction rate finally began to slow, as most of the vulnerable species had already either died out or been reduced to isolated refugees where they were relatively safe. The Wisent went extinct in much of western Europe, but aside from this, relatively little else disappeared. What was left had been heavily diminished however. A few species which previously inhabited open habitats, such as the Aurochs, had managed to adapt to living in forest clearings and marshes, though they were still in continual decline. At this point, the primeval woodlands typically associated with the early Holocene had developed, as in the absence of any meaningful grazing pressure, the trees grew tall and dense. As the great canopies shaded out the forest floor, biodiversity began to decrease significantly, and many species were restricted to the coasts and wetlands, where woods were still more open. 

This was the landscape that would remain for over two millennia, until our ancestors finally began cutting down the woods to make way for primitive agriculture. The Atlantic was in many ways something of a gloomy and dour period, marked by declining wildlife and dark woods. It is interesting in that it serves as a particularly good example of the danger that undergrazing can pose. While overgrazing is a big issue in many parts of the world, Europe included, the Atlantic period shows what happens when you go to the opposite extreme. 

An old-growth forest similar to the ones that covered most of Europe during the Atlantic

In many ways, the early Holocene stands as something of a lost eden. While the last interglacial is certainly a better example of a pristine and healthy nature, the periods of the early Holocene have the distinction of feeling particularly close. One can still see the echos of this time, when we walk through the old-growths of Slovenia or Poland, or when we uncover the remains of long gone giants, preserved in bogs and marshes. In a way, it was a more innocent time, one in which our control over the planet was not yet quite as apparent. In today's gloomy and often stressful world, there is some relief to be found in imagining yourself walking through those long lost woods, ones that had never seen neither axes nor bulldozers. But at the end of the day, even during the early Holocene, the world was already heavily altered by our actions. Many species that would have been if not for us were gone, and we can be certain that the world would have been a very different place, even back then, had it not been for us. As alluring as the notion of a pristine wilderness just a few thousand years ago is, it was nothing of the sort, an illusion, hidden deep in the woods.

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