Europe - Part I: Prelude and Climate

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This is the first installment in our series on the European extinctions. The second entry, focusing on the arrival, culture and impact of modern humans to the continent, can be found here. The third entry, exploring the continuation of the extinctions into the Holocene, can be found here.

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Introduction

The original state of Europe’s nature has long been an issue of contention. The issue has not been eased by disagreements, not merely regarding Europe’s landscape and fauna in its original condition, but also what period even constitutes a “pristine” Europe. The typical answer, common in literature and conservation-projects, has been the early Holocene until the introduction of agriculture. The true nature of the early-Holocene landscape has seen many contending hypotheses: Generally, the claim has been that the period was dominated by dense, closed-canopy woodland and temperate rainforest (14), while others have claimed, not uncontroversially, that the landscape was predominantly open and park-like (34) (13). However, we are perhaps mistaken in our place of investigations. If the original state of Europe is the object of our query, the early Holocene is not it. Modern humans had by then already been present on the continent for over 30,000 years, a period during which much of Europe’s original megafauna—animals quite alien to our modern eye—had vanished. The degree of human influence in these extinctions has been hotly debated and will be explored as the primary focus of this article. One thing, however, is evident. Europe in the early Holocene was already an unusual landscape, even a post-catastrophic one, with a megafauna greatly diminished, to a degree not seen since the end of the dinosaurs (4). If our object is a truly original baseline, clear of any possibility of human influence and showing the full flora and fauna under a climate as similar to the present as possible, we will need to look back further. This will take us to the last interglacial, or warm period, before the long cold of the Weichsel. It will take us to the Eemian.

Figure 1. Reconstruction of a Mesolithic (middle stone-age) hut from Ireland. Europe in the early Holocene had already been a human-settled landscape for many millennia.

Terms of use: This image is licensed under a creative commons attribution-ShareAlike 3.0 Unported. It is attributed to Notafly. The image is unedited and the original can be found here

The Eemian baseline

The Eemian interglacial lasted from around 130-115,000 kya (5), preceded by the Saalian glacial and followed by the Weichsel. Its faunal and floral assemblage was similar to that of Europe today, being near enough in geological time that evolution has not yet produced substantial change. No discussion of the Eemian can take place without first addressing the climatic and environmental conditions at the time. The climate was similar to that of today, but warmer—closer to the warmest part of the Holocene than the present. Temperatures were about 4° warmer in summer, with a similar mean in winter (10) (26). The winter-mean cannot have been below -1.5°, due to the presence in England of Hedera and Ilex, both relatively cold-intolerant plants (9). Rainfall and aridity are difficult to estimate but seem to have been overall similar to conditions in the Holocene. Precipitation in England was enough to sustain flowing rivers throughout the year (10), while Sandom et al (2000)’s survey of beetles in Eemian Britain found no fire-adapted species, compared to 9% of early-Holocene finds. This could potentially mean that the Eemian was on average less arid than the Boreal and Atlantic phases of the Holocene, but might also plausibly be a result of herbivorous megafauna reducing woodland cover, and thereby affecting the fire-regime. Precipitation in Central Europe seems to have varied substantially through the Eemian, but was similar to the current epoch, only with a slightly more oceanic climate (35). In conclusion, then, the climate in the Eemian was one quite closely resembling ours today.

But what, precisely, was the large-animal fauna of this foregone interglacial? Certainly, it is important that we introduce our players, so to speak, before analysing their later fates. The megafauna of Europe today is obviously much diminished, even since the Medieval Ages, so a better comparison is probably the Eemian to the early-to-mid Holocene. During the latter period, the continent was home to a relatively diverse guild of large animals, with the aurochs (Bos primigenus), wisent/European bison (Bison bonasus), red deer (Cervus elaphus) European elk (Alces alces), wild horse (Equus ferus), European wild ass (Equus hydruntinus) and wild boar (Sus scrofa) as the principle herbivores. Of the predators in early Holocene Europe can be counted principally the grey wolf (Canis lupus), Eurasian lynx (Lynx lynx), Brown bear (Ursus arctos) and, in the southeast, the European jackal (Canis aureus moreoticus) and lion (Panthera leo) (12) (11) (28).

This may seem a sizeable guild of megafauna—and it is, by modern standards—but it falls far short of the Eemian diversity. In addition to the aforementioned Holocene species, to the Eemian fauna can be added the European water-buffalo (Bubalus murrensis) and steppe-bison (Bison priscus), the giant deer (Megaloceros giganteus) and long-antlered deer Haploidoceros mediterraneus, as well as, perhaps surprisingly, the hippopotamus (Hippopotamus amphibius). There were three species of rhinoceros, the woolly rhinoceros (Coelodonta antiquitatis) in the north, and the temperate Mercks’ and Narrow-nosed rhinoceroses (Stephanorhinus kirchbergensis and hemitoechus) to the south. Two species of elephants inhabited Europe: the woolly mammoth (Mammuthus primigenius) in the colder regions of the Ukraine and Russia, and the temperate European elephant in the south (Palaeoloxodon antiquus). Not even listed are the predators, which beyond merely the lion counted also the leopard (Panthera pardus) and spotted hyena (Crocuta crocuta) as traditionally “African” characters, as well as the dhole (Cuon alpinus) and globally extinct cave bear (Ursus spelaeus) (32) (29) (17). The Pleistocene lions were of a separate species, the cave lion (Panthera spelaea), whose extinction left an ecological vacuum filled by Asiatic lions part-way through the Holocene.

Figure 2. An Eemian scene of Northwestern Europe, circa 120kya. Animals from left-to-right are aurochsen (Bos primigenius), grey partridges (Perdix perdix), European elephants (Palaeoloxodon antiquus), dholes (Cuon alpinus) and a robin (Erithacus rubecula).

Terms of use: Art commissioned by The Extinctions from Hodari Nundu.

These, then, give a good impression of the guild of megafauna we are dealing with as we enter the Weichsel glaciation. A further, significant subdivision can be drawn between those species which are typically considered as “Ice Age faunas”—that is, those species which are either adapted to, or capable of tolerating, the cold and treeless steppe-conditions of the continent’s north during glacial times—and those species which are, broadly speaking, “temperate”. To the former can be counted quite obviously animals such as the woolly mammoth, saiga antelope (Saiga tatarica) and reindeer (Rangifer tarandus), as well as predators like the grey wolf, lion, and hyena. To the temperate fauna, incapable of surviving on the steppes, and enduring during the glacials in isolated southern refugias, can be added the European elephant, hippopotamus, Mercks’ and narrow-nosed rhinoceros and many more. The drawing of this distinction already now will be very useful going forward, as it will be seen that the fates of these two faunas—broadly speaking a southern and a northern—are ecologically separate, and that this distinction is key to the broader question of what, most likely, happened to Europe’s vanished megafauna.

The Climate & Chronology

“Climate” is as recurring a word in discussions of past megafauna as it is of the world today. Long, it has been posited as the primary cause driving the extinction of the Pleistocene megafauna. The debate regarding the Quaternary extinctions has divided largely into two camps: those favouring a human-driven explanation (incarnated variously as the “overkill hypothesis”, “blitzkrieg hypothesis” or “anthropogenic hypothesis”) and those promoting a climatic cause (15). As such, a thorough investigation of the available evidence will be needed before a conclusion can be reached. It would be helpful to begin by presenting a more-or-less uncontroversial chronology of events, which may later be interpreted and analysed:

As noted previously, the Eemian interglacial lasted from about 130-115,000 years ago. Following its ending began the Last Glacial Period, referred to in Northern Europe as the Weichsel (19). It lasted for approximately 100,000 years, reaching its greatest extent generally between 30-20,000 years ago, during what is known as the Last Glacial Maximum (LGM) (16). It was marked by numerous stadials and interstadials—that is, periods of relative cold and warmth (6). Within the Greenland ice cores, these are termed Dansgaard-Oeschger events (37), and will be returned to later. Near the end of the LGP was a particularly warm interstadial, called the Bølling-Allerød (BAIC), which was followed by an abrupt and severe cold-snap, the Younger Dryas, and then the onset of the Holocene (21). The next aspect of the chronology which must be understood is the dating of extinctions.

The traditional picture has been, as noted, that there existed during the Ice Age a series of refugia in the southern regions of Europe (31). Here, the temperate, heat-craving species survived, and later spread out after the end of the glacial. Newer studies, however, have indicated that this picture lacks nuance, and that some trees (particularly the hardy species such as Salix caprea) had potential ranges far greater than hitherto assumed.  And yet, even granting this, it does seem apparent that the old idea was not entirely wrong. The refugia in southern Europe did act as sorts of “Noah’s arks” for a range of species, including more frost-intolerant trees (31). This included nearly the entirety of the temperate Eemian fauna, which had survived the onset of the glacial period, but of which many species were now restricted to the continent’s southern fringes. (30) (24) (4). Here the temperate fauna endured for the first circa 60,000 years of the glacial period, suddenly vanishing around 50-40ky (4). This wave of extinctions did not affect the northern, “ice age” megafauna, which survived until about 15-11,000 years ago, when it too began to decline (30) (21). It is worth noting already here that “the ice-age ended and then the ice-age megafauna died out” is a simplification bordering on error. Several species typically associated with the ice-age fauna survived well into the Holocene in parts of Europe and Eurasia.

The Climatic evidence

Now, when considering the effects of the last glacial upon species survival, it is useful to consider the previous glacials preceding it. As per Lang et al. (2018), the Saalian—the second-to-last glacial, preceding the Eemian—was even more extreme in its glacial advance than the Weichsel, extending into central Germany and creating large ice-dammed lakes as far south as Dortmund and the Münsterland. The Saalian reached its maximum extension in its last phase, just before the transition into the Eemian. (9). Despite its larger icesheet, the Saalian was warmer than the Weichsel. (9). The Eemain, however, was also warmer than the Holocene, with the result being that the temperature shift from Saalian to Eemian was still larger than that from the Last Glacial Period to the Holocene. This immediately appears as fairly damning evidence against any solely climatic explanation for the end-Pleistocene extinctions, since the previous and more dramatic Saalian-Eemian shift did not cause any comparable spike in extinctions. More substantial yet, however, is the presence of numerous glacial-interglacial cycles even before the Saalian. In Scandinavia, there was a total of ten separate periods of glacial advance (23), which renders any notion of the Last Glacial Period as particularly unique rather untenable, and certainly needing firm evidence to support it. The evident conclusion from these studies is that, unless evidence can be produced that the LGP was exceedingly abnormal in some hitherto undiscovered way, the failure of the preceding nine glacials to produce any comparable extinction to that of the end-Pleistocene constitutes a severe blow against the climatic hypothesis.

Figure 3. The maximum extent of respectively the Saalian (red line) and Weichsel (dotted blue line) glacials.

Terms of use: This image is licensed under a Creative Commons Attribution-ShareAlike 4.0 International. It is attributed to Juschki. The image is unedited and the original can be found here

Even within the context of the LGP alone, it is unclear why the end of the glaciation should be a more damaging event than the beginning of it—surely the end of the Eemian and rapid collapse of Europe into near-arctic conditions was an at least comparably devastating event, in terms of range-contractions and lost habitat, to the climatic amelioration that was the dawn of the Holocene. Interesting in this context is Salonen et al. (2018), who show that a series of dramatic and abrupt climatic shifts occurred at high latitudes during the Eemian, in one case returning the July mean temperature to pre-interglacial levels. Equally relevant here are the numerous stadials and interstadials mentioned in our chronology of events, of which one, the Bølling-Allerød, is often posited as a key player in the extinction of the ice-age megafauna (21) (20). Yet the BAIC, though the warmest interstadial during the Weichsel, was only the last of eight (5), which is not even counting the stadials or the oscillations during the previous glacials. Cooper et al (2015) explore the relationship between Dansgaard-Oeschger events (interstadials) and population-transformations, and do find a clear correlation (37). As per their results, climatic fluctuations, particularly warming events, are correlated with the decline and/or collapse of many sub-populations. It is evident, then, that such events do have severe ecological impacts, up to and including regional extinctions. Even so, the authors note that there were numerous interstadials, both within and prior to their studied period, and that the great majority of these failed to cause any species-level collapse. Local populations were disrupted, but only in the presence of human settlers do we see total extinctions at the metapopulation-level (37).

If megafauna is not simply vulnerable by nature to climatic shifts, that means the event that destroyed them must have been unusual. Yet the issue now becomes not only to show that it was unusual, but that it was, in fact, an event. Unfortunately, the dating, even by studies affirming a climatic cause, quite clearly confuses this. The temperate megafauna, restricted to the southern refugias, vanished suddenly around 45-40ky. This was not the coldest period thus far, nor the most climatically unstable (5). It also does not correspond well to any other extinctions world-wide, save possibly that in Australia, though even that is unclear (4). What follows is then a lull of nearly 30,000 years, before the extinctions once again begin around the very end of the Pleistocene. Yet even in the studies affirming a climatic role, the supposed dates of extinction seem very haphazard. The decline of the muskoxen and steppe-bison in Europe seems to correlate with the Bølling-Allerød interstadial, but even then, they hold out in Siberia for many millennia more, and see no corresponding decline in North America, despite similar climatic conditions (21). The woolly rhinoceros likewise may have declined around the Bølling-Allerød, but as noted, the mammoth survived into the early Holocene in northern Europe (20), and until 2000 BC on Wrangel island off Siberia (33). Why an inbred population on a small island should be more resilient than their mainland counterparts is unclear at best. The spotted hyena disappears from the record about 26,000 years ago, the cave bear about 24ky (20), while the leopard may have survived into the Holocene (27), and the dhole and giant deer quite confidently seem to have done so (25) (20).

The Extinction of the Mammoth – a case study

A case-study may be helpful in our investigations. The woolly mammoth is one of—if not the most—well-known members of the Ice-age fauna. It had a circumpolar range, stretching from the Atlantic coast of Doggerland through Asia, over Beringia (the present-day Bering Strait) and to the opposite Atlantic coast in North America (22). Throughout this area, it inhabited a vast grassland biome termed the mammoth steppe, which was, during the LGP, the most widespread biome on earth. (36). It was with the disappearance of its steppe that the mammoth vanished, or so it is traditionally held (22). As such, the woolly mammoth, beyond merely being an emblem of the ice-age, is also in many ways an indicator-species for the glacial steppe. Determine the reason for the extinction of the mammoth, and you may know better the fate of its cohabitants.

So why did the mammoth disappear? The climate is, as ever, an intuitive place to look. It has often been claimed that the mammoth was strongly specialised to a diet of tundra- and steppe-vegetation (2) (7). At the end of the Ice-age, temperatures rose dramatically over only a few millennia, while dramatic sea-rises (up to 80 metres) drowned much of what had previously been the mammoths’ habitat (8). The transition from Pleistocene to Holocene saw a marked change in the northern biotopes towards marshier and wetter conditions. Vast areas of previously dry steppe were gradually replaced by watery tundra and wet boreal swamps (22). One study, Nogués-Bravo et al (2008), models a 90% range-decline between 42-6ky BP, and it was indeed within this period than the great majority of mammoth-populations died out.

Figure 4. Landscape from glacial northern Spain, showing wild horses (E. ferus ferus), woolly mammoths (Mammuthus primigenius), Eurasian cave-lions (P. Spelaea) and a woolly rhinoceros (C. antiquitatis).

Terms of use: This image is licensed under a Creative Commons Attribution 2.5 Generic. It is attributed to Mauricio Antón. The image is unedited and the original can be found here

This explanation, however, begins to meet severe problems upon closer inspection. Let us begin by looking at Nogués-Bravo et al (2008), and the assumptions baked into their model: For them, the replacement of the dry mammoth-steppe by watery tundra was a key driver of the mammoth’s decline. Yet a new study has found that northern Siberia was already dominated by wet forest-tundra during the Pleistocene (3), even during the days of the mammoth steppe. The idea that the transformation of the steppe into this habitat should be an existential crisis for the species seems quite suspect, then, given that the animal had seemingly already been inhabiting such tundra for the last 100,000 years. One must also ask, regarding the decline of the steppe: What did the mammoth do in the previous interglacials? Are we to assume that the mammoth steppe did not see a comparable retraction during the Eemian, a period even warmer than our own? Indeed, given this broader perspective, it is perhaps unsurprising that the mammoth does seem to have been more habitat-flexible than often assumed. During the late Pleistocene, its range extended into Iberia, where they inhabited a mixed assemblage of temperate and cold-adapted species (1). Even clearer evidence of this is Stuart, A. J. (1976), which shows that mammoths, as well as other typically tundra-associated species, such as the woolly rhinoceros, coexisted with completely temperate faunas during parts of the Eemian. Taken together, these studies indicate that the mammoth had an extremely broad and adaptable distribution throughout the northern Palearctic. Its primary habitat was the dry mammoth steppe, but it also occurred in wet, semi-forested tundra, as well as warm, temperate pastures. This renders traditional explanations of its extinction by diminishing habitat implausible. A consequence of this is that studies like Nogués-Bravo et al (2008), which claim to model a 90% reduction of suitable mammoth habitat between 42-6ky BP, are almost certainly operating off of an incorrect estimation of what qualifies as “suitable” habitat.

A final study that must be raised here is Drucker et al (2015), which shows that mammoths began declining in Europe before their habitat shrank. For a time, their niche was occupied by expanding wild horses, who shared their lifestyle and environment. How this can be squared with a climatic explanation is difficult to see.

Conclusion

Two great takeaways arise from our investigation: The first is the issue of precedent: these same climatic shifts had already occurred countless times before—why did they have no comparable effect? The latest glaciation was but one of many similar events—the most recent link in a chain of glacials and interglacials that has stretched back for more than a million years. Any hypothesis touting a climatic explanation must account for this enigma, or else be of very little value. Yet if one does attempt to explain how this latest event was unique, we encounter the second takeaway: the lack of synchronicity. Even within the European context alone, the extinctions occur in two separate, apparently unconnected waves, 30,000 years apart. Even narrowing in on the second wave, those extinctions surrounding the end of the glacial period, closer inspection reveals them as less of a singular event, and more of a haphazard trickle, scattered across time and space, rarely with clear correspondences to climatic shifts. This will become even clearer in the third part of this series, where we investigate the Holocene, not as a separate epoch in earth’s history, but as a seamless continuation of the Quaternary extinctions. Before part three must come part two, however, and there, we will turn our attention to the gaping omission from our narrative thus far—Homo sapiens.

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