Research led by Eske Willerslev reveals that the sharp decline in the number of plant and animal species in the Northern Hemisphere over the past 50,000 years has not been caused by human activity but by climate change, primarily driven by changes in the quantity of precipitation.
The decline in the diversity of plants and large mammals has been caused by changes in precipitation – and only slightly by temperature. This is a conclusion of a study led by Eske Willerslev, Professor, Department of Zoology, University of Cambridge, United Kingdom that examined ancient Arctic and subarctic plant and mammal communities. The research involved metagenomic analysis of permafrost and lake sediment samples spanning the past 50,000 years from across the Arctic and the subarctic zone.
“Our study shows that nature responds unpredictably to climate change. Knowing how much the global mean temperature will rise does not predict how much ecosystems are going to change. And even if the climate could be brought back to its original state, ecosystems would not revert to what they once were,” explains Eske Willerslev.
He adds that how animals and plants react to climate change largely seems to depend on local conditions.
“We should not expect all northern temperate areas to react uniformly to climate change. We cannot simply say that global warming will cause “southern Europe” to move further north, such as the concept that the nature in France will move to Denmark. It is not that simple,” he says.
Precipitation more important than temperature
The research investigated how climate change during the last glacial–interglacial cycle changed the biodiversity of plants and animals. Eske Willerslev and colleagues collected 535 permafrost and lake sediment samples from across the Arctic and subarctic zone spanning the past 50,000 years and analysed their DNA. To identify and map the distribution the plant species that have characterised the flora through the ages, the researchers compared their analysis with 1,541 contemporary plant genome assemblies, which they collected from both the Arctic and the subarctic zone.
“The study was comprehensive and delivers a wealth of results. The most interesting result is that climate change clearly drives the loss of plant and animal diversity. Climate change determines how the environments change the composition of flora and fauna,” explains Eske Willerslev, who goes on to outline the close interaction between climate and the composition of flora.
During the Ice Age 55,000–30,000 years ago, the Arctic was a homogeneous plain with many plant species. But around 20,000 years ago, the abundant species richness of the steppe declined markedly. The assumption so far has been that declining temperatures caused the number of species to shrink. But then diversity would have increased as the climate became warmer again 10,000–11,000 years ago. According to Eske Willerslev, this did not happen.
“Our metagenomic study shows that the warmer climate led to a complete change in plant composition, so that it came to resemble the current situation with a not very diverse cover of shrubs and grasses. Even though the plant cover was dense, the diversity was not great, and most plant species adapted to a humid climate. This suggests that the number of species depends on the quantity of precipitation rather than temperature. The vegetation became more adapted to the humid conditions instead of the previous dry conditions,” he says.
Good climate models needed
Eske Willerslev and his team not only discovered that the quantity of precipitation controls species richness but also found considerable regional divergence. Whereas plant composition in the steppe in North America already began to change 20,000 years ago, this change occurred significantly later in northeastern Siberia and very late in northern central Siberia. Ice Age plants and animals such as mammoths survived there until just 4000 years ago. Only then did the composition of flora and fauna change, he says.
“Our study shows that biology responds over time in regionally specific and unpredictable patterns. This has major implications for the models developed for the current and future projections for species composition. Assuming that we can predict the distribution of plant and animal species in 20, 50 or 100 years based on future climate projections is incredibly foolish,” explains Eske Willerslev.
He hopes that the new data can help researchers adapt the models based on an assessment of how good they are at predicting how climate will affect nature. Eske Willerslev says that if the models cannot describe the past, they will not be very useful at predicting the future.
Humans have not directly caused the sixth mass extinction
In addition to mapping how climate change affects Arctic and subarctic flora and fauna, Eske Willerslev and colleagues also tried to determine what has driven the sixth mass extinction in the Holocene Epoch: the increased losses of plant and animal species after humans colonised the globe. Since then, the number of insects, vertebrates and plant species has plummeted, which a 2019 United Nations report calls a potential disaster for biodiversity. Up to 1 million species are threatened with extinction, including two thirds of all the large mammals in the Northern Hemisphere.
“Researchers have been discussing what drives this mass extinction for more than a century. The extinction of animals coincides with both climate change and human activities, which have therefore formed the basis of each theory,” says Eske Willerslev, who adds that the research appears to show that the extinction of animal species is correlated with climate change – for example, finding the last mammoths in northern central Siberia because the steppe continued to exist for much longer than elsewhere.
“I think that this is strong evidence that the sixth mass extinction is driven by change in vegetation and climate and not by humans. And here, too, it is not the temperature that controls the trend but the quantity of precipitation,” explains Eske Willerslev.
Influence of climate change on ecosystems is irreversible
According to Eske Willerslev, the fact that the quantity of precipitation controls the loss of biodiversity among flora and fauna is food for thought in the current struggle to limit climate change, despite everyone only being concerned about temperature.
“Our research shows that the current biodiversity crisis is therefore extremely closely linked to the climate crisis, which can clearly have a gigantic impact on our environment without us being able to predict exactly how. The only thing we can say with certainty is that the regional differences are quite extreme and can lead to specific extinctions of animals and plants, and once this happens, it is irreversible,” says Eske Willerslev.
Since nature cannot revert to its old state, Eske Willerslev thinks that it does not matter how much we try to mitigate climate change by, for example, extracting carbon dioxide from the atmosphere and storing it underground.
“Politicians are currently focusing on how to regulate the climate. But our results show that they should be thinking to a much greater extent about the impact climate change is going to have and when. If they do not inject resources into improving the prediction of what is going to happen, then we will not be able to prepare in time for the changes in nature that are going to take place, because we cannot predict them. And when people do not know what is going to happen, then they can easily react in desperation and panic,” explains Eske Willerslev, who concludes:
“Politicians and foundations may well find themselves investing huge amounts in health research, and health is important. But climate change is the greatest crisis facing humanity, and how little is actually invested in research on the green transition is therefore alarming. I hope that our results may be a wake-up call.”
