Rewilding Arctic with mammals ‘will do little’ to slow impact of climate crisis, study suggests

Changing climate made landscapes less able to support megafauna, rather than loss of animals due to human hunting allowing an expansion of shrubs, scientists say

Changes to tundra landscapes due to warming climates left mammoths with little to eat, research suggests (Daniel Eskridge)

Returning large grazing mammals to tundra environments may not have a significant impact on landscapes and the environment, new research suggests.

A study looking at the demise of large animals following the end of the ice age, indicates that the warming climate allowed the expansion of plant species such as shrubs, which ultimately resulted in the loss of iconic mammal species, rather than the other way round.

The researchers said their findings have “major implications” for proposals to prevent soils in the Arctic from thawing by reintroducing animals such as bison and horses.

Evidence shows that about 14,000 years ago, grassy landscapes that had extended eastwards from France all the way across the Bering Sea to Canada, were transformed by the rapid spread of shrubs.

At the same time, several iconic mammal species such as the woolly mammoth became extinct from what is now Alaska and the Yukon, while archaeological evidence has revealed the presence of humans in these regions.

The research team described these findings as “ancient coincidences”, but said they lead to the suggestion that human hunting caused the demise of the mammals, and this in turn led to the expansion of shrubs, as the animals were not there to trample down the vegetation and keep the nutrients in the soil.

Now, due to the climate crisis which is particularly powerful in the Arctic, shrubs are spreading even further north into tundra regions.

Meanwhile there is increased advocacy for rewilding where animals are returned to their original ecosystems to restore more “natural” conditions.

It has been hypothesised that increasing numbers of mammals might reverse the trend of increasing shrub cover, resulting in the possible benefit of keeping carbon stored in the ground.

This is because low-growing vegetation exposes the ground to colder conditions than shrub cover does, and therefore the ground and the carbon it contains remain well frozen.

Another theory is that the changing climate drove the vegetation and landscape changes, and these led to the loss of the animals as their habitat disappeared.

To test these alternative hypotheses, an international research team examined records of fossil pollen preserved in lake sediments across Alaska and Yukon for thousands of years.

They examined records that met strict dating criteria which meant the team could accurately pinpoint the timing of shrub expansion across this region. They then compared this with how the numbers of radiocarbon-dated bones from horse, bison, mammoth and moose changed through time – which provided them with an estimate of their changing population sizes.

Their results showed that willow and birch shrubs began to expand across Alaska and Yukon around 14,000 years ago, when records of dated bones indicate that large grazing mammals were still abundant on the landscape.

Professor Mary Edwards of the University of Southampton who was part of the study team, said: “Our study uses a clear predictive test to assess two opposing hypotheses about large animals in ancient and modern tundra ecosystems: that the animals disappeared before the shrubs increased, or that the shrubs increased before the animals disappeared.“

Dr Ali Monteath, the lead author from the Universities of Alberta and Southampton, said: “The results support the idea that at the end of the last ice age a major shift to warmer and wetter conditions transformed the landscape in a way that was highly unfavourable to the animals, including mammoths”.

The findings indicate that climate change was the primary controller of northern ecosystems and that the large herbivores were not able to maintain their environment as the shrubs spread.

“While humans may have compounded population declines, our results suggest climate-driven vegetation change was the primary reason the mammals disappeared,” said Professor Edwards.

Rewilding the north with large mammals that are currently absent from the region, “would probably not transform the vegetation over large areas” and so do little to curtail release of carbon from the Arctic permafrost, the team concluded.

Study co-author Professor Duane Froese of the University of Alberta said: “Rewilding experiments at the scale of local paddocks, as has been done for example at Pleistocene Park in north-eastern Siberia, show that megaherbivores can alter their environment, drive changes in vegetation and even cool soil temperature, but these animal densities are much higher than we would expect for Pleistocene ecosystems.

“Our study shows that the effect of megafauna grazing is small at sub-continental scales even with the presence of mammoths, and climate, once again, is the main driver of these systems.”

Benjamin Gaglioti of the University of Alaska-Fairbanks said: “The hypothesis that reintroducing megafauna will prevent or slow warming-driven permafrost thaw and vegetation change in the Arctic has been bolstered by the idea that Pleistocene megafauna were instrumental in maintaining ice age ecosystems.

“In contrast to this prediction, our results show that high-latitude ecosystems responded sensitively to past warming events, even though megafauna were abundant on the landscape. These results lend support to the hypothesis that reintroducing megafauna today will do little to desensitize high latitude ecosystems to human driven warming.”

The research is published in the Proceedings of the National Academy of Sciences.