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How extinction of five-tonne sloths slows up growth along the Amazon

Region now lacking the fertilising effect provided by South America's distinctive 'mega-fauna' - which has led to a serious imbalance of soil minerals
  • @SteveAConnor

Vast regions of the Amazon are growing more slowly than they were several thousand years ago because they lack the fertilising effect provided by South America’s distinctive “mega-fauna” – the very large mammals that went extinct soon after the arrival of humans.

A study of how soil nutrients are distributed within the Amazon basin has revealed there is a dearth of vital minerals such as phosphorus because large mammals no longer roam the region to fertilise the soil with their dung.

Scientists believe that the extinction of large herbivores, such as five-tonne ground sloths and armadillo-like glyptodonts the size of small cars, led to a serious imbalance of soil minerals which is still having an impact today.

“It is having a big effect. The Eastern Amazon in particular is phosphorus-limited which means that if you added phosphorus to the region the trees there would grow faster,” said Christopher Doughty of the University of Oxford, the lead author of the study.

“When you had big animals roaming more than 12,000 years ago, there would have been wider dispersal of minerals such as phosphorus and the trees there would have been growing faster than they are today,” Dr Doughty said.

Minerals are continually washed down from the high Andes by the Amazon and its tributaries, but the elements tend to remain within the muddy soil of the flood plains rather than being dispersed more widely.

This was not the case for tens of thousands of years previously when generation after generation of the mega-fauna fed on the plants and trees growing in the flood plains before roaming to higher ground where they fertilised the soil with their dung and dead bodies, the scientists said.

The study, published in Nature Geoscience, calculated 98 per cent of nutrient-dispersal has been lost since the extinction of the mega-fauna, which happened around the same time that humans first colonised South America from the north.

“While 12,000 years may be a timescale that is beyond most people’s understanding, through this model we show that extinctions back then still affect the health of the planet to this day,” Dr Doughty said. “Put simply, the bigger the animal, the bigger its role in distributing nutrients that enrich the environment.

“Most of the planet’s large animals have already gone extinct, thereby  severing the arteries that carried  nutrients far beyond the rivers into  infertile areas,” he said.

Deforestation of the Amazon is hindering the dispersal even more because although human activity leads to increased fertilisers being added to the land, these become concentrated as a result of farming practices such as the fencing-in of livestock.

Wild mega-fauna that roamed freely acted in the same way as the arteries of the human body by distributing nutrients further and further from the source, the scientists said.