The giant ground sloth, imagined in happier days (Image: Jaime Chirinos/SPL)

It’s not just humans that still feel the effects of a trauma many years later: ecosystems do too. Thousands of years after human hunters wiped out big land animals like giant ground sloths, the ecosystems they lived in are still feeling the effects.

Many ecosystems rely on big animals to supply them with nutrients, mostly from dung. “If you remove the big animals from an ecosystem, you pretty much stop nutrients moving,” says Chris Doughty of the University of Oxford.

Doughty and colleagues simulated the distribution of phosphorus, a nutrient that plants need to grow, in the Amazon basin in South America. This area was once home to spectacularly large animals, including the elephant-like gomphotheres and giant ground sloths.


But 12,500 years ago, around the time humans moved into South America, these huge animals all died out, hit by a double whammy of being hunted and a changing climate. Nowadays the Amazon is still home to a huge diversity of animals. “But these extinctions cut out all the big animals,” says Doughty.

Dung shippers

It seems the mass extinction had a profound effect on how phosphorus is spread around the Amazon basin. Nutrients are released when rocks are eroded, and then get distributed onto flood plains by rivers. In South America, the most phosphorus-rich soils are found near the Andes mountain chain in the west, and the rivers flowing from it – especially the Amazon.

Using the relationship between animal size and nutrient distribution seen in living animals as a guide, Doughty estimated how much phosphorus South America’s larger extinct animals would have transported 15,000 years ago, before they started to decline.

The model suggests that megafauna would have spread nutrients 50 times further in the same time than animals today do. Or to put it another way, killing off the massive animals reduced the movement of nutrients by 98 per cent. This is because big animals carry a disproportionately large amount compared with small animals as they travel further in search of food, and keep that food in their guts for longer.

Doughty compares big animals to the arteries that carry blood around the human body. “When you get rid of big animals, it’s like severing the nutrient arteries.” He thinks the same thing happened in North America, Europe and Australia, where most big animals have also been wiped out.

“The idea that herbivores redistribute nutrients is not new, but the scale of this thinking is much, much bigger,” says Tim Baker at the University of Leeds in the UK.

Enduring effect

If Doughty is right, the Amazon is still changing in response to the extinction. He estimates the spread of nutrients will keep getting patchier for another 17,000 years, although the effect will likely be dwarfed by the impacts of deforestation and climate change in the short term.

In the absence of massive herbivores, humans now dominate the movement of nutrients. But we do the opposite of what the lost animals did. “These megafauna would disperse nutrients, whereas humans concentrate them,” says Doughty. We spread fertiliser on small plots of productive farmland, and keep large animals like cows fenced rather than letting them roam freely. “There are probably more nutrients because of people, but they are very poorly distributed.”

In a separate study, the team estimates how much large animals help to spread nutrients around in Kruger National Park in South Africa. The park is divided into a nutrient-rich basalt area and a nutrient-poor granite zone, but elephants and rhinos transfer nutrients between the two, helping plant growth in the granite zone. Wiping out these animals would have a severe effect. “You would see nutrient distribution drop by about 50 per cent,” say Doughty.

Journal references: Nature Geoscience, DOI: 10.1038/ngeo1895; PLoS One, DOI: 10.1371/journal.pone.0071352