Large Animal Extinctions and Soil Fertility

By The Editors

During the final millennia of the Pleistocene Epoch, roughly 100 genera of megafauna became extinct worldwide.

In other words, between 50,000 and 10,000 years ago, the large majority of animals weighing more than 100 pounds died out

Examples of these extinct animals are mammoths, the saber-toothed tigers, Diprotodon (an Australian marsupial the size of a hippopotamus) and Coelodonta (a woolly rhinoceros found in Europe).

Hotly debated theories have been proposed to explain why megafauna died out. These are the overill (died because of diseases), overchill (died because of colder temperature caused by climate change), and overkill (died because of human hunting) theories.

Now, results from a new study (published on August 11, 2013, in the scientific journal Nature Geoscience) highlight the worldwide environmental value that big animals provide by spreading, long-distance, essential nutrients that contribute to soil fertility: large animals transport nutrients into areas where the soil is otherwise infertile.

For their study, the researchers used a novel mathematical model to calculate the effect of mass extinctions of the big animals that lived on our planet 12,000 years ago, when much of the world looked like an African savannah. In South America, the megafauna included several species of elephant-like creatures, giant ground sloths, and armadillo-like animals the size of a small car. On the basis of the average size and distribution of the animals, the model allows to calculate the ability of animals to distribute nutrients anywhere on the planet at any time. It can also forecast the effects of potential events thousands of years in the future, such as calculating how much the fertility levels of the soil would fall following elephant extinction in Africa.

The researchers focused on a case study of the Amazon forest and estimated that, back then, extinctions reduced the dispersal of phosphorus (a major nutrient) in the Amazon forest by 98%, with far-reaching environmental consequences that remain to this day.

In South America, most nutrients originate in the Andes mountain range and are washed into the forests through the river system.  However, on dry land, these nutrients are in short supply unless they are transported through animal dung and bodies. While small animals distribute nutrients over small distances, large animals have a much greater range.

According to the study, the extinctions of large animals 12,000 years ago wiped out one of the main means of transporting nutrients far from the rivers. Vital nutrients, such as phosphorus, were no longer spread around the region but became concentrated in those areas bordering the floodplains and other fertile areas. The Amazon basin has not yet recovered from the change caused by megafauna extintion, and nutrients may continue to decline in the Amazon and other global regions for thousands of years to come.

Christopher Doughty (from the Environmental Change Institute at the School of Geography and the Environment, University of Oxford), lead author of the study, said: “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. We can also predict the effects of further extinctions – a fate fast approaching many of the large animals that remain – mainly in Africa and Asia – and examine the likely impact thousands of years into the future.”

Hopefully, we’ll be able to rescue the remaining African and Asian large animals from extinction.