Mass extinction of most of the world's large mammals some 10,000 years ago liberated roughly 1.4 petagrams of plant life previously consumed as food. The surplus endured until human populations grew to fill the void.
Christopher Doughty, now at the University of Oxford, UK, and Christopher Field at the Carnegie Institution in Stanford, California, estimated consumption by the extinct Pleistocene megafauna and by humans, and compared the results with net primary plant production around the globe. Averaging the figures out worldwide, they found that liberated plant resources — about 2.5% of net terrestrial productivity — had been used up by humans by about 1700.
The duo also showed that by 2000, humans were consuming roughly six times more than the megafauna had done. Meanwhile, human agriculture had reduced global primary productivity by about 10% as a result of factors such as land degradation.