The legacy of the lost giants

We have a couple of new papers out this week, presenting an intriguing new idea. They are focussed on the idea that large animals play a disproportionate role in transferring nutrients around landscapes through their dung.

The papers can be downloaded as pdfs from my publications list here or directly from here (Nature Geoscience paper) and here (PLOS One paper).
There is coverage of these papers on the BBC here, and also in New Scientist and the Independent among others.

Until very recently in Earth history, just before the dawn of farming, large animals were everywhere. The landscapes of North and South America, Eurasia and Australia looked very much like how African safari parks do today, teeming with giant creatures, the megafauna. In most continents these megafauna disappeared around the time of the arrival of the first human hunters. There is still some debate as to the relative role of humans vs. climate change (particularly in North America) but from my perspective the evidence looks pretty overwhelming that humans played a major role. In Africa the megafauna held out, perhaps because they co-evolved with humans and learnt to become cautious of these puny-looking but dangerous primates.

In a paper in Nature Geoscience we explore the consequences of this megafaunal loss on nutrient cycling in the Amazon forest. We develop a mathematical model, calibrated off modern data from Africa, showing that large animals act like major nutrient arteries pushing nutrients through landscapes. Large animals matter because they eat more, the food sits longer in their guts, and they travel further each day. Using this model we estimate that the nutrient pump in Amazonia diminished 50-fold after the megafaunal extinction.

In a parallel paper in PLOS One we look at the contemporary world. We test our model against data from Kruger National Park in South Africa. Then we use global animal size and range data to look at the size of the modern nutrient pump across the world. Apart from bright patches in parts of Africa and southern Asia, the pump is effectively switched off across the world, a legacy of the megafaunal extinctions.

More generally, we argue that in the world of the megafauna nutrients moved freely across landscapes, mixed around by the large beasts, and that the current patchy distributions of nutrients in weathering regions and floodplains is a legacy of megafaunal extinctions. If humans played a major role in these extinctions, as I suspect, our alteration of biogeochemical cycling and planetary function at global scales began long before even the dawn of agriculture.

Today, some of the remaining the megafauna are still being slaughtered, particularly the forest elephants of the Congo Basin. Over half of the remaining elephants have been slaughtered in the last decade because of the illegal ivory trade. In March I was in Gabon in Ivindo National Park, one of the last strongholds of the forest elephant. I gained an immense respect for these giant, intelligent and complex creatures. Our new papers show that their ongoing loss may have consequences for more than the elephants alone.