It's the largest, most spectacular animal migration on our planet. Every year, some 1.2 million wildebeest trample through the Serengeti and the perilous Mara River crossings where thousands of them succumb to the river rapids.
Ecologists know that animal migration, especially on grand scales, affects land ecosystems. But now for the first time they have measured the ecological contribution of mass drownings in the iconic Kenyan river. What they have found is an astonishing example of the circle of life that sustains the natural world.
When immense numbers of wildebeest embark on their annual migration across the savannas of East Africa, the stream of animals takes some 200,000 zebra and antelope along for the trip.
You have probably seen a natural documentary depiction of this epic pilgrimage, especially its most dramatic part around the Mara River where the animals have to make several crossings. Most of them brave the waters with success, but unlucky ones are often shown drowning or being eaten by crocodiles.
All those carcasses eventually pile up in the river, and are slowly consumed by the various creatures that inhabit the ecosystem, both in the river, on land and in the skies.
A team lead by ecologist Amanda Subalusky from the Cary Institute of Ecosystem Studies wondered about the crazy amount of biomass these drownings must represent.
"We used historical reports from 2001 to 2010 and field surveys from 2011 to 2015 to quantify the frequency and size of wildebeest mass drownings in the Kenyan portion of the Mara River," the team writes in the paper.
Armed with data from field surveys and biochemical analysis, they calculated the fate of an animal carcass as it drowns and enters the river ecosystem.
The researchers found that, on average, 6,200 wildebeest drown each year in the Kenyan portion of Mara River, amounting to 1,100 tons of biomass.
"To put this in perspective, it's the equivalent of adding ten blue whale carcasses to the moderately-sized Mara River each year. This dramatic subsidy delivers terrestrial nitrogen, phosphorus, and carbon to the river's food web," says one of the team, ecologist Emma Rosi.
The team used cameras to track scavenger birds, and a common aquatic ecosystem nutrient tracking method, stable isotope analysis, which allowed them to trace the nutrients from the drowned animals all the way down the food chain.
It turns out that each mass drowning represents a massive boon to the local river ecosystem, feeding everyone in the river. Only a small proportion - some 2 percent - of the wildebeest feast is eaten by crocs.
On land, up to 9 percent of the corpses are devoured by several vulture species. But the biggest winners are the various species of common fish in the river. When carcasses are abundant, they will make up half of the diet for these fish.
And once the drowned bodies have been picked clean, the bones end up leaching even more nutrients into the waters, continuing to feed the ecosystem for years to come.
As dramatic as it is to have thousands of animals go down in the turbulent waters every year, ultimately the gain for the ecosystem is much greater than the loss to the herd.
"These mass drownings have little impact on the wildebeest herd, comprising only 0.5 percent of the total herd size, but they provide huge short-term and long-term sources of nutrients to the Mara River," write the researchers.
As humans have encroached on animal habitats, mass migration routes have altered. The researchers point out that loss of widespread drownings could be responsible for fundamentally altering river ecosystems.
But each year, the wildebeest still travel across the Serengeti plains, with unlucky ones still drowning in troves, providing sustenance to myriad river creatures long after their death.
"What is happening there is a window into the past, when large migratory herds were free to roam the landscape, and drownings likely played an important role in rivers throughout the world," says Subalusky.
The study has been published in PNAS.