Thousands of Drowning Wildebeest Keep a Serengeti Ecosystem Humming

In the largest animal migration on Earth, every year about 1.2 million wildebeest run through the Serengeti and the Mara River crossing in Kenya, where thousands drown in the river rapids.

For the first time, ecologists have measured the contribution that these mass drownings make to the ecology of the Mara River, contributing nutrients and providing a food source to other animals in the food chain. It is an example of the 'circle of life' that constitutes ecological processes in nature.

Thousands of wildebeest embark on their migration across the savannas of East Africa every year, and they usually take about 200,000 zebra and antelope who get caught up in the herds with them. When they get to the Mara River and have to make several crossings, many of the unlucky ones do not make it because they drown or are eaten by crocodiles.

A team of ecologists analyzed the number of wildebeest that drown in the river every year, noting the amount of biomass that accumulates as a result. The carcasses pile up in the river and are slowly consumed by animals that inhabit the ecosystem, including animals from the river, on land, and in the air.

The researchers found that on average 6,200 wildebeest drown each year, which amounts to 1,100 tons of biomass, or the equivalent of adding ten blue whale carcasses to the Mara River each year. "This dramatic subsidy delivers terrestrial nitrogen, phosphorus and carbon the river's food web," says ecologist Emma Rosi, a member of the team.

The methods used by the team to track the carcasses as they flowed through the ecosystem were cameras to track scavenger birds such as vultures, and stable isotope analysis, which allowed the team to trace the nutrients as they travelled elsewhere through the acquatic ecosystem food chain.

They found that about 9% of the carcasses are devoured by vultures species, however the biggest recipients of the nutrients were fish found in the river. During the migration season, wildebeest carcasses make up half of the diet for these fish. Lastly, once the carcasses have been stripped of thei rmeat, the bones break down and release more nutrients into the water, continuing to feed the ecosystem for years afterward.

Thus, although thousands of wildebeest are lost from the herd, ultimately the gain for the ecoystsem is much greater than the loss to the herd, according to Science Alert. "What is happening there is a window into the past, when large migratory herds were free to roam the landscape, and the drownings likely played an important role in rivers through the world," said Subalusky.