Every year, hundreds of thousands of wildebeest thunder across the Serengeti chasing the rains and a promise of fresh, sweet grass. But along the way, thousands of the animals drown as they have to cross the Mara River. For the first time, researchers have now quantified exactly how this mass dying every year impacts the ecosystem.
Over the last six years, researchers from the Cary Institute of Ecosystem Studies have been tracking how this huge mass of nutrients flows through the ecosystem of the Mara River in Kenya, building up a picture of how the large volume of wildebeest killed maintains the environment. They found that while the soft tissue is consumed or decomposed within a matter of weeks, the bones of the animals continue to leach nutrients into the system for years.
“The Mara River intersects one of the largest overland migrations in the world,” said Amanda Subalusky, co-author of the paper published in PNAS. “During peak migration, the wildebeest cross the Mara River multiple times, sometimes resulting in drownings of hundreds or thousands of wildebeest. Our study is the first to quantify these mass drownings and study how they impact river life.”
On average, roughly 6,200 wildebeest drown during the migration each year, which equates to around 1,000 tonnes (1,100 tons) of biomass. Almost every year there is at least one mass drowning event, with one in 2007 that saw an estimated 10,000 animals dying in just a few days. This is an extraordinary amount of food and nutrients, taking nitrogen phosphorous and carbon from the terrestrial environment and moving it into the aquatic.
First to scavenge on the remains are often the fish, who can get up to 50 percent of their diet from the wildebeest soft tissue. Birds, such as Marabou storks, white-backed vultures, and hooded vultures eat up to 10 percent of the flesh, while crocodiles only consumed around 2 percent. But even once the juicy bits had all been eaten away, the bodies continued to fuel the ecosystem.
“Once carcasses disappear, bones – which make up nearly half of biomass inputs – continue to feed the river,” explained co-author Emma Rosi. In fact, they found that the bones of the wildebeest can continue adding phosphorus to the Mara River for up to seven years after their owner drowned.
The results raise an interesting question about how large migratory herds affected ecosystems in the past. The mass movements of bison in North America and quagga in South Africa were sure to have had similar impacts, and the wildebeest migration across the Mara can give a rare insight into what these may have been before the large herds were all killed off.