Quirks & Quarks

Mass drownings of wildebeest nourish Serengeti river ecosystem

Thousands of wildebeest drown every year during their annual migration in the Serengeti. Now scientists have discovered that what looks like a tragedy for the herd offers a rich source of key nutrients to creatures in the river and beyond.
Wildebeest follow the rains in search of green grass to feed on in the Serengeti. (Pixabay)

The amazing spectacle of 1.2 million wildebeest crossing the Mara river on their Serengeti migration, often ends in drownings for thousands of members of the herd. But those carcasses feed an array of creatures in the rich river ecosystem.

Migrating wildebeest follow the rains to find long, green grass to eat. Crossing the Mara River is an arduous part of the journey.  

Wildebeest crossing the Mara River. The animals are not strong swimmers and the steep banks can be too hard to handle for thousands every year. (David Post)

Dr. David Post is a professor of ecology and evolutionary biology at Yale University in New Haven, Conn. Post and his colleagues at the Cary Institute of Ecosystem Studies in New York released a new study that quantifies the mass drownings of wildebeest and looks at how drowned carcasses provide nutrients to creatures in the river and on land.

Wildebeest are poor swimmers and the steep river banks can be too much for their hooves to handle. If they cross at a tricky section, it can lead one animal to panic, leading to trampling. Thousands go down. 

Marabou stork and vultures immediately feed on wildebeest flesh, transporting nutrients from drowned wildebeest back to the land. (Amanda Subalusky)

The scientists found that on average, 6,200 wildebeest succumb every year. That's the equivalent of 10 blue whale carcasses. It all adds key nitrogen, phosphorus and carbon to the river's intricate food web. 

First, crocodiles race to the drowning scene while vultures and storks fly in to feast, too.  The scavengers help transport soft tissue back to the plains. At the same time, the beasts' flesh is a "perfect food" for fish and insects, Post says.

"From their death comes a lot of life."

And bones from carcasses continue to decompose over years, acting as slow-release fertilizer for algae and fish.

Post sees the mass drownings of wildebeest as a window into the migratory history of herds like bison that have been driven to extinction or dwindled in numbers.