Every year thousands of wildebeest drown or are eaten by crocodiles when they cross Kenya’s Mara river during their annual migration.

The mass annual journey of 1.2 million wildebeest (also known as gnus) from the Serengeti in Tanzania to the Mara in Kenya in Africa is the largest mammal migration in the world, and certainly the largest annual mass drowning of wildebeest.

Amanda Subalusky at Yale University has measured the nutrients released into the river ecosystem from the 1100 tonnes of biomass from about 6,200 wildebeest carcasses (dead bodies) that float downstream in the Mara river each year. That includes 100 tonnes of carbon, 25 tonnes of nitrogen and 13 tonnes of phosphorus.

Subalusky says that crocodiles and birds benefit from the carrion (decomposing bodies), particularly vultures.  But the slow release of nutrients benefits everything in the river from fish to insects.

The drownings don’t reduce the wildebeest numbers by much. While the animals can only swim for a few minutes, Subalusky says, only one in 200 fails to cross the river.

The crocodiles may look like the biggest threat, but they don’t eat much. Crocodiles have a relatively low metabolic rate, and they easily become full. She says that crocodiles take about only 2 per cent of meat.

Mass drownings of mammals transfer nutrients from terrestrial (land) ecosystems to aquatic (water) ecosystems.

blue wildebeest

Journal reference: PNAS, DOI: 10.1073/pnas.1614778114

Photographer: Martina Nicolls

Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM