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Kenya's annual wildebeest migration is good for the ecosystem, say scientist


Every year during the annual mass migration of wildebeest, thousands drown or are eaten by crocodiles when they try to cross Kenya’s Mara river. The mass annual journey of an estimated 1.2 million wildebeest (also known as gnus) from the Serengeti in Tanzania to the Mara in Kenya is probably the largest surviving mammal migration, and certainly the largest annual mass drowning of wildebeest.

The deaths, say scientists, are good for the river ecosystem. Amanda Subalusky at Yale University has measured the fate of nutrients released into the local ecosystem from the 1100 tonnes of biomass from about 6,200 wildebeest carcasses that float downstream in the Mara river in a typical year. That includes 100 tonnes of carbon, 25 tonnes of nitrogen and 13 tonnes of phosphorus – the equivalent, says Subalusky, of the weight of 10 blue whales.

The article documented in New Scientist (19 June 2017) is taken from Subalusky’s research.

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

“These are large and very clear effects on the nutrient cycles in the Mara river,” says Grant Hopcraft at the University of Glasgow in Scotland. “The actual event of a herd crossing the river happens very quickly, in a matter of minutes, and yet the ecological repercussions last for months and over a much larger space.” This creates “ecosystem resilience”, he says.

First, the drownings don’t take much of a toll on wildebeest numbers. While the animals can only swim for a few minutes, Subalusky says, only one in 200 fails to make the crossing.

Second, the crocodiles lurking in the waters may look like the biggest threat, but they don’t eat much. “Crocodiles have a relatively low metabolic rate, and they easily become satiated,” she says. ‘’Their take of the bushmeat bonanza is only about 2 per cent.’’

In the past, “mass drownings of mammals moved large amounts of resources from terrestrial (land) to aquatic (water) systems”, she says. The loss of such migratory events “may fundamentally alter how river ecosystems function” in many places, says Subalusky.

Two centuries ago, there were around 60 million free-roaming bison in North America – compared with about half a million now, mainly contained on fenced ranches. “Early explorers documented large drownings of thousands of bison during spring river crossings,” says Subalusky. These animals must have left behind lots of bones in rivers, she says, releasing a constant supply of nutrients into those ecosystems. Subalusky thinks many river ecosystems currently have low levels of phosphorus possibly as a result of the loss of migration drownings.


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


MARTINA NICOLLS is an international aid and development consultant, and the author of:- Similar But Different in the Animal Kingdom (2017), The Shortness of Life: A Mongolian Lament (2015), Liberia’s Deadest Ends (2012), Bardot’s Comet (2011), Kashmir on a Knife-Edge (2010) and The Sudan Curse (2009).


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