Mass wildebeest drownings bolster Mara River ecosystem

"This dramatic subsidy delivers terrestrial nitrogen, phosphorus and carbon to the river's food web," explained researcher Emma Rosi.
By Brooks Hays   |   June 19, 2017 at 5:06 PM

June 19 (UPI) -- Scientists have quantified the ecological impact of mass wildebeest drownings in the Kenyan reach of the Mara River. Drowned wildebeest provide a dramatic boost to the Mara River ecosystem, they say.

"The Mara River intersects one of the largest overland migrations in the world," Amanda Subalusky, a postdoctoral researcher at the Cary Institute of Ecosystem Studies, said in a news release. "During peak migration, the wildebeest cross the Mara River multiple times, sometimes resulting in drownings of hundreds or thousands of wildebeest."

Researchers used field surveys to estimate the number of wildebeest who drown each year during river crossings. The data suggests roughly 6,200 wildebeest succumb to the raging waters each year.

"Our study is the first to quantify these mass drownings and study how they impact river life," said Subalusky, who conducted the research while at Yale University.

Subalusky and her colleagues tracked the fates of wildebeest carcasses by studying camera footage of scavengers and conducting stable isotope analyses of Mara River fish. Isotope analysis of bacteria, fungi and algae biofilms also helped scientists follow the trail of nutrients provided by decomposing wildebeest.

The annual influx of more than 1,100 tons of biomass is a boon to a variety of plant and animal species in the river.

"To put this in perspective, it's the equivalent of adding ten blue whale carcasses to the moderately-sized Mara River each year," said Emma Rosi, an aquatic ecologist at the Cary Institute. "This dramatic subsidy delivers terrestrial nitrogen, phosphorus and carbon to the river's food web."

While wildebeest flesh only provided sustenance for a few weeks, their bones can continue providing the ecosystem with valuable nutrients for several years.

"Mass drownings present a striking picture," said Rosi. "Rotting animal flesh spikes the aquatic ecosystem with nutrients. But once carcasses disappear, bones -- which make up nearly half of biomass inputs -- continue to feed the river."

Observations provided wildebeest flesh comprised nearly half the diet of common fish in the Mara River when carcasses were present. Biofilms growing on wildebeest bones also served as fish food. Scavenging bird species -- including Marabou storks, white-backed vultures, Rüppell's vultures and hooded vultures -- also feasted on wildebeest flesh. Surprisingly, just 2 percent of the wildebeest biomass ends up as crocodile food.

Researchers detailed their analysis of mass wildebeest drownings in the journal PNAS.

"The Mara River is one of the last places on Earth left to study how the drowning of large migratory animals influences aquatic ecosystems," said David Post, an aquatic ecologist at Yale University. "Many migratory herds, like bison, quagga, and springbok have been driven to extinction or remnant populations."

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