Every year, over a million wildebeest make their way across the Serengeti in the largest remaining land migration. New research shows the thousands of wildebeest that drown crossing the Mara River in the process are an important source of nutrients for the entire Serengeti Mara ecosystem.

What they found: Wildebeest carcasses take about 28 days to decompose, and as they did they were integrated into the food chain at every level, from scavenging birds to fish to bacteria. Crocodiles, possibly because of their slow metabolism, only ate about 2% of the carcasses. After the meat was consumed, the bones leached nutrients downstream.

How they did it: The scientists used historical reports of mass drownings from 2001-2014 and conducted field surveys from 2011-2015. They studied the carcasses as they decomposed, measured nutrient levels downstream of the drownings, used cameras and chemical analysis to estimate how much of the carcasses were eaten by each scavenger.

By the numbers: According to the researchers, up to 1600 tons of wildebeest end up in the river each year - that's roughly the equivalent of ten blue whales. As the carcasses decompose, they raise the phosphorous levels in the river by as much as 451%, the carbon by up to 191%, and the nitrogen by up to 78%.

For comparison, this is four times the mass moved by the salmon runs of the Pacific Northwest. Some scientists think the Northwest owes its lush forests to these salmon, which scavengers distribute through the forest like fish fertilizer. It's possible wildebeest play a similar role in the Mara.

Bottom line: River systems that no longer have large migrations may have lost a vital source of nutrients, say the researchers.

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