It’s one of the iconic sights of Africa: hundreds of thousands of wildebeest thundering across the Serengeti in an annual mass migration. But when the animals come to the Mara River, the scene can turn deadly. Unable to scramble up steep banks, thousands drown in a mass panic or get picked off by crocodiles. It turns out, however, that what’s bad for the wildebeest is good for the ecosystem, say Amanda Subalusky and Emma Rosi, ecologists at the Cary Institute of Ecosystem Studies in Millbrook, New York.
For the past 6 years, Subalusky and her husband, Christopher Dutton, also at the Cary Institute, have studied the scale and effects of this mass carnage. They have taken stock of the pileup of carcasses, surveyed the parade of scavengers assisting in their decomposition, and tracked where nutrients from the dead animals wind up in the food chain. “It’s a pulse of nutrients, but then you have a legacy of bones, which are acting as a slow release fertilizer” with multiple effects downstream, Subalusky says. “The sheer amount of organic matter that is made available is astonishing,” says deep-sea ecologist Paulo Y. G. Sumida at the University of São Paulo in São Paulo, Brazil, who studies the ecological role of whale carcasses. It “is likely to make a big difference for the whole trophic web and for animals as well.”
The wildebeest migration is the world’s most massive animal movement: 1.2 million animals cross the savanna in an 1800-kilometer circuit between Kenya and Tanzania as they follow the rains. They consume more than 4500 tons of grass every day and deposit heaps of dung, transforming the landscapes they cross. “The migration affects every single process in this ecosystem,” says J. Grant Hopcraft, a landscape ecologist at the University of Glasgow in the United Kingdom who has studied wildebeest for decades. But the impact on the Mara River had not been as closely assessed.
Subalusky, then a Yale University graduate student working with David Post, decided to take a closer look when she first saw the aftermath of the mass drownings: massive flocks of vultures and storks picking over the smelly carcasses. She checked the historical records and in 2011 began surveying the Mara River annually, measuring the carbon, phosphorus, and nitrogen content of carcasses; counting the numbers of scavengers; testing water quality; and capturing fish for chemical analyses of the sources of their nutrients.
As Subalusky and her colleagues report in this week’s issue of the Proceedings of the National Academy of Sciences, about 6500 animals drown each year, dumping 10 blue whales’ worth of meat into the river. The fresh carcasses, which accumulate at bends and in the shallows, feed crocodiles and provide up to 50% of the diet of local fish. As they decay, they annually add about 13 tons of phosphorus, 25 tons of nitrogen, and 107 tons of carbon to the ecosystem in half a dozen pulses that each last about a month. During those weeks some nutrient levels can quadruple temporarily.
The bones, which make up half the biomass, are the last to decay, taking 7 years. Along the way they support a film of microbes that in turn become food for fish and other river-dwellers.
“I am stunned by the extent of the annual mass wildebeest drownings and their large contribution of [carbon, nitrogen, and phosphorus] to the energy budget of the Mara River,” says Gary Lamberti, an aquatic ecologist at the University of Norte Dame in South Bend, Indiana. The boon likely extends beyond the river, as vultures and storks move wildebeest-derived nutrients tens of kilometers inland.
The study, which was quite challenging and dangerous to do, adds to a growing body of evidence that mass mortality can have ecosystem impacts. Researchers like Sumida have found, for example, that dead whales provide a pulse of food to nutrient-starved ocean floors, enabling a specialized ecosystem to flourish on the decaying carcasses. Others have tracked how salmon that die after they finish their final upstream journey to spawn add nutrients to river ecosystems. The impact of the wildebeest appears to be larger, however; they contribute four times more biomass to the Mara than dying salmon add to British Columbia’s rivers, Subalusky notes.
“These phenomena highlight the multiple pathways—nutrients, direct consumption, food web transfers—by which animal tissue can influence food webs,” Lamberti says.
On a broader scale, “the [wildebeest] findings have implications for understanding the ecological role of past and present animal migrations,” says David Janetski, an aquatic ecologist at Indiana University of Pennsylvania. The bison in North America, the saiga antelope in central Asia, and many caribou in the Arctic once migrated by the millions, sustaining ecosystems in the rivers they crossed. When the migrations dwindled, the organisms that relied on the carcasses of animals that came to grief may have declined or vanished, he says.
On the positive side, the wildebeest drownings kill only about 0.7% of the total herd each year. Illegal harvesting, starvation, and predation kill many more. “Although drowning events are horrendous and graphic, they should not be our primary concern for the long-term sustainability of this population,” Hopcraft says. “If anything,” he says, “the Serengeti shows us what an ecosystem should look like.”