Wildebeest drownings feed a river ecosystem for years

More than a million wildebeests migrate each year in East Africa, moving from Tanzania to Kenya and back again. These antelopes, also known as gnus (NEUZ), follow the rains and abundant grass that will later spring up. Their path takes these animals across the Mara River. Some of its crossings are so dangerous that hundreds or thousands of wildebeests drown as they try to reach the other side. But those lost lives also provide a brief, free buffet for crocodiles and vultures. And, a new study finds, those carcasses also nourish an aquatic ecosystem for years to follow.

The Mara flows for more than 13,000 kilometers (8,000 miles) through Tanzania and Kenya. Many of Africa’s iconic animals — such as lions, zebras and elephants — roam the region. Some of the lands, here, have been set aside as protected areas. People live and farm elsewhere.

Amanda Subalusky is an ecologist. She works at the Cary Institute of Ecosystem Studies in Millbrook, N.Y. Several years ago, she had been studying water quality in the Mara River. That’s when she and her colleagues noticed something odd. Commonly used indicators of water quality include levels of dissolved oxygen. Yet such indicators sometimes were poorest at sites within a protected area. They quickly realized that it was because of the animals that flourished there. Hippos, which eat grass at night and defecate in the water during the day, were one contributor. Dead wildebeests were another.

“Wildebeest are especially good at following the rains. And they’re willing to cross barriers to follow it,” says Subalusky.

These animals tend to cross at the same spots year after year. Some of those spots are more dangerous than others. “Once they’ve started using a site, they continue, even if it’s bad,” she notes. The result: Each year, on average, more than 6,000 wildebeests drown. (That may sound like a lot, but it’s only about 0.5 percent of the herd.) Annually those carcasses add mass into the river equivalent to the bodies of 10 blue whales. Those are the biggest animals on the planet.

Tracing the effects of dead animals

Subalusky and her colleagues set out to see how all that meat and bone affected the river ecosystem. When they heard about drownings, they would head off to count the carcasses. They retrieved dead wildebeests to test what happened to their various body parts over time. And they measured nutrients up and downstream from river crossings to see what the wildebeest carcasses added to the water.

“There are some interesting challenges working in this system,” Subalusky says. For instance, in one experiment, her team put pieces of wildebeest carcass into mesh bags and placed them in the river. The plan was to retrieve the bags over time to see how quickly the pieces decomposed. “We came back the next day to collect our first set of samples,” she recalls. And surprise! “At least half the bags with wildebeest meat were just gone. Crocodiles and Nile monitors had plucked them off the chain.”

Eventually, the researchers learned that the wildebeests’ soft tissue breaks down in about two to 10 weeks. This provides a pulse of nutrients — carbon, nitrogen and phosphorus — to the aquatic food web. It also provides nutrients to the nearby terrestrial system.

A succession of scavengers will feast on the wildebeests. They include crocodiles, vultures, marabou storks, egg-laying bugs and things that eat the bugs that hatch.

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Subalusky and her colleagues are still figuring out the sequence in which those critters devour the soft tissue. But they have discovered some interesting bits already. For instance, vultures prefer to tear into the wildebeest through the anus. (Yes, you read that right.) That’s because the animal’s hide is tough, especially if it’s been baking in the sun for a couple of days.

Once the soft tissue is gone, the bones remain. Sometimes the bones pile up in bends in the river or other spots downstream. “They take years to decompose,” Subalusky says. Over that time, they slowly leach out most of the phosphorus that had been in the animal. The bones also can become covered in a biofilm of algae, fungi and bacteria that provides food for fish.

What initially looks like a short-lived event actually provides resources for seven years or more. That’s what Subalusky and her colleagues reported June 19 in the Proceedings of the National Academy of Sciences.

Such migrations becoming more rare

The wildebeest migration is the largest terrestrial migration on the planet. Others have largely disappeared as people have killed off animals or cut off their migration routes.

Only a few hundred years ago, for instance, millions of bison roamed the American West. There are accounts in which thousands of bison drowned in rivers, similar to what happens with wildebeests. Those rivers may have fundamentally changed after bison were nearly wiped out, Subalusky and her colleagues argue.

But there still are places where scientists sometimes may be able to study effects of mass drownings on rivers. A large herd of caribou reportedly drowned in Canada in the 1980s. And there are still some huge migrations of their European kin, the reindeer. Like the wildebeests, these animals might be feeding an underwater food web that no one has ever considered.