Wildlife deaths don’t come much more dramatic. Every year, thousands of wildebeest drown or are eaten by crocodiles when they try to cross Kenya’s Mara river on their annual migration.
Most years, camera crews are on hand to witness the slaughter in the Serengeti. But the good news is that the carnage is a massive boost to local ecosystems.
So says Amanda Subalusky at Yale University, who has braved hippo charges and lurking crocodiles to measure the fate of nutrients released into the local ecosystem from the 1100 tonnes of biomass that float downstream from some 6200 wildebeest carcasses 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.
Crocodiles and birds benefit from the carrion, particularly vultures. But the slow liberation 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, UK. “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.
For those that view the spectacle on TV, there may be two surprises, however.
Firstly, the drownings don’t take much of a toll on wildebeest numbers. While the animals can only swim for a few minutes, she says, only one in 200 fails to make the crossing.
And secondly, 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.
The bigger picture
For Subalusky, the mass drownings tell a bigger story about what has been lost. In the past, “mass drownings of mammals moved large amounts of resources from terrestrial to aquatic systems”, she says. But in our fenced and fragmented landscapes, such events are largely confined to history.
The mass annual journey of an estimated 1.2 million wildebeest from the Serengeti in Tanzania is probably the largest surviving mammal migration, and certainly the largest annual mass drowning. The loss of such 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 some 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 beds of bones in rivers, she says, releasing a constant supply of nutrients into those ecosystems. “We now think of many of our river ecosystems as being limited by low levels of phosphorus. Perhaps this is a result of the loss of those drownings.”