Every year during the annual mass migration of wildebeest,
thousands drown or are eaten by crocodiles when they try to cross Kenya’s Mara
river. The mass annual journey of an estimated 1.2 million wildebeest (also known
as gnus) from the Serengeti in Tanzania to the Mara in Kenya is probably the
largest surviving mammal migration, and certainly the largest annual mass
drowning of wildebeest.
The deaths, say scientists, are good for the river ecosystem. Amanda
Subalusky at Yale University has measured the fate of nutrients released into
the local ecosystem from the 1100 tonnes of biomass from about 6,200 wildebeest
carcasses that float downstream in the Mara river 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.
The article documented in New Scientist (19 June 2017) is taken
from Subalusky’s research.
Subalusky says that crocodiles and birds benefit from the
carrion (dead bodies), particularly vultures. But the slow release 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 in
Scotland. “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.
First, the drownings don’t take much of a toll on wildebeest
numbers. While the animals can only swim for a few minutes, Subalusky says,
only one in 200 fails to make the crossing.
Second, 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.’’
In the past, “mass drownings of mammals moved large amounts of
resources from terrestrial (land) to aquatic (water) systems”, she says. The
loss of such migratory 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 about 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 lots of bones in rivers, she says, releasing a
constant supply of nutrients into those ecosystems. Subalusky thinks many river
ecosystems currently have low levels of phosphorus possibly as a result of the
loss of migration drownings.
MARTINA NICOLLS is an international
aid and development consultant, and the author of:- Similar But Different
in the Animal Kingdom (2017), The Shortness of Life: A Mongolian Lament
(2015), Liberia’s Deadest Ends (2012), Bardot’s Comet (2011), Kashmir on a
Knife-Edge (2010) and The Sudan Curse (2009).
Comments
Post a Comment