told me, explaining this thinking.
“Otherwise they would just go crazy
and destroy everything.”
Sinclair wasn’t convinced. “It
occurred to me that we could
demonstrate why that was not the
case in the Serengeti wildebeest
population.”
He went back to the Serengeti,
and over the next several years,
he and his colleagues began to
notice significant changes. The
first was that predator populations
were growing. This wasn’t all that
surprising— more prey meant more
food for lions, hyenas, cheetahs,
and leopards. But Norton-Griffiths
also noticed that there were fewer
fires. He and Sinclair figured out
that the large wildebeest herd was
keeping the grass shorter, so fires
didn’t burn as frequently or as hot,
which allowed trees to grow. Sud-
denly, large areas that had been
grassland for nearly a century were
being reforested.
More trees meant more insects,
more birds, and more animals that
eat the leaves of trees, including
giraffes and elephants. And as the
wildebeests traveled, they spread
their dung, improving the soils and
producing more grass for them-
selves and other species. Elephant
populations grew, butterflies pro-
liferated, even lowly dung beetle
species flourished.
Sinclair realized the Serengeti
was being transformed into a place
that few, if any, living humans could
remember. And the driver for this
change was the humble wilde-
beest. At the time, the concept of a
keystone species—an animal that
was singularly crucial to the struc-
ture and health of an ecosystem—
was relatively new. Until then, all
the identified keystone species
had been top predators. But in the
Serengeti, the lion wasn’t king; its
prey was.
To put it bluntly, Sinclair told
me, “there’s no Serengeti, at least
Zebra
Oribi
Impala
Topi
Buffalo
Giraffe
Elephant
Rhino
Hippo
22,000 lbs
330 lbs
2,200 lbs
220 lbs
Deaths due
to predation
Fewest Most
Body
weight
1961
1.3 million
1977 1985 1995 2005 2018
NUMBERS LEVEL OFF
Migrant wildebeests are
smaller than resident ones
and more agile. Their risk
of predation is lowered by
moving in large groups.
Resident wildebeests
are larger and don’t
migrate. These herds
must share territory year-
round with predators.
UNIVERSITY OF BRITISH COLUMBIA; GRANT HOPCRAFT AND THOMAS MORRISON,
UNIVERSITY OF GLASGOW; SEAN CARROLL, THE SERENGETI RULES, 2016
POPULATION CONTROL
Serengeti animals under 330 pounds typically
die from predation; those heavier than that
threshold, from lack of food. Wildebeests, mid-
size, attempt to avoid both threats by migrating.
After peaking in 1977, the wildebeest
population stabilized at a level the food
supply could support, about 1.3 million
animals. Their numbers can still fluctuate
with variations in annual rainfall.
The Serengeti’s
worst drought of
the 20th century