THE MIGRATION OF WILDEBEEST and other lager grazers across
the Mara-Serengeti ecosystem has boldly been called the ‘Greatest Show on
Earth’. Not that there aren’t other contenders (the migration of white-eared
kob ,topi and Mongalla gazelles sports similar numbers in the Sudan, while the
600-kilometer round trip taken by elephants in Mali is equally epic), but the
ceaseless movement of ungulates across these plains is undoubtedly Africa’s
best-known parade of animals. Players include a cast of up to 1.4 million
wildebeest, with the supporting act coming from lager numbers of zebras and
Thomson’s gazelles. There seasonal movement between wet and dry seasons ranges
take them full circle over some 30,000 square kilometers.
What exactly drives
this extravaganza has been debated for decades (we’re still not sure, for
example, of the role played by social knowledge of migration routes), but at
its heart lies an interesting dichotomy. In the south-east corner of Tanzania’s
Serengeti National Park, the volcanic soils support nutrient-rich plains of
short and highly nutritional grasses. These are the wildebeest’s favourite
food, although the forage is only available for a few months of the year. In
the opposite corner, in the northern areas, grass quality is poorer (although
the higher rainfall guarantees a longer supply),So at the end of the wet season
as the short-grass plains of the south-east dry up, the wildebeest are
forced elsewhere in search of better
grazing. Their quest leads them to the tall-grass woodlands and savanna further north.
Quite what prompts
them to leave these northern woodlands en masse at the onset of the rains remains unclear. To head south
again, the wildebeest must be able to pick up changes in vegetation some 80 to
100 kilometer distant from their current location (relying on cues from the
quality of their present grazing
conditions would fail to drive the migration ever onwards). We’re not sure how
they do this, but in all probability they follow rain, responding to localized
flushes of green grass. This would explain why the migration doesn’t observe a
fixed route or time frame but changes from year to year, depending on just
where and when rain falls.
