The Grass is Always Greener

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.