According to Official Guinness Records,
The last great land migration on Earth sees between 1 and 2 million wildebeest (Connochaetus taurinus) travelling a circuit between the Serengeti of Tanzania to Kenya and across the Mara River. Millions of other animals such as zebras, antelopes and gazelles plus their predators join the wildebeest on their annual roundtrip in search of water.
Such a large movement of animals has a huge impact on the ecosystem, as an enormous amount of plant life is consumed, seeds are trampled (preserving the grasslands) and the land fertilized by vast quantities of dung and urine.
For a complete list of 2011 records, please visit 2011 Guinness Records in Tanzania.