We visited the Serengeti during the dry season. The migration was meant to be in the Northern part of the park. But then it rained (briefly) and the animals started to turn back. This led to a larger than normal (for this time of year) group of antelope, zebra and wildebeest.
The flow on impact will be interesting, particularly if this happens season after season. Some immediate effects were the big cats were no longer hungry and the very real threat that if the early rains didn't continue the herbivores would run out of food.