As natural wonders go, perhaps the most awe-inspiring is the annual migration of 1.2 million wildebeest flowing across East Africa’s vast Serengeti grassland. It would be a tragedy to lose these animals. But we almost did in the mid-20th century when, decimated by disease and poaching, their numbers crashed to 300,000. The consequences of that collapse were profound. Much of the Serengeti ecosystem remained ungrazed. The accumulating dead and dried…