"If left unchecked, populations (of any organism) tend to grow exponentially. But they never grow forever. Eventually resources are depleted and the population declines. When the population is again small enough that resources are plentiful, exponential growth resumes. The result is a boom-bust cycle — stability through fluctuation. Of course, if the fluctuations are huge, we can hardly call this ‘stability’. But in most healthy ecosystems, population fluctuations are small. The key to dampening boom-bust cycles appears to be diversity. When there are many species in an ecosystem, each keeps the population of others in check. Prolonged exponential growth never gets a foothold."