Everybody have heard about lemming years, when a large number of lemmings follow each other over a cliff, falls down into the sea below and drown. Political analogies abound, and so this has become common knowledge: That’s what lemmings do. But why? By the way, 2011 was a lemming year in Northern Sweden, it repeats itself every 20 years or so. To explain why, there is an even better example with rats. When food is plentiful they multiply fast and after about 4 years there are far too many rats for the food supply. No, they do not die of starvation, virtually all the rats get stressed out and die, all in short order, and the cycle starts anew. It used to be a fairly limited supply of food for the rats, people ate what the land produced, and the number of rats were kept below the stress level. Enter modern day landfills. There is an overabundance of food in a very limited area, so rats congregate there and multiply. Eventually they will hit the stress level die out and the cycle is renewed. Enter the division of rodent control. In their wisdom they put out rat poison to keep the population down. In so doing they manage to keep the rat population just below the stress level, thereby ensuring that every year is a year of plenty of rats rather than every fourth year.
I refrain from tempting political analogies.
Here is a video of lemmings from BBC Nature