Hudson's Bay fur trading records show these fluctuations stretching back for hundreds of years. In 1965 Krebs tried a simple experiment in an Indiana pasture. He simply fenced in an area of grassland the size of a soccer field to see what would happen to the population of voles living inside the fence. Amazingly, within a year, he found the population of voles increased about 5 times. You don't do anything to the animals.
Lemmings are small rodents that look like guinea pigs.
They live throughout the world in Northern latitudes. Every 4 years their population increases up to 500 times and then crashes to almost nothing. A population explosion on the left side fenced in area has occurred and the voles have eaten everything except the thistles. Graph showing the population explosion and crash caused by the fence. The gray line is for voles in a similar unfenced area.