Wild meadow vole populations can fluctuate a lot, and scientists wanted to figure out why. They did experiments by changing the number of voles in a group and tracking their survival and growth. They found that when there were a lot of voles, fewer babies were born, and the population didn't grow as fast. This effect changed throughout the breeding season and from year to year. This shows that how many voles there are can affect how fast the population grows, and this effect can happen quickly and last for years.