Voles, mice, shrews and owls:

In a woodland community, three species of rodents coexist: voles, field mice, and shrews. All three species eat seeds and nuts. Each species has a preference for seeds of the most appropriate sizes for their teeth and mouths; however, all three species compete for the same kinds of nuts.

An owl species also lives in this woodland community. The owl preys on all three rodent species.

During one particular year, a parasite that causes pneumonia in birds is introduced into the community. This parasite dramatically reduces the owl population, which remains low for several years as a result. Following the initial reduction in the owl population, there is a dramatic increase in the population of field mice and a dramatic decrease in the populations of both voles and shrews.

What is going on here - what is the ecological concept that is demonstrated by this situation? Explain.