The graphic below is from a series of Wildlife Field Notes that I published through the Purdue University Cooperative Extension Service.
Rodents are at the larger end of the spectrum of the 'little things that make the world work'-- in biologist E. O. Wilson's words. They, along with microbes, protists, arthropods (especially insects), little fish, and small mammals provide critical ecological functions upon which larger animals depend.
The rodents serve as important "energy transformers" that convert hard-to-digest plant foots into rations for a wide array of meat-eaters. Foxes, weasels, coyotes, hawks, and owls are just some of the carnivores that include rodents (especially mice and voles) in their diets.
Their generally small size makes them very prolific, for mammals. Their population densities are often very high. Some vole (meadow mice) populations in good habitat, and in peak years, may number 100 individuals per acre (or 64,000 per square mile hypothetically). Their small size is counteracted by their large numbers and important roles in ecological communities.