Cats are especially devastating on island ecosystems where birds, small mammals, reptiles, fish and amphibians have few natural predators. Small mammals such as mice, voles, shrews, moles, bats, pikas, and yes even rats are valuable in ecosystems as an abundant source of food for birds of prey and other predators or good controllers of insects and other things we consider pests. Without small mammals larger predators have a greatly reduced food supply. Plus some small mammals are cute as fuck.