When invasive species become common or dominant, populations of native species typically decline. The new invader often out-competes native animal species for food, light and space.
- English starlings (Sturnus vulgaris) compete with native Western bluebirds (Sialia mexicana) and Tree Swallows (Tachycineta bicolor) for nest cavities.
- Competition may be more direct, as in the case of bullfrogs (Lithobates catesbeianus) that actively prey on native amphibians.
- An invasive mollusc (or mollusk), the grey garden slug (Deroceras reticulatum), prefers to eat native, annual wildflower seedlings, which encourages the spread of non-native plants that many native animals have difficulty eating.
Invasive animal species can also threaten human well-being. Aquatic animal invasives such as the New Zealand mudsnail (Potamopyrgus antipodarum) and quagga mussel (Dreissena bugensis) are established in parts of Oregon and other Western states. Efforts to control these invasive species represent a significant cost to fish hatcheries and hydroelectric power plants.
Releases of non-native fish can harm native fish stocks. Invasive animals such as nutria (Myocastor coypus) can carry diseases such as Salmonella spp. that can sicken humans.