Accidentally or intentionally, people often relocate species to habitats where they aren't native. For example, the invasive zebra mussel was probably transported to North America in the ballast water of transatlantic ships. These relocated species are known as exotic or non-native species, and those likely to cause economic or environmental harm are called invasive species. These biological invaders can cause problems for native wildlife by preying on it, competing for essential resources, or destroying habitat. Once invasive species move in, they are very difficult to control or eliminate.