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You may have noticed that many of North America's common, urban birds are actually not native to North America. They were originally found elsewhere in the world, such as Europe or western Asia, were transported to North America by humans or through human agency, and have since established breeding populations in their new range. Species that are not native to a particular area, but now have established populations in that area, are called by a variety of names. They are called "introduced species", "non-native" species, "exotic" species, and sometimes "invasive" species. Introduced species are currently one of the primary threats to biodiversity and human economies worldwide.

These species may be not terribly common or problematic in their native ranges, but when they establish populations in a new area they are often suddenly freed from their native predators, parasites, and diseases. Animals such as these that also compete effectively with native animals (in their new range) or exploit niches not currently exploited in that new range often expand in numbers very quickly and can wreak havoc on these newly colonized ecosystems. Under such circumstances introduced species become "invasive" species.

There is also a set of animal species that tend to be found throughout much of the world in association with humans. These are called "commensal" species. The word commensal actually has a much broader, ecological meaning, which is: an organism participating in a symbiotic relationship where one benefits and the other is unaffected. So, it is more correct to call these animals "human commensals". Typically, human commensals are also introduced species, where they are occurring outside of their original range. But, human commensals are often limited in distribution to areas where humans live, so usually pose less of a threat to native wildlife. Human commensals have huge impacts on human health and economies, though. Just think of the food destruction and disease transmission facilitated by the many hundreds of millions of rats that live near humans throughout the world.