Biological invasion is the process by which species (or genetically distinct populations), with no historical record in an area, breach biogeographic barriers and extend their range. Invasions occur as a natural ecological process. For example, after North and South America collided 3 million years ago, numerous mammals (deer, rodents, cats, skunks, weasels, llamas) extended their ranges south, and a few (armadillo, porcupine, oppossum) extended their ranges north. However, over the course of human history, the pace of biological invasions has accelerated because, through their travels and trade, humans tend to move other species. The immediate consequence of biological invasion is a local increase in the number of introduced species, also called alien, invasive, exotic, or nonindigenous species.