Alien or naturalized species are those species which are not native to an area but established, and those that are a threat to native species and biodiversity are often called invasive species. The term "invasive" is poorly defined and often very subjective, Invasive species may be plants, animals, fungi, and microbes, some also include native species that have invaded humane habitats such as farms and landscapes. Some broaden the term to include indigenous or "native" species that have colonized natural areas. The definition of "native" is also sometimes controversial. For example, the ancestors of "Equus ferus" (modern horses) evolved in North America and radiated to Eurasia before becoming locally extinct. Upon returning to North America in 1493, during their human-assisted migration, it is debatable as to whether they were native or exotic to the continent of their evolutionary ancestors.