An invasive species is a species that is considered non-native or alien to a given ecosystem and whose introduction has caused, or is likely to cause, significant changes to the ecosystem. Invasive species can be seen as a subgroup of non-native species with characteristics that make them spread easily within their host ecosystem. Their impacts on ecosystems are often perceived as negative and harmful. For example, invasive species are typically described as crowding out populations of native species or harming them in other ways such as through predation. Some conservationists are critical of categorising species as native, non-native and invasive species. They point out that species have always travelled from one ecosystem to the other, in many cases enabled by the travels of humans. Some species that are perceived to be invasive in one area can also be near extinction in places where they are considered to be ‘native’ to.