Invasive species are a major cause of biodiversity loss in Mexico, altering ecosystems, affecting native species, damaging environmental services and public health, and causing economic losses. An invasive species is one native to a particular area that has been introduced into a new habitat, adapting and altering to suit its new conditions. Due to its geography, a convergence of Nearctic and Neotropical regions, Mexico is a megadiverse country, with a high number of species. This has favored the existence of a considerable number of habitats with diversely distant species which inhabit various aquatic and terrestrial ecosystems. Economic, social and cultural exchange between Mexico and other countries has facilitated the entry of exotic and invasive species.
Terrestrial animals In Mexico the effects of invasive species has been documented since the arrival of the Europeans. Colonization brought about the systematic introduction of animals for economic purposes, but also the occasional arrival of unwanted visitors that became pests (Challenger, 1998). On account of these problematic intruders, many ecosystems were being impoverished and many species driven to extinction. There are three divisions:
Exotic species (introduced or non-native): a species established outside its natural area in differing timescales and potential spread, outside the area it occupied naturally, or that could not continue without human intervention (CBD, 2009 area; IUCN, 2000;) Invasive exotic species: a non-native species or population outside its natural range, which is able to survive, reproduce and become established in natural habitats and ecosystems and threaten native biodiversity, economy and public health (DOF, 2010) Invasive species: an exotic species is established in natural or semi-natural habitats or ecosystems, an agent of change that threatens native biodiversity (CBD, 2009) Not all exotic species become invasive immediately, and potential effects of a non-native species are unpredictable (DET). The problems related to invasive exotic species are complex and vast, the biggest obstacle being that it is a little-known and underestimated topic.
On Mexican islands Island biodiversity is vulnerable to introduced species, because in these environments there is a high proportion of endemic species that lack defense mechanisms against exotic species with which they did not co-evolve. In particular, for island birds the risk of extinction is forty times higher than for continental species. It is the leading cause of biodiversity loss in that region. 62% of mammals, 88% of birds, 54% of amphibians, 86% of reptiles and 68% of mollusks that have been reported extinct are island species. On Mexican islands 12% of endemic birds and 20% of endemic mammals have become extinct because of introduced species. Over the last decade forty populations of introduced mammals have been eradicated on 28 islands, a step toward the conservation of over a hundred endemic species and subspecies that inhabit the islands. One exemplary result is the Gulf Islands of California, where the initiative of some individuals and civil society organizations is supported by an inter-institutional collaboration and international cooperation.
Guadalupe Island Guadalupe Island is a volcano with a height of 5800 m. Its surface area is approximately 250 km². It is a center of endemism with 34 species of plants, including two classes (Redman and Moran, 1997); eight land birds; one sea bird; eleven land snails; and at least 18 species of insects (Gonzales, 1981). The island is an important breeding site for marine mammals such as the Guadalupe fur seal (Arctocephalus townsendi), elephant seal (Mirounga angustirostris), and numerous species of birds, including the Laysan albatross (Phoebastria immutabilis). In the nineteenth century, goats were introduced, reducing tree coverage of pine, cypress, juniper and oak palm to only 6% of their original coverage. Feral cats caused the extinction of six bird species, and threaten the permanence of large populations of native birds. Eradication of goats started in 2003, concluding successfully in 2006. Confirmatory monitoring took place from 2007-2009. Following the eradication, for the first time in over 150 years germination and survival of tree species was recorded, more than 130,000 in 2009, as well as observation of six native species believed extinct or extirpated from the island. The project for environmental restoration of Guadalupe Island, coupled with advances in other islands, has national and global significance, placing Mexico as an international leader in the conservation of ecosystems.
Feral cats Cats are associated with humans as a communal species, and impact an area of 5 km around each center of human population. Their presence is particularly important in oceanic islands (CONABIO, 2008), where they are the predator with the greatest impact on seabird populations. Some examples of species that have been reduced almost to extinction are the Mexican shearwater (Puffinus opisthomelas), Cassin's auklet (Ptychoramphus aleuticus), and Xantus's murrelet (Synthliboramphus hypoleucus). Extinct species include the Guadalupe storm petrel (Oceanodroma macrodactyla), the Socorro dove (Zenaida graysoni) and an endemic sparrow of Todos Santos Island (Aimophila ruficeps sanctorum). Cats are a risk factor for the populations of many endemic rodents, like the Chaetodipus anthonyi and Peromyscus interparietalis mice, as well as the Neotoma bryanti rat, and the extinction of the Neotoma anthonyi and Neotoma martinensis endemic rats. It is possible that Peromyscus guardia has become extinct due to the introduction of cats in Ángel de la Guarda Island (, CONABIO 2008). The feral cat has also reduced the rabbit population (Sylvilagus bachmani cerrosensis) on Cedros Island. Cats are one of the most important risk factors for waterfowl populations of Ángel de la Guarda Island, San Marcos, Carmen, Santa Catalina and Cerralvo (Velarde and Anderson, 1994). Cats also carry and transmit numerous diseases and parasites, some of which can be transmitted to humans (CONABIO, 2008).
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