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;Pilsbry, H. A. (1927). Expedition to Guadalupe Island, Mexico, in 1922: Land and Freshwater Mollusks. 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).
Introduction of non-native bees The introduction of pollinating bumblebees (Bombus) significantly affected native pollinators and both native and introduced plants. The introduced bees are highly pollinating, are abundant in invaded areas, and in some cases, dominate the communities in which they live. In general, they acquire resources more efficiently than native species, despite using the same flowers; they are mutually exclusive both spatially and temporally, suggesting potential competition with native species.
Colonies of B. impatiens, originally from the northeast United States, have been imported to Mexico. B. terrestris, a species native to Europe, North Africa, and Asia was introduced in other areas as a pollinator of commercial crops.
Introduction of geckos The common house gecko (Hemidactylus frenatus) (Dumeril & Bibron 1836) synonymous with H. nigriventris, also known as house gecko or bridled house gecko, is native to the islands of the Pacific Ocean. It is currently found throughout the tropics of the Old World and is distributed discontinuously in Central America. Its introduction was probably accidental and repeated, as a stowaway on commercial cargo ships coming from the Pacific Islands. This gecko could potentially have a negative effect on local populations of other gecko species, either because their nocturnal eating habits may be similar, or because adult geckos can prey on juveniles of other lizards and even their own kind. Under specific conditions, they may be vectors of disease or parasites to the native fauna.Alvares et al. (2008). Animales exóticos en México: una amenaza para la biodiversidad. Comisión Nacional para el Conocimiento y Uso de la Biodiversidad, 12, 41, 277- 280 Revisado en: http://www2.inecc.gob.mx/publicaciones/download/590.pdf
Amphibia The American bullfrog (Lithobates catesbeianus) is native to Canada and the eastern United States, and was introduced in the early 1900s to Colorado and California. The first records of the species in Mexico date from 1853. It is a very aggressive species and an unspecialized predator, which has a negative impact on the populations of virtually any animal it can capture or ingest. Its introduction has caused the extinction of native amphibians. One can anticipate a negative effect on native vertebrate and invertebrate populations, aquatic and terrestrial, in any location. This problem has been known for thirty years. The species is considered among the 100 most important invasive exotic species at a global level. The main concern is due to the potential it has to compete with and prey on other species.
Reptiles Japanese and red-eared turtles (Trachemys scripta) are native to the country and sold in many pet stores, in most cases released when the owners see the size at which they reach adulthood. The turtle is considered one of the 100 most dangerous invasive species by the Specialist Group on Invasive Species of the IUCN for its omnivorous diet and being inclined to compete with other species of turtles for basking places, besides being an important vector of potentially harmful pathogens for humans and other species (Bringsoe, 2006). Trade of this species should be regulated more effectively to urge the public not to release their pets in nature when they no longer want to be responsible for them.
Urban environments Several invasive bird species are found in urban areas. The house sparrow (Passer domesticus) was originally distributed in the Middle East to the Mediterranean region, and was introduced in 1850 in North America. It competes for food and nesting sites with other species. The rock dove (Columba livia) is native to Europe and has been introduced worldwide. It causes damage to buildings and transmits diseases to other species. Similarly, the monk parakeet (Myiopsitta monachus) is native to Argentina and Uruguay and is a recognized pest of crops. It is easily adapted to urban environments so it has high demand as a pet in Mexico. People unaware of the potential damage liberate and help feed them, as well as pigeons and house sparrows.
Terrestrial plants Invasive plants are fast-growing pioneer species that take advantage of disturbed areas to colonize bare soil. Once established, they inhibit seed germination or development of native species, and can dominate the vegetation, preventing the disturbed ecosystem from restoring itself.
In Mexico there are various efforts to identify and monitor invasive plants. CONABIO (2015) listed 145 species and established a monitoring network. It also created a hierarchy of species' invasiveness according to their ability to spread, their establishment, and their impact on biodiversity (UNIBIO, s.a.). Subsequent date produced by the network UNIBIO (s.a.) listed 226 invasive species across Mexico, of which 2% have a degree of "extreme invasiveness", 46% "high invasiveness", and 15% "moderate invasiveness". Combining these three categories, there are a total of 184 species identified as invasive plants.
Invasive species with an extreme degree of invasiveness identified by the Network are: Eucalyptus globulus Ricinus communis Pennisetum clandestinum Eragrostis lehmanniana Cenchrus ciliaris Rhynchelytrum repens Tamarix ramosissima
Of these 7 species, 4 belong to the grass family (Poaceae). This group contains the largest number of invasive plants in Mexico, with 82 species of 226 total registered so far. The family with the second highest number of invasive species is the Asteraceae with 14 species, and joint third and fourth are the Fabaceae and Brassicaceae, containing 12 species each (UNIBIO, s.a.).
Distribution