Species in an ecosystem can be classified as natives or exotics. Native species are those that have coevolved in the ecosystem, while exotic ones have not. The introduction of exotic species into an ecosystem is usually associated with human influence, which can be intentional or accidental. Some exotic species do not survive, at least not without artificial assistance. But some others do quite well on their own in a new environment. Exotic species may have no natural predators in the new environment or they may make better use of the natural resources than the natives, so they spread in the new territory and compete with some of the natives, who eventually become extinct. Exotic species that successfully establish and spread in an ecosystem are called invaders. The process by which an invader arrives and spreads into the new territory is called biological invasion. It is worth mentioning that, although invaders are usually exotic species, sometimes native species may also behave like invaders. That is, if an ecosystem suffers a strong disturbance, like fire or heavy grazing, some native species whose populations were originally stable may start to grow, outcompeting other native species. There are many examples of introduced species that became invaders, ranging from bacteria to cattle. Accidental or intentional introductions by humans are responsible for most of the present biological invasions, threatening the structure and functioning of many ecosystems. There are many effects associated with biological invasions, perhaps the most important one being the possible loss of biodiversity in the long term. But biological invasions may also introduce changes in different environmental traits, like climate, hydrology (invaders may consume more water than natives), and soil composition (for instance, some plants take up salt from soil and deposit it on the surface, making it unsuitable for some native species). All these changes have strong economical impacts, considering their influences in agriculture, forestry, and public health [9]. Hence, it is of interest to understand this phenomenon in order to predict the potential invasiveness of a species before its introduction in an ecosystem, and to develop strategies of control for invasive species that have already been introduced.
Contribución al conocimiento de las xenófitas en España: Catálogo provisional de la flora alóctona de Almería.
