Biological invasions

Introduction
Biological invasions have captured the attention of the scientific community since the eighties decade. They are considered one of the most serious threats for global biodiversity. Many human activities can help to the intentional and accidental spreads of species (for example agriculture, aquaculture, recreation, transport) breaking the natural barriers of dispersion. Although the natural distribution of species can change with time, those activities can greatly increase the speed and spatial scale of movement of species around the world. Invasive species can act as “ecosystem engineers”; the environmental changes that cause can have strong effects on the community structure. Control and prevention of biological invasions are the main factors to consider for the conservation of the environment in general.

But not every introduced species is considered invasive. A species is considered as introduced or not native when it spreads directly or indirectly outside its natural range for any human activity, whereas a species is considered invasive when it is dispersed widely, colonizing and invading the natural ecosystem, becoming very abundant and causing harm to the ecosystem, its goods and services.

Process of invasion
Vilches et al, 2010 described the invasion process as a series of stages that species must face to overcome the natural barriers of distribution. To detect invasions the date, history, habitat and their ecological attributes should be considered in order to evaluate the feasibility and effectiveness of the vector. For that purpose there are six categories or types of vectors.
 * 1) Transport.
 * 2) Establishment or naturalization.
 * 3) Dispersion.
 * 4) Human perception.
 * 1) Ships.
 * 2) Aquaculture.
 * 3) Bio-control.
 * 4) Escape of transported species for the following purposes: ornamentation, agriculture or research.
 * 5) Channels.
 * 6) Vector combination.

Impact
Pereyra et al (2007) recognized many direct and indirect effects of bioinvaders.


 * Direct effects: The deepest change that introduced species produce is on the habitat, since it can affect the entire ecosystem. Other examples include parasitism, predation, or competition with the native species.


 * Indirect effects: An introduced species can affect the interaction between other species on the ecosystem, sharing dams, hosts and parasites.

Evaluation
The problem of bioinvasions has regional importance and the degree of impact can be evaluated in a standard and objective manner through simplified protocols developed by specialists, for example the Invasive Species Environmental Impact Assessment (ISEIA) (Vanderhoeven, S. & Branquart, 2010). This protocol is composed of compatible sections with the stages of the invasion process: potential spread; colonization of natural habitats; impacts on native species and ecosystems. By combining biological history, impacts on neighboring areas and ecological profiles of alien species, the protocol allows to classify species on one of the following three categories of risk:


 * Category A: (blacklist) includes species with high environmental risks.
 * Category B: (watch list or alert) includes species with moderate environmental risk based on existing knowledge.
 * Category C: includes other non-native species, which are not considered a threat to biodiversity and native ecosystems.

Vilches et al (2010) argue that detecting species introduced at an early stage of the invasion process is the only way to eradicate the problem. It is at this moment when population growth and economic risks are lower. Once the species has settled it is virtually impossible to eradicate it from the environment. The only possibility it is to try to control it, restrict its development and stop its dispersion, with a very high economic cost.