Biopollution is a dynamic process: for instance, the spread of a species within a biogeographical region after having been introduced from elsewhere has raised special interest. In this case, the primary inoculation consists of an arrival from a distant source. Secondary spread is defined as the transport to new regions within the same biogeographical area following establishment.  Secondary spread may disperse alien biota more efficiently and so could compromise the ability to manage the invasion. The establishment and spread of NIS seem directly linked to propagule pressure, which has been recognised as the major factor in the success of an invasion and depends on the quality, quantity, and frequency of invaders.

2. Impacts of NIS

Adverse effects of NIS (especially invasive species) on ecosystems have been shown as biodiversity loss, alteration of food webs, physical habitat disruption, and import of parasites and diseases. Moreover, a progressive homogenization of biota across regions and continents, as a consequence of increased translocation of species, is raising concern among ecology students

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While information on the total number of non-indigenous species, their traits, invasiveness potential, and vectors of spread has accumulated in many parts of the world, scientific evidence is still generally scanty to draw general conclusions on the real magnitude of their ecological impact . Communities of organisms can change over historical (ecological) time in three ways: species can be deleted (extinctions), added (invasions), or can change in relative abundance. Ecological functions can change dramatically over time, or else changes can become evident only after long periods of innocuous presence of the alien species. Such cryptic processes may lead to an underestimation of the long-term impacts and constrain the effectiveness of management actions.

Biological introductions have implications for environmental quality. As with chemical pollutants, consequences can be detected at the individual level (as in the case of attacks by parasites or pathogens), or at the population (by genetic change, i.e., hybridization), or at the community level (by structural shifts). Habitat (by modification of physical–chemical conditions) and ecosystem changes (by alteration of energy flow and organic material cycling) are also possible. Some biopollution assessment methods have been proposed  in order to classify the impact on natural ecosystems, while measures of the more complex assessment of economic impact are rare. Functional consequences of invasive NIS have not been explored extensively, yet published studies show that species addictions affect ecosystem functioning (e.g., productivity, biogeochemical cycles, and decomposition) and biotic relations (e.g., prey–predator interactions, and introductions of parasites and diseases). These effects will contribute to ecosystem functioning in complex ways and range from almost negligible to dramatic proportions.

In order to describe the relevance of the impacts caused by alien species the IUCN’s Environment Impact Classification for Alien Taxa (EICAT) is being proposed as a standard for categorizing alien species’ impact. It is used to classify individual alien taxa according to the magnitude of their impacts on native taxa, based on the organizational level in the affected community (

https://www.iucn.org/theme/species/our-work/invasive-species/eicat

). Impact categories range from Minimal Concern to Massive. If only individual performance is affected, it is considered a Minor impact; if a native taxon is removed from the community (locally extinct or extirpated), it is considered Major or Massive, based on the reversibility of the change. The IUCN EICAT Standard is the product of a long process of developing and adapting frameworks to quantify impacts. One evolution is SEICAT, a standardized method for classifying alien taxa in terms of the magnitude of their impacts on human wellbeing, based on the capability approach from welfare economics. The core characteristic of this approach is that it uses changes in people’s activities as a common metric for evaluating impacts on wellbeing.

The INvasive Species Effects Assessment Tool (INSEAT) also contributes to the current scenario of invasive species assessment. INSEAT considers both positive and negative impacts of IAS on ecosystem services (ES) and uses the ES framework, commonly classified into provisioning, regulating, and cultural services. This differs from SEICAT, which uses the constituents of human wellbeing, and from EICAT, which defines its own categories of environmental impacts .

Adverse effects of invasive NIS, or biological pollution, is an increasing problem and is getting the attention of managers and decision makers. The need for assessing the size of the problem and the decision to put in place effective mechanisms to prevent introductions—as well as to contain, control or eradicate the introduced species that may cause damages—are more-and-more felt as an imperative. However, it is very difficult to predict which of NIS introductions may result in detrimental effects on environmental quality by way of changes to the biological, chemical, and/or physical properties of an invaded ecosystem.

Risk assessment and management strategies have been in many instances derived from the practice of chemical pollutants. Moreover, environmental quality criteria have encompassed both biological, physical, and chemical conditions of the environment in many legislations based on scientific knowledge, including the presence and the effects of NIS.