Invasive alien species are recognised as one of the leading threats to biodiversity and also impose enormous costs on agriculture, forestry, fisheries, and other human enterprises, as well as on human health. Rapidly accelerating human trade, tourism, transport, and travel over the past century have dramatically enhanced the spread of invasive species, allowing them to surmount natural geographic barriers. Not all non-indigenous species are harmful. In fact the majority of species used in agriculture, forestry and fisheries are alien species.
Thus, the initial step in a national programme must be to distinguish the harmful from the harmless alien species and identify the impacts of the former on native biodiversity.
Development of a national strategy summarizing goals and objectives should be the first step in formulating an alien species plan. The ultimate goal of the strategy should be preservation or restoration of healthy ecosystems. An initial assessment, including a survey of native and alien species (and their impacts) will help define the starting-point and serve as a base for comparison as the programme progresses. The support of all stakeholders must be engaged during the entire programme, ideally using a social marketing campaign. Legal and institutional frameworks will define the basic opportunities for prevention and management of invasive alien species. There are four major options (or better, steps) for dealing with alien species:
- early detection,
- eradication, and
Prevention of introductions is the first and most cost-effective option. This lesson has been learned the hard way from several cases of highly destructive and costly invasive organisms such as the zebra mussel in the Great Lakes. Had such species been intercepted at the outset, an enormous loss of native species and/or money could have been prevented. Exclusion methods based on pathways rather than on individual species provide the most efficient way to concentrate efforts at sites where pests are most likely to enter national boundaries and to intercept several potential invaders linked to a single pathway. Three major possibilities to prevent further invasions exist:
- interception based on regulations enforced with inspections and fees,
- treatment of material suspected to be contaminated with non-indigenous species, and
- prohibition of particular commodities in accordance with international regulations. Deliberate introductions of non-indigenous species should all be subject to an import risk assessment.
Early detection of a potential invasive species is often crucial in determining whether eradication of the species is feasible. The possibility of early eradication or at least of effectively containing a new coloniser makes investment in early detection worthwhile. Early detection in the form of surveys may focus on a species of concern or on a specific site. Species-specific surveys are designed, adapted or developed for a specific situation, taking into consideration the ecology of the target species. Site-specific surveys are targeted to detect invaders in the vicinity of high-risk entry points or in high value biodiversity areas.