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Alien species and the water framework directive

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About this book

Alien species constitute a major pressure in aquatic environments, both ecologically and economically. This recognition has initiated a debate on the role of alien species in ecological status classifications. We distributed a questionnaire to review how EU Member States (MSs) deal with alien species in their national status assessments under the Water Framework Directive (WFD). The questionnaire was filled and returned by 23 EU MSs and Norway. Analysis of the questionnaire returns and referred methods revealed the existence of a wide range of approaches: (1) The majority of MSs do not take alien species explicitly into account for classification under the WFD. This implies that the biological methods that were developed for assessing anthropogenic pressures are assumed to be able to pick up pressures by alien species, but this remains to be verified. Some of these MSs do acknowledge the need for a more direct or supplementary account of alien species, but fail to do so because of the lack of information on the impact and distribution of alien species within their territory. (2) Few MSs assess ecological status using the pressure-based tools, and subsequently downgraded the status in case of presence of predefined high-impact alien species. The main critiques to this approach are that it puts too much weight on alien species, compared to other pressures, and that it may results in downgrading of many water bodies, even in the absence of any perspectives for remediation of the problem. (3) Others account for alien species by attributing aliens a different score than native species in a metric, or by including alien species in comparisons with alien-free reference communities. Such approaches may blur the impact assessment of other pressures, and may obscure the magnitude of the alien species problem. (4) Almost all MSs support the idea of a supplementary biopollution index. Such an index would uncouple alien species and anthropogenic pressure assessments, and allow for a correct appraisal of the problem without affecting the WFD classification. A pan-European index may not be feasible because of different trade-offs between practicability and accuracy across MSs. A better harmonization of the views on alien species and water body classification within Europe is desirable, but a challenging task. It requires an agreed interpretation and usage of alien species related terms, and an increased compatibility and completeness of national and regional alien species lists. These topics have been added for discussion to the 2010-2012 mandate of the WFD Working Group on Ecological Status.