Summary 1.Pathways describe the processes that result in the introduction of alien species from one location to another. A framework is proposed to facilitate the comparative analysis of invasion pathways by a wide range of taxa in both terrestrial and aquatic ecosystems. Comparisons with a range of data helped identify existing gaps in current knowledge of pathways and highlight the limitations of existing legislation to manage introductions of alien species. The scheme aims for universality but uses the European Union as a case study for the regulatory perspectives. 2. Alien species may arrive and enter a new region through three broad mechanisms: importation of a commodity, arrival of a transport vector, and/or natural spread from a neighbouring region where the species is itself alien. These three mechanisms result in six principal pathways: release, escape, contaminant, stowaway, corridor and unaided. 3. Alien species transported as commodities may be introduced as a deliberate release or as an escape from captivity. Many species are not intentionally transported but arrive as a contaminant of a commodity, for example pathogens and pests. Stowaways are directly associated with human transport but arrive independently of a specific commodity, for example organisms transported in ballast water, cargo and airfreight. The corridor pathway highlights the role transport infrastructures play in the introduction of alien species. The unaided pathway describes situations where natural spread results in alien species arriving into a new region from a donor region where it is also alien. 4. Vertebrate pathways tend to be characterized as deliberate releases, invertebrates as contaminants and plants as escapes. Pathogenic micro-organisms and fungi are generally introduced as contaminants of their hosts. The corridor and unaided pathways are often ignored in pathway assessments but warrant further detailed consideration. 5. Synthesis and applications. Intentional releases and escapes should be straightforward to monitor and regulate but, in practice, developing legislation has proved difficult. New introductions continue to occur through contaminant, stowaway, corridor and unaided pathways. These pathways represent special challenges for management and legislation. The present framework should enable these trends to be monitored more clearly and hopefully lead to the development of appropriate regulations or codes of practice to stem the number of future introductions.

I In n a a n nu ut ts sh he el ll l: :• Ecological and economic impacts of alien species are usually studied separately, but they are likely to be highly correlated • Few studies have compared these impacts, so their effects are probably underestimated for species-rich taxa or across large regions • Although aliens may affect all categories of ecosystem services, current economic valuations focus primarily on "provisioning" services, because of limited available data relating to impacts on other services • Nature conservation, agriculture, forestry, and fisheries are the main economic sectors where alien species cause marked direct costs in Europe • Europe has the most up-to-date information on numbers of aliens and their impacts, but lags behind North America with respect to current knowledge of mechanisms underlying impacts; researchers from both continents can profit from each other's experiences and work toward reliable and comparable estimates of costs from alien species invasions Alien species impacts in Europe M Vilà et al. 136w ww ww w. .f fr ro on nt ti ie er rs si in ne ec co ol lo og gy y. .o or rg g

The brackish Baltic Sea hosts species of various origins and environmental tolerances. These immigrated to the sea 10,000 to 15,000 years ago or have been introduced to the area over the relatively recent history of the system. The Baltic Sea has only one known endemic species. While information on some abiotic parameters extends back as long as five centuries and first quantitative snapshot data on biota (on exploited fish populations) originate generally from the same time, international coordination of research began in the early twentieth century. Continuous, annual Baltic Sea-wide long-term datasets on several organism groups (plankton, benthos, fish) are generally available since the mid-1950s. Based on a variety of available data sources (published papers, reports, grey literature, unpublished data), the Baltic Sea, incl. Kattegat, hosts altogether at least 6,065 species, including at least 1,700 phytoplankton, 442 phytobenthos, at least 1,199 zooplankton, at least 569 meiozoobenthos, 1,476 macrozoobenthos, at least 380 vertebrate parasites, about 200 fish, 3 seal, and 83 bird species. In general, but not in all organism groups, high sub-regional total species richness is associated with elevated salinity. Although in comparison with fully marine areas the Baltic Sea supports fewer species, several facets of the system's diversity remain underexplored to this day, such as micro-organisms, foraminiferans, meiobenthos and parasites. In the future, climate change and its interactions with multiple anthropogenic forcings are likely to have major impacts on the Baltic biodiversity.

There are about 100 nonindigenous species recorded in the Baltic Sea. Invasive species have resulted in major changes in nearshore ecosystems, especially in coastal lagoons and inlets that can be identified as "centres of xenodiversity". Fewer than 70 of these species have established reproducing populations. Dominant invasion vectors include unintentional introductions via ballast water, tank sediments, and hull fouling, aquaculture, and the construction of canals that have facilitated active or passive natural dispersal. Of the approximately 60 unintentionally introduced species with a known invasion history, 38 are transoceanic (including 19 Atlantic species of American origin) and 18 of Ponto-Caspian origin. Species that have caused economic damage to fisheries, shipping, and industry include the hydrozoan Cordylophora caspia, the barnacle Balanus improvisus, the cladoceran Cercopagis pengoi, and the bivalve Dreissena polymorpha. The Baltic Sea has served as a secondary source of nonindigenous species to the North American Great Lakes. Further study is warranted to quantify large-scale ecosystem changes in the Baltic associated with establishment and population growth of nonindigenous species and to prevent future invasions.