Difference between revisions of "Non-native species invasions"
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Revision as of 14:50, 2 March 2009
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Contents
Introduction
The introduction of harmful aquatic organisms to new marine environments is believed to be one of the four greatest threats to the world's oceans. An alien or non-native species is one that has been intentionally or accidentally transported and released into an environment outside of its historic or resident geographical range or habitat. Such species are described as 'invasive' if they are ecologically and/or economically harmful. Most accidental marine species invasions are transported in ships' ballast water, and associated sediments, from fouling on ships' hulls and through aquaculture. Ballast water is the water taken in by ships for stability and is usually released on arrival at the destination port. Invasive species can dramatically change the structure and function of marine ecosystems by changing biodiversity and eliminating vital components of the food chain.
Over 100 non-native species have been recorded in the NE Atlantic, mainly in the North Sea, the Celtic Sea, the Bay of Biscay and along the Iberian coast. Over the past twenty years, the number of alien species transported into the Baltic has increased and poses a significant threat to the region given its naturally low species diversity. At least 50 non-native species have entered the Black Sea in the last century. Some have been invasive e.g. the comb jelly-fish Mnemiopsis leidyi was the primary cause of collapse of the fisheries in the area in the early 1990s.
Functional integrity of indigenous commuities
Even though all seas are interconnected, there are certain dispersion barriers such as salinity, temperature gradient or ocean currents that keep local communities distinct. Two identical habitats can have different species communities with different types of interactions contributing to the global richness of biodiversity. Indigenous, or native species are those living within their natural range (past or present) including the area that it can reach and occupy using its natural dispersal system. By contrast, introduced species are transported either intentionally or accidentally by human-mediated vectors into habitats outside their native range. These species are also termed as alien, exotic, invasive, foreign, non-native, immigrant, neobiota, naturalized, or non-indigenous. Some biota cannot be sufficiently proved to be neither native nor exotic and these are termed cryptogenic species (Carlton, 1996)[1]
Through the evolutionary fitness processes, species adjust to each other and adapt to available resources by occupying different ecological niches within communities. Virtually all resources are utilized optimally and all available niches are filled, maximising the biodiversity value. Various factors influence the functional integrity of a community. If changes are occurring gradually over a long timescale species have enough time to adapt and fill available niches. In turn more rapid shifts create niche openings and this has been identified as the main prerequisite of species invasion. Non-native species are found primarily in disturbed areas, such as harbours, bays, estuaries and semi-enclosed seas where the communities are weekened by various types of pollution (Elton, 1958[2]; Cohen, 2004[3]).
Vectors of introduction
Establishment of non-indegenous species
Not all translocated species become established in new environments. If the population is relatively small it will be vulnerable to stochastic threats such as demographic or genetic drift. It is hard to predict what is the minimum viable population size. A generally accepted rule is that 50 individuals are needed to prevent excessive inbreeding and a minimum population of 500 hundred is required to keep a high level of heterozigosity (Franklin, 1980). Yet, those numbers may very greatly between different taxa and even if the population is large enough to prevent loss of alleles, it still might be prone to environmental threats such as poor food or oxygen availability, impacting recruitment success or juveniles development, or catastrophic events (El Niño, tsunami, etc.)
If the founder population is large enough to overcome stochastic threats and manages to establish itself in a new environment, it will join the network of interactions within the receiving environment. Alien species have evolved in different environments and it is hard to predict what their interactions with indigenous biota will be. Not all introduced species can be termed as invasive. Some of them may well coexist with native species and share the resources. But every now and then an introduced species becomes an invader and impacts the host community, with the ultimate result being eradication of native species and destabilization of the system.
Impacts
The scale of the problem
References
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