Non-native species invasions
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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
Whether deliberately or accidentally, people have been transporting whole range of organisms, breaking natural distribution boundaries and interfering with community structures. The unwanted hitchhikers are usually either well hidden or too small to be noticed – for the entire lifespan or just for its part.
For most coastal species the open ocean environment is inhospitable, preventing them from spreading into habitats similar to their own but located elsewhere. Distances separating such habitats might be too long to overcome either through their active swimming abilities or passive floating in water currents. Mechanisms by which humanity aids introduction of exotic species are called vectors of introduction and these are chiefly associated with shipment activities, marine aquaculture or ornamental species trade. Other vectors include the international transport and sale of live marine bait, live seafood, and live organisms for research and education (Cohen, 2004).
- Shipment
Organisms can travel either attached to the submerged part of the vessels hull or contained within their ballast waters. Ballast water has been extensively used since 1870s (Gollasch, 2004) and certain species are able to complete their life cycle and breed in ballast waters, meaning that they can be translocated far away from their native range. Studies on ballast tanks found more than 1,000 taxa ranging from phytoplankton to small fishes up to 15 cm in length (Gollash, 2004).
- Mariculture
There has been a growing interest in the search for fish, shellfish, and plant species whose biology was well known and which either already had achieved or could achieve success in cultivation. Depending on the type of mariculture, the organisms can be either allowed to establish in the wilderness (intentional species introduction) or kept in enclosures from which they occasionally escape (accidental release) either directly or through their dispersal system. Very often those new introduced species carried along parasitic organisms that could later establish themselves in indigenous species. Since the latter have not developed any defence strategy against non-indigenous parasites, they could be greatly impacted.
- Ornamental species trade
Saltwater species are a rapidly growing sector of the aquarium industry. Species can be released by their owners either accidentally or deliberately when no longer wanted. This vector is responsible for one of the major invasions launched up to date, namely the release of Caulerpa taxifolia from Oceanographic Museum of Monaco.
- Indirect introduction
Occasionally, species introduction can result not from their physical re-location but by offering a way for its dispersal to areas that it wouldn’t be able to reach if the conditions were not changed. Such opportunity is offered by alteration of hydrological regimes, like canal and reservoir construction (Grigorovich et al., 2000).
- Secondary introduction
If species are introduced and become established in a new geographic setting, they might continue to spread by both natural and anthropogenic mechanisms, colonizing habitats that they wouldn’t be able to reach without the initial, human-mediated translocation. Such introduction is termed a secondary introduction.
Very rarely we can connect an invasion event with one particular vector. Instead it is more common to assess the probability of a given vector being responsible. In British waters, Eno et al. (1997) identified accidental release associated with mariculture as the main vector (31% of introduced species). Fouling accounted for 26%, ballast water for 18% and deliberate commercial introduction 8%. Additionally, 12% species were equally likely to be introduced by any of the shipping vectors.
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
Consequences of human-induced altering of species composition are sometimes detrimental to local communities and their magnitude may range from limitation or exclusion of single species or to destabilization of the whole system. Species introduction has been identified as one of the main causes of species extinction. The contribution of human-mediated species translocation toward animals extinction based on 484 extinct species has been estimated as 17% (World Conservational Monitoring Centre, 1992).
- Competition
Introduced species might have evolved in the presence of much more aggressive competitors than the ones present in the receiving environment and they might be extremely successful in colonising new areas. The most common competition is for resources, whether it is food, solar energy or space. In California bay the native shore crab (Hemigrapsus oregonensis) declined in mean abundance by 10 times as a consequence of the European green crab (Carcinus maenas) introduction and competition for food (Nutricola sp.) (Grosholz et al., 2000).
- Diffuse competition
The impact on native organisms can be greater through diffuse competition. A population might resist competition along one axis if the resource is plentiful, but its realised niche can virtually disappear in the presence of several species competing along many axes (Giller, 1984).
- Positive indirect interactions
Even with a stable or decreasing rate of new introductions, it can be assumed that systems will become increasingly destabilized through positive indirect interactions and diffuse competition, as more and more interactions within the community become altered. Even early, non-invasive introductions may become a nuisance due to a change induced by another introduced species (Grosholtz, 2005). It is possible for a new alien to transform an older exotic species into an invader.
- Predation and herbivory
Some organisms are more effective competitors for resources than others and have the potential to dominate the area, excluding other species from the access to viable resources. In such cases organisms feeding on it (predators or herbivores) act as a biocontrol agent keeping their abundance low and maintain a high level of stability and productivity. If those predators or herbivores are introduced to a new environment, local community members might be lacking the evolutionary-driven ability to resist them. On the other hand if a species limited in number in its native range by biocontrol agents is introduced to a new environment, it might thrive and outcompete indigenous species.
- Parasitism
Very often introduced organisms are accompanied by parasitic species. Mass transfer of large numbers of animals and plants without inspection, quarantine, or other management procedures has led to the simultaneous introduction of pathogenic or parasitic agents.
- Genetic impact
If transferred species are closely related to native counterparts, they may hybridise affecting the original gene pool. Introduced or hybridised individuals may prove to be more successful in mating than the indigenous organisms and the original genetic diversity might be lost.
- Multiple negative interactions.
Two species may interact with one another in a several ways. A species confronted by one-way interactions can compromise and trade-off one component of fitness for the sake of the survival. If it is forced to compromise too many components it may be pushed into an extinction vertex (Russel et al., 2004).
The scale of the problem
Further reading
References
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