In a wide range of terrestrial, marine and freshwater ecosystems, accidental or voluntary introduction of non-native species by humans has altered local biological community interactions, triggering dramatic and often unexpected changes in ecosystem processes and causing large monetary and cultural losses.
Alien species can act as vectors for new diseases, change biodiversity, disrupt cultural landscapes, reduce the value of land and water for human activities and trigger other socio-economic consequences.
Invasive species themselves come in all sorts of shapes and sizes and occur in all major taxonomic groups, including viruses, fungi, algae, mosses, ferns, higher plants, insects and other invertebrates, fish, amphibians, reptiles, birds and mammals.
In some cases non-native species have been deliberately imported, such as kudzo vine plants introduced to control soil erosion in North America, or game fishes stocked for sport fisheries. Many imported species were intended only for agricultural, horticultural or aquarium use, but subsequently moved into ‘natural landscapes’, some with significant ecological or economic impacts. Crops and trees have escaped plantations and become pests. Non-native ornamental plants used in landscaping often disperse into surrounding untended areas. And aquatic species reach new environments when unwanted pets or bait fish are set free in natural waterways.
Within ten years of the introduction of 24 individuals for sport in 1859, rabbits had multiplied so rapidly that two million could be shot or trapped annually without having any noticeble effect on the population. It was the fastest spread ever recorded of any mammal anywhere in the world. The effects have been devastating, with rabbits suspected of being the most significant known factor in the extinction or major decline of native animal species in Australia (since the arrival of Europeans in the 18th century, one eighth of all mammalian species in the country have become extinct). More recently, since the mid-1990s, rabbit populations have crashed in many parts of the country, through the impact of rabbit haemorrhagic disease.
Many other invasions are accidental and unexpected. A striking illustration flagged in the UN’s Millennium Ecosystem Assessment is in the Caribbean Sea, where dust blown from the Sahara across the Atlantic introduced new pathogenic bacteria that were at least partially responsible for coral reef diseases in the last two decades. The increase in transport associated with global trade is seen as a major indirect driver for increases in species invasions.
Not all new arrivals are able to establish themselves. Aliens that are successful invaders are species that have some advantage over native species. These adavantages are often enhanced when aliens move into ecological niches and thrive because, outside their naturral environments, they are not held back by natural predators, parasites, disease or competition in the way that native species are.
Susceptibility to invaders
Different types of ecological systems seem to have different susceptibilities to invasion. According to the Global Biodiversity Assessment (page 449) , islands and ecosystems with relatively few component species, such as boreal forests, would appear to be more susceptible to species invasions than species-rich biomes such as tropical forests. On the other hand, some ecologists have pointed to highly diverse but heavily invaded ecosystems, and have argued that ecosystems with high species diversity seem to be more susceptible to invasion. In general, areas of ecosystems that have been subjected to disturbance or stress from other environmental factors – such as fire, drought, overgrazing or extensive clearing – can provide open habitat and resources that allow invaders to become established. In the end, this debate seems to hinge on the spatial scale at which invasion studies are performed.
In all climatic zones, freshwater ecosystems seem to be especially sensitive to invasions and introductions, which are major causes of species extinctions. And the forecast of the Millennium Ecosystem Assessment is for a very rapid increase in the impact of invasive species on inland waters.
Whatever the route of transport, alien species can trigger massive changes in the structure, functioning and dynamics of ecosystems. For example, the introduction of the comb jelly fish (Mnemiopsis leidyi) in the Black Sea mnemiopsis feeds voraciously on small fish and plankton and can reach very high densities and biomass (up to 10-12 kg/m2 in some coastal areas). It caused the loss of 26 major fisheries species and have been implicated (along with other factors) in subsequent growth of the anoxic ‘dead zone’. Water hyacinth, a wayward ornamental from South America, has become a scourge of waterways worldwide. In Africa, for instance, the weed has exploded across the continent since the 1950s, threatening rice cultivation, fisheries, navigation, hydroelectric power generation, tourism, and even human health (by providing habitat for snails and mosquitoes that vector schistosomiasis, malaria and other diseases).
The economic impact of invasive alien species can be substantial. One estimate suggests that the global economic damage may be as much as US$1.4 trillion , which is equivalent to China’s foreign trade in 2005. A 2004 overview by the Ecological Society of America reports that the economic costs of non-native species invasion in the United States exceed $137 billion each year, at the time ‘more than the combined total of all other natural disasters.’
Despite the great diversity of invasive species and their impacts, the species are transported by an identified group of pathways, with a common set of biological processes – introduction, establishment, spread and impact – operating in all invasions. Policy and management solutions become clearer when these common pathways and processes are recognized.
There are three major approaches to control:
Mechanical control entails the removal of invasive species by hand or with machines. It is often successful at the small scale but can be expensive and labour-intensive. Modifying habitat conditions (through such means as burning, flooding) is an option when removal of individuals is impracticable.
Chemical control includes the application of chemical compounds (herbicides, pesticides, fungicides). Use may be problematic due to impacts on non-target organisms. It can be expensive and may be ineffective when target organisms evolve resistance to certain chemicals.
Biological control involves the release of a specific species to restrict the spread of the invasive species, and with the proper research can be both environmentally safe and successful. The introduction of a natural enemy (predator or parasite, generally from the invader’s natural range) is often the only approach for controlling an invasive species that has established dense populations over large areas. It can be an environmentally sound way to control invasive species with minimum expense, but some control agents do not survive and others attack non-target organisms. An example concerns the rosy wolf snail (Euglandina rosea) in Hawaii. It was imported in 1958 to control another alien predator, the giant african snail, and is an ideal killing machine, tracking its victimes by their slime trails. Howewer, Euglandina quickly developed a taste for the native snails too and went on a binge. Local biologists comment thait it is very difficult to prove that Euglandina is responsible for the extinction of native snails, but the weight of evidence virtually forces this conclusion.
But as in many things, prevention is better than cure, and the best method of control is by preventing the establishment of invasive species. Many governments have taken measures to limit the entry of potentially harmful species into their territories, through inspections of international import shipments, customs checks and quarantine regulations. The creation of lists of safe and potentially harmful species can be helpful in regulation. The general public can also participate in invasive species prevention by educating themselves about threats, making informed decisions and taking appropriate actions in their own communities.
Proposals for a global strategy to address the problem of invasive alien species include recommendations on preparing national strategies and plans and building alien species components into global change initiatives.
Though there may be very good ecological and economic reasons to control specific exotics in particular situations and circumstances, it would be imparcticable and indeed wrong to protect a given state against evolutionary process. Living communities cannot be ocked up in a sort of metaphoric jar, and we jsut have to recognize that ours is a time of rapide and wholesacle reshuffling of species. It must also be recognized that, unlike chemical pollution, invasive species are self-regulating and may quickly evolve mechanisms to overcome control efforts.
Human-caused invasions by exotic species are a world-wide phenomenon but they are particularly frequent and significant on oceanic islands and archipelagoes, where native species may not have the means to protect themselves against non-native newcomers. One historical example is the flightless dodo (Raphus curcullatus) of Mauritius, which became extinct in the 17th century in the face of the combined onslaught of dogs, pigs, rats , cats… and humans.
Many examples of biological invaders have been documented on the Hawaiian Islands, the most isolated archipelago on Earth. Before the arrival of human beings, it is calculated that a new species arrived only every 100,000 years, while in the last few decades some 20 new species of invertebrate arrive annually. The native flora of ± 1,200 species (> 90% endemic) has been swelled by ± 4,600 exotic plant species brought by Polynesian, European and Asian colonists. At least 800 of these species have established breeding populations. Among the invaders is Myrica faya, a small evergreen tree native to the Canary Islands, Azores and Madeira. Invasion of this nitrogen-fixing tree into such sites as Hawaii Volcanoes National Park has been promoted by by its prolific seed production, effective dispersal by exotic birds and rapid growth rates. The wide-rannging impact of invaders such as Myrica, the brown tree snake and the red fire ant has led to a major research effort and extensive public involvement, including the setting up a multi-stakeholder group (the Hawaii Invasive Species Council) aimed at providing leadership, coordination and action for ‘Stopping the silent invasion’.
– Baskin, Y. 2002. A Plague of Rats and Rubber-Vines. The Growing Threat of Species Invasions. Island Press, Washington, D.C.
– Drake, J.A.; Mooney, H.A.; di Castri, F.; Groves, R.H.; Kruger, F.J.; Rejmánek, M.; Williamson, M. (eds). 1989. Biological Invasions: A Global Perspective. SCOPE (Scientific Committee on Problems of the Environment) 37. John Wiley, Chichester.
– McNeely, J.A.; Mooney, H.A.; Neville, L.E.; Schei, P.J.; Waage, J.K. (eds). 2001. Global Strategy on Invasive Alien Species. Global Invasive Species Programme (SCOPE/CAB International/IUCN). IUCN, Gland and Cambridge.
– Mooney, H.A.; Mack, R.N.; McNeely, J.A.; Neville, L.E.; Schei, P.J.; Waage, J.K. (eds). 2005. Invasive Alien Species: A New Synthesis. SCOPE 63. Island Press, Washington, D.C.
– Perrings, C.; Williamson, M.; Dalmazzone, S. (eds). 2000. The Economics of Biological Invasions. Edward Elgar, Cheltenham.
– Shine, C.; Williams, N.; Gündling, L. 2000. A Guide to Designing Legal and Institutional Frameworks on Alien Invasive Species. Environmental Policy and Law Paper No. 40. IUCN, Gland and Cambridge.
– Wittenberg, R.; Cook, M.J.W. (eds) . 2001. Invasive Alien Species: A Toolkit of Best Prevention and Management Practices. Global Invasive Species Programme (SCOPE/CAB International/IUCN). CAB International, Wallingford.