Summary
Biodiversity and national economies are being threatened by the invasion of non-native species
In the early 1980s, a biologist at the tropical aquarium in Stuttgart, Germany, noticed that the beautiful green alga Caulerpa taxifolia (Fig. 1) would make an ideal aquarium decoration. It does not wither, it is fast growing and it withstands cool temperatures. In the following years, many aquariums acquired Caulerpa plants from Stuttgart, among them the Oceanographic Museum of Monaco, from which it escaped into the Mediterranean Sea. In 1984, the alga covered an area of around 1 m2 on the coast of Monaco. Since then, it has invaded large areas of the French and Italian coasts and has spread as far as the coasts of Spain, Croatia and North Africa. The alga grows everywhere, from the surface to the lower limits of underwater vegetation, forming huge monocultures that displace other plant species and, being highly toxic to many fish species, also threatens aquatic fauna wherever it roots.
Invasive species are the second biggest cause of loss of biodiversity after habitat destruction
Figure 1.
The green alga Caulerpa taxifolia
In the summer of 1999, New York City witnessed an unusual spate of bird deaths, particularly of crows. During the same summer, on August 23, a physician from a hospital in northern Queens reported two encephalitis patients to the New York City Department of Health, and further surveillance identified other cases. A month later, scientists from the Centers for Disease Control and Prevention (CDC) in Atlanta, GA, USA, identified the link between these seemingly unrelated events. They discovered that the causative agent was the West Nile virus, which had never before been seen in North America. To contain the outbreak and prevent transmission of the virus by mosquitoes, New York's mayor, Rudy Giuliani, ordered the spraying of whole neighbourhoods in New York with pesticides. It was in vain. During 2001, 149 cases of encephalitis due to West Nile virus were reported to the CDC from all over the USA, 18 people died and, within two years, migrating birds had spread the virus as far as California (Fig. 2).
Figure 2.
The spread of the West Nile virus in the USA in 2001
Caulerpa taxifolia and West Nile virus are what biologists call invasive species, and many governments are investing considerable efforts into eliminating, or at least controlling, them. Invasive species are the second biggest cause of loss of biodiversity after habitat destruction. And scientists from Cornell University in Ithaca, NY, USA, estimate the annual economic damage to the USA to be US $138 billion (BioScience, 50, 53–65; 2000), which is more than the US government is spending in total on scientific research this year. The
...devising policies to curtail or prevent species invading has turned out to be a challenge that is almost as complex as nature itself
Australian Department of Agriculture, Fisheries and Forestry, puts the damage to the Australian economy at AUS $420 (US $260) million per year, and there is a further AUS $80 (US $50) million price tag for control efforts and research. And, as the Australians learned soon after the introduction of European rabbits (Oryctolagus cuniculus), it is almost impossible to get rid of them. “Invasive species are the least reversible form of pollution there is,” said David M. Lodge, Professor of Biological Sciences at the University of Notre Dame in Notre Dame, IN, USA, and a member of the US National Invasive Species Advisory Committee to the US Government. “Once a species has become invasive, and has spread and established, it is nearly impossible to eradicate it.” However, Daniel Simberloff, Professor at the Department of Ecology and Environmental Biology at the University of Tennessee in Knoxville, TN, USA, takes a more positive view, citing the successful eradication of smallpox and the malaria-transmitting mosquito Anopheles gambiae in Brazil.
As the impact of invasive species is expected to increase with growing global travel and trade, ecologists, farmers and government agencies around the world are increasingly concerned about the threats this creates to the local ecology, economy and health of the population. But devising policies to curtail or prevent invasive species has turned out to be a challenge that is almost as complex as nature itself. And the problems start with their definition. US Executive Order 13112 (www.invasivespecies.gov/laws/execorder.shtml) defines them as “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” But even this, seemingly simple, definition has raised considerable discussion among scientists, politicians and economists. “This is the most difficult scientific question embedded in this debate,” Lodge acknowledged. And there is no clear scientific definition, he thinks, because “harm is a judgement that only humans can make.” Most ecologists thus disregard 'harm' and describe invasive species as species that move into, and sustain themselves in, more or less natural habitats, which, according to Simberloff, is now widely accepted among ecologists and invasion biologists. The situation in Australia is even more complicated, commented Quentin Hart, project officer at the Australian government's Bureau of Resource Sciences in Kingston, ACT, Australia, because most legislation and management does not occur at the federal level but is left to the states and territories.
The definition of what is 'invasive' is important because it has repercussions for all other decisions: identifying an alien species as invasive, assessing the potential risks, and drafting strategies to eradicate or control the new immigrants. Consequently, a lot of
“Once a population has established, eradication chances are likely to be low or non-existent due to high costs, lack of political will and, for many species, the extreme difficulty of the task”
research is needed just to define risk assessment procedures that quantify and qualify the potential harm posed by alien species. The outcome of such risk assessments then forms a basis for deciding whether to prohibit the introduction of a certain plant or animal, or whether to eradicate it once it has arrived. Current models are based on a threestep process for explaining invasion: arrival, establishment and spread into other habitats. Species usually arrive at a new location either intentionally, by import, or unintentionally, by trade or travel. The newcomers first have to survive in the new environment and establish a sound population base before they can spread into other areas, which explains why many introduced species show a lag period between arrival and invasion. But even if a species spreads, it does not necessarily pose a threat to native fauna and flora, but may peacefully coexist with resident species in the new environment. According to Lodge, ecologists in the USA use a 'trade-based' approach that qualifies and quantifies the various factors for each step of the invasion process. The aim is to forecast whether a newcomer will spread and cause harm, but this process clearly cannot take too long. “Once a population has established, eradication chances are likely to be low or non-existent due to high costs, lack of political will and, for many species, the extreme difficulty of the task,” Hart commented. “It is therefore critical that there are contingency plans and associated resources for early detection and eradication of newly escaped individuals or small localised populations that become established.” But, given the speed with which animals in particular can spread, it is equally important to be aware of the risks even before they arrive. “It's just so irreversible [...] that it makes prevention of so much greater value than for any other forms of pollution,” Lodge said, pointing out that it is now possible to reverse most kinds of chemical but not biological pollution (Table 1).
Table 1.
Invasive species in Australia, Europe and the USA
| Species (common name) | Type and habitat | Area invaded |
|---|---|---|
| Achatina fulica (giant African snail) | Land invertebrate | USA |
| Anoplolepis gracilipes (yellow crazy ant) | Insect | Australia, Europe, USA |
| Anoplophora glabripennis (Asian longhorned beetle) | Insect | Europe, USA |
| Ardisia elliptica (shoebutton ardisia) | Land plant | Australia, USA |
| Arundo donax (bamboo reed) | Land plant | USA |
| Bufo marinus (cane toad) | Amphibian | Australia |
| Carcinus maenas (European shore crab) | Aquatic invertebrate | USA |
| Caulerpa taxifolia (caulerpa) | Aquatic plant | Europe |
| Chromolaena ordorata (bitter bush) | Land plant | Australia, Europe, USA |
| Cercopagus pengoi (fishhook waterflea) | Aquatic invertebrate | Europe, USA |
| Cinchona pubescens (quinine) | Land plant | USA |
| Clidemia hirta (soapbush) | Land plant | Europe, USA |
| Diabrotica virgifera (Western corn rootworm beetle) | Insect | Europe |
| Dreissena polymorpha (zebra mussel) | Aquatic invertebrate | Europe, USA |
| Eriocheir sinensis (Chinese mitten crab) | Aquatic invertebrate | Europe, USA |
| Euglandina rosea (cannibal snail) | Land invertebrate | Europe, Hawaii |
| Fallopia japonica (Japanese bamboo) | Land plant | Australia, Europe, USA |
| Heracleum mantegazzianum (giant hogweed) | Land plant | Europe |
| Herpestes javanicus (small Indian mongoose) | Mammal | Oceania, USA |
| Imperata cylindrical (speargrass) | Land plant | USA |
| Lantana camara (flowered sage) | Land plant | Australia, Europe, USA |
| Leptinotarsa decemlineata (Colorado potato beetle) | Insect | Europe |
| Leucaena leucocephala (wild tamarind) | Land plant | Australia, USA |
| Linepithema humile (Argentine ant) | Insect | Australia, Europe, USA |
| Lymantria dispar (gipsy moth) | Insect | USA |
| Miconia calvescens (velvet tree) | Land plant | Europe, USA |
| Mikania micrantha (Chinese creeper) | Land plant | Australia, Europe |
| Mimosa pigra (catclaw mimosa) | Land plant | Australia, Europe |
| Myocastor coypus (nutria) | Mammal | Europe |
| Oryctolagtus cuniculus (rabbit) | Mammal | Australia |
| Pheidole megacephala (big-headed ant) | Insect | Australia, USA |
| Pinus pinaster (cluster pine) | Land plant | Australia |
| Plasmodium relictum (avian malaria) | Microorganism | Hawaii |
| Pomacea canaliculata (apple snail) | Aquatic invertebrate | Europe, USA |
| Psidium cattleianum (Chinese guava) | Land plant | Australia, USA |
| Pueraria Montana var. lobata (kudzu) | Land plant | UK, USA |
| Rattus rattus (rat) | Mammal | Australia |
| Schinus terebinthifolius (Brazilian pepper tree) | Land plant | USA |
| Sciurus carolinensis (grey squirrel) | Mammal | Europe |
| Solenopsis geminata (tropical fire ant) | Insect | Australia, Europe, USA |
| Solenopsis invicta (red imported fire ant) | Insect | Australia, USA |
| Wasmannia auropunctata (little red fire ant) | Insect | USA |
| West Nile virus | Virus | USA |
Source: Invasive Species Specialist Group's Global Invasive Species Database (www.issg.org/database/welcome)
To deal with invasive species, Simberloff makes a distinction between eradication of every individual and 'maintenance management' of populations of invasive species at levels that are not problematic. He identified four general approaches to maintenance: mechanical, “by pulling out plants or shooting animals”; chemical; biological; and ecosystem management. “But [there is] no one answer,” he said, and “one has to think carefully [and] has to look at the specifics of the case.” The chosen approach is not always effective and sometimes even backfires, particularly in the case of biological control. The USA introduced various species to control the Russian wheat aphid (Diuraphis noxia) with no effect. For example, the seven-spot lady beetle (Coccinella 7-punctata), introduced from Eurasia, did not control the aphids but competed with, and decreased, the population of several native lady beetle species. The cane toad (Bufo marinus), introduced to Australia in 1935 to control frenchi and greyback beetles in sugarcane fields, has turned into a major pest for the local fauna and may soon reach the Kimberley National Park in Northwest Australia. Thus, Simberloff favours a response that is as early and as brutal as possible, which he describes as “shoot first and ask questions later” (Conservation Biology, 17, 83–92; 2003). “My point is, in an early invasion [...] even unsophisticated approaches can deal with that, but it's more difficult when the invasive species is established,” he explained. Some successes have proven him right. After the Caribbean blackstriped mussel (Mytilopsis sallei) was discovered in 1999 in Darwin Harbour in Australia, the Australians reacted immediately and eradicated the non-native species by killing every living organism in the bay with 160,000 litres of liquid bleach and 6,000 tons of CuSO4.
However, eradication is not always met with unequivocal support. Many New Yorkers expressed concern about their neighbourhoods being sprayed with pesticides to control the West Nile virus. Furthermore, eradication efforts are often resisted for economic or aesthetic reasons—the culling of feral horses in Australia's Guy Fawkes' River National Park in New South Wales in 2000 provoked sharp criticism from local residents. Conversely, some species may not be worth the effort, despite public demand—since raccoons (Procyon lotor; Fig. 3) set foot in Central Europe they have become a major nuisance for homeowners when the animals rummage through their rubbish at night, but the German government does not yet consider them a threat to the environment or to the economy. Any eradication programme therefore has to muster broad public and governmental support, both Lodge and Simberloff stressed. “Of course, these are social issues,” Simberloff acknowledges, and “all the stake-holders should be represented in those forums that make decisions.” Science definitely has an important role, because it provides the biological data, but “this is simply not purely a scientific issue,” Lodge said.
Figure 3.
The raccoon Procyon lotor
The main obstacles to controlling invasive species are still economic interests, particularly agreements to ensure free trade and travel. Although clearly beneficial for the global economy, free trade creates the threat of new invasions. Jonathan Levine and Carla d'Antonio from the University of California in Los Angeles and Berkeley, CA, USA, respectively, investigated the relationship between trade volumes and invasive species to forecast new biological invasions. Their most careful estimate predicts that at least 115 new insect and 5 new plant species will reach the USA within the next two decades (Conservation Biology, 17, 322–326; 2003). “International trade will inevitably lead to a greater number of environmental disasters caused by invasive species,” Lodge commented. Most new species hitchhike through trade, for example, in the wooden pallets that carry the Asian longhorned beetle (Anoplophora glabripennis; Fig. 4), which has caused great concern in the USA that it will become a serious forest pest. Many species, imported as pets or for horticulture, later turn out to be invasive. Simberloff thus names trade agreements and organizations, such as the World Trade Organization (WTO), as the biggest problem for efforts to prevent and control invasive species. “They have been uniformly hostile to arguments based on the environment,” he said. But it is not the WTO alone; national agencies have an important role as well. US agencies that support eradication or prevention efforts, such as the Environmental Protection Agency or the Department of Agriculture, regularly find their efforts blocked by other departments, such as the Department of Commerce or the Trade and Development Agency. Species invasions are not a scientific or economic problem but a political one, Simberloff believes, therefore more public education and awareness about the various dangers is needed. This could be achieved by relating ecological damage to economic costs, Lodge noted. “We must extend our perception from the agriculture to the tremendously important function of ecosystems,” he said.
Figure 4.
The Asian longhorned beetle Anoplophora glabripennis
Although invasive species are a growing problem for every country, there seems to be greater awareness in the USA, Australia, New Zealand and South Africa. According to Lodge, these countries have seen more man-made invasions than Europe, but Europe certainly has its own problems. The Colorado potato beetle (Leptinotarsa decemlineata; Fig. 5) arrived in Western Europe during the First World War and quickly became a major pest for potato farmers. More recently, the American Western corn rootworm beetle (Diabrotica virgifera) hitched a ride to Europe and has since been munching through the continent's farmlands. The European Union banned the import of red-eared slider turtles (Trachemys scripta elegans; Fig. 6) from the USA amid fears that animals released from private aquariums could outcompete native fauna. But it is also a matter of public perception. The impact of invasive species has been more prominently highlighted in the USA and Australia, and there is greater concern about the health of pristine landscapes in these countries. “We in America put a higher value on wilderness than in Europe,” Lodge said, which is not necessarily due to the fact that Europeans are less concerned about the environment, but that they simply have less wilderness left. Furthermore, the 'cultural landscape' of Europe, shaped by humans over millennia, seems to be less vulnerable to alien invaders than the largely wild areas of North America and Australia.
Figure 5.
The Colorado potato beetle Leptinotarsa decemlineata
Figure 6.
The red-eared slider turtle Trachemys scripta elegans
However, some critics maintain that efforts to prevent or eradicate invasive species are not justified at all, as they represent a normal evolutionary process. Mark Sagoff, a philosopher at the Institute of Philosophy and Public Policy at the University of Maryland, MD, USA, believes that the whole concept of invasive species is based on “a logical flaw” and that efforts to control them are not justified. “From an ecological or economic perspective, we have no grounds that native species are less costly or damaging than non-native species,” he said. Potential harm to the environment is also not an argument for Sagoff. “If by biodiversity we mean the variety of the gene pool, then invasive species add to the gene pool,” he explained. “On that criteria, there is no question that invasive species add to biodiversity.” Sagoff attributes ecologists' concerns about invasive species, particularly in the USA, to American puritanism in the tradition of Jonathan Edwards or Henry David Thoreau, coupled with religious and aesthetic motives. He described this attitude as “Nature is God's creation and everything that man touches gets corrupted.” In addition, it serves as an argument for government agencies that “like bigger budgets. And they find any excuse to justify bigger budgets.” Although Sagoff stressed that he supports any measures to control or eradicate what he calls “black-listed” species—pests and pathogens that cause damage to health and agriculture—he thinks that the concept of invasive species does not justify intervention or money.
Their most careful estimate predicts that at least 115 new insect and 5 new plant species will reach the USA within the next two decades
But most ecologists do not subscribe to this view. “It's simply not true,” Simberloff criticized Sagoff's arguments. No one is trying to eliminate or control all introduced species, he commented, but only the minority of ∼10% that is invasive or potentially invasive. Furthermore, invasive species have already caused a global decrease in biodiversity, so any efforts to preserve local biodiversity are justified. “By and large, the biota has not been homogenized,” Simberloff said, but “if we give up, we're surely going to lose”. And the goal is not necessarily a reduction of non-native species, as Lodge pointed out, but a reduction in the damage they cause. The problem is not so much the question of whether to fight invasive species, but whether to educate the public and convince politicians and economists about the long-term dangers of unlimited trade in plants, animals and their associated diseases. “The technology and the science are sufficient to do a good job,” Simberloff said. “The impediments are political and economical.”