Why invasive species are a problem

Erosion releases sediment to streams, increases stream turbidity, and impairs water quality. Invasive plants provide less streamside cover and shade, which increases stream temperatures.

Invasive plants, such as Japanese knotweed or Himalayan blackberry, form monocultures areas entirely dominated by one species next to streams, which prevent tree establishment. Habitat loss and invasive plants are the leading cause of native biodiversity loss. Invasive plant species spread quickly and can displace native plants, prevent native plant growth, and create monocultures.

A healthy plant community has a variety of herbs, shrubs, and trees. Invasive plants cause biological pollution by reducing plant species diversity. Changes in plant community diversity reduce the quality and quantity of fish and wildlife habitat. Invasive plants are a leading cause of declines in native plant and animal numbers, and are a factor in Endangered Species Act listings.

Invasive plants outcompete and displace native plants that many native wildlife species depend on for food and cover. A variety of food and cover fosters more animal species and larger populations. In addition to displacing native species, invasive plant monocultures and simplified habitat often provide habitat for non-native wildlife. Non-native song sparrows, for example, often nest in Himalayan blackberry patches.

Increase soil erosion and fire hazard. Decrease the quality of understory habitat in forests and facilitate the spread of other invasive species. One example includes wild pigs in the Hawaiian forests.

Decrease the quality and amount of range for wildlife and range animals. Degrade aquatic habitats and clog waterways. Examples include: Giant salvinia, in the south, can quickly cover an entire water body. This prevents sunlight from getting to the aquatic plant and phytoplankton and preventing new oxygen from entering the waters. Decaying organic material then uses up the existing oxygen thus causing harm to organisms that need oxygen.

The salvinia also prevents many forms of aquatic recreation such as fishing and boating. Other plants that cause similar problems across the nation include alligatorweed, Brazilian elodea, water hyacinth, hydrilla, Eurasian water-milfoil, water lettuce, and Caulerpa taxifolia an invasive seaweed which was eradicated from San Diego, CA waters. Invasive Species Impacts to the Economy In addition to harming the natural world, invasive species also have serious effects on our economy.

Invasive species can: Cause reduced revenues to natural resource based businesses. Giant salvinia completely covers water bodies making it impossible to go fishing and allowing no space for waterfowl to land thus making hunting much less profitable. Aquaculture and rice production could also be severely impacted by giant salvinia. The sea lamprey and round goby have caused decreases in native fish populations in the Great Lakes.

The Indian mongoose damages some papaya and banana crops in Hawaii Honeybee mites can kill bees, thus damaging honey crops Affect boaters and fisherman by changing fish habitat and clogging waterways.

Hydrilla and water hyacinth are clogging waterways in the south. Giant salvinia may soon join in if it cannot be contained. Act as hosts for other damaging organisms. Buckthorn is an invasive shrub that also carries an oat rust that damages oat crops. Johnsongrass, an invasive grass native to the Mediterranean, harbors viruses that affect corn. Decrease the quality and quantity of rangeland. Leafy spurge and yellow starthistle are problems in the northwestern states. Decrease land values and cost the landowner time and money.

Many perennial weeds knapweed, leafy spurge are known to reduce production and thus ultimately reduce land values. Cause soil erosion. This is common when native plants with fibrous roots are replaced with invasive broad-leaved plants with taproots.

Cause damage and increased maintenance costs to power plants and industrial water systems. Zebra mussels, quagga mussels, and Asian clams. Aquatic nuisance plants like giant salvinia and water hyacinth. Have a negative impact of tourism. In Colorado, many invasive plants are replacing the beautiful blend of grasses and wildflowers in our refuges and parks that visitors come to photograph.

In Hawaii, the tourism and real estate industries are being affected by a combination of invasive species. The disturbingly loud call of a Puerto Rican frog known as the coqui is keeping tourists awake at night, while invasive seaweeds are damaging coral reef ecosystems and washing up on the beaches in huge amounts, causing foul odors as they decay. Invasive Species Impacts to Human Health Although most of the impacts caused by invasive species are to our ecology and economy, invasive species can also have severe impacts on human health.

Invasive species can: Serve as vectors carriers for human diseases The Asian tiger mosquito can carry the West Nile virus. The Indian Mongoose can carry rabies. At least one species of invasive snail carries schistosomiasis. Cholera has been found in the ballast water of ships. Be poisonous or caustic to humans. Approximately half of the poisonous plants in the eastern U.

African honey bees and fire ants can bite or sting. Examples include: the gypsy moth, nutria a muskrat-like rodent , exotic plants via the nursery trade such as kudzu and multi-flora rose, plants from the seed trade such as crabgrass and johnsongrass, escapes from aquaculture facilities, and the mongoose in Hawaii.

Examples of unwanted pets that get released into the wild include aquatic organisms dumped from unwanted aquariums and snakes and lizards that get too large for their owners to take care of. Q: What can I do about invasive species? Visit the Habitattitude Web Site for more information on how the pet and aquarium owner can prevent the spread of invasive species.

Aquatic recreationsists should always clean and drain their boats and gear before leaving a site. Aquatic invasive plants can get stuck on various parts of a boat, and aquatic organisms can be transported in water.

The seeds of invasive plants can easily get transported in mud and dirt. Always remember to clean the dirt off of your hiking boots or off of your vehicle before you leave an area. You could: Consult local natural resource guidebooks and field guides. Consult web pages on the Internet that can help one identify invasive species The Plants Database provides information about the plants of the U.

NatureServe Explorer is an authoritative online source for information on more than 65, plants, animals, and ecosystems of the United States and Canada. Explorer includes particularly in-depth coverage for rare and endangered species, but can also help one determine whether a species is native to a particular region. The Invasive Species Information Node has links to many identification resources. Try and locate some help at your local park, nature center, nursery, university, aquarium, or zoo.

Fish and Wildlife Service Office. These offices, part of the Cooperative State Research, Education, and Extension Service, provide answers to commonly encountered problems on topics such as: agriculture, animal and plant health, nutrition, and environmental issues. To report an aquatic invasive species by phone, the U.

Fish and Wildlife Service and the U. The National Council for Science and the Environment. Carlton, J. Introduced Species in U. Congress, Office of Technology Assessment. Westbrooks, R. Online version available. There are also many books and publications that can be purchased on the subject of invasive species: Bright, Chris.

Life Out of Bounds. Worldwatch Institute. Devine, Robert, S. National Geographic Society. Cox, George, W. Alien Species in North America and Hawaii. Island Press. Mooney, H. Invasive Species in a Changing World.

Simberloff, et. Williamson, M. Biological Invasions. Chapman and Hall. Consider these statistics: 92 to 98 percent of the linear miles of rivers and stream are so altered that they do not fit criteria for National Wild and Scenic Rivers or United States Geologic Survey Benchmark Streams. Only 42 Rivers across the United States still flow unimpeded by dams.

Approximately ten thousand non-indigenous aquatic species are currently present in U. At least major populations of salmon and steelhead trout on the west Coast have been extirpated, and an additional salmon, steelhead trout, and sea-run cutthroat trout stocks are at risk of extinction. As for the discussion above that focused on differences in invasibility among regions, invasion biologists have traditionally left aside introduction and focused on establishment and spread when looking for differences between the traits of invasive and non-invasive species.

Recent studies have challenged this by showing that those species most likely to establish are often those introduced in the highest numbers and most frequently. This does not mean that the search for traits of invasive species is not worthwhile, but does indicate that additional factors are important. Recent studies of the characteristics of invasive terrestrial animal species have shown that invasive species tend to have been introduced in higher numbers and more frequently than non-invasive species [ 69 , 78 ].

Mammals and birds that are hunted by humans are more frequently invasive than other species of mammals and birds because they have been more frequently introduced than other species, even though their establishment success is not higher than that of other species [ 69 ].

The same is true for mammals and birds with large native ranges which also become invasive more frequently than species with smaller native ranges. Their establishment success has not been shown to be higher than that of non-native species with smaller native ranges, meaning that this pattern is best explained by their increased frequency of introduction [ 69 , 79 ].

Terrestrial animals living in association with humans tend to become invasive more often than other species [ 69 ]. Good examples are the Norway rat Rattus norvegicus which reaches extremely high population densities in cities; the rose-ringed parakeet Psittacula krameri , native to Africa and Asia and also often very abundant in human settlements; and the harlequin ladybird Harmonia axyridis , native to Asia and infamous for its large aggregations in buildings during winter [ 6 ].

Thus, a clear understanding of human activities, both in terms of propagule pressure and the location of human settlements, is very important for understanding patterns of establishment, spread, and harm for non-native terrestrial animals. There are also species-level biological traits linked to terrestrial animal invasiveness. For example, behavioural flexibility as expressed by brain size was among the best predictors of invasiveness in a study of non-native birds [ 80 ].

Mammals and birds with high ecological flexibility indicated by the number of different types of food they consume or the number of different types of habitat they use also tend to be more invasive than other species [ 69 , 81 , 82 ]. Thus, species that are relatively more behaviourally and ecologically flexible tend to become invasive more often than other species.

Several factors are related to the invasion success of individual plant species. Second, residence time i. This is an effect of having the opportunity to fulfil more life cycles and also simply having the time to spread further.

The importance of residence time is also associated with propagule pressure, as species that were introduced a long time ago are likely to have been introduced many times since the first introduction. Third, species with larger native ranges are more likely to successfully establish beyond their native range. Similar to terrestrial animals, this is presumably associated with a higher probability that the species will be accidentally transported [ 85 ].

Additionally, species with a large native range are more likely to have a strong climate match to at least part of Europe, making them pre-adapted to survive there. Fourth, once terrestrial plant species have been introduced to the new range, traits of the species are important for determining whether they will successfully establish, spread, and cause harm. Traits known to promote passage through the invasion sequence include long flowering season, being an annual, vegetative spread, having multiple dispersal vectors [ 85 ], high maximum relative growth rate, and high resource allocation to shoots and leaves [ 86 , 87 ].

Many studies that have attempted to relate biological traits to invasiveness have explained little of the variation and have neglected trait interactions. Including interactions among traits i. Interestingly, long flowering season was beneficial for self-pollinated species, but was disadvantageous for wind pollinated species, and had no effect on insect-pollinated species.

Furthermore, the effect of timing of the end of the flowering season on invasion success differed among plant species with different vegetative reproduction strategies or different levels of ploidy number of chromosome sets in the cell. Thompson and Davis [ 89 ], however, argue that such analyses tell us very little because successful invaders do not differ in their traits from those of widespread native plant species.

Despite this critique, incorporating statistical interactions among traits should increase our knowledge of characteristics that make a species likely to expand or contract its range, whether non-native or native. As for terrestrial animals and plants, there are some general rules that separate non-native aquatic species that successfully pass through the invasion sequence from those that do not.

Some of the characteristics that influence invasion success are associated with biological traits whereas others are closely linked to interaction with humans.

For example, species introduced intentionally and hitchhiking species associated with them because they have desirable attributes tend to be more successful than undesired species. Prominent examples in marine environments include the introductions of alien shellfish species e. Crassostrea gigas introduced to France from Japan for mariculture, which have arrived with several associated parasites and algae.

Additionally, many of the most widespread non-native aquatic species in Europe are generalists that can tolerate a wide range of environmental conditions such as water temperature and salinity. European brackish water systems hold a great diversity of invaders which may be due to their poor native species richness [ 90 ] and the great ecological plasticity of the non-native species that have established. In addition to the breadth of ecological niches, similarity of environmental conditions in the donor and the receiving region can also be crucial [ 28 ].

For example, most of the non-native species in the Mediterranean are thermophilic and originated from tropical waters in the Indo-Pacific, the Indian Ocean, the Red Sea, and pan-tropical regions [ 36 ].

Differences in life history and reproduction can differentiate between invaders and rare species. This is evident in freshwater unionid mussels, which are among the most critically imperiled freshwater taxa both in North America and Europe [ 91 , 92 ]. These species produce glochidia larvae that need to attach to a suitable fish host to survive. The high degree of specialization and the complex life cycle of unionids probably contribute to the decline in this group.

In contrast, invasive mussels of the genus Dreissena are less specialized and produce free veliger larvae, allowing for a higher rate of dispersal through passive transport e.

Non-native marine species, which have been predominantly introduced to Europe through shipping, are also more likely to have larval stages that are tolerant of conditions in ships.

Reproduction rates tend to be higher in invasive aquatic species compared to those in most non-invasive species e. High reproduction can facilitate rapid spread and secondary introductions into other areas. Mode of feeding can also be important, with filter-feeding freshwater macroinvertebrates in Europe and North America known to be more successful at invading than predator macroinvertebrates [ 94 ].

This has the impact of enhancing energy flow between benthic i. For intentional introductions of fishes, where large predator species tend to be most popular, competition and top-down regulation may be more important. Overall, effects of non-native species tend to be greater when they establish in high abundance and have strong functional distinctiveness from native species [ 95 ].

At least several thousand non-native species are now established in Europe [ 6 ]. These include species not native to any part of Europe, as well as species native to one part but now established in another. The following sections give estimates of the number of species in different habitat categories that are established in Europe and not native to any part unless stated otherwise.

These figures should be seen as low estimates of the true numbers of established species because only recorded species are included; it is likely that many additional species are established but not yet recorded. According to the DAISIE database, there are 33 non-native established mammal species [ 31 ] and 77 established bird species in Europe [ 32 ].

These figures are probably quite accurate because these taxa are relatively large and easy to distinguish from native species. For the same reason, the estimate of 55 established reptiles and amphibians in Europe [ 32 ] is also probably quite accurate. In contrast, estimates for invertebrates are likely to be more severe underestimates because these species are more difficult to collect and identify. Within terrestrial invertebrates, data for insects tend to be more accurate than those for other invertebrates [ 33 ].

Terrestrial plants are generally well sampled, but it can be difficult to assess total numbers of established non-native species because the same species is often given different scientific names in different parts of Europe. According to the DAISIE database [ 10 ], 5, plant species have been recorded from the wild not necessarily established in at least one European country to which they are not native. These species come from families and 1, genera, and include 2, species not native to any European country.

A total of 3, plant species are known to be established in at least one European country to which they are not native, and 1, of these species are entirely non-native to Europe. We note that the numbers just given include all species recognized as non-native, irrespective of their date of introduction [ 97 ]. Traditionally, in those countries where records are available, botanists distinguish between species introduced before the European discovery of the Americas and those introduced later.

It is estimated that non-native multicellular animal species from marine environments and non-native freshwater animal species are now established in Europe [ 36 , 37 ]. These comprise a wide range of taxa, including fishes, arthropods, mollusks, platyhelminthes, and annelids.

For aquatic plants, it is estimated that at least species not native to any part of Europe are established in inland waterways [ 34 ]. The number and diversity of non-native species is variable across different regions of Europe. For example, in Great Britain the established non-native species in freshwater ecosystems are dominated by plants 31 , fishes 18 , non-decapod crustaceans 17 , platyhelminths 15 , and amphibians 11; full list in [ 8 ].

In Italy, the patterns are somewhat different, with the non-native species from inland aquatic systems being dominated by fishes 38 , non-decapod crustaceans 28 , and gastropods 7; full list in [ 98 ]. In each case, it is reasonable to expect that non-native species from groups such as fishes and crayfishes are better represented in the data than records of species that are smaller and less often sampled e.

Just seven non-native vascular plants have been identified in European marine ecosystems [ 34 ]. In contrast, numbers of non-native marine species i. The three main marine biogeographic regions of Europe are the Mediterranean, the Atlantic coast, and the Baltic Sea; these contain , , and 62 established non-native species, respectively [ 36 ].

These species cover a large taxonomic range, from fishes to barnacles to plants. Invasive species have a large and diverse range of impacts in Europe. This diversity of impacts is mainly driven by the diversity of species, and makes generalized statements about types of impact difficult. However, it is clear that invasive species have significant negative impacts on many native species and almost all ecosystems, on the European economy, and on human health recently reviewed by [ 7 ].

Economic impacts alone are estimated to be at least Economic effects include impacts on human infrastructure, human health, human social life, livestock, plant production, and forestry [ 7 , , ]. For example, Norway rats Rattus norvegicus predate many native species and have caused declines in native bird species and small mammal species. They are also a reservoir and vector of many diseases, including hepatitis E, leptospirosis, hantavirus, and Q fever.

Another example of an invasive competitor of a native species is the North American grey squirrel Sciurus carolinensis , Figure 1A that threatens the native red squirrel Sciurus vulgaris , especially in the U. The Canada goose Branta canadensis , Figure 1B is also an abundant invader. It hybridizes and competes with native geese, and its droppings can cause human health hazards and algal blooms. The Asian tiger mosquito Aedes albopictus competes with native mosquito species; its bites are a nuisance to humans, and it is a vector for diseases such as West Nile virus.

Another invasive terrestrial invertebrate with severe impacts is the harlequin ladybird Harmonia axyridis. Its tendency to overwinter in large aggregations inside buildings is a nuisance to many people, and the unpleasant odour of its body fluids can destroy the taste of wine. It also threatens native ladybirds and other European insect species [ 6 ]. Overall, invasive terrestrial invertebrate species cause costs of at least 1.

Examples of high-impact invasive species in Europe. Many invasive plant species in Europe are primarily recognized as agricultural or forestry weeds. Additionally, 17 out of the 18 plant species recorded among the most damaging invasive species in Europe [ 6 ] are known to reduce the habitat of native species [ 34 ]. Eight of them are reported to disrupt community assemblages, for example by impacting plant pollinator networks [ 34 ]. Non-native plant species can also hybridize with closely related native species so that distinctive genotypes of native plants are lost [ ].

Species such as Japanese knotweed Fallopia japonica and Himalayan balsam Impatiens glandulifera grow and are nuisance species along railway lines the former and waterways both. Other plant species can cause severe health problems. For example, giant hogweed Herracleum mantegazzianum , Figure 1C produces sap that causes skin lesions to humans upon contact [ ]. The pollen of invasive ragweed Ambrosia artemisiifolia , Figure 1E is highly allergenic to humans, and estimates of associated medical costs in Germany range between 17 and 47 million EUR annually [ ].

These produce annual costs for control and eradication in Spain of approximately 0. Overall, invasive terrestrial plants cause costs of at least 3. Invasive species are considered one of five major threats to aquatic biodiversity worldwide [ 4 ], with particularly large impacts on freshwater habitats [ , ].

The isolated nature of most freshwater habitats means that natural spread of aquatic organisms into new habitats occurs at low frequencies.

In turn, this means that aquatic communities tend to be more different to each other, and thus that the increased rates of species movement caused by human pathways have large potential for impacts on biodiversity. Traditionally, the study of invasions in aquatic ecosystems has had a strong focus on economically important and visible species, whereas invasive populations of small taxa e. As a larger and more visible species, North American signal crayfish Pacifastacus leniusculus , Figure 1F have been relatively well monitored and recorded.

They were introduced to Europe primarily for aquaculture, have spread rapidly, and are now considered one of the major threats to the indigenous crayfish fauna [ ]. In addition to their competitive behaviour [ ], North American crayfish are hosts of the crayfish plague Aphanomyces astaci , an oomycete fungus that causes a lethal disease to European crayfish [ ].

Also, the introduction of non-native salmonids and gobiids e. Neogobius melanostomus , Figure 1D has resulted in the decline and even extinction of indigenous species, and caused ecosystem shifts in lakes and streams [ 6 , ]. In economic terms, the zebra mussel Dreissena polymorpha , which can completely block cooling systems in hydropower plants, probably has the greatest impact of all freshwater invaders.

In marine habitats, negative effects of non-native species include declines in native species richness and abundance. These impacts have been associated with the invasion of Caulerpa taxifolia into the Mediterranean [ ], and with the high mortality rates of European oysters Ostrea edulis due to competition with introduced Pacific oysters Crassostrea gigas and damage from introduced parasites.

Despite these examples, there is little comprehensive evidence for most impacts of invasive marine species, and there are some examples of economic benefits. For example, the release of the red king crab Paralithodes camtschaticus into the Barents Sea and its southward spread along the Norwegian coast has provided an additional fishery and income for fishermen in the order of 9 million EUR per year [ 36 ]. Nonetheless, negative impacts of invasive aquatic species in Europe are high and have been estimated to cost at least 2.

Compared to other environmental problems, invasive species present at least six particular challenges. First, their impacts tend to increase over time as populations become larger and spread [ ]. In contrast, other environmentally damaging activities, such as the release of chemical pollutants, generally decrease in severity over time after the activity is ended.

This means that populations of invasive species can best be managed through rapid eradication of new populations [ ]. This is most feasible soon after a species becomes established, but the type of biological information needed to support eradication and the resources and political will to eradicate a harmful invader are generally not available until the species has spread and become invasive.

By this point there are fewer options to control the population, and the level of resources required, and the possibility for undesirable side effects on other ecosystem components, are usually prohibitive. Second, many other issues for environmental policy, such as forestry and mineral exploration, can be effectively managed with little concern for the policies followed by one's neighbours. In contrast, invasive species readily cross borders as long as there is suitable habitat on the other side [ 1 ].

This means that efforts to prevent invasions need to be mindful of the efforts being made in neighbouring jurisdictions [ 1 ]. As European economies become more integrated there are fewer barriers, such as border checks, to the spread of invasive species.

Third, invasive species often travel as contaminants of valuable trade. For example, most large ships carry ballast water to increase their weight when they are not fully loaded with cargo. This ballast is taken on at one port, along with any organisms in the water, and discharged at subsequent ports as cargo is loaded.

Unintentional transfer of organisms in ballast tanks is now the largest single cause of marine invasions [ ], and a strong factor in freshwater invasions [ 28 , ]. Reducing the spread of invasive species via this pathway would either require constraints on where ships travel, or the installation onto all ships of expensive ballast treatment technology. Either of these options would increase the cost of shipped goods.

Thus, control of invasive species often involves trade-offs with other activities, complicating decisions about how impacts can best be managed [ ]. Fourth, it is usually impossible to determine the exact conditions that will lead to an invasion. Risky activities, such as collecting ballast water in one port and dumping it in another, may occur many times before an invader becomes established. The invasion of the Laurentian Great Lakes by zebra mussels Dreissena polymorpha in the late s, for example, had been predicted for over a century before it actually occurred [ ].

These predictions were based on shipping connections between the Great Lakes and regions where zebra mussel was established, and on the fact that the species is well suited to Great Lakes conditions. Why it took so long for the invasion to occur and why it occurred when it did remains unknown. This general uncertainty about invasions complicates management because economic activities with definite and well-quantified immediate benefits must be balanced against risks from the stochastic invasion process [ ].

Fifth, controlling the spread of invasive species requires international cooperation, and this is often difficult to achieve [ 1 ]. This convention requires the establishment of ballast water management systems on ships, with the goal of preventing the movement of live organisms.

The dates at which each ship would be required to have ballast water treatment facilities differ based on ship size and age, but fall between and Despite this convention being finalized for several years and despite a widespread consensus that its implementation could reduce rates of invasion, it has not yet come into effect because too few countries have ratified it.

Many other pathways of species invasion could be best managed at an international level, but policies have rarely been negotiated and implemented at this level [ 1 ]. Finally, there is often a considerable time lag, occasionally as much as several centuries, between the introduction of non-native species and their spread. These time lags appear to be especially long for terrestrial plants [ 12 ]. In a recent paper, Essl et al. In their paper, Essl et al. This means that current activities in Europe may be setting in motion many future invasions.

Ecological studies of species invasions have increased in number and sophistication over the last several decades [ 19 ]. This has created a much clearer picture of the number of species that are established beyond their native range, and the types and severity of impacts that they cause.

Recognition of these impacts has led to increased calls for management and policy e. Although many nations now have policies to reduce the impacts of invasive species, invaders continue to arrive at an increasing rate, indicating that the results from ecological studies have not prompted sufficient changes in policy and management.

Concurrent with advances by ecologists, economists have taken an interest in the problems of invasive species.

Much of their interest centres on the trade-offs that must be made to manage invasive species [ ]. For example, economists might make the link between a beneficial trade route and the risks from invasions that it causes. They can then ask if and how the trade route can be restricted or modified to reduce risks from invasive species while maximizing overall societal welfare e.

Although ecologists and economists have often addressed similar concerns i. This stems in part from a lack of understanding between the disciplines. On the economist side, there is often a lack of willingness to work with ecological systems because they are so complex, in many cases are poorly understood even by ecologists, and because the relationships among ecosystem components may not fit into traditional economic models. As described above, the introduction of non-native species is a consequence of the globalization of trade and travel.

The benefits to society from globalization are large, as are the benefits from non-native species used in agriculture, horticulture, forestry, fisheries, landscaping, and the pet trade, etc. Thus, the benefits from policy that prevents and manages invasions must be balanced against the costs of the policy, including the costs of restricting trade. In practical terms, this requires a good understanding of both the ecological and economic dimensions of invasions, and requires that economists and ecologists work together to design appropriate policies.

It is generally accepted, and has often been shown, that the best way to reduce total impacts from invasive species is to prevent their introduction [ , , ]. For example, studies have shown that preventing the spread of an invader among lakes [ , ], and the spread of species among countries through trade [ ], is economically preferable to managing species after they arrive.

If prevention of all introductions is not feasible, efforts to reduce propagule pressure should be the goal as this may effectively prevent a proportion of invasions. Prevention is considered more effective than managing established invaders for at least three reasons. First, it is the only sure way to avoid impacts of invasive species, which as noted above are rarely eradicated.

Second, there are more management options available prior to invasion. These include a variety of laws and regulations, quarantine efforts, and the threat of punitive action against individuals or companies that do not comply. Finally, prevention efforts give the opportunity to shift the costs of reducing impacts to the trades that derive greatest benefits from the movement of species [ 1 ]. For example, although the pet trade has introduced many harmful invaders around the world, the exporters, importers, and retailers who derive greatest benefits from the trade have not been required to pay the costs.

Instead, those costs are generally borne by society as government agencies work to control invaders. Prior to invasion, however, it is possible to impose regulations that require the trades to ensure that they are not transporting potential invaders. Thus, the externality of trade can be internalized.

Post-establishment, the broad range of invasive species and the ecosystems they become established in, and the heterogeneous ways that humans value those ecosystems, means that few generalizations can be given for management and policy. That is, appropriate management and policy for harmful invasive species is highly context dependent.

Options include poisoning e. As the applications of these methods are context dependent, and because different countries within Europe have different standards for the application of these methods, they are not explored further here. The general principle of prevention still applies, however, and efforts to slow or eliminate population spread will often be the most cost-effective ways to reduce total impacts.

There is a large diversity of national approaches to invasive species within the EU see review in [ ]. These range from relatively advanced policies and procedures, such as in the Netherlands where there is a national policy framework for addressing invasive species, to nations that do relatively little to prevent the arrival and spread of invaders. Open internal borders, however, create a weak-link problem in the EU where even nations with the most stringent policies remain at risk because some of their neighbors and trading partners are doing relatively little [ 1 ].

At the international level there are several instruments that address the threats from invasive species. There is no history, however, of nations being sanctioned for not following the invasive species guidelines, and there is little evidence that the CBD has led to improved invasive species management at the global scale.

While these instruments have proven effective for restricting the spread of some invaders, they have most often been used to address diseases of livestock and pests of crops, with less attention paid to invasive species that cause primarily environmental harm [ ]. Within the framework of the IPPC is the European and Mediterranean Plant Protection Convention which develops regional measures to prevent the spread of invasive plants and plant pests for its 50 member nations i.

Its focus has also been largely on pests of agriculture.