Additionally, the difficulty of sampling in marine and freshwater environments means that a species may be well established, and may have spread from its initial site of introduction, before it is recorded.
The shipping network creates connections among aquatic ecosystems across the globe, and organisms are frequently transported in the ballast water of ships, or attached to hulls as fouling organisms [ 28 ]. Ballast water is taken on to increase a vessel's weight when it is not fully laden with cargo. As this water is taken on, any organisms in the water are also sucked in. Vessels then travel to subsequent ports, and surviving organisms can be discharged with ballast water if the vessel takes on more cargo.
The opening of canals that link previously isolated water bodies has created many opportunities for the introduction and spread of non-native species. Canals have also had a profound impact on the establishment and spread of non-native freshwater species in Europe, and this impact is tightly linked to shipping. There are now river and canal connections running from the Black Sea across Europe to the mouth of the Rhine River, and north to the Baltic Sea [ 38 ].
These connections have served as invasion corridors for many species native to the Ponto-Caspian into Western and Northern Europe.
In addition, many of the species that arrived through shipping see previous paragraph could only do so because of the existence of canals. Stocking has been largely of fish to create new wild populations, while aquaculture introductions have arisen from the unintended escape of farmed species and their associated organisms e.
The final pathways mentioned here are the trades in ornamental predominantly aquarium and watergarden and aquaculture species.
Ornamental introductions also appear to be by far the dominant pathway for introduction of aquatic plants. For example, in Great Britain 22 of 31 established non-native freshwater plant species were introduced for the ornamental trade [ 8 ]. The aquaculture trade has unintentionally introduced a large number of non-native aquatic species as contaminants of intentionally introduced species such as fish or shellfish.
This is true for both marine and freshwater habitats. For example, the unintentional introduction and spread of the brown algae Sargassum muticum , the Japanease kelp Undaria pinnatifida , and the snail Ocinebrellus inornatus , as well as the oyster parasites Mytilicola orientalis and Myicola ostreae , all occurred because these species inadvertently arrived associated with marine shellfish imported from Asia to Europe for aquaculture [ 36 ].
The number of invasive species found in a region depends on the number of species that have been introduced, the proportion of introduced species that have become established, and the proportion of established species that have gone on to cause impacts.
When investigating differences among regions, invasion biologists have generally left aside the pathways and process of introduction and focused instead on the proportions of introduced species that become established, and of established species that become invasive [ 39 ]. Ecosystems where these proportions are high have been assumed to be highly invasible, while others have been deemed relatively resistant. Different theories have been proposed to explain why some regions appear more invasible than others.
Perhaps the most influential has been the biotic resistance hypothesis, an early champion of which was Charles Elton, often referred to as the scientist who founded the field of invasion biology e. This hypothesis holds that regions with high biodiversity and a relatively low level of disturbance, especially disturbance from humans, are more resistant to establishment by non-native species [ 42 ].
Although this is an intuitively appealing argument, there has been little empirical evidence generated to support it. In fact, especially at larger spatial scales, there is increasing evidence that highly diverse habitats are actually more prone to non-native species establishment e. Several authors have attempted to reconcile the contrasting theory and evidence, but no consensus has yet been reached e. A large obstacle to finding this consensus comes from the difficulty of quantitatively assessing levels of disturbance and the presence of vacant niches.
As well as trying to reconcile theory and observed patterns in species establishment, ecologists are now paying more attention to the introduction process. Recent results show that the number of species introduced to a region may be at least as important as the invasibility of the region in determining how many species become established [ 48 — 52 ]. This is discussed further in the following sections. There is a particular lack of support for the biotic resistance hypothesis when terrestrial animals are considered see [ 52 ] and references therein.
It has become clear in recent years that the key difference among regions with different numbers of established terrestrial animals is the number of species that have been introduced. But which regions are the ones that receive more introductions than others? The answer is that regions with high human impact typically receive more species introductions than other regions, and that this leads to them containing more established species.
For example, 12 non-native mammal species have established in France, nine in Germany, but just two in Portugal. Contrary to what would be predicted from the biotic resistance hypothesis, it is not easier for introduced mammals to establish in countries with high human impact, but these countries host more non-native mammals than other countries because they have received more species introductions [ 52 ]. Differences among the number of established birds in European countries can also be best explained by differences in number of introduced species [ 51 ].
Patterns of plant invasion in Europe offer little support for the biotic resistance hypothesis e. Instead, apart from broad habitat type, the number of species introduced and their propagule pressure appear to be the most important determinants of the number of established non-native species in any given region e. The most invaded habitats in Europe are in heavily transformed landscapes such as agricultural land, coniferous forests, urban areas, and dump or construction sites [ 57 ].
In contrast, natural and semi-natural environments such as broad leaved and mixed forests, pastures, natural grasslands, moors, heathlands and peatbogs have remained relatively uninvaded [ 57 ]. This pattern is consistent with that observed for terrestrial animals - that sites experiencing high levels of human disturbance and high propagule pressure tend to be the most invaded. Disturbance increases plant invasions because it leads to loss of native species that could compete with introduced non-native species, and because it increases availability of resources [ 58 ].
High propagule pressure occurs in the same regions because human activities lead to many plant introductions [ 59 ]. The highest proportions of established terrestrial plant species in Europe occur in agricultural landscapes, especially in eastern Britain, northern France, Central and Eastern Europe, and the Po floodplain in Italy. In contrast to a global pattern of Mediterranean-type ecosystems being highly invaded, the European Mediterranean biogeographic region is relatively uninvaded, probably due to a long history of human presence and prehistoric introductions in the Mediterranean Basin, which may make its ecosystems relatively resistant against recently introduced species [ 57 ].
Additionally, the Mediterranean Basin acted as more of a donor than recipient region for species introductions during the colonization of the New World [ 60 ]. It has been argued that harsh environments e. However, these are also often the habitats that experience low propagule pressure [ 61 ]. Hence, it is clear that the intensity of human activities that increase or facilitate propagule pressure, pathways of introduction, intensity of disturbance, and eutrophication, are important determinants of non-native plant invasions.
For many taxa in Europe, it is even more important than climate or other features of the physical environment [ 62 ]. European aquatic ecosystems containing the highest numbers of non-native species tend to be those with high connectivity to other ecosystems, high frequency of human access e.
These include boat harbours, recreational areas at lakes jetties etc. More remote water bodies, including mountain lakes and headwater streams, tend to be least and last invaded. Thus, propagule pressure can largely explain the intensity and diversity of established non-native species in aquatic environments [ 3 , 20 ].
In marine ecosystems, the number and frequency of pathways, tidal movements, availability of empty niches, and availability of different substrate types for settlement are the main factors that determine susceptibility to invasion, with highest rates of non-native species establishment typically found in shallow coastal zones [ 36 ]. Consequently, marine ecosystems with high numbers of established species in Europe include the eastern Mediterranean with hundreds of introductions through the Suez Canal [ 63 ], as well as the Gulf of Finland, the Gulf of Riga, the coastal lagoons [ 64 — 66 ], and the Oosterschelde estuary [ 67 ].
Of the non-native multicellular animal species recorded from European seas, have been found in the Mediterranean, along the Atlantic coast Norway to the Azores, including the UK and Ireland , and 62 in the Baltic Sea [ 36 ]. Numbers in the Mediterranean are highest because of the Suez Canal, the role of the Mediterranean as a long-time hub of international shipping, and a surge in development of mariculture [ 36 ]. An alternative perspective comes from asking whether there are traits of non-native species that are associated with successful passage through the invasion process.
Ecologists have been asking this question for several decades e. This work has recently become more important because many nations, including several in Europe and the European Union, have begun to develop risk assessment programs for non-native species [ 71 — 75 ].
The development of risk assessment tools begins with the search for patterns in species traits that are associated with successful passage through the invasion process. In this way, knowledge of non-native species traits can support pro-active efforts to prevent new invasions.
Although the search for traits of invasive species has been fruitful, recent results have shown that propagule pressure can be confounded with invasiveness [ 39 ]. 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 ]. Accidental introductions are usually the result of contaminated freight or movement of contaminated wood products including shipping pallets, bracing and other dunnage , plants, or food products. Individuals or propagules including its seeds, eggs, spores, or other biological material capable of propagation of these invasive species can be contaminants or hitchhikers in these shipments.
Are all exotic species invasive? No, actually only a small percent of introduced species ever become invasive. However, it is nearly impossible to predict which species will become invasive and new species are being introduced every day. Some species are present for many years before they exhibit invasive characteristics. What type of harm does an invasive species do? Since invasive species are in a new environment, free from natural predators, parasites, or competitors, they often develop large population sizes very rapidly.
These high populations can out-compete, displace or kill native species or can reduce wildlife food and habitat. Some also have the potential to disrupt vital ecosystem functions, such as water flow, nutrient cycling, or soil decomposition. Other invasive species cause massive amounts of economic damage to the agricultural business by destroying crops and contaminating produce. Some invasive species can cause direct harm to humans or domestic animals. Why are the species on this website of concern to North America?
Why don't our native plants have resistance to these exotic pests? Plants developed resistance to pests through their interaction with the pest over many generations. The plants that were resistant survived to pass their survival characteristics on to the next generation. While a small percentage of organisms transported to new environments become invasive, the negative impacts can be extensive and over time, these additions become substantial. A species introduction is usually vectored by human transportation and trade.
However, it must first subsist at low densities, when it may be difficult to find mates to reproduce. For a species to become invasive, it must successfully out-compete native organisms, spread through its new environment, increase in population density and harm ecosystems in its introduced range.
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