Alien species are common and widespread in fresh water
Comprehensive inventories of aliens have been attempted for just a few freshwater ecosystems (Mills et al., 1993, 1996a; Ricciardi, 2006), which contain tens to hundreds of alien species (Table 1). Comprehensive regional inventories of alien species in fresh waters also are scarce and incomplete (García-Berthou, Boix & Clavero, 2007; USGS, 2008; Gherardi et al., 2009), but support the idea that the world’s fresh waters have been heavily invaded (Table 2). Inventories of specific, well-studied parts of the biota, usually fishes (e.g., Whittier & Kincaid, 1999; Leprieur et al., 2008), are more common than comprehensive inventories, and confirm that aliens often constitute a large fraction of the species, individuals, or biomass of freshwater ecosystems (Fig. 1). Finally, range maps of well-known freshwater alien species (Fig. 2; see Table 3 for additional examples of widely distributed aliens) show that high-profile invaders now occupy countless sites beyond their original ranges. Very few freshwater sites are beyond the current or projected range of at least one high-profile invader.
Table 1. Numbers of known or suspected alien species in the Laurentian Great Lakes (Ricciardi, 2006) and Hudson River (updated from Mills et al., 1996a and Waldman et al., 2006) basins. Figures in parentheses are the percentage of species in the basin that are alien. Except perhaps for fishes and molluscs, the numbers of alien species probably are underestimated, sometimes severely
|Taxon||Great Lakes||Hudson River|
Table 2. Numbers of alien species recorded as established in the fresh waters of North America and Europe (Gherardiet al., 2009; USGS, 2008). For poorly studied taxa, the number of actual introductions may be substantially larger than the number recorded here
|Transplants within North America||Introductions from outside North America||Total||Transplants within Europe||Introductions from outside Europe||Total|
Figure 2. Potential range of the alien freshwater snail Potamopyrgus antipodarum in North America based on its existing range in North America (circles). Darker shades show areas where more models predict occurrence (Loo et al., 2007).
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Table 3. Characteristics of some important classes of alien species in fresh waters
|Group||Major impacts||Major vectors||Examples|
|Herbivorous molluscs||Reduction of biomass and production of edible primary producers, with consequent effects on composition and abundance of all biota, water chemistry, water clarity||Usually inadvertently introduced by ballast water, releases from aquaria and water gardens, and contamination||Dreissena spp., Corbicula spp., Limnoperna fortunei, Potamocorbula amurensis among the bivalves; Potamopyrgus antipodarum, Pomacea canaliculata among the snails|
|Fishes (and other vertebrates)||Loss of large, active prey, including native fish||Often deliberately stocked; releases from aquaria and bait buckets||Various salmonids, centrarchids, and cichlids; Cyprinus carpio Linneaus, Ctenopharyngodon idella, Hypophthalmichtys spp., Lates niloticus, Gambusia affinis (Baird & Girard), Petromyzon marinus, silurid and ictalurid catfishes, Rana catesbeiana Shaw|
|Aquatic plants||“Ecosystem engineering” effects on current, air-water-sediment exchanges and the amount of surfaces for chemical reactions and biotic attachment; changes in the amount and quality of primary production and detritus; effects ramify through ecosystem||Horticulture, releases from aquaria and water gardens, contamination||Alternanthera philoxeroides (Mart.) Griseb., Azolla spp., Egeria densa Planch., Eichhornia crassipes, Elodea spp., Hydrilla verticillata (L.f.) Royle, Lythrum salicaria, Myriophyllum spicatum, Phragmites australis, Pistia stratiotes Linneaus, Nasturtium officinale, Salvinia spp., Tamarix spp., Trapa natans, Typha spp.|
|Decapods||Loss of macrophytes, snails, and other benthic animals, with consequent effects on other parts of the food web||Deliberate stocking, bait bucket releases, ballast water (Eriocheir only)||Orconectes rusticus, O. limosus (Rafinesque), O. virilis (Hagen), Procambarus clarkii (Girard), Pacifastacus leniusculus (Dana), Eriocheir sinensis|
|Diseases||Loss of affected species, with consequent effects on ecosystem||Ballast water, contamination of stock||Chytridiomycosis, Aphanomyces astaci (crayfish plague), Myxobolus cerebralis (whirling disease), viral hemorrhagic septicaemia, various diseases of humans|
Alien species are a highly nonrandom subset of the freshwater biota
Although existing inventories of freshwater alien species are scarce and incomplete, it is clear that introduced species are a highly nonrandom subset of the freshwater biota (Fig. 3; see also García-Berthou et al., 2007; Gherardi et al., 2009). In particular, although insects dominate the world’s freshwater fauna, they are almost unrepresented in lists of alien species. Vertebrates and molluscs, on the other hand, are overrepresented among alien species.
There are several possible reasons for the very uneven representation of different animal groups among freshwater aliens. First, some groups may be undersampled by invasion ecologists (e.g., Demoor, 1992). Surely invasion ecologists have not adequately sampled the entire freshwater biota, and all published inventories must underestimate the actual number of alien species. For instance, no invasion ecologists have sampled for freshwater gastrotrichs, nor would they be likely to recognise an alien gastrotrich if they saw one. However, it seems likely that the same groups that have been overlooked by invasion ecologists have been overlooked by taxonomists. The mismatch shown in Fig. 3 would require that a taxonomic group have very different detection probabilities by taxonomists and invasion ecologists, which seems unlikely. Second, the pathways that transport alien species (described below) all are highly selective (cf. Hulme et al., 2008), which partially explains Fig. 3. For instance, humans have deliberately stocked many fishes but few chironomids, which (along with releases from aquaria, aquaculture, and bait buckets) accounts for the overrepresentation of vertebrates among alien species. Third, because different taxonomic groups are differentially susceptible to the barriers that set up differences between the biotas of different basins or continents in the first place, they should respond differentially to the breaching of those barriers. Generally, species that disperse poorly on their own but are readily moved by humans would be expected to respond most dramatically, which is consistent with the observed dominance of fishes and molluscs among invaders.
The non-random selection of invaders must apply to ecological traits as well as taxonomic composition, although this has not been well documented (but see Olden, Poff & Bestgen, 2006b; Statzner, Bonada & Dolédec, 2008). For instance, 45% of the alien freshwater fish species in the Hudson River basin are substantially piscivorous, compared with just 14% of the natives, so species introductions have greatly increased the number and distribution of piscivorous fish in the basin (Mills et al., 1996a), which may have had large ecological effects (see below). Consequently, the highly selective transport of alien species by humans changes the taxonomic and ecological character of the local and regional freshwater biota, as well as its size.
Alien species are moving between the world’s fresh waters by known vectors
Early inferential studies of the likely vectors by which alien species were transported (e.g., Mills et al., 1993, 1996a) have been supplemented recently by direct studies of the species moved by different vectors (e.g., Padilla & Williams, 2004; Duggan et al., 2005; Gertzen, Familiar & Leung, 2008). Consequently, we can identify the major vectors that transport alien species between the world’s fresh waters, as well as the kinds of species that are most likely to be transported by each vector (Table 4). Direct studies of vectors have great potential for improving procedures and policies to prevent the spread of alien species.
Table 4. Major vectors thought to transport freshwater alien species. Modified from Mills et al. (1993, 1997)
|Vector||Typical scale||Taxa typically transported|
|Stocking||Local to intercontinental||Sport fishes, forage animals (e.g., crayfishes, Mysis, small fishes), ornamental plants|
|Aquarium releases||Local to intercontinental||Ornamental fishes, invertebrates, and plants|
|Garden escapes||Local to intercontinental||Ornamental fishes, invertebrates, and plants|
|Bait bucket escapes||Local or interbasin||Bait fishes or crayfishes|
|Stocking contaminants||Local to intercontinental||Contaminants of stocks of sport, forage, or bait species, or of horticultural or aquarium stock|
|Escapes from commercial aquaculture||Local to intercontinental||Fish or large crustaceans grown in outdoor facilities|
|Ballast water||Local to intercontinental||Nekton, plankton, or species with free-living larvae or resting stages|
|Canals||Interbasin||All species, but especially motile or fouling species|
Primary consumers, especially molluscs
One of the most important classes of freshwater invaders includes molluscs that suspension-feed on phytoplankton and seston, graze on periphyton, or browse on vascular plants. These species can develop massive populations in all kinds of fresh waters, consuming so much primary production that they substantially affect the amount and composition of primary producers. Interactions radiating out from the primary producers can affect nearly every part of the ecosystem.
Possibly the best-known of these species is the zebra mussel (Dreissena polymorpha [Pallas]), a native of the Ponto-Caspian region that has been introduced widely into lakes and rivers in western Europe and North America. Populations of zebra mussels often are so large that they dominate heterotrophic biomass and clear large volumes of water. For instance, the population of zebra mussels that appeared in the Hudson River in 1991 usually has constituted >50% of all heterotrophic biomass in the river and had a growing-season clearance rate equal to 25–100% of the river’s volume each day (Strayer et al., 1999). As a result, phytoplankton biomass in the river fell by ∼80%, the pelagic part of the food web withered, and the littoral part of the food web flourished in response to increased water clarity (Fig. 4). Similar changes have been documented in other ecosystems invaded by zebra mussels (Strayer, 2009). Other suspension-feeding alien bivalves that have had large effects on freshwater ecosystems similar to those described for the zebra mussel include the quagga mussel (Dreissena bugensis Andrusov) from southeastern Europe, now spread widely through western Europe and North America (Mills et al., 1996b; Vanderploeg et al., 2002; Orlova et al., 2005); Corbicula fluminea (Muller) and possibly other species in this genus from east Asia, now widely distributed in North America, western Europe, and the Plata River system of South America (e.g., Cohen et al., 1984; Hakenkamp & Palmer, 1999; Sousa, Antunes & Guilhermino, 2008); Potamocorbula amurensis (Schrenck), also originally from east Asia but now in brackish waters in California (e.g., Alpine & Cloern, 1992; Kimmerer, 2002); and Limnoperna fortunei (Dunker), an Asian mytilid that is ecologically similar to the dreissenid mussels and which has developed large populations in the Plata system of South America (e.g., Ricciardi, 1998; Darrigran & Damborenea, 2005; Boltovskoy et al., 2006). All of these species are poised to spread around the world with careless global trade.
Other alien molluscs have had large effects on freshwater ecosystems through their consumption of benthic primary producers. The New Zealand mud snail (Potamopyrgus antipodarum [Gray]), which has invaded large areas of Australia, Europe, and North America (Loo, MacNally & Lake, 2007), can reach very high densities (>10 000/m2) in lakes and hydrologically stable streams. At such sites, this periphyton feeder can consume almost all algal production and dominate nutrient cycling (Hall, Tank & Dybdahl, 2003). These basal impacts must propagate to other parts of the ecosystem, but have not yet been fully investigated (but see Kerans et al., 2005; Riley, Dybdahl & Hall, 2008). Other snails that feed on periphyton have been introduced widely outside of their native ranges (e.g., Physa acuta Draparnaud, Bellamya [=Cipangopaludina] spp., Melanoides tuberculatus [Müller]), and may affect ecosystem functioning at least occasionally.
Herbivorous molluscs also have had large impacts on ecosystems into which they were introduced. The South American golden apple snail (Pomacea canaliculata [Lamarck]) has spread widely through southeastern Asia as an escape from aquaculture (Hayes et al., 2008). It reaches high population densities, and feeds voraciously on a wide range of aquatic plants (Carlsson, Brönmark & Hansson, 2004). Golden apple snails nearly eliminate macrophytes from the wetlands that they invade, causing concentrations of nutrients and phytoplankton to increase enormously. Again, impacts on other parts of the ecosystem, including economically valuable fisheries, seem likely to occur but have not yet been well documented. The effects of herbivorous molluscs like the golden apple snail thus cause a regime shift similar to that of severe eutrophication in shallow lakes (Carlsson et al., 2004).
In general, all of these molluscs graze down some primary producers severely. Their activities favour primary producers (if any) that can live in a given habitat but avoid being eaten (e.g., macrophytes or toxic cyanobacteria in the case of zebra mussels –Vanderploeg et al., 2001; phytoplankton in the case of the golden apple snail –Carlsson et al., 2004). The shift in the amount and quality of primary production usually raises concentrations of dissolved nutrients, and may produce large effects that ramify through the entire food web. The size and breadth of these effects arising from even a single mollusc species (Fig. 4) may rival or exceed those produced by any human stress on freshwater ecosystems.
Many species of fishes have been deliberately introduced around in world to provide food or sport. In addition to these deliberate introductions, a large number of fish species have been spread beyond their native range by releases from aquaria, bait buckets, and water gardens, as contaminants of fish intended for stocking, or in ballast water. Some of these fishes have had large ecological effects.
Especially in mountainous, glaciated terrain, many lakes, ponds, and small streams were naturally fishless (e.g. Knapp, Matthews & Sarnelle, 2001; Hesthagen & Sandlund, 2004; Schilling et al., 2008). The widespread introduction of fishes into these habitats brought large, active predators into highly vulnerable communities for the first time. As often is the case when a new functional group is introduced into an island community, the establishment of fishes into these formerly fishless habitats has had large effects on the behaviour, distribution, and abundance of native species, as well as ecosystem functioning (Simon & Townsend, 2003). The most obvious effects of fish introductions to formerly fishless sites include the near-disappearance of large, active prey species (Fig. 5) and behavioural changes in remaining prey species to avoid daytime use of microhabitats frequented by fish (reviewed by Simon & Townsend, 2003). Less direct changes to the community and ecosystem must be common and sometimes strong as well, as a result of nutrient excretion by fish and cascading effects from the loss of the most vulnerable prey species (Simon & Townsend, 2003).
Figure 5. Abundance (number per 15 standard sweeps, log10-transformed) of various kinds of benthic macroinvertebrates in lakes of the Sierra Nevada that never contained fish, that had been stocked but are now fishless, and that were stocked and still contain fish. Bars show means +1SE, NS = not significant (P > 0.05), *P < 0.05, **P < 0.01, ***P < 0.001, according to a pairwise Wilcoxon rank-sum test. From Knapp et al. (2001).
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Humans also often introduce fishes into fresh waters that already contain fish, either accidentally or in a deliberate attempt to improve the fish community. Again, the most obvious impacts have been losses of favoured prey species, especially in cases where the alien has no native trophic analogue in the system. Perhaps the most dramatic example is the global extinction of ∼200 species of cichlids from Lake Victoria following the invasion of the Nile perch (Lates niloticus [Linneaus]) (Lowe-McConnell, 1993), but many other examples exist, including the decline in lake trout (Salvelinus namaycush [Walbaum]) from the upper Great Lakes after the arrival of the sea lamprey (Petromyzon marinus Linneaus), the near-disappearance of galaxioids from Southern Hemisphere streams after salmonids were introduced (McDowall, 2006), and the decline or disappearance of cyprinids from American lakes after any of several large piscivores were introduced (Whittier, Halliwell & Paulsen, 1997; Findlay, Bert & Zheng, 2000).
Alien fishes that are not piscivores may also have large effects on their food. For example, introduced alien planktivores such as Alosa spp. and kokonee salmon (Oncorhynchus nerka [Walbaum]) can greatly alter zooplankton (Brooks & Dodson, 1965), introduced salmonids have had large direct and indirect effects on stream invertebrates (Flecker & Townsend, 1994; Simon & Townsend, 2003; Baxter et al., 2004), even when introduced to sites where fishes already live; and even tiny mosquitofish (Gambusia spp.) have reduced densities of native invertebrates and outcompeted native fishes (Pyke, 2008). Herbivorous alien species such as grass carp (Ctenopharyngodon idella [Valenciennes]) likewise can have strong effects on the amount and composition of aquatic vegetation (Bain, 1993; Cudmore & Mandrak, 2004; Pipalova, 2006).
As is now well appreciated, indirect effects of alien fishes can be propagated through the food web and affect many parts of the ecosystem. Thus, both alien piscivores and alien invertivores can have large effects on primary producers (Carpenter et al., 1987; Flecker & Townsend, 1994; Simon & Townsend, 2003) and exchanges with neighbouring ecosystems (Baxter et al., 2004), and bioturbation and nutrient excretion by alien fishes may alter light and nutrient availability (e.g., Vanni, 2002; Parkos, Santucci & Wahl, 2003; Simon & Townsend, 2003).
Thus, as was the case with alien molluscs, alien fishes have had large, far-reaching effects on almost all parts of freshwater ecosystems, both lentic and lotic. It is possible that these effects have been so dramatic because most fishes are able to disperse so poorly (if at all) on their own between continents and drainage basins that many sites support naturally depauperate fish faunas. Introductions of new species into such sites are therefore likely to bring in functionally distinctive species, which often have large ecological effects in insular ecosystems (Vitousek, 1990; Cox, 1999; Lockwood et al., 2007) such as remote lakes and drainage basins.
Countless aquatic plants (including macroalgae) have been introduced around the world, either deliberately because they were thought to be ornamental or otherwise desirable, or inadvertently as releases from aquaria or water gardens or contaminants of solid ballast or agricultural stock. Some of these plants have spread and flourished in the wild, and have had large ecological impacts. Important invaders cover all of the major guilds of aquatic plants, including riparian species (Tamarix spp.), emergent plants (Phragmites australis [Cav.] Trin. ex Steud, Typha spp., Lythrum salicaria Linneaus), submerged species (Elodea spp., Myriophyllum spicatum Linneaus), floating-leaved species (Trapa natans Linneaus), and floating plants (Azolla spp., Salvinia molesta Mitchell, Eichhornia crassipes [Mart.]). The most obvious impacts of hypersuccessful alien plants have been to outcompete or hybridise with native plants (e.g., Boylen, Eichler & Madsen, 1999; Ailstock, Norman & Bushmann, 2001; Moody & Les, 2007; but see Houlahan & Findlay, 2004), increase the amount of plant biomass and primary production (Fig. 6; Farnsworth & Ellis, 2001; Kelly & Hawes, 2005), and change the quality of that primary production. Increases in primary production alone can affect rates of many biogeochemical processes in the ecosystem. In addition, the nutrient content and physiology of the alien plant may differ greatly from that of the native plants that it replaces, causing changes in nutrient cycling (Wigand, Stevenson & Cornwell, 1997; Templer, Findlay & Wigand, 1998; Angeloni et al., 2006), rates of herbivory and decomposition, and consumer growth (Going & Dudley, 2008; Moline & Poff, 2008). Some alien plants (e.g., watercress, Nasturtium officinale Aiton) contain potent chemicals that prevent herbivores from using the alien as effectively as native plants (Newman, Kerfoot & Hanscom, 1996).
In addition, the high biomass and often-distinctive physical structure of alien plants frequently cause strong and varied engineering effects (in the sense of Jones, Lawton & Shachak, 1994). The high surface area provided by dense beds of alien plants offers colonisation space for epiphytic algae, invertebrates, and fishes, and can greatly increase the diversity and populations of these organisms (e.g., Strayer et al., 2003; Kelly & Hawes, 2005; Troutman, Rutherford & Kelso, 2007). However, alien plant species do not always support more animals than their native counterparts (Keast, 1984; Toft et al., 2003; Theel, Dibble & Madsen, 2008), although community composition of the fauna usually differs. Even in cases where alien species increase animal densities, their beds can be so dense that they inhibit foraging of predatory fishes (Valley & Bremigan, 2002; Theel & Dibble, 2008), preventing them from taking advantage of the high productivity of these beds. Likewise, the invasion of alien plant species can change wildlife use of the area (e.g., Benedict & Hepp, 2000; Maddox & Wiedenmann, 2005; Rybicki & Landwehr, 2007). Dense plant beds reduce current speed and increase water depth in running waters (Wilcock et al., 1999), prevent sediment resuspension (Huang, Han & Liu, 2007), trap suspended particles, and lead to greatly increased sedimentation rates (Rooth, Stevenson & Cornwell, 2003), and protect shorelines from erosion (Coops et al., 1996). The dense shade produced by stands of alien plants can inhibit understory species (Angeloni et al., 2006) and reduce temperature. In the case of floating or floating-leaved species, shading can cause hypoxia or anoxia in the underlying water (Thomas & Room, 1986; Caraco & Cole, 2002). Alien riparian plants like Tamarix spp. can colonise and stabilise floodplain soils, ultimately affecting channel morphology (e.g., Graf, 1978; Birken & Cooper, 2006). Thus, the establishment of even a single alien plant species can radically transform the entire character of an aquatic ecosystem, affecting nearly every aspect of ecosystem structure and function, and having effects that reach far beyond the boundaries of the plant bed itself.
The invasion of the Hudson River by the water-chestnut (Trapa natans) provides a good example of the strong, varied effects of a successful alien plant (Fig. 6). This plant was deliberately released into North America as an ornamental in the late 19th century, and appeared in the Hudson in the 1930s. By the 1950s, it was abundant and widespread in the river, forming large, nearly monospecific beds with biomasses of 100–1000 g DM m−2 (Hummel & Kiviat, 2004). These beds are c. 10× denser than those of the native water-celery (Vallisneria americana Michx.) that they replaced (Fig. 6). The combination of dense shade and high respiration in water-chestnut beds greatly reduces dissolved oxygen concentrations, leading to frequent and severe hypoxia or anoxia (Caraco & Cole, 2002; Goodwin, Caraco & Cole, 2008). Nitrate is greatly depleted in water-chestnut beds, through a combination of denitrification and plant uptake (Caraco & Cole, 2002). The invertebrate communities in water-chestnut beds are denser and have a different species composition than those of Vallisneria (Strayer et al., 2003). It is difficult to study fish use of water-chestnut beds (the dense canopy defeats most conventional sampling gear), but there at least strong hints (reviewed by Strayer, 2006) that it has altered littoral fish communities. In addition to these ecological effects, water-chestnut is regarded as a serious nuisance in the Hudson because its dense stands prevent recreational use of hundreds of hectares of shallow-water habitat and block access to the river channel from the shore.
Unlike other alien species in fresh waters, there have been many successful programs to control or locally eradicate alien plants using mechanical removal, herbicides, or biological control (McFadyen, 1998; Cuda et al., 2008).
More than 20 species of freshwater decapods (chiefly crayfish) have been introduced around the world for human food, fish forage, and bait (Hobbs, Jass & Huner, 1989). Decapods are adaptable omnivores that feed on algae, macrophytes, benthic invertebrates, fishes, and fish eggs. Alien crayfish species often reach high densities (>1 m−2, Bobeldyk & Lamberti, 2008), and so may have strong direct and indirect ecological impacts on several parts of the food web (Hobbs et al., 1989; Lodge et al., 2000; Gherardi, 2007b).
The rusty crayfish (Orconectes rusticus [Girard]) is one of the best-studied of the alien freshwater decapods. Native to parts of the American Midwest, it has spread widely to lakes and streams elsewhere in North America through bait-bucket releases and intentional stocking (Olden et al., 2006a,b). It is aggressive, and displaces or kills native crayfish (Klocker & Strayer, 2004; Olden et al., 2006a,b; and references therein). Because they are active, omnivorous, and often abundant, rusty crayfish and other crayfish species have strong and wide-ranging effects. Rusty crayfish can greatly reduce macrophyte biomass and species richness (Fig. 7, Lodge & Lorman, 1987; Lodge et al., 1994; Wilson et al., 2004; Rosenthal et al., 2006), which must in turn affect the animals that live among macrophytes (Wilson et al., 2004). They also decimate populations of snails (Lodge et al., 1994; Wilson et al., 2004) and possibly other molluscs (Klocker & Strayer, 2004), and change the abundance and community composition of other benthic macroinvertebrates (McCarthy et al., 2006). In addition to indirect effects on fish arising from their destruction of macrophytes, crayfish may be important predators of fish eggs (Dorn & Wojdak, 2004). Possibly as a result of diminished grazing by macroinvertebrates, periphyton biomass increases in at least some invaded sites (Bobeldyk & Lamberti, 2008). Crayfish also may increase rates of litter breakdown (Bobeldyk & Lamberti, 2008). Crayfish have been shown to increase rates of sediment suspension and transport (e.g., Statzner et al., 2000). Thus, alien crayfish are capable of large effects on several parts of freshwater ecosystems in streams and lake littoral zones. Indirect effects arising from macrophyte destruction must be especially important, but have not yet been fully investigated.
Figure 7. Response of macrophytes and gastropods in experimental enclosures containing the rusty crayfish (Orconectes rusticus) (black circles, solid lines) versus exclosures (open circles, dashed lines) run for 11 weeks over the summer in Plum Lake, Wisconsin. Graphs show means ±1SE; note that some y-axes are logarithmic. From data of Lodge et al. (1994).
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The Chinese mitten crab (Eriocheir sinensis Milne-Edwards) is another widely introduced decapod with strong ecological impacts. This catadromous species can migrate inland for several hundred km into rivers, creeks, and lakes. In addition to producing effects on the food web broadly similar to those produced by crayfishes (Rudnick & Resh, 2005), mitten crabs produce extensive burrow systems in some sites, which may cause erosion of muddy creek banks (Rudnick, Chan & Resh, 2005).
Alien diseases have not been as well-studied as other freshwater invaders, but several examples show that may have strong ecological effects. Perhaps the best-known is amphibian chytridiomycosis, which now appears to be responsible for breathtakingly dramatic declines of amphibians around the world (Lips et al., 2006, 2008): “the most spectacular loss of vertebrate biodiversity due to disease in recorded history” (Skerratt et al., 2007). Amphibians play important roles in small-stream and pond ecosystems, so the near-disappearance of once-abundant amphibians probably has important, varied ecological consequences (Whiles et al., 2006). Crayfish plague (Aphanomyces astaci Schikora) is a fungus that was introduced from North America into Europe in the 19th century (Edgerton et al., 2004). Crayfish native to Europe are highly susceptible to this disease, so many European populations have declined or disappeared. Because crayfish play such important roles in freshwater ecosystems (see above), these losses probably have led to other ecological changes (Matthews & Reynolds, 1992). Other diseases that may be ecologically important in fresh waters include whirling disease (Myxobolus cerebralis Hofer), probably originally from Europe, but now widespread around the world, and capable of killing large numbers of salmonids in hatcheries and possibly affecting wild populations (Bartholomew & Reno, 2002; Kerans & Zale, 2002); viral hemorrhagic septicaemia (VHS), which has appeared in several sites in eastern North America, presumably as a result of ballast water releases, and has caused large fish kills involving several species (Groocockl et al., 2007; Lumsden et al., 2007); an enigmatic infectious pathogen that was carried by the alien fish Pseudorasbora parva (Temminck & Schlegel) into Europe, where it now endangers the native Leucaspius delineatus (Heckel) and other cyprinids (Gozlan et al., 2005); and the Asian tapeworm Bothriocephalus acheilognathi Yamaguti, which has been widely introduced throughout the world with cultured fish, and which may harm many fish species, including endangered cyprinids in the American Southwest (Henja, 2009). Finally, several important human diseases associated with fresh waters have been moved outside their native ranges by humans (e.g., introduction of malaria, schistosomiasis, onchocerciasis, and lymphatic filariasis into the New World –Cox, 2002; Lammie et al., 2007) and have affected not only human populations, but human impacts on fresh waters.
Diseases constitute a much more heterogeneous group than the other classes of aliens just discussed: vectors carrying diseases are highly varied (e.g., ballast water for VHS, contaminated stock for crayfish plague and whirling disease, infected humans for our diseases), and the ecological effects of diseases depend entirely on which species are affected. Nevertheless, like the other classes of invaders, diseases have the potential to have strong effects on many aspects of freshwater ecosystems. Because non-human diseases have received so little attention, the effects of alien diseases on freshwater ecosystems probably have been underestimated.
Other aliens in fresh waters
There is not space to list all of the alien species in fresh water or describe their ecological effects in detail, but I will mention briefly some of the important freshwater invaders that do not fit neatly into my rough classification. Several predatory zooplankton (Mysis, Cercopagis, Bythotrephes) have been widely introduced and have strong effects on zooplankton that ramify to other parts of the ecosystem (e.g., Spencer, McClelland & Stanford, 1991; Yan & Pawson, 1997; Strecker & Arnott, 2008). Likewise, benthic amphipods (e.g., Dikerogammarus, Gammarus tigrinus Sexton, Echinogammarus ischnus [Stebbing], Corophium curvispinum Sars, Gammarus pulex [Linneaus]) have been widely established in Europe and North America outside of their native ranges, where they may affect at least macroinvertebrates and fish (e.g., Kinzler & Maier, 2003; Kelly & Dick, 2005; Berezina, 2007). Fur-bearing aquatic mammals such as beavers, muskrat, mink, and nutria, and ornamental waterfowl such as the mute swan (Cygnus olor [Gmelin], alien to North America) and the Canada goose (Branta canadensis [Linneaus], alien to Europe and New Zealand) have established many populations beyond their native range, and surely have affected many aspects of freshwater ecosystems.