Echinogammarus ischnus has a moderate environmental impact in the Great Lakes. Realized:
Following its initial establishment, Echinogammarus ischnus became one of the most abundant non-dreissenid benthic invertebrates in the Lake Ontario, Lake Michigan, and Lake Erie watersheds, where it locally displaced the native amphipod Gammarus fasciatus from many sites (Dermott et al. 1998; Stewart et al. 1998a,b; Nalepa et al. 2001; van Overdijk et al. 2003; Ratti and Barton 2003; Haynes et al. 2005; Limén et al. 2005). Differences in fecundity and generation time, which appear to favor G. fasciatus, are not sufficient to explain species replacement by E. ischnus (Dermott et al. 1998). Rather, it has been hypothesized that such displacement is due to intraguild predation and competition for resources (Witt et al. 1997; González and Burkhart 2004; Limén et al. 2005; Palmer and Ricciardi 2005; Kestrup and Ricciardi 2009b).
Studies in the St. Lawrence River have shown that E. ischnus and G. fasciatus are mutual (intraguild) predators. Echinogammarus ischnus is generally the superior predator of adult gammarids in waters of higher conductivity (Kestrup and Ricciardi 2009b), but this advantage is offset by G. fasciatus preying more efficiently on E. ischnus juveniles (Kestrup et al. 2011a). Research in central Europe also reports the invasive E. ischnus to be a stronger predator over native gammarids in cases of intraguild predation, suggesting that predation is a probable mechanism of species replacement (Kinzler and Maier 2006).
A mechanism for competitive exclusion of G. fasciatus by E. ischnus is less clear and may be influenced by total or relative amphipod densities (van Overdijk et al. 2003; Kestrup and Ricciardi 2009a) or by differences in the physical environment (Palmer and Ricciardi 2004). For instance, the initial replacement of G. fasciatus by E. ischnus occurred in primarily rocky and dreissenid-covered habitats, while G. fasciatus populations continued to persist on algal and macrophyte-covered substrates (Dermott et al. 1998; Duggan and Francoeur 2007). These two amphipod species may also differ in their responses to abiotic factors such as current velocity or pH, which could affect their relative fitness in different environments (Palmer and Ricciardi 2004). Echinogammarus ischnus typically numerically dominates high flow sites, and its abundance in the St. Lawrence River has been more positively correlated with current velocity than with any other physical attribute (Palmer and Ricciardi 2004). Kang et al. (2007) also encountered E. ischnus more frequently at high energy coastal sites throughout the Great Lakes.
It has been suggested that E. ischnus has potentially benefited from a co-evolved relationship with dreissenid mussels (Ricciardi and MacIsaac 2000). Available nutrition from mussel biodeposits, in combination with the structural complexity of Dreissena mussel substrate, may have given E. ischnus a competitive advantage, stimulating its population expansion in the lower Great Lakes (van Overdijk et al. 2003). However, at some sites, native amphipods have been found to consume more Dreissena pseudofeces than E. ischnus (González and Burkhart 2004). Furthermore, carbon isotopic composition data indicated that the diets of E. ischnus and native Great Lakes amphipod G. fasciatus differ, suggesting that competition for food is an unlikely mechanism of the species replacement (Limén et al. 2005).
It is possible that E. ischnus evades predators more easily than G. fasciatus, particularly on dreissenid-covered substrate (González and Burkhart 2004). In laboratory feeding trials, G. fasciatus was more heavily consumed by Yellow Perch (Perca flavescens) and Round Goby (Neogobius melanostomus) on dreissenid-covered substrate than E. ischnus, while E. ischnus was consumed more heavily on macrophyte beds (González and Burkhart 2004). In contrast, other studies have found no difference between the two species in their vulnerability to predation on dreissenid-covered substrate (Palmer and Ricciardi 2005; Kestrup and Ricciardi 2009a).
Interestingly, E. ischnus and G. fasciatus appear to coexist in the St. Lawrence River, where G. fasciatus remains the dominant amphipod, but E. ischnus remains abundant at some sites (Palmer and Ricciardi 2004, 2005; Kestrup and Ricciardi 2009a,b). Research has suggested that the use of mussel habitat in this location was equal between the two species, and that both species were equally as vulnerable to Round Goby predation, negating two possible mechanisms of species replacement (Kestrup and Ricciardi 2009a). Instead, coexistence in the St. Lawrence River may be the result of habitat heterogeneity, which is thought to allow the species to segregate along physicochemical gradients (Palmer and Ricciardi 2004; Kestrup and Ricciardi 2009b).
Potential:
This species has also been introduced to Western Europe and the Baltic Sea (Cristescu et al. 2004). In parts of Germany and Poland, it has reduced or replaced native gammarids (such as Gammarus fossarum, G. roeseli, and G. pulex) (Jazdzewski et al. 2004; Kinzler and Maier 2006). In Central European streams, introduced E. ischnus and Dikerogammarus villosus appear to have contributed to declines in certain native macroinvertebrates through predation, E. ischnus being a stronger predator than native gammarids (Krisp and Maier 2005). However it was deemed unlikely that macroinvertebrate predation by E. ischnus has a significant negative impact overall (Krisp and Maier 2005).
It is possible that with increasing colonization of Dreissena bugensis in deeper waters of the Great Lakes, E. ischnus may follow (Nalepa et al. 2001), though 2015 survey data collected from 140 stations in Lake Michigan did not indicate that E. ischnus occurred at deeper sites that contained D. bugensis (Nalepa et al. 2020). Additionally, canal systems have promoted the dispersal of this species throughout Europe and may aid its further dispersal in North America (Witt et al. 1997).
Echinogammarus ischnus has been found to host a parasitic water mold (oomycete) in the St. Lawrence river. This oomycete also parasitizes the Great Lakes native amphipod G. fasciatus, but the effects are less severe, potentially preventing E. ischnus from becoming dominant (Kestrup et al. 2011b).
There is little or no evidence to support that Echinogammarus ischnus has significant socio-economic impacts in the Great Lakes.
There is little or no evidence to support that Echinogammarus ischnus has significant beneficial effects in the Great Lakes.