Displacement and loss of native crayfish biodiversity
The aggressive behavior and rapid growth of invasive crayfishes make them considerable threats to native crayfish biodiversity. In fact, many studies have posed that introduced, non-indigenous crayfishes are the single greatest threat to global crayfish biodiversity (Lodge et al., 2000). In the Great Lakes Region, invasive crayfishes, such as the rusty crayfish, have displaced native species from large portions of their natural ranges (Momot, 1996; Taylor and Redmer, 1996; Olden et al., 2006).
Other aquatic invertebrates
The impacts invasive crayfishes can have on aquatic invertebrates go far beyond crayfishes themselves. Declines in non-crayfish aquatic macroinvertebrates have been correlated with the introduction and establishment of invasive crayfishes (Charlebois and Lamberti, 1996). Invasive crayfishes have also been shown to reduce overall macroinvertebrate species richness in stream ecosystems (Stenroth and Nyström, 2003). Further, a long-term study in Wisconsin showed a considerable decline in snail densities over a period of nearly two decades following the introduction of rusty crayfish (Faxonius rusticus). The same study also reported significant declines in the abundances of multiple insects, including dragonflies and caddisflies (Wilson et al., 2004).
Declines in native fish populations
Invasive crayfishes have been shown to adversely affect both native fish assemblages and recreational sport fisheries. Invasive crayfishes can cause declines in fish populations through a variety of different mechanisms. For example, invasive crayfishes can compete with native fishes for similar prey resources, and they can reduce the density of aquatic plants used by juvenile fish as cover (Wilson et al., 2004). There is also evidence that invasive crayfishes may directly affect the breeding success of some fishes by consuming their fertilized eggs (Dorn and Mittelbach, 2004).
Adverse effects on amphibian breeding
Invasive crayfishes have been associated with population declines in native amphibian species around the world. One mechanism through which this can occur is breeding interference. For example, the red swamp crayfish (Procambarus clarkii), a prevalent invader in the Great Lakes, has contributed to the decline in some amphibian species through consumption of fertilized amphibian eggs (Gamradt and Kats, 1996). In ecosystems where native crayfishes are present, P. clarkii can consume amphibian eggs at a higher rate than the native crayfish species (Renai and Gherardi, 2004). Invasive crayfishes can also greatly alter resource availability, potentially influencing amphibian growth and survival (Cruz et al., 2006).
Destruction of aquatic plants
Aquatic plants provide shelter for fishes, amphibians, and aquatic macroinvertebrates. Crayfish have been shown to reduce macrophyte biomass and species richness through both non-consumptive and consumptive grazing (Lodge and Lorman, 1987; Wilson et al., 2004; Rosenthal et al., 2006), with invasive crayfishes consuming aquatic plant material at a faster rate than some native crayfish species. Such changes can strongly affect ecosystem structure and function, potentially resulting in the decline or displacement of other freshwater taxa within the ecosystem.
Invasive crayfishes are also a potential vector for transmitting devastating diseases. For example, the ‘Crayfish Plague’, a devastating disease-causing population decline and range reductions in native European crayfishes, was introduced to Europe through invasive North American crayfishes (Lodge et al., 2000). While introduced diseases are not yet a major concern for native crayfishes in the Great Lakes, the potential for disease transmission still exists, especially from species introduced from outside North America.