A global perspective on crayfish invasions

Crayfish, also known as “crawfish”, “mudbugs”, and “crawdads”, are freshwater crustaceans known more for their tasty tail meat than their destructive behaviors. In fact, crayfish are so well liked as a culinary item that two species indigenous to the United States have been introduced to Europe and Asia for aquaculture. Unfortunately, these non-native crayfish escaped the lakes and aquaculture ponds where they were introduced and are now widespread in places as far away as Italy and Japan. Non-native crayfish are now a major concern throughout the world because they threaten freshwater organisms ranging from aquatic plants to native crayfish and fish. A recent meta-analysis by the Olden lab draws from a large body of crayfish research to summarize the extent of damage that non-native crayfish cause to freshwater ecosystems.

Nearly all components of freshwater food webs, ranging from plants to insects to fish, are impacted negatively by non-native crayfish. We found that non-native crayfish cut and consume aquatic plants, reducing growth and survival at all life stages, from seedlings to adult plants. Crayfishes’ destructive behaviors directly reduce output of rice crops that grow in wetlands in Spain and Portugal where non-native Red Swamp Crayfish have been introduced. In addition, plant cutting can reduce survival of fish and insects that rely on aquatic plants for shelter from predation and food production.

Northern Crayfish (top), native to the North Central United States, and Red Swamp Crayfish (bottom), native to the Southeastern United States. Both have been introduced widely, and are aggressive and hardy species that threaten native crayfishes in areas where they were introduced.

Northern Crayfish (top), native to the North Central United States, and Red Swamp Crayfish (bottom), native to the Southeastern United States. Both have been introduced widely, and are aggressive and hardy species that threaten native crayfishes in areas where they were introduced.

Non-native crayfish consume large numbers of prey animals like snails and aquatic insects, which can limit prey availability for fish, amphibians, and native crayfish. Juvenile fish and amphibians are also preyed on directly by non-native crayfish. Together, competition and predation are responsible for declines in fish and amphibian populations in areas where crayfish have been introduced. Many fish and amphibian species are already threatened by pollution, habitat destruction, and global climate change; and non-native crayfish further undermine the viability of already vulnerable populations.

By contrast, we did not find that non-native crayfish significantly influence native crayfish through aggressive behaviors or competition. However, many European crayfish have been extirpated from their native habitats due to the contraction of fungal diseases from resistant North American crayfish. For example, North American Signal Crayfish (Pacifastacus leniusculus) have eradicated white-clawed crayfish (Austropotamobius pallipes) in England through transmission of crayfish plague. Therefore, despite our findings about aggression, we recommend that managers and individuals focus on preventing introductions of crayfish beyond their native range.

Notably, our research indicates that negative effects of non-native crayfish are consistent regardless of crayfish species. These consistent impacts are likely because crayfish are ecosystem engineers, animals that can dramatically modify the environment they inhabit. A well-known example of an ecosystem engineer is the beaver that transforms stream channels into shallow ponds by building dams. Crayfish engineer by removing aquatic plants, in turn releasing algae from competition with plants. Thus, crayfish transform clear, plant-dominated ecosystems into murky, algal-dominated ones. In addition, crayfish are omnivores that eat a wide variety of foods, from decaying animals and plants, to larval frogs and fish. These two mechanisms, ecosystem engineering and omnivory, are responsible for non-native crayfishes’ dramatic ecosystem-scale effects throughout their introduced range.

- Laura Twardochleb