The American crayfish, also known as signal crayfish, is invading and displacing native crayfish species around the world and becoming dominant. Reasons include:

- Prolific Reproduction: American crayfish reproduce rapidly and produce multiple offspring, allowing them to quickly establish large populations.

- Adaptability: They can survive in diverse habitats, including polluted waters with low oxygen levels, where native species struggle.

- Competitive Advantage: American crayfish are aggressive and can outcompete native species for resources, such as food and shelter, leading to declines in native crayfish populations.

- Disease Transmission: American crayfish carry a fungal pathogen called Aphanomyces astaci, which is fatal to many native crayfish species. This pathogen can spread quickly and cause epizootics, leading to population crashes of native crayfish.

- Human-Mediated Spread: American crayfish have been transported intentionally or unintentionally through the release of bait, the aquarium trade, and human activities, facilitating their spread to new regions.

These factors have contributed to the successful invasion and dominance of American crayfish in various ecosystems worldwide, displacing native crayfish species and altering ecological dynamics.