The rusty crayfish is harmful to native species of crayfish because it clear cuts the bottom of waterways leaving native species without a food supply.

The above statement is true.

Rusty crayfish is harmful to native crayfish because they displace the native crayfish. They takeover the food and shelter of native crayfish making the native crayfish vulnerable to their predators.

Rusty crayfish also eat greedily on native aquatic plants. Reducing their numbers and diversity and robbing invertebrates of their food source.