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22 (snapping, painted, slider), and snakes as well as other crayfish and humans. 109, 184 A few snakes, such as the Graham's Crayfish Snake (Regina grahami), specialize in crayfish as prey. 78, 86 Crayfish have been described as ecosystem engineers because their actions can alter their environment. The Devil crawfish got that designation through its construction of burrow systems. In the process of burrowing they are moving soil to the surface and mixing the upper soil layers. At the same time, the burrows increase the infiltration of water and nutrients. 190 They can alter the distribution of sand and gravel and, in the process, alter the structure of stream bottoms. 219 Crayfish burrows are used by many species besides crayfish as refugia. Several species of snakes have been documented to use crayfish burrows for winter hibernation. These include the Common Garter Snake, Thamnophis sirtalis sirtalis) and the Northern Water Snake, Nerodia sipedon sipedon) 23, 201 , the Massasauga, Sistrurus catenatus 152, 154 , the Diamondback Water Snake, Nerodia rhombifer 130 and the Copperbelly Water Snake, Nerodia erythrogaster neglecta 201 . Frogs like the Striped Chorus Frog, Pseudacris nigrita triseriata) 23 and Northern Cricket Frog, Acris crepitans 120 used crayfish burrows during winter hibernation. There are instances where the Massasauga, Sistrurus catenatus), used crayfish burrows to survive fires. 51, 154 The larvae of the endangered Hines Emerald dragonfly regularly used the burrows of the Devil crawfish as summer refugia. 190 . AS AN INVASIVE SPECIES Wildlife species are often being transported and introduced into new areas. The nature of the introduction can range from global to local. Many such introductions don't "take" and the introduced species dies out. Some introductions are relatively benign or even beneficial. A few cause problems and subsequently become a pest. These are the ones we call "invasive". Several crayfishes have fallen into this latter category. Potential sources of introductions include deliberate stockings, the bait trade, aquarium and pet hobbyists, aquaculture and schools. Impacts can be positive, neutral or negative or a combination of these based on different points of view. A crayfish farmer might consider an alien crayfish as a positive whereas a biologist would consider the same species, after it escapes the farm, a negative. Burrowing crayfishes can cause problems in golf courses, lawns, irrigation canals, flood control levees and earthen dams. Crayfishes feed on snails, insects, fish and fish eggs as well as aquatic plants. Nonnative crayfishes often reach very high densities and this, combined with their food habits, can change the food web of waterbodies. They can compete with native crayfishes directly (predation) or indirectly (competition for hiding spaces) causing the natives to decline. They can also carry diseases into new areas which has been a particular problem in Europe. 113 A possible positive impact has been seen in Africa where there are no native crayfishes. In this case, the alien Red Swamp Crayfish eats the snails that carry the Schistosomaisis parasite. At the same time and in the same areas, the crayfish interfere with fisheries