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Ecological roles

Being highly efficient predators, crocodilians tend to be top of the food chain in their watery environments. The nest mounds built by some species of crocodilian are used by other animals for their own purposes. American alligator mounds are used by turtles and snakes, both for basking and for laying their own eggs. The Florida red-bellied turtle specialises in this, and alligator mounds may have several clutches of turtle eggs developing alongside the owner's eggs. Alligators modify some wetland habitats in flat areas such as the Everglades by constructing small ponds known as "alligator holes". These create wetter or drier habitats for other organisms, such as plants, fish, invertebrates, amphibians, reptiles, and mammals. In the limestone depressions of cypress swamps, alligator holes tend to be large and deep. Those in marl prairies and rocky glades are usually small and shallow, while those in peat depressions of ridge and slough wetlands are more variable. Man-made holes do not appear to have as large an effect.

In the Amazon basin, when caimans became scarce as a result of overhunting in the mid-20th century, the number of local fish, such as the important arapaima (Arapaima gigas), also decreased. These are nutrient-poor waters, and the urine and faeces of the caimans may have increased primary production by contributing plant nutrients. Thus the presence of the reptiles could have benefited the fish stock; the number of crocodilians in a stretch of water appears to be correlated with the fish population.