There are 15 species of bat which are considered native to Britain. Bats are known to be sensitive to changes in the environment, their numbers having declined significantly during the present century through a combination of loss of habitats, roost sites and food supply, and by increased frequency of disturbance. Bats are therefore believed to be useful indicators of environmental change. The method is based on that used in a Bats and Habitats Survey (Joint Nature Conservation Committee). It uses a tunable bat detector to record bats in 1 km squares, noting the positions along a pre-determined transect of all bats seen or heard. Habitat features along the transect, and changes in those features, are also recorded using a standard list