Brownsea Island It is within the South Dorset constituency of the House of Commons and the South West England constituency of the European Parliament. Brownsea Island has built up on a bare sand and mud bank deposited in the shallow harbour. Ecological succession has taken place on the island to create topsoil able to support ecosystems. The nature reserve on the island is leased from the National Trust by Dorset Wildlife Trust. This reserve includes a brackish lagoon and area of woodland. Other ecosystems on the island include salt marsh, reedbed, two freshwater lakes, alder carr, coniferous woodland, deciduous woodland and arboretum.
Brownsea Island island has a heronry, in which both grey heron and little egret nest. There is a large population of non-native sika deer on the island. In the past the numbers have been higher than the island can sustain and have overgrazed. To try to limit damage to trees and other vegetation by deer, areas of the island have been fenced off to provide areas of undamaged woodland to allow other species such as red squirrels to thrive. The lagoon is noted for the large population of common tern and sandwich tern in summer, and a very large flock of avocets