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 Castle another device in 1544 to further improve the country's defences, particularly along the south coast. The castle was built on the south-east corner of Brownsea Island between 1545 and 1547 to protect the entrance of the busy Poole Harbour. The island belonged to the Crown, having been confiscated from Cerne Abbey during the Dissolution of the Monasteries a few years before. It was a conservative design, being a one-storey, square blockhouse, reported in 1552 to be across, able to support guns on its roof and sub-divided into three rooms; the blockhouse was originally intended to have been two storeys in