Brownsea Island by third parties, is owned by the National Trust. Most of the buildings are situated near the small landing stage. The northern portion of the island is a Nature Reserve managed by Dorset Wildlife Trust and an important habitat for birds; this part of the island has limited public access. A small portion to the southeast of the island, along with Brownsea Castle, is leased to the John Lewis Partnership for use as a holiday hotel for staff, and is not open to the public. The island forms part of the Studland civil parish in the Purbeck local government district.
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