Obedska bara is a nature reserve in the province of Vojvodina in Serbia . It is a swamp and forest area , which extends along the Sava about 30 km west of Belgrade over an area of 175,000 hectares. It is also one of the oldest nature reserves in Serbia, founded in 1874 . There are over 220 species of birds , 50 species of mammals , 13 species of aquatic animals, 11 species of amphibians , 16 species of fish , 180 species of mushrooms and some species of moss . This nature reserve is therefore the leader in biodiversity in the Pannonian Basin . Because of these natural treasures Obedska bara is viewed internationally and was also the list of UNESCO - World Heritage nomination.