There are 70 species of native ferns in the British Isles with some 30 hybrids between them. The diversity ranges from the large bracken, Pteridium, which can dominate vast tracts of hillside as grazing pressures decrease, through a wide range of rock-face or wall ferns, to the unique Pillwort, Pilularia globulifera, and the free-floating Water Fern, Azolla filiculoides.