Culm grassland on the formation's slates and shales is composed of purple moor grass and rush pasture. It is noted for a wide diversity of species, some extremely rare including the marsh fritillary butterfly. Some 92 percent of Culm grassland has been lost in the past 100 years, 48 percent being lost between 1984 and 1991 alone. There are a number of organisations trying to halt the decline including Devon Wildlife Trust with its Culm Natural Networks project, Butterfly Conservation, and Natural England with its Environmental Stewardship Scheme.