The Semidi Islands were designated part of the Refuge System in 1932 and the Simeonof Islands in 1958. Most of this unit is accessible by boat, but due to the rocky, rugged shoreline on most of the islands, access is difficult. This unit has a moderate polar maritime climate characterized by high winds, mild temperatures, cloud cover, and frequent precipitation. Fog and drizzle are frequent in summer and severe storms are possible year-round. Vegetation on the Sanak Islands consists primarily of lyme grass, as well as other grasses, and umbellifers. Vegetation on the Semidi Islands consists of maritime tundra communities. The Semidi Islands are extremely important to seabirds. About 2,400,000 birds nest there during summer, of which 1,730,000 are murres. About 370,000 horned puffins nest on the Semidis. Aleutian Canada geese (Branta canadensis leucopaceia), a threatened species, nests on two islands in the Semidi Island group.