Beneath Santa Rosa's non-native grasslands are the remains of a rich cultural heritage. More than 600 archeological sites have been mapped. These include several associated with early human presence in North America. Chumash Indian villages and camps of early explorers and fur hunters are evident.

In the 1840s and 1850s, Santa Rosa was a cattle rancheria. After the cattle industry of old Spanish California collapsed in the 1860s, sheep were brought to Santa Rosa and soon became its economic mainstay. Sheep grazing continued into the early 1900s, but when the island was sold to Vail & Vickers Company in 1902, the sheep were removed and cattle reintroduced. Though the impacts of introduced grains, insects, sheep, pigs, deer, elk and cattle were severe, examples of Santa Rosa's native plant communities survive. These tend to be restricted to rocky canyons and upper slopes. Native plants include the tree poppy, island manzanita, and endemic sage. Native Island Oaks grow on protected slopes, and two groves of Torrey pine are visible near Becher's Bay.

More than 195 bird species are found on Santa Rosa. With its extensive grasslands, the island supports large populations of European starlings, horned-larks, meadowlarks, house finches and song sparrows. Shore birds and waterfowl favor the brackish habitat found on Santa Rosa's eastern tip. This marsh and the island's running streams provide habitat for tree frogs and Pacific slender salamanders.

Other terrestrial animals include the gopher snake, deer mouse, and two species of lizard. The island fox may be frequently seen. The endemic spotted skunk--found only on Santa Rosa and Santa Cruz Islands--is only rarely observed. Among the islands extinct terrestrial mammals is the pygmy mammoth. In 1994 a nearly complete pygmy mammoth skeleton was excavated from a dune.

High mountains with deeply cut canyons give way to gentle rolling hills and flat marine terraces. Vast grasslands blanket about 85 percent of the island, yet columnar volcanic formations, extensive fossil beds, and highly colored hill slopes are visible. Rocky terraces on the west end provide superb habitat for intertidal organisms. Harbor and elephant seals breed on the island's sandy beaches. On the eastern tip of the island, a unique coastal marsh is among the most extensive freshwater habitats found on any of the Channel Islands. The entire island is surrounded by expanses of kelp beds. Consequently, its surrounding waters serve as an invaluable nursery for the sea life that feeds larger marine mammals and the sea birds that breed along the coastal shores and offshore rocks of all the Channel Islands.