Wetlands are among the most important ecosystems in the world. They produce high levels of oxygen, filter chemicals, reduce flooding and erosion, recharge groundwater, and provide critical habitat for plants and animals. They can be permanent or seasonal. Prominent wetland plants, such as cattails, reeds and tules, are effective biofilters, breaking down harmful pollutants. Wetlands provide a resting stop for migrating birds in the fall and spring, and support thousands of wintering birds in the City.
The 509-acre Lake Merced, located in the southwestern corner of San Francisco, is the largest freshwater wetland between Pt. Reyes in northern Marin County and Pescadero Marsh in southern San Mateo County. The lake provides critical habitat for more than 200 species of birds, and nesting habitat for 70 species when you include its surrounding uplands. A number of bird species that are considered locally vulnerable or threatened are found in this habitat, including the Common Yellowthroat, the Yellow Warbler and the Green Heron.
Lake Merced was once home to deer, elk and a number of other mammals, but today the only mammals commonly found around the lake are the California Vole and House Mouse. However, muskrat, striped skunk and Virginia opossum have been observed at Lake Merced. Historically, the Lake was valuable hunting territory for the local Ohlone villages, as well as a source of non-food necessities. The Ohlone people made boats, houses, baskets and sleeping mats from the tule rush at Lake Merced. Following the original poisoning of native fish species in the lake, more than 20 species of fish have been introduced or re-introduced to Lake Merced, providing food for humans, cormorants, herons, kingfishers and osprey. Reptiles and amphibians present at Lake Merced include the western skink, the northern alligator lizard, the introduced red-eared slider and soft-shell turtles, California slender salamander and the California red-legged frog.
Friends of Mountain Lake, the Golden Gate National Parks Conservatory, the Presidio Trust, the National Park Service, and the California Academy of Sciences are working to help restore Mountain Lake through toxics remediation and the removal of non-native species of plants, and planting native species. The 14.2 acre lake and its vegetated banks provide habitat for fish, birds, mammals, reptiles, amphibians and numerous invertebrates.