Yerba Buena Island Natural History and Profile
Situated only a few miles between San Francisco and Oakland, Yerba Buena Island is a remarkable ecological resource, with noteworthy natural features including an intriguing pygmy grove of native coast live oaks shaped by unrelenting on-shore winds from the Golden Gate; a buckeye grove of large mature specimens; and a willow ‘mangrove’, accessible from a sandy beach, with tree branches hanging in salt water and covered with barnacles. Intact native habitats such as valley wild rye grassland, northern coastal scrub and tide pools used for haul out by harbor seals round out the remarkable ecology that still exists on Yerba Buena Island.
"View of San Francisco from Yerba Buena Island" ca. 1852
Copyright 2000 Cartography Associates. Used under Creative Commons license
Yerba Buena Island is approximately 150 acres in size. Historically, topography was broadly sloping from the Island’s summit at 338 feet above mean sea level, becoming steeper further from the summit. Current topography includes a series of terraces engineered for development beginning at the top of the Island, with steep slopes and cliffs down to the Bay on all sides.
YBI is thought to have been uplifted by faulting along a branch of the Hayward Fault approximately 1 million years ago. The Island is a rock outcrop comprised of greywacke sandstone interbedded with shale and siltstone of the Franciscan formation. Bedrock on the Island is covered by thin sandy deposits from the Pleistocene (over 20,000 years B.P.). The soils on YBI are mapped as a complex consisting of the Candlestick, Kron, and Buriburi series. The texture of these soils ranges from fine sandy loam to gravelly loam, reflecting the underlying bedrock.
Climate and Ecology
Marine influences exert a strong control on YBI’s climate, which is characterized by frequent nightly fog, which sometimes persists all day during the summer, and strong prevailing winds coming from the northwest through the Golden Gate. The Island supports several microclimates that influence the distribution of the plant species and assemblages present. Thin soils over rocky substrate support the pygmy oak stands on the wind-swept western part of the Island. Coastal scrub is found on steeper slopes. Taller oaks and toyons, along with lush fern grottoes are present on the shady north-facing slope above Clipper Cove, sheltered from the prevailing winds. Stands of willows have evolved at the base of sandy slopes where groundwater is present near the surface. Rocky outcrops on western exposures support coastal scrub species such as seaside woolly sunflower and live forever, or dudleya, a native succulent.
Native Plant Species
Yerba Buena Island is home to a variety of native plant species including the fiesta flower, Dutchman's Pipevine, Blue Dicks lily and California Hazelnut. The Island is also home to a multitude of defined vegetation communities, including:
- California Annual and Valley Wild-rye Grasslands
- Central Coast Riparian Scrub
- Northern Coastal Scrub
- California Buckeye Woodland
- Coast Live Oak Woodland
- Eucalyptus Woodland
While understanding and cataloging of the Island's wildlife species is an ongoing endeavor, surveys and observations to date indicate a diverse variety of fauna present on Yerba Buena Island. Species previously surveyed and understood to inhabit Yerba Buena Island, temporarily or permanently, include:
- Butterflies - including the Pipevine swallowtail, Monarch, Orange tortrix moth, Umber skipper and Rural Skipper
- Invertebrates - including Western fence lizard, San Francisco alligator lizard, and garter snake
- Birds - including Double-breasted cormorant, Anna's hummingbird, Wilson's warbler, Red-tailed hawk
- Mammals - including California ground squirrel, pocket gopher, raccoon, harbor seal