But what makes the 2.8 acre rocky promontory unique is that it is a remnant of San Francisco's indigenous landscape, containing 60 species of native plants. The easiest access is a stairway at Twin Peaks and Clarendon. May. The site also exposes some beautiful Franciscan radiolarian chert rock outcrops, the city's oldest natural features, formed on the ocean floor 130 million years ago. Tank Hill is a fragmented natural habitat, meaning that it is an island of natives surrounded by an ocean of exotic plants. The greatest threats to this native plant heritage are the invasive exotic plants commonly called weeds.
Weeds invade when the ecological balance is altered.
Soil disturbance, extreme chemical change or erosion, elimination of the sunlight, or increased moisture can create a niche for a weed to thrive and spread. The hill's worst weed threat comes from non-native trees, mainly eucalyptus. Planted after the Pearl Harbor attack to conceal the water tank from Japanese planes, the trees now shade the natives, collect and drip moisture, drop tons of debris, and alter the soil composition. Cape ivy, sheep sorrel, oxalis, and erharta grass. Erosion allows weeds to attack native habitats rather like the way a wound on the body can become infected.