South of the Ft. Funston native plant nursery in the Golden Gate National Recreation Area, the air is full of the warbles of robins, thrushes and white-crowned sparrows. The birds patrol the hillsides in search of insects among the lupines, lizard tail and coyote brush that crowd the landscape.
But turn around to face the flats directly to the north, and all is silent. Instead of a textured, silvery-green palate of native plants, the area is dominated by one species: ice plant.
"It looks green, but to wildlife it may as well be concrete," said George Durgerian, a park ranger at Ft. Funston.
Ice plant carpets California's coastal bluffs from Baja California to the Oregon border, where its yellow and pink flowers and fleshy green leaves are familiar features of the landscape.
In fact, ice plant is so familiar that for decades, many assumed it was a native California plant. But the springy succulent is not. Ice plant (Carpobrotus edulis) is an invasive species, which out-competes native flora. It is crowding out several threatened and endangered plant species and menacing the birds and small mammals that depend on those plants. It also alters soil chemistry and may hasten erosion in certain areas.
"It's a well-recorded threat to indigenous ecosystems," said Jake Sigg, chairman of the invasive exotics committee of the California Native Plant Society. "People concerned about saving biological diversity are worried about whole biological communities being invaded and displaced."
Ice plant, native to South Africa, is not the biggest threat to California's ecosystems. Other invaders, such as scotch broom and tamarisk, imperil native plants more directly and are harder to root out. But in places where ice plant has taken hold, particularly in sensitive ecosystems such as coastal dunes and prairie, it can devastate indigenous species.
"Within those coastal areas, it's a very serious problem," said botanist Peter Connors, reserve manager at the UC Davis Bodega Marine Laboratory. "It sends runners out over the surface and it will cover over all the surrounding grassland, and exclude everything else. It's almost a total loss of whatever plant community was there in the first place."
At Ft. Funston, rangers say that as recently as 70 years ago, the 250-acre area was covered by native plants. But when the area became an active Army base in mid-century, the Army plowed over the sand dunes and planted ice plant. By the late 1980s, it carpeted nearly all of Ft. Funston.
Now rangers and volunteers are removing it from large swaths of land and reseeding native plants. The need to eradicate was especially urgent on some of the coastal cliffs, which provide homes for the bank swallow, a threatened species in California.
This brown and white bird, North America's smallest swallow, nests on near-vertical banks and cliffs. At Ft. Funston, ice plant had overgrown most of the seaside cliff faces, disrupting the bank swallow's habitat.
Volunteers are also removing ice plant from the El Segundo dunes along the Santa Monica Bay. There, ice plant has displaced the once-abundant coast buckwheat, the primary food source for the endangered El Segundo blue butterfly. As the coast buckwheat population has dwindled, so have the butterflies.
Ice plant can crowd out sensitive plant species as well. The showy Indian clover, which produces bright purple-and-white flowers, was thought to be extinct until Peter Connors of the Bodega Marine Laboratory discovered one. Since then, a small population has grown in Marin County. But that would likely disappear if Connors and others didn't vigilantly remove the encroaching ice plant.
One reason ice plant is so insidious is that it changes the soil chemistry in ways that are still not fully understood.
"It's not entirely clear how persistent the changes are and how bad they are for restoration," said Carla D'Antonio, associate professor of integrated biology at UC Berkeley.
D'Antonio said ice plant makes the soil more acidic and reduces levels of calcium, magnesium and other minerals. This could make it more difficult to replace ice plant with native plants.
Ice plant also spreads quickly. Connors estimated that the circular patches spread at a rate of about 3 feet in radius each year.
Until recently this was considered a good thing.
Caltrans used to plant ice plant along highway and road embankments, partly because it was cheap and spread quickly.
In the 1930s, the California Conservation Corps planted it widely to control erosion. But on steep hill faces, ice plant can exacerbate erosion. As the plant covering builds, the weight of it can pull down whole sheets of topsoil, conservationists say.
"It builds up tremendous biomass, and the cliffs sometimes collapse under the weight," said Sigg, adding that native plants, with their variety of root systems, anchor the soil better. "These native plants have been here for thousands of years, and they've been doing very well, thank you."
No regulations prevent cultivating or selling ice plant, and it is available at many nurseries.
"It's blamed right now for being a terrible plant. We can't see the logic to that," said Jack Wick, regulatory consultant for the California Assn. of Nurserymen. "It doesn't make sense to stop selling something that's already scattered all over."
Wick said the association cooperates with coordinated efforts to eradicate certain species. The difficulty, he said, is that species like ice plant are problems only in certain areas, and should not be restricted statewide.
Meanwhile, removal efforts remain localized and relatively small. Most consist simply of volunteers and even schoolchildren yanking the plants out of the ground.
Ice plant pulls up relatively easily. Its runners can extend for yards and yards, but are anchored only by shallow roots and one large tap root at its base.
"It's fun. I like it," said Larisa Stephan of Dune & Prairie, a Los Angeles-based group working to restore the El Segundo Dunes. "You feel instant gratification. You can look at the square yards of cleared ground and the huge mound of ice plant that is about to be hauled away and feel that you really did something that day."