How non-native species are disrupting Sonoma County’s valuable ecosystem

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To some, they’re a little like invaders from space in a science fiction movie: nearly invisible, hard to detect, and with a mission to destroy. Many are already among us. And thousands more are approaching. They are invasive plant and animal species, and stopping them is a full-time job, particularly in places with a lot of agriculture at stake, like Wine Country.

But trying to manage, or at least slow them down, is a challenge, because, according to state and local authorities, most of the invaders are getting a free ride in: from humans.

Take the imported red fire ant, Solenopsis invicta. As anyone who’s met one can testify, it’s a notoriously bad actor. They pack a burning, painful sting, react aggressively if their nest is disturbed, and have the nasty habit of stinging and biting repeatedly in swarms after climbing your legs. Originally native to South America, imported red fire ants are now well-established in large colonies across many places in the southern United States. But they still haven’t turned up in Sonoma County and Northern California.

How long that lasts is an open question.

Last summer agricultural biologist David Jagdeo was on duty in Petaluma inspecting shipments when he opened an unmarked package from Texas on its way to a local pet store. Inside, he found 10 vials holding live ants, one with eggs and adults of the red fire ant, invicta (ants are the diet of choice for some reptiles, including the horned lizard).

That import was seized. But Sonoma County is under constant siege from a dizzying array of invasive plants, animals, insects and microbes.

And the problems they cause can be huge. Invasive species disrupt habitats, crowd out and endanger native plants and animals, choke waterways and deplete water resources, spread disease, and even increase fire risk. Insect pests brought into California from other states or countries can have devastating effects on citrus groves, grapevines and other economically important agricultural crops.

According to the Center for Invasive Species Research at UC Riverside, invasive pests already cost California as much as $3 billion a year.

Invasive species

People have been importing nonnative plants and animals into Sonoma County since the Spanish arrived in the 1700s, and clearly, not all are considered invasive. Many are welcome. The honey bee, for example, was transplanted from Europe because relatively few of California’s 4,000 native wild bees make honey or can be conveniently managed in hives.

Unfortunately, according to the California Native Plant Society, roughly 450 plants that were originally imported to California as ornamentals have “jumped the fence,” and are now rooted and competing in native habitats.

Pet fish, turtles, snakes and cats, released by their owners or escaped, are also now part of the invasive species problem in many regions.

To biologists and government agencies, invasive species are nonnative, aggressive colonizers that are quick to spread and hard to eradicate, and can have a substantial negative impact on other species or entire habitats.

Officially, the California Department of Fish and Wildlife, which manages the State’s Invasive Species Program, considers a species invasive if it threatens the diversity or abundance of native species. Martha Volkoff, the department’s program manager, points out that many invasives, from ice plant to oat grass to Norway rats, are now so widespread, they’re here to stay. The state’s efforts are largely concentrated on reducing their impact and further spread, and especially, keeping new aliens out. But that’s an ongoing challenge.

“The primary mechanism of introduction,” she says, “is human-mediated pathways.” Aliens are either intentionally or accidentally spread by car, truck or airplane, by shippers or travelers, in boat ballast water, soil or firewood, or in shipments intended for plant nurseries and pet shops.

Altering ecosystems

Public land managers are also engaged in efforts to control invasive species. According to Sonoma County Regional Parks Deputy Director Melanie Parker, since park resources are limited, they’re only able to target the worst of the offenders, those that may significantly alter the health of an ecosystem. Those are primarily species whose spread can change a complex ecosystem into a highly simple one.

Some aggressive invaders can turn a rich habitat with a diverse network of life into one entirely dominated by themselves.

A number of invading species are fairly nightmarish. The voracious American bullfrog, which eats anything it can fit in its wide mouth, including birds, fish, insects and other frogs, was first imported to California in 1898 to meet demand from hungry miners and settlers, who’d nearly wiped out the smaller native Red-Legged Frog.

Sonoma County’s initial bullfrog population was traced to a single dealer from New Orleans in 1925. Now well-established in the wild, the bullfrogs are steadily encroaching on what’s left of the endangered Red-Legged Frog habitat.

Then there’s the Brown Headed Cowbird. Native to the Great Plains, they once followed wandering bison herds, which scared up a rich diet of insects. Instead of building nests, they laid their eggs in the nests of other birds, who then unwittingly fed and raised them. To ensure they do, cowbirds often remove the host bird’s own eggs. Cowbird chicks tend to hatch early and grow large, so they usually crowd out any remaining nestmates.

Cowbirds happily followed when humans brought cattle to Sonoma County, and two native birds, the Least Bell’s Vireo and Willow Flycatcher, are currently endangered because of Brown Headed Cowbirds.

Increasing fire threats

In addition to disrupting habitats and driving native species into endangered status, scientists and managers are also concerned because many invasive plants increase the risk and severity of California wildfires.

Eucalyptus trees, imported near the turn of the last century as windbreaks and as fast growing, get-rich-quick lumber schemes, have since spread far and wide. They form dense groves, and their leaves contain oils that are natural herbicides, preventing other plants from growing. Those same oils burn fiercely, turning towering eucalyptus trees into virtual blowtorches. The Oakland firestorm in 1991, which destroyed about 2,800 homes and killed 25, was largely fueled by hillsides covered in mature eucalyptus.

Many Sonoma County hillsides bloom in spring with yellow thickets of Scotch and French brooms, imported for their scented flowers in the 1800s. The blooming shrubs have since escaped into the wild, where they form dense, spreading hillside thickets that choke out native plants and insects, birds and animals. A mature bush can produce 8,000 seeds a year, dispersed by spring-loaded pods. And once established, they are also extremely tough to remove. They easily re-sprout when cut down. And the woody skeletons of older plants are highly flammable.

Medusahead is another alien that’s an increasing fire hazard in the West. A fast-spreading grass species, it has a unique defense. It takes up silica (the material used to make glass) from the soil and builds sharp spikes on its seed heads and stems. The spines can injure grazers’ mouths and eyes, making the grass and seeds so unpalatable even rodents avoid eating them. Worse, because the silica-rich grass decays very slowly, it forms dense mats that smother other plants, and create hot-burning fuels for wildfires. Medusahead has already overtaken millions of acres of western grasslands, including in Sonoma County.

Not all invasive species are plants or animals. Some are pathogens, like Phytophthora, a fungus-like water mold that was set loose in California on the roots of nursery plants imported from France.

Its spores are spread by raindrops, or humans, and infect the water carrying tissues of certain trees, primarily Tan Oaks, which eventually die of thirst. Known as Sudden Oak Death, this invader has killed over 1 million trees in 12 coastal counties of Central and Northern California, leaving stands of wildfire-prone dry wood.

Managing the risk of invasive species is a monumental challenge for local and state agencies, who are increasingly reliant on the public to help stem the threat.

The California Department of Fish and Wildlife offers guides and education to discourage pet owners from setting animals “free” into the wild, and encourage the public to help identify emerging threats early, before invasive species become deeply established. Gardeners should avoid plants that can naturalize easily, or are known to be invasive.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker. Contact him at

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