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Removing broom at Sonoma Valley Regional Park

Why invasive species are a threat in Sonoma County parks

By Stephen Nett

They’re coming!

They’re here!

No, it’s not the zombie apocalypse. This threat is from invasive plant and animal species that have wormed their way into Sonoma County despite best efforts to stop them. Some are just as scary, gruesome and strange as fantasy creatures. But unlike the fictional walking dead, these invaders can do actual harm.

Take the brown-headed cowbird (Molothrus ater.) Cowbirds originally lived on the Great Plains, following bison that scared up a rich diet of insects. Because their food source was mobile, cowbirds, didn’t have the luxury of sitting on nests to raise their young. Instead, they developed the strategy of laying eggs in the nests of other birds, who then unwittingly feed and raise them. To ensure they do, cowbirds often remove the host bird’s own eggs.

Brown-headed cowbird

When humans brought cattle to Sonoma County, the cowbirds followed. Today, two of California’s native birds, the least Bell’s vireo (Vireo bellii pusillus) and willow flycatcher (Empidonax traillii), are listed as endangered because of brown-headed cowbirds.

This is just one case of the silent ongoing battles between natives and invasive species in Sonoma County. 

Fighting an alien invasion

Then there’s medusahead (Taeniatherum caput-medusae.) Medusahead is a fast-spreading grass with a nasty survival skill: The plant incorporates exceptionally large amounts of fine silica (the raw material used to make glass) into its leaves, stems and spiky awns, the needle-like crowns (pictured below) that give medusahead its fearsome name. 

Medusahead seed

The sharp silica helps keep the grass from being eaten, by biting back. The stiff spines can injure the mouths, nostrils and eyes of grazers. The silica even slows the grass’s decay, so it quickly forms dense mats that keep native grasses and flowering plants from germinating.

As a result, medusahead has overrun millions of acres in the West, including Sonoma County, and continues to spread.

What is an invasive species?

Both cowbirds and medusahead are officially designated "invasive," a term biologists and government agencies use to identify species that are non-native, aggressive colonizers and may have substantial negative impacts on other species or entire habitats. 

Not all non-native species are considered invasive. And being non-native doesn’t mean a plant or animal is automatically unwelcome here. California has more than 4,000 species of wild bees, but relatively few of them make honey, which is why European honey bees were imported and are happily protected.

For Sonoma County Regional Parks, the threshold for whether a species is invasive, and requires some type of management action, has to do with whether it alters the function of the ecosystem it’s occupying.

As Regional Parks’ Deputy Director Melanie Parker explains, the agency’s historically limited resources have meant it could target only the worst offenders. Those are the species whose spread might change a complex ecosystem into a highly simple one or which reduces or impedes the survival of native species. 

Why invasive species thrive

Some invasives are so successful and aggressive, they can turn a habitat filled with dozens of interdependent species into one filled exclusively with themselves.

Scotch and French brooms

Scotch and French brooms, introduced in the 1800s for their scented yellow flowers, are notorious for this. Left alone, their dense thickets cover hillsides, choke out native plants, and displace insects, birds and animals. And they’re particularly tough to remove. They resprout easily after being cut down. And when the broom dies after seven or eight years, its woody skeleton is highly flammable. 

French broom in forest

American bullfrog

Another notorious species already in the parks is a particularly voracious predator. It’s an ambush hunter and will eat anything that will fit into its mouth, which is as wide as its head. It dines on birds, fish, turtles, mice, lizards, snakes, newts, ducklings and more.

The large, broad-bodied American bullfrog (Lithobates catesbeianus) was imported to California in the late 1800s for diners who enjoyed frog legs, after hungry miners and settlers nearly wiped out the smaller native red-legged frog (Rana draytonii.)

American Bullfrog

Because bullfrogs eat and compete with the rare and threatened red-legged frog here, they’re high on the invasive species watch list. Fortunately, according to Regional Parks, bullfrogs aren’t a serious problem yet.

They need year-round bodies of water, while the red-legged does not. In the parks, red-legged frogs breed in vernal pools, seasonal water ponds that form with winter rains. Because they dry out by summer, these habitats haven’t been invaded by bullfrogs.

Sudden oak death

Some of the worst invasive species aren’t plants or animals. They’re pathogens, like Phytophthora ramorum, a fungus-like water mold that was set loose in California on the roots of imported nursery plants. Its spores are spread by raindrops or humans.

Sonoma County Sudden Oak Death

When it infects certain hosts, it spreads into water-carrying tissues, eventually filling and clogging them. Phytophthora’s victims, primarily tanoak trees, are unable to move water to leaves and die of thirst. The infection, well-known as sudden oak death, has so far killed over 1 million trees in 12 coastal counties.

What can be done?

Managing the risk of invasive species is a monumental challenge. In an era of global transportation, international animal and plant trade, and human travel, new threats can easily move in. The best hope for keeping invasives under control is public awareness. Pets like fish, snakes, turtles should never be released into the wild, and garden plants that can get loose and naturalize should be avoided.

Efforts to address invasive species in Sonoma County’s Regional Parks are included in the spending plan for the parks funding measure approved by voters in November 2018. Monies from that measure will help fund early warning and rapid response projects and staff.

Meanwhile, public vigilance, learning how to recognize the invaders, and keeping them from establishing new footholds, will help protect native communities from being overwhelmed.

More information on Regional Parks' efforts to control invasive species is available on its natural resources program page. To participate in an invasive species management project, contact Regional Parks' volunteer program

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

Originally published Dec. 1, 2018.