Close up view of a bin of green crabs pulled from a single trap. In some cases there can be close to 100 crabs per trap in years of very high numbers. Photo courtesy of Ted Grosholz.


It’s nothing less than an invasion. Interlopers are coming into California by land, by sea…and by FedEx.

That’s what happened with the European green crab, a voracious cannibal that stowed away in packages of worms sent by overnight delivery to commercial fisherman in California. Unknown to anyone, the tiny crustaceans were concealed in seaweed that wrapped the cargo and were freed into the Pacific when fishermen tossed it overboard.

Then the green crabs, which a century ago decimated the East Coast’s shellfish industry, began to dine out in the Pacific, munching nearly everything in sight. Authorities made plans to rid the ocean of the pests.

But, as a research team from UC Davis discovered, invasive species don’t go quietly. Nor do they react well to full-on assaults. In fact, years of digilent and costly crab removal from a Bay Area lagoon went terribly wrong, triggering an unexpected population explosion.

Still, this serendipity has led to a new, live-and-let-live approach to combat invasive species: forget about trying to wipe them out, and get them down to a manageable population instead.

The new strategy could be a game changer. An army of scientists and state biologists are spending millions of dollars annually in California to combat an increasing scourge of invasive species — more than 1,700 types of plants, bugs and marine animals that are out-competing, elbowing out and, in some cases, devouring native plants and animals.

California has “unique things that make us susceptible,” given the enormous diversity of its environment, said Martha Volkoff, who manages the state Department of Fish and Wildlife’s invasive species program. “We have a lot of risks that states that are more homogenous wouldn’t have.”

Costly to control, these invasives have damaged some California crops and critical flood control and water delivery systems.

“Eradication is rarely successful and control is an unending process.”
— State Lands Commission, January report

California spends $3 million a year attempting to eradicate nutria, a large, homely, orange-toothed rodent that destroys wetlands and bores holes into levees. Another $3 million a year goes to educating boaters about quagga mussels, which hitch rides on hulls and cling to equipment in the state’s vast water transport system. And, for the last 20 years, authorities have spent more than $34 million to manage Atlantic cordgrass in the San Francisco Bay-Delta.

These costs represent only a fraction of the costs “because eradication is rarely successful and control is an unending process,” according to a report that state officials presented to the Legislature in January.

The environmental damage in the United States is estimated at $120 billion to $137 billion per year. One of California’s most destructive foreign pests was the Mediterranean fruit fly, which infested fruit orchards around the state beginning in the 1970s and cost hundreds of millions to combat.

The economic and environmental impacts are getting worse, abetted by a changing climate and a smaller world where exotic creatures can hitch a ride across the globe.

Efforts to get rid of invasives have mixed results, and sometimes make things worse, as when animals or insects are introduced to eradicate pests, and instead wind up becoming a new pest.

As with the stubborn little European green crab, attempts to erase them can backfire. Big time.

A case of crabs

For creatures with seemingly limited mobility, it’s remarkable how easily invasive species move around the world.

The state has had a Marine Invasive Species Program for more than 20 years, a recognition that about 80% of non-native pests arrive in North America via international commercial ships. Much of the dispersal is accomplished with the help of unwitting humans, for example, in ballast water when seagoing vessels take on water then disgorge it along its path.

Then there’s the panicked disposal of a once-cute pet, such as an alligator that’s outgrown the family bathtub and released into a local canal or park lake. Or the silent menace of classroom aquariums, which it turns out, are abetting in the trafficking of invaders from Ukraine — zebra mussels, near the top of California’s most-wanted list.

Zebra mussels filter out algae that native species need for food and they glom on to native mussels, incapacitating them, according to the U.S. Geological Survey. The fingernail-sized mussels also congregate and clog water intake areas of power plants.

After years of an all-out campaign by state agencies to fend off the introduction of zebra and quagga mussels, a highly-efficient commercial distribution chain unleashed the pests in the state.

Moss balls, which are placed in home aquariums, are infested with tiny zebra mussels,” Volkoff said. “They were found in Washington, (in moss balls) imported from Ukraine. Then they came into California from a distributor that supplied two national pet store chains across 49 states. Now we have zebra mussels on shelves of big-box pet stores. We didn’t see that coming.”

Invasive marine animals can move about by attaching themselves to marine equipment and hidden in bait buckets. Sometimes they don’t even need human help: Green crab larvae can bob along in ocean currents for as long as three months. They are models of adaptability, growing and molting wherever they wash up, immediately establishing themselves as the new local bullies.

Green crabs, which are native to Europe, have decimated shellfish industries in South Africa, Brazil, Asia and Australia. They made their way to California, where they were first noted in the 1980s, and are moving up the coast to British Columbia and off Alaska, threatening the Pacific shellfish catch. The crabs are too small — 3 to 5 inches at full growth — to wind up on dinner plates as a viable commercial harvest.

In 2009, researchers mounted a project to remove European green crabs from Seadrift Lagoon, at the northern end of Stinson Beach in Marin County.

In Seadrift Lagoon, crabs damage eelgrass beds, which are critical for young fish. They also have pushed out (or eaten) native crabs that provide food for shore birds.

So far they’ve had no significant impact on the area’s lucrative commercial crabbing, officials say, but the Dungeness Crab Task Force is keeping a wary eye on them. Green crabs are not picky eaters and can mow through scallops, soft shelled crabs, mussels and clams.They use their outsized claws as shovels and then crack the shells.

Funded by about $500,000 in federal grants, Ted Grosholz, a professor and ecologist at the UC Davis Department of Environmental Science and Policy, has spent more than a decade trying to evict green crabs from the lagoon.

His team used a straightforward approach: absolute eradication of the adult crab population in Seadrift Lagoon. Aided by platoons of volunteers, scientists baited traps with smelly leftovers from fish processing operations. Then they waited.

The adult population was estimated at about 125,000 in 2009. Four years later, the trapping had reduced them to about 10,000.

With that news, Grosholz and colleagues at the Smithsonian Environmental Research Center and Portland State University were preparing to write an academic paper detailing their success.

The crabs had different plans.

Like house guests overstaying their welcome, they proved to be nearly impossible to boot out.

When biologists say that non-natives “out-compete” natives, they don’t intend it as a metaphor. Almost across the board, invasive species have singular attributes that allow them to land on their feet and take over.

Grosholz is almost admiring when he describes the characteristics of European green crabs that allow them to thrive wherever they wash up. “It has a suite of traits that make it a good invader,” he said. “They are physiologically tougher than a lot of other crabs. They are more tolerant of variable salinity. They are very tolerant of terrible conditions.”

Kate Bimrose, with the Greater Farallones Association, supervised volunteer data recording for the green crab research, but also managed her own invasive emergency at nearby Bolinas Lagoon. European beach grass and ice plants were pushing out native dune and saltmarsh plants on an island there.

The invasive plants were classic: “Admirable, adaptable and ferocious,” Bimrose said.

Ice plants don’t enjoy salt water, so Bimrose and her crew set up pumps with the idea to drown the ice plants with salty water, a project operated by the Marin County Parks Department. “It was a great idea, but it proved to be difficult,” she said, repeating a familiar refrain among those trying to eradicate invaders.

The team also tried pulling up the plants and piling them in a massive heap, allowing them to dry out and decompose naturally. But the disembodied plants, in an effort to survive, sent out creepers in search of ground to re-establish themselves. Like zombie ice plants.

Unlike native animals and plants that don’t have weapons or immunity to fend off predators, invasive species are willing to kill to maintain a foothold in a new ecosystem.

Black Mustard plants, leggy green plants with showy yellow flowers, sprout in areas where wildfires have scorched vegetation. Native plants can be picky, temperamental and slow to grow, but not so the mustard, which, given an inch, will take over a hillside.

Even its dead stalks produce a chemical that inhibits the ability of other plants to grow around it.

“They are physiologically tougher than a lot of other crabs. They are more tolerant of variable salinity. They are very tolerant of terrible conditions.”
— Ted Grosholz, ecologist at the UC Davis Department of Environmental Science and Policy

Volkoff has a special disdain for the stout American bullfrog, which was brought to California from the Midwest during the Gold Rush for food and insect control. They carry a fungal disease that infects native amphibians, including rare species.

Some troublemakers are so appealing that their beauty masks their malevolence. Take the Pacific purple sea urchin — the state of California dearly wishes someone would.

The colorful, spiny creatures take advantage of twin calamities: warming seas and a disease that killed starfish, its main predator. Purple urchins have been decimating much of the coast’s kelp forests, triggering a state of emergency.

The urchins are an example of a native species becoming invasive, a distinction critical to understanding invasive species management. It’s not as important to identify a species as native or non-native, Volkoff said, but rather focus on its ability to do damage. The federal definition cites the potential to cause harm to the environment, the economy or public health.

Ted Grosholz examines a trap just pulled out of the water holding green crabs removed from Seadrift Lagoon, near Stinson Beach. Photo courtesy of Ted Grosholz

Not home alone

The business of eradicating invasive species is nothing if not humbling. The UC Davis team was elated in 2013 after removing 90% of Seadrift Lagoon’s green crabs. But then, a census the next year revealed a surprise: A green crab baby boom, with about 300,000 swimming around, nearly triple the pre-eradication population.

Grosholz and the team were dumbfounded. “This was so unexpected. We thought, ‘What happened, what did we do?’,” he said. “We were almost at the finish line. We’ve wasted all this money, all this effort. We’ve failed miserably.”

Upon reflection, the scientists realized they had not taken into account that the crabs are cannibals. They are not sentimental when it comes to devouring their own young, or those of their neighbors. So when the volunteers eliminated the adults — mature crabs that would have culled the population — juvenile crabs had the run of the place. And their populations surged.

“This was so unexpected. We thought, ‘What happened, what did we do?’”
— Ted Grosholz, after the first eradication attempts

In a paper published in March, Grosholz and his co-author identified a “hydra effect,” a reference to the mythical serpent that grew two heads for each one that was removed. Their all-out eradication approach was heavy-handed, they concluded.

Grosholz said the findings could become a game plan for fish and wildlife authorities: Rather than attempting to wipe out an entire population and possibly unleash unintended consequences, try “functional eradication,” reducing invasives to a level low enough to protect native species, but not so low that it triggers a population explosion.

The work to control the crabs is ongoing, this time using the new technique. The lagoon now is home to about 50,000 — less than half as many as a decade ago.

“Whenever you get something that’s unexpected in science, you need to look at it,” Grosholz said. “This is an important result, the management lesson is going to be important.”

In the war against invasive species, you learn to take what victories you can get.

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