A UC Davis ecologist reports today on the cautionary tale of a transplant from America's East Coast that moved to California and lived peacefully among the natives for a half century -- until a new invader radically altered the community dynamics, pushing out the natives.
The setting is the sandy bottom of Bodega Harbor, on California's northern coast. The new arrivals, European green crabs, have accelerated the invasion of the eastern clams at the expense of native clams. The takeover drama has implications reaching far beyond these animals, says the ecologist: It is one of the first documented cases of its kind and may portend an "invasional meltdown," in which invaders create ecosystem changes that hasten other invasions in a positive feedback cycle.
"Here we have a newcomer that quickly changed a benign, introduced species into an aggressive invader," said UC Davis ecologist Edwin "Ted" Grosholz. "There already are more than 500 non-native species in the coastal waters of the United States, with more arriving every day. If a remarkable shift like this can happen in Bodega Harbor, maybe it can happen anywhere."
Grosholz has studied invasive species in San Francisco Bay and surrounding waters for 10 years. His new study appears this week in the online and print editions of the journal Proceedings of the National Academy of Sciences.
Scientists generally agree that invasive plants and animals constitute one of the greatest threats to Earth's natural webs of life, called ecosystems. Invasions are second only to habitat loss as a cause of species endangerment and extinction. In the United States, of an estimated total of 150,000 species, 7,000 are aliens.
Invasions have tremendous economic impacts as well: It's estimated that invasive species cost the United States more than $138 billion a year. Across the United States, farmers, ranchers and other resource managers battle exotic species that include West Nile virus, fire ants, sudden oak death, cheatgrass, yellow star thistle, kudzu, Dutch elm disease, citrus canker, gypsy moths, glassy-winged sharpshooters, Asian tiger mosquitoes, purple loosestrife, water hyacinth and tomato bushy stunt virus.
In recent years, ecologists studying invasive species have come to fear that interactions between the invaders (like those Grosholz describes today) may disrupt ecosystems so extensively that human health and well-being will be affected.
Grosholz is one of many UC Davis researchers with long-standing monitoring programs of water conditions and plant and animal species in Bodega Harbor, which is a small, protected inlet on the Pacific Ocean 70 miles north of San Francisco. The harbor is also home to UC Davis' Bodega Marine Laboratory, which studies the coastal environment and seeks solutions for sustaining local fisheries, improving degraded marine and terrestrial habitats, and controlling invasive species.
In the case reported this week by Grosholz, the native species are two small clams with white shells, named Nutricola confusa and Nutricola tantilla. (They have no common names.) These species have lived in Bodega Harbor for millennia, feeding on phytoplankton and serving as an important food source for shorebirds, native crabs and other predators.
Another small, white clam, called Gemma gemma or eastern gem clam, was transplanted from the East Coast to the West Coast in the late 1800s. It escaped into San Francisco Bay from shipments of oysters and spread north to Bodega Harbor by about 1960.
Grosholz says the gem clams lived there in small numbers among the abundant native Nutricola until about 1994, when another newcomer radically altered the neighborhood dynamics.
This newcomer was the European green crab, Carcinus maenas, which arrived in San Francisco Bay from Asia. Taste tests Grosholz conducted in his laboratory showed that the crabs preferred the native Nutricola clams over gem clams 2 to 1, because the Nutricola are larger.
Before the green crab showed up, Grosholz estimates, there were 10,000 Nutricola clams in a square yard. Today there are fewer than 1,000 -- only one-tenth as many. In field experiments, Grosholz found that the native clams had kept the gem clams in check for nearly 50 years through direct competition. However, once the green crab reduced the numbers of natives, the invasive gem clam was able to spread through the harbor rapidly.
Clearly this is a bad situation for the native Nutricola clams in Bodega Harbor. But, Grosholz said, the new study points to a much larger problem for this and other ecosystems: When non-native species facilitate the spread of other invaders in a positive feedback loop, there is a danger of rapid and extensive ecosystem change and possible collapse.
"We don't know how many more invasions are necessary before we alter the balance," he said.
The new study was funded by the National Oceanic and Atmospheric Administration's Sea Grant Partnership Program.
UC Davis has experts on many of the invasive species currently changing the face of California and the world. For more information, see the Spring 2003 issue of UC Davis Magazine at http://www-ucdmag.ucdavis.edu/sp03/feature_2.html.
The University of California is one of the world's foremost research and teaching institutions, and UC Davis is the University of California's flagship campus for environmental studies. UC Davis is a global leader in environmental studies relating to air and water pollution; water and land use; agricultural practices; endangered species management; invasive plants and animals; climate change; resource economics; information technology; and human society and culture. One in six of UC Davis' 1,500 faculty members specializes in an environment-related subject.
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