The Aliens Among Us

Invasive plants, animals and other organisms are wreaking havoc in the United States, and UC Davis researchers are on the hunt for ways to alleviate the damage.

By Clifton B. Parker

Under a cold, bright November sky, a couple dozen students and researchers trek through Putah Creek Riparian Reserve on the edge of the UC Davis campus. They’re here to investigate the aliens among us—invasive plants, animals and pests that constitute one of today’s biggest ecological challenges.

Peter Moyle, a fish ecologist at UC Davis and expert on native and non-native species in California, peers through a pair of binoculars at a Putah Creek that is now receiving increased water flows, which benefit native species.

“We’re surrounded by invasive species,” Moyle says. “We’re trying to understand them better, find out how to cope with future invasions and manage the ecosystems where trouble currently exists.”

Today, alien invasions are second only to habitat loss as a cause of species endangerment and extinction. Over the past few hundred years, as immigrants from Europe and Asia arrived in North America, thousands of non-native plant and animal species have become entrenched on the continent. It’s estimated that invasive species cost the United States more than $138 billion a year. That’s a mind-boggling number, but one commonly agreed upon by the government and academic researchers.

Safeguarding diversity

Beyond economics, safeguarding the earth’s diversity is the reason behind the concern about invasive species. Native species are often driven to extinction by competition from aggressive invaders. Invasive species can cause harm to the economy, environment or even humans, as the outbreak of West Nile Virus has demonstrated.

Ecologists generally define an alien species as one that people inadvertently or deliberately carry to a new location. In the New World, exotic species are those introduced after the first European contact. They’re called “invasive species” when they cause trouble in their new environments.

Worst case scenarios? Ask Britain and Europe about their devastating encounter with foot-and-mouth disease, a highly contagious virus that wiped out a large percentage of the cattle, sheep and pigs in that region. Another example is the Mediterranean fruit fly or “Medfly.” Only through vigilant eradication programs has the United States been able to prevent the establishment of this highly destructive agricultural pest.

The glassy-winged sharpshooter is an intruder that poses a serious threat to California’s agricultural system, especially its grape and wine-making industries. The sharpshooter can transmit the deadly Pierce’s disease to grapevines, and other diseases to almond trees, alfalfa, citrus and oleanders—endangering more than 100 species of plants.

Then there’s Sudden Oak Death. UC Davis plant pathologist David Rizzo has identified how Phytophthora, a fungus-like organism, has killed thousands of coastal oak trees and threatens the state’s treasured redwoods.

Sheer peskiness is also a problem. How about the “plant that ate California,” the thorny yellow starthistle, which has spread extensively throughout 10–15 million acres in California and throughout the Pacific Northwest. The prickly plant is believed to have come to the West Coast from Eurasia as a contaminant in alfalfa seed.

California also faces invasions by red fire ants, “killer algae” or Caulerpa taxifolia, cheatgrass, rice blast, Africanized honey bees, Asian clams, Northern pike, the Chinese mitten crab—and these are only a few. Across the United States, the zebra mussel, Dutch elm disease, leafy spurge, kudzu, the citrus canker, feral pigs, the brown tree snake, purple loosestrife, the tomato bushy stunt virus and the gypsy moth are on the march.

Salmon dreams

Back at Putah Creek, it’s apparent that environmental changes can favor one species or another. Building a dam or diverting water flow for land-use purposes has consequences for plants and animals.

Moyle points out that decreased water flows along Putah Creek—once an active salmon stream—have made it hospitable for non-native species and less so for salmon. Some of these non-native species, for example, feed on the young salmon. In recent years, the university, the city of Davis, and the Putah Creek Council have worked cooperatively with the Solano Water Agency to manage flows in the creek to benefit native fishes and discourage aliens.

“Increased water flows benefit native species such as salmon because they seek waters that run deeper, faster and cooler,” Moyle says. “Relatively small but strategic increases in flow have turned what was once a stream dominated by alien fishes to one dominated by natives.”

Moyle says the science on invasive species is in its “infancy” and is “enormously complex.” Compounding the problem is the role of humanity in this interactive drama.

“We’re the ultimate invasive species,” he says. “Along with ourselves we’ve brought many favored species into new environments. It’s a form of cultural imperialism that has scientific ramifications.”
Many times species were introduced with good intentions, only to have disastrous outcomes; for example, the Nile Perch resulted in the extinction of more than 200 other fish species in East Africa. Today, some question the wisdom of selling Caulerpa taxifolia for aquariums (it’s a popular seaweed) or garden seeds or owning pets that are non-native.

Crossing borders

David Robertson, an English professor, is leading this trip along with Moyle. Robertson is a faculty member in the Nature and Culture program, an interdisciplinary set of studies allowing students to explore the complex relationships between human cultures and the natural world through a heavy dose of the basic sciences wed to literature courses.

As he tells it, Robertson is seeking a “secret at the heart of the universe.” It’s a search that blends nature and literature. In his 1997 book Real Matter, Robertson retraced the steps of such writers as Fitz Hugh Ludlow, Clarence King, Gary Snyder, Jack Kerouac and John Muir who found solace in the mountains and the natural world.

“Our environment cannot be understood without looking at all the ways that human beings have tried to make sense out of the world: literary, religious, social, economic, scientific, political,” Robertson says.

He notes that the humanities and social sciences need to join with the sciences to increase our understanding of complex issues such as bioregions and invasive species.“When things are done in isolation and they don’t work, then we often need to broaden our scope and try something else,” he says. “Creative ways to explain this problem exist beyond spreadsheets and data—we have stories to tell to people.”

Robertson motions to some familiar-looking eucalyptus trees on the banks of the nearby Putah Creek. The tree seems as common in California as coastal oaks and redwoods. But in this Ellis Island of the plant and animal world, common doesn’t equate to native.

“Many Californians are surprised to find out that eucalyptus trees are non-native species from Australia,” Robertson says. “We almost accept alien species as native because they’ve been here so long and are part of the landscape many of us grew up with.”

The IGERT

Robertson and a dozen or so other faculty members are infusing this “holistic” approach into the campus’s new Biological Invasions IGERT or “Integrative Graduate Education and Research Traineeship.” Made possible by a National Science Foundation grant, this interdisciplinary program is training students in the ethical, political, legal, economic and scientific issues connected to biological invasion.

Kevin Rice, an ecology professor and co-chair of the IGERT, talks about “bringing together” those interested in invasive species through this two-year program that involves courses, retreats and conferences, fellowship, internships, workshops and mentors. “We’re training students from all backgrounds,” he says—backgrounds that include the life sciences, social sciences, engineering, physical sciences and the humanities.

Caz Taylor, a doctoral student in ecology and a participant in the Biological Invasions IGERT, walks along a weed-strewn path on the creek’s sloping edge. “Some of the questions we’re asking are why invasions happen where they do and why some species are more prolific and others weaker,” she says. “We have to engage the public and explain why it’s important to care about native species. I’m not sure if we’ve reached that critical mass yet where it becomes a societal priority—though it needs to be.”

Investigating invasives

While the Biological Invasions IGERT is new, it has a solid foundation to build upon. UC Davis is among the world’s leading environmental research institutions with a host of resources in the environmental and biological sciences.

The Putah Creek Riparian Reserve was set aside by the university in 1983 with the aim of preserving native plants and animals while providing research opportunities. The reserve is a melting pot of both native and non-native species. Side-by-side can be found large valley oaks (native), Fremont cottonwoods (native) and an American hackberry tree (non-native). The plants at ground level are mostly non-native grasses and thistles, introduced from Europe, which have almost replaced California’s native bunch grasses and other plants.

The landscape also contains a few large dead oaks full of holes drilled by woodpeckers (native). Such holes are important nesting places for a number of native bird species. But it’s likely these holes are used by European starlings, an alien species. Also common are holes in the ground made by ground squirrels, a native species that has thrived in the presence of humans despite being considered an agricultural pest. Perhaps their abundant numbers are due to the fact that their chief predators—hawks, owls and grey foxes—are declining in numbers.

Down in Putah Creek, the water is green and murky—but swimming with life. The small guppy fish in the shallowest waters are “mosquito fish,” a non-native species introduced in California to help with mosquito control.

Along the bank are dark green tamarisk or saltcedar bushes, a species from Asia that now dominates streams and riparian habitats in the western United States. As Moyle explains, it’s one of the hardiest plants around and fills in the banks of water systems quickly.

Toward the South Fork of Putah Creek, Moyle and Robertson lead their group to “Restoria,” which is the product of a restoration effort led by Dan Leroy, M.S. ’01, when he was a graduate student. Though it recently suffered a fire, the site includes native trees and shrubs planted by volunteers and students. Before this project, Restoria was just a litter-filled area marred by off-road vehicles and dominated by non-native species.

Restoration of such a site, Robertson says, requires much dedication and work in watering and weeding. “It’s about making a vision of native species come true.”

McLaughlin Reserve

A similar vision is at work a hundred miles north at the Donald and Sylvia McLaughlin Reserve. The reserve, which at 6,800 acres is the largest of 34 such reserves in the UC system, is located on land belonging to the Home-stake Mining Co. and is managed by the university. From 1985 to 2002, Homestake operated a gold mine at the site and subsequently reached an agreement with UC to develop an innovative environmental program on the land. In 1998, UC built a modest field station and has since used the reserve for 47 research projects and 19 field courses—including now the Biological Invasions IGERT. A few weeks after the Putah Creek trip, UC Davis researchers and students headed to the reserve.

Susan Harrison is an environmental professor and campus director of the Natural Reserve System. Once or twice a week during spring, she makes the long, winding drive from Davis around Lake Berryessa to the reserve.

“The reserve offers students in this program a unique opportunity to actually manage native and exotic species and thus to examine the real-world applications of their academic knowledge,” she says. “The IGERT students are designing an invasive species management plan that will be implemented by the reserve’s managers, Paul Aigner and Cathy Koehler.”

Kara Moore (shown right) and Sarah Elmendorf, doctoral students in ecology, are conducting an experiment to understand the establishment and spread of invasives. On a large hill created by the rocks pulled from the mine and now covered by non-native grasses, Moore and Elmendorf have planted 100 small patches with mustard seeds to see how much the plant spreads and how much its invasion success depends on “initial conditions,” as Harrison says. That means determining how many mustard seeds arrive at a site and how disturbed the site was when those seeds arrived. One year from now, they will introduce another batch of seeds.

“We’d like to know if the game is over once the invasive species arrives,” says Elmendorf. “In other words, how much does repeated invasion matter?”

Harrison notes all the subjects that could be studied at McLaughlin, including restoration ecology, fire ecology, conservation biology, water and watersheds, recreational impacts, tectonic geology and volcanology, mining effects, limnology (the study of lakes), landslides, hydrology and the humanities-science interface.

“All around us are potential topics or courses in art, writing and interdisciplinary studies of the environment,” she says.

Harrison would like to get support for a visitor center with artistic and historical exhibits, public events and docent-led tours, workshops and seminars on land-management issues, and K–12 activities for students in neighboring Napa and Lake county schools, including day trips and in-residence classes.

Another scientist on the McLaughlin trip is John Randall, a plant ecologist and the director of the Nature Conservancy’s invasive species team. In one project, Randall and his colleagues are helping eradicate a troublesome insect from the Mianus River Gorge Preserve in the state of New York. The woolly adelgids from Japan have no natural predators in the United States—and they kill hemlocks, which play a critical role in the ecology of many northern forests.

In June 2000, Randall and his crew reviewed the situation and recommended the introduction of Japanese ladybird beetles—a natural predator of adelgids—into the Mianus River Gorge.

“This was the first time we did an intentional release of this insect on this target,” says Randall. “We didn’t want the Mianus sites to become guinea pig sites, but in this case we realized the hemlock woolly adelgid is a horrible problem.”

Randall does worry a little, noting that “We didn’t want the cure to be worse than the disease.”

It will take some time before the outcome is known, but it appears the beetles have survived the winters and have been seen eating the adelgids. In a monitoring session held in June 2002, preserve staff found beetles in the hemlock underbrush, though they also found that the year had been an especially good one for the adelgids.

Tough questions

Research on invasive species occurs throughout campus by faculty from the Division of Biological Sciences, College of Agricultural and Environmental Sciences, School of Veterinary Medicine, College of Letters and Science and even the School of Law.

Holly Doremus is a professor specializing in environmental law. Originally a research scientist before her law career, Doremus has explored how legislators can use scientific information to bolster laws like the Endangered Species Act. The legal implications of trying to control invasive species raise intriguing issues.

“How do we decide if it’s worth the effort?” she says. “And who sets the standards on controlling invasive species? Clearly, this issue can overlap international, national and state areas of concern.”

Doremus points out that the international shipping and import business is a major factor behind the introduction of unwanted invasive species. When the U.S. government set up a regulatory program to control the zebra mussel that was so destructive in the Great Lakes region, the goal was to control how these aquatic creatures are carried in ships’ ballasts. In the case of zebra mussels, no obvious solution exists, apart from using other species to feed on it and one day perhaps finding a chemical control that’s safe for other aquatic life.

“What are the enforcement mechanisms, and what do we know about these species? This mixes both law and science,” she says.

What kind of world do we want and what kind of species do we want to include?

“One could argue that we want native species around us; another might say we need species that serve humanity in some way,” Doremus says. “What is special about native species?”

Defining which species belong in an ecosystem is sometimes based less on science than on historical, cultural, moral, geographic or theological arguments. Some scientists question the scientific wisdom of trying to roll back exotic species. They even contend that alien species often fit into their environments.

Michael Rosenzweig, a professor of evolutionary biology at the University of Arizona, says that the distinctions between exotics and native species are artificial. It’s a case of someone picking a date and calling the plants and animals that show up after that exotic.

“You can’t roll back the clock and remove all exotics or fix habitats,” Rosenzweig told the New York Times. “Both native and exotic species can become invasive, and so they all have to be monitored and controlled when they get out of hand.”

Of the country’s 7,000 alien species—out of a total of 150,000 species—only about 10 percent are invasive. The other 90 percent have fit into their environments and are considered “naturalized.” Still, some ecologists would say appearances are deceptive and many of these aliens may be considered acceptable only because no one has documented their harmful effects.

Fish ecologist Moyle agrees that society cannot get rid of all invasive species. Rather, it’s a matter of degrees, and that means using science and policy to prevent or diminish future invasions and managing the environment to favor natives over non-natives.

He points to the Chinook salmon as one example. “There used to be a time in California when people living in the Sierra Nevada could literally hear thousands of salmon tails smacking the water at night. That’s how plentiful they were. Now, salmon numbers are a fraction of what they were.

“If we lose the salmon,” Moyle adds, “we lose part of ourselves.”

Clifton B. Parker is associate editor of Dateline UC Davis, the campus’s faculty and staff newspaper. Photography by Debbie Aldridge/UC Davis Mediaworks.