Smithsonian National Zoological Park l Friends of the National Zoo



Weedy New World
by Mary-Russell Roberson

If you like to spend time outdoors, you have no doubt noticed a change in the scenery over the years. Where before you saw ferns and spring wildflowers on the forest floor, you now see Japanese stiltgrass (Microstegium vimineum). Where before you saw an old field or pasture, you now see a tangle of autumn olive bushes (Elaeagnus umbellata). Where before you saw a pond ringed with cattails, you now see an expanse of purple loosestrife (Lythrum salicaria). These new colonizers are invasive exotic plants: plants from other parts of the world that spread rapidly and become dominant in their new home. Other well-known invasive exotic plants include kudzu (Pueraria lobata), mile-a-minute (Polygonum perfoliatum), Japanese honeysuckle (Lonicera japonica), and garlic mustard (Alliaria petiolata).

purple loosestrife
With breathtaking beauty and stunning speed, introduced purple loosestrife overruns many North American wetlands, evicting wildlife along the way. (Randy Westbrooks, U.S. Geological Survey, www.forestryimages.org)

Invasive plants are very much on the mind of Bill McShea, a research scientist at the Smithsonian’s National Zoo, based at its Conservation and Research Center (CRC) in Front Royal, Virginia. He says, “I’m here walking in the woods every day and these woods are not the same woods I grew up in; they’re not the same woods I became an ecologist in.” He’s curious about the role of the new plants in the ecosystems he’s been studying for years.

“These invasive species seem to really be changing the rules out there,” McShea says. “I just wonder, while I’m trapping small mammals or listening to birds, how can I make sense of my hypotheses without incorporating these large-scale changes that are happening?”

In the face of rapidly changing habitats, McShea and other ecologists are studying how invasive plants are affecting the ecosystems they are invading; they are also searching for the best ways to restore native vegetation in places where invasive plants have taken over.

Attack of the Invasives in Potomac Gorge

McShea is studying the effects of invasive plants on native vegetation in Great Falls National Park and the Chesapeake and Ohio Canal National Historical Park. Both lie in the Potomac Gorge, a 15-mile stretch of the Potomac River in and near Washington, D.C.

“In this project,” McShea says, “we are trying to answer the question, ‘Which should the park worry about more: deer browsing or invasive plant species?’” McShea and Norman Bourg, another National Zoo ecologist at CRC, are monitoring more than 200 four-foot by four-foot plots in the parks. Some of the plots were left alone to serve as controls. The others were treated in one of three ways: The scientists put up a fence to keep white-tailed deer (Odocoileus virginianus) out, or removed all invasive plant species by hand, or put up a fence and removed all invasives.

McShea and Bourg tracked the numbers and types of plant species in each plot before and during the treatments. They have already analyzed the first two years of data and have found that the number and diversity of non-woody native species increased the most in the two treatments where invasives were removed. The natives included wildflowers such as spring beauty (Claytonia virginica), jack-in-the-pulpit (Arisaema triphyllum), and showy orchis (Galearis spectabilis). (It’s too early to say whether the same is true of the slower-growing woody plants, but the project is scheduled to run through 2009.) Bourg says, “Particularly for the herbaceous layer, it seems that the invasives are more of an inhibitor to the species richness and diversity than deer.” That’s surprising, especially when you consider that the density of deer in the study sites is very high—between 91 and 104 per square mile—and the finding illustrates just how powerful invasive plants can be.

Japanese stiltgrass
Japanese stiltgrass, a prevalent invasive exotic plant in Mid-Atlantic woodlands. (Chris Evans, River to River CWMA, www.forestryimages.org)

The most numerous invasives McShea and Bourg encountered in the Potomac Gorge were Japanese stiltgrass, garlic mustard, and Japanese honeysuckle. Japanese stiltgrass was used as packing material in crates coming from Asia. It was first recorded growing wild in the United States in Tennessee in 1919. Garlic mustard arrived in the United States in the late 1800s from Europe, likely brought by immigrants for use as food or medicine. Japanese honeysuckle was imported from Japan and Korea in the mid-1800s as an ornamental plant with good potential for erosion control.

Today, exotic plants continue to enter the United States, but the vast majority never gain a toehold. Of the thousands of exotic plants used in landscaping, only a small percentage escape the bounds of the cultivated garden, and most escapees don’t cause problems. But about ten to 15 percent of the estimated 5,000 exotic plant species that now live in the wild in the United States are considered invasive. “A lot of wildflowers we all like are exotic, but they are not invasive,” McShea says. “You don’t see a whole world of Queen Anne’s lace.” In order to become invasive, an exotic plant needs to land in a place where soil quality and climate are suited to its needs. It also needs pollinators, seed-dispersers such as birds (unless its seeds travel on wind or it spreads via shoots and runners), and an absence of predators.

Some exotics are kept in check by native plant predators, whether insects, deer, or fungi or other pathogens. Some exotics bring their own tiny predators with them when they arrive. But some exotic plants have few or no predators in their new homes, a situation that gives them a competitive advantage over native plants, all of which live with a host of organisms that attack their leaves and other tissues.

Taming the Purple Threat

Some scientists think that when plants don’t have to spend energy defending themselves from predators, they grow and reproduce more vigorously. Many scientists have noticed that invasive exotic plants are often larger or produce more seeds than their cousins at home. A 1991 study by Bernd Blossey, now an assistant professor and director of the Ecology and Management of Invasive Plants Program at Cornell University in Ithaca, New York, showed that seeds gathered from purple loosestrife in New York, where there were no purple loosestrife predators, produced taller and heavier plants in a laboratory than seeds from purple loosestrife gathered in its native Switzerland.

Purple loosestrife is a striking plant that grows four to ten feet tall and is found around ponds and in wetlands. Magenta blossoms cover its spike-like stems from June until September. Purple loosestrife came to North America from Europe and Asia in the 1800s as an ornamental plant and also as a hitchhiker on imported wool and in ship ballasts. For decades it was sold and planted extensively. Now it is widespread in the Northeast, Midwest, Pacific Northwest, and is present in every state except Florida, Alaska, and Hawaii. It tends to grow in extensive stands, pushing out a more diverse native population of plants as it spreads. Some states have made it a crime to sell purple loosestrife, but it is still available for sale in some other parts of the United States.

black tern
Black terns will not nest in purple loosestrife-ridden wetlands. (Terry Spivey, USDA Forest Service, www.forestryimages.org)

When purple loosestrife moves in, native wildlife moves out. Blossey describes walking through areas of purple loosestrife infestation and seeing very few amphibians or wetland birds besides red-winged blackbirds (Agelaius phoeniceus) and maybe a common yellowthroat (Geothlypis trichas) or two. In the mid-1990s, graduate students at Cornell documented that black terns (Chlidonias niger), bitterns, grebes, and rails—all of which build nests in native wetland vegetation—do not build nests in purple loosestrife.

Blossey and others at Cornell University have also done studies demonstrating one way that purple loosestrife is bad for some native amphibian species. The group raised tadpoles of American toads (Bufo americanus) and gray treefrogs (Hyla versicolor) in the laboratory and discovered that only about a third of the American toad tadpoles survived when they were raised in water containing extracts from dried purple loosestrife leaves. The water was designed to mimic natural waters surrounded by purple loosestrife plants. About 90 percent of the American toad tadpoles survived in water containing extracts from leaves of the broadleaf cattail (Typha latifolia), a common native wetland plant. The treefrogs did well in both kinds of water, perhaps because treefrog tadpoles can use lungs or gills to breathe, whereas the toad tadpoles rely only on gills. Blossey speculates that some of the chemicals in purple loosestrife leaves damage gills.

American toads
In a lab test, about a third of American toad tadpoles raised in purpe loosestrife-tainted water survived. (David Cappaert, Michigan State University, www.forestryimages.org)

In the last decade, purple loosestrife has been subdued in some areas by tiny but powerful insects. Because there were no purple loosestrife predators in the United States, scientists went looking in Europe to find some. In the mid-1990s, several insects that eat purple loosestrife leaves, flowers, or roots were approved by the U. S. Department of Agriculture (USDA) as biological controls for the plant, meaning that the insects could be brought over from Europe and released into the wild here. Four of the approved species have been released across much of the country, with good but mixed results. In some sites, as much as 95 percent of the purple loosestrife plants have been destroyed.

The process of approving an insect for biological control is long and arduous because everyone wants to avoid the nightmare of importing an insect that would attack native plants or crops. The first step is to identify potential insect predators in the plant’s native land. The insect is tested in a laboratory there to see whether it has an appetite for other plants besides the targeted invasive. If the insect is found to be single-minded in its food preferences, it is brought to the United States and kept in laboratory quarantine for more tests. Scientists offer the insect different native plants and crops in the absence of the invasive target to determine if extreme hunger will induce the insect to eat other plants. If the insect continues to snub all food except the invasive, it is released within a vegetated, insect-proof enclosure outdoors. If it proves to be a voracious attacker of the invasive plant, yet well-mannered around native plants and crops, it will be approved for widespread release in the wild.

Biological controls are not expected to completely eradicate an invasive. Judith Hough-Goldstein, professor of entomology at the University of Delaware, explains, “The goal is to reduce the population of the invasive plant to a point where it’s integrated into the ecosystem and not dominating all the native plants.”

It is harder to find an appropriate biological control for some plants than for others, says Hough-Goldstein. For example, insects that like to eat kudzu also enjoy eating soybeans. And she expects it would be hard to find a biological control for Japanese stiltgrass, because corn and wheat, both of which are grasses, would likely appeal to the same insects that eat stiltgrass.

root-mining weevil
A root-mining weevil, which feeds on garlic mustard. (Hariet Hinz & Ester Gerber, CABI Biosciences, www.forestryimages.org)

In the case of purple loosestrife, two leaf-eating beetles in the genus Galerucella, a root-mining weevil in the genus Hylobius, and a flower-eating weevil in the genus Nanophyes have been released in more than 30 states since 1992. Blossey says that at high population densities, leaf-eating beetles have been observed “nibbling” on other plant species, but the nibblers are usually recently emerged beetles and soon find their way to purple loosestrife.

“The success is not complete,” Blossey says. “There are some places where [the beetles] don’t work as well.” Still, he expects the insects will continue to spread and gradually increase their impact. In the meantime, he notices big changes in places where the insects are already established. “We see all our frogs come back when the insects defoliate the purple loosestrife,” he says. “We get loosestrife at a very low level and it doesn’t appear to be a problem for amphibians.” Preliminary laboratory results show that American toad tadpoles do well in waters with a very small of amount purple loosestrife leaf extract. In areas of defoliated purple loosestrife in New York, Blossey also notices an increase in bird diversity, including black terns, American coots (Fulica americana), marsh wrens (Cistothorus palustris), American bitterns (Botaurus lentiginosus), and herons.

A Tangled Tale of Trees and Birds

The well-entrenched saltcedar now raises vexing conservation questions. (Steve Dewey, Utah State University, www.forestryimages.org)

Out West, one of the most infamous invasive exotics is the saltcedar or tamarisk tree (Tamarix spp.). Saltcedar is native to central Eurasia, from Turkey, Iraq, and the Ukraine to Mongolia and central China. It was brought to the American West in 1823, where it was popularly used for landscaping, windbreaks, and erosion control. By the late 1800s, it was spreading on its own. A single saltcedar tree produces as many as 500,000 seeds from April to October. If axed or burned, the stump sends up new shoots.
By the 1940s, saltcedar was growing along most of the riparian (streamside) habitats in the Southwest, pushing out native cottonwoods (Populus spp.) and willow (Salix spp.) trees. In many cases, the native trees were already struggling due to the fact that dams were modifying or eliminating natural flooding cycles.

Today, saltcedar covers between one and 1.6 million acres from Mexico to Montana and from Kansas to California. In the arid and semiarid West, vegetation and wildlife tend to concentrate around streams and rivers, and this is precisely the habitat that saltcedar now dominates. Jeffrey Lovich, the deputy center director at the U. S. Geological Survey’s Southwest Biological Science Center in Flagstaff, Arizona, says, “The impact is magnified far beyond the mere acreage because of the fact that the acreage affected is some of the most sensitive and valuable in the West.”

Saltcedar trees don’t just take over river banks, they change them. They are able to extract more water from the soil than willows and cottonwoods, lowering water tables and sometimes drying up spring-fed pools in the process. These pools serve as habitat for desert pupfish (Cyprinodon spp.) and supply bighorn sheep (Ovis canadensis) and other wildlife with a reliable source of water. Further, as the common name implies, these trees can make use of salty water. They excrete excess salt through special pores in their leaves. As the leaves fall off, salt accumulates under the trees, making the soil too salty for some native plants. 
In the 1970s, USDA entomologists began traveling to China, Kazakhstan, Uzbekistan, Greece, and other countries looking for insects that attack saltcedar trees. There were hundreds to choose from, but one leaf-eating beetle became the top contender because of its strict saltcedar diet and its potential for completely defoliating the trees. The beetle is called Diorhabda elongata, although some entomologists are now discussing whether the name actually encompasses several different species.

Luckily, saltcedar has no close relatives in the United States and entomologists discovered that Diorhabda elongata could not be tempted to eat any native plants or crops. It appeared to be the perfect biological control. However, a complication arose while the beetle was still being tested: Birdwatchers and ornithologists discovered that the southwestern subspecies of the willow flycatcher (Empidonax traillii extimus) was nesting in saltcedar in Arizona. This bird was added to the federal Endangered Species List in 1995. If the southwestern willow flycatcher needed saltcedar, the Endangered Species Act would block the use of saltcedar biological controls in areas with flycatchers.

Saltcedar or tamarisk has become the predominant tree in many riparian, or streamside, habitats in the Southwest, displacing willows and other native vegetation. (William M. Ciesla, Forest Health Management International, www.forestryimages.org)

The southwestern willow flycatcher’s wintertime range is from Mexico to northern South America. It breeds and raises its young in spring and summer in Arizona, western New Mexico, and the southern parts of California, Nevada, Utah, and Colorado. About one-third of the approximately 1,300 known pairs now nest in habitats dominated by saltcedar. Mark Sogge, an ecologist at the Southwest Biological Science Center in Flagstaff wanted to know whether saltcedar helped or hurt the willow flycatcher, which once nested primarily in willows.

“When I started studying birds in the Southwest in the early 1990s, I was pretty much convinced that saltcedar was public enemy number one for birds and the flycatcher in particular,” he says. “But it’s really become clear to me that saltcedar is serving as bird habitat in a lot of areas.”

From 1996 to 2005, Sogge and his colleagues studied breeding flycatchers around Theodore Roosevelt Lake in central Arizona. The lake is a reservoir created by a dam on the Salt River, and its shores host the largest known breeding population of the endangered flycatcher. The group of scientists monitored the physiology, immunology, site fidelity, productivity, and survivorship of flycatchers around the lake for ten years. The habitats used by the birds varied from predominantly saltcedar to predominantly native, and included habitats that were a mixture of both. The bottom line: “The flycatchers seem to be suffering no negative consequences from breeding in saltcedar,” Sogge says. He found that the birds that lived in saltcedar ate a diet of different insects than those that lived in the native trees; however, the productivity and survivorship of the two groups were essentially the same.

“Probably everyone would agree that in a perfect world, we’d have native riparian habitat everywhere, but that’s not the world we’re in,” Sogge says. “There are some places where the native habitat is not going to come back, and there are some places where it’s not going to come back without hugely expensive intervention. In some of those places, saltcedar has a high habitat value.”

Sogge emphasizes that the southwestern willow flycatcher is not the only bird species that uses saltcedar. “There are about 50 bird species that breed in saltcedar in different parts of the West,” he says. “The willow flycatcher gets all the press because it’s endangered, and that mistakenly leads people to believe it’s the only species involved.”

Sogge’s colleague Charles van Riper, III, who is an ecologist at the Southwest Biological Science Center’s Sonoran Desert Research Station in Tucson, has studied how a variety of bird species use saltcedar. Van Riper and his colleagues compared the abundance and diversity of bird species in saltcedar and native habitats in two national wildlife refuges on the lower Colorado River along Arizona’s western border. Between 1998 and 2002, team members visited the refuges once every seven to ten days from March to May and August to November and recorded all bird species seen or heard during a five-minute period at each of 30 observation sites. They also described and quantified the vegetation at each of the 30 sites. The team analyzed data from several dozen different bird species in each wildlife refuge. At Cibola National Wildlife Refuge, most of the bird habitat was either saltcedar or native, with very little mixing of the two. The scientists found more birds and more bird species in the native habitat than in the saltcedar habitat.

At Bill Williams National Wildlife Refuge, the story got more interesting. There, the habitats were more varied, including mixtures of saltcedar and native trees. Again, the all-saltcedar habitats had the lowest bird abundance and diversity of species. However, the mixed habitats actually contained slightly more birds than the all-native stands.

Van Riper speculates that the mixed habitats attract more birds because they support a wider variety of insects, especially when saltcedar is blooming. Also, when saltcedar forms an understory beneath cottonwood, birds have more choices of places to forage and nest at varying heights. He suggests that adding native trees to stands of saltcedar may be just as beneficial to birds as the current strategy in Arizona—bulldozing saltcedar and planting rows of native trees.

“Birds prefer native plants and I’m sure they would prefer an all-native habitat,” van Riper says. “But they sometimes prefer a mix over the pure natives, because a lot of what’s left on the Colorado River is re-established, what I call ‘orchards’—trees in a line, cottonwood trees with no understory. The tamarisk comes in and provides an understory.”

“People have spent so long trying to show that invasives are bad, and in most situations they are, there’s no question about that,” van Riper says. “But nothing is ever simple. That’s the problem. Everyone wants a silver bullet and there is none.”

Sogge agrees. “It’s great to have natives be the sheriff in the white hat and the exotics always be the bad guy in the black hat,” he says, “but the real world is more complex than that. The biggest danger is that we will make management decisions with these overly simple concepts in mind. That’s when you have the potential to make a mistake.”

In 1999, after a lengthy biological assessment, the U.S. Fish and Wildlife Service ruled that no saltcedar beetles could be released in Arizona or other areas near the flycatchers. The USDA began releasing the beetles into the wild in 2001, leaving a buffer zone of at least 200 miles around saltcedar-nesting flycatchers.

Jack DeLoach, research entomologist with the USDA Agricultural Research Service in Temple, Texas, has worked on the saltcedar biocontrol project for years. He says the beetle has been released and has established itself in California, Nevada, Utah, Colorado, Wyoming, Texas, and New Mexico. Beetles have also been released in Montana and Oregon, but have not yet become established there. The beetles’ most spectacular performance so far has been in Nevada. Fourteen hundred leaf-eating beetles were released in the desert near Lovelock, Nevada, in the spring of 2001. “In August 2002, the beetles defoliated almost two acres of saltcedar,” DeLoach says. “By the next year, it was 500 acres, the next year it was 5,000 acres, and the next year, 50,000 acres.”

It takes several years of defoliation to kill saltcedar trees, and even then, not all of them die.  “Eradication has never, never happened in any biological control weed project in the world,” DeLoach says. “What you expect and what you want is not eradication but reduction of the weed down below a threshold of damage, either economic or environmental. But the beetles are always there too. Even if the weed does reproduce and spread, the beetles are always there to take care of it.”

Although the beetles have not been released in Arizona, there’s no reason to think they won’t make it there anyway. The beetles spread quickly up and down rivers. Ranchers and citizen gardeners are also moving the beetles by hand. DeLoach thinks it’s almost certain the beetles will make it to flycatcher habitat. Sogge does too: “There’s no biological or ecological barrier to stop their progress.”

Sogge has some concerns about biological control because, as he says, “It’s not a very precise tool. You can’t target it.” He’d like to see more monitoring of wildlife before and after the beetle comes in to find out more about how, or even whether, native wildlife rebounds. “The saltcedar beetle has the potential to really dramatically alter the habitat,” he says, and native vegetation does not always spring back on its own. If there are no natives nearby to provide seed, the landscape may remain barren or even be taken over by another invasive plant. Sogge, DeLoach, and other scientists would like to see restoration efforts begin before the saltcedar trees are defoliated. “Long before the beetles get there,” Sogge says, “you could begin to start replacing parts of that habitat with native vegetation.”

The questions and issues being raised by the saltcedar in the American West will continue to come up in other contexts. Humans are intentionally and unknowingly spreading so many plants and animals so far and wide that, according to Lovich, some scientists have taken to calling the current epoch the Homogecene. “We’re running one of the largest experiments the world has ever seen,” he says. “We’re moving things all over the world and things are coming into contact that have never been in contact.” The best way to deal with invasive plants is to find them early and respond quickly. “Once they become established, they are more or less here,” he says.

“I used to get very upset about tamarisk but now I realize that tamarisk is pretty much here to stay, certainly for my lifetime,” Lovich says. “I try to take a deep holistic view to ecology. This is just a blip on the radar screen today. Another ice age or two and it’s all going to be different. Things will adjust and evolve and co-evolve and it will be a new world.”

But Lovich’s holistic view does not lead him to complacency: “There are areas we should focus our efforts—high biodiversity areas like oceanic islands and nature preserves—where we should do everything we can to restore ecosystems.”

In Your Own Backyard

Meanwhile, back on the East Coast, Bill McShea says people can help restore ecosystems by the choices they make in their own backyards. “People can plant native plants and they can become aware of the invasive species and pull those things out,” he says. “Any invasive that is seeding is a potential source of problems because it’s going to be bird-dispersed and mammal-dispersed.” For example, if you have Japanese honeysuckle, autumn olive bushes, or privet bushes (Ligustrum spp.) in your yard, birds could be carrying seeds from these plants into nearby natural areas.

For his part, McShea is continuing his work in the Potomac Gorge and is also beginning work on a new project at CRC in Front Royal to determine the most efficient ways of ridding old fields of autumn olive, a shrub or small tree that produces red berries in the fall. “An old field is something you have to look at quick in this part of the world, because it’s a transition on its way to something else,” he says. Abandoned farm fields, grassy mountain knolls, or burned-out areas of a forest turn into old fields when they become colonized with dogwoods, persimmons, black locust, raspberry, and grape vines. “It’s a jungley, viney, thorny mess, but what’s a mess to me is not a mess to a lot of birds and small mammals. There’s a lot of productivity in an old field.” An old field eventually becomes a forest—until the next forest fire or hurricane or ax-wielding human comes along and starts the process again.

Invasives such as autumn olive can hijack the process by taking over an open area and preventing hardwood trees from becoming established. McShea is testing several different methods for getting rid of the tenacious autumn olive shrubs and restoring native species in old fields. Within a couple of years, he plans to have a demonstration site in Front Royal to share his findings with the public.

Four other conservation centers in other parts of the United States are also participating in the project, and each site is trying to eradicate or tame a different combination of invasive plants. “We all have different ecosystems, but we all are dealing with the same general problem,” McShea says.

He is also hoping to come up with a new prescription for keeping invasives at bay while restoring native forests; if he gets funding for the project, he’ll be wrestling with questions about invasive plants for years to come. As an animal ecologist, he is somewhat surprised to find himself focusing on plants. But, as he puts it, “It’s hard to study white-tailed deer ecology when this big problem is slamming you in the head every day. The world is changing fast.”

Mary-Russell Roberson is a contributing editor who last wrote about South America’s camel family members in the January/February issue of ZooGoer.