A marshland invader crowds out life along the West Coast.
By Elizabeth Brusati ’95, Ph.D. ’04
Standing in the mud, with one foot half out of my boot, I faced a dilemma. I had taken too large a step in the soft muck at the marsh in Point Reyes National Seashore, and my back foot had slipped out of its mudder, a snowshoe-like piece of footwear that wetland biologists wear to keep from sinking into the mire. How was I supposed to re-insert my foot without dropping the metal pipe I was using to collect cores? But I managed to reach back, get my foot in, and pull the mudder forward with a sucking sound to take another step.
When I dreamed of becoming a marine biologist, I didn’t picture quite this much mud. I’m standing here in the mire and fog of a Point Reyes morning to measure vegetation and collect some of the small creatures that make their home within the marsh. I take a core a few centimeters deep, which will be washed through a sieve to separate out the small worms (called polychaetes and oligochaetes), crustaceans and other organisms. They live among the stems of California cordgrass (scientific name Spartina foliosa) in the low intertidal zone, an area that is covered with water at high tide but at low tide plays host to small shorebirds that feed in open ponds and channels throughout the marsh. This is all part of my Ph.D. research in ecology at UC Davis, funded largely by a fellowship from the Canon National Parks Science Scholars program. It provides money for graduate students to study problems in biology, geology and the social sciences that are of interest to the National Park Service. But my muddy Point Reyes project is only one aspect of a larger collaboration that reaches from San Francisco Bay to Willapa Bay, Wash. Its goal is to track the impacts of an invasive plant that threatens the ecological well-being of West Coast estuaries and may limit their use and enjoyment by people.
The focus of this study is the invasive hybrid Spartina, actually a cross between the native California cordgrass and a species from the East Coast, which is transforming the open mudflats of San Francisco Bay into a completely different type of habitat. Researchers at UC Davis, along with colleagues from Scripps Institution of Oceanography at UC San Diego and the nonprofit San Francisco Estuary Institute, are using tools that range from DNA analysis to global positioning systems to study hybrid Spartina. Pronounced Spar-TYE-nah, it is one of many non-native invasive species encroaching upon parks and other natural areas throughout the United States. A particular problem in Willapa Bay, cordgrass invasions have also altered thousands of acres of delicate estuarine ecosystems in Britain, China and Australia.
Not every non-native species is invasive, but those that are can wreak both ecological and economic havoc by displacing native species or by changing a habitat in such a way that the natives can no longer survive. For example, Scotch broom and eucalyptus increase the frequency and intensity of wildfires, while Arundo, known as the giant reed, clogs river channels and increases flooding.
Hybrid Spartina is a classic example of good intentions gone awry. An eastern species called smooth cordgrass that was transplanted for a shoreline stabilization project in the 1970s crossbred with California cordgrass, creating a hybrid that is capable of growing farther onto the mudflats than either parent plant, gradually transforming open expanses of mud into dense meadows. San Francisco Bay has lost 90 percent of the marshes that once lined its shoreline to development or agriculture. How, then, can a grass that adds more vegetation be bad? The answer lies in the delicate balance among species that call the bay home.
The problem with hybrid cordgrass is that it destroys some of the interconnections among marsh species. It grows unlike any native plant. Not only can it survive lower in the intertidal zone than California cordgrass, spreading into mudflats and channels where California cordgrass can’t stand to have its roots wet, but it sends out dense rhizomes (root runners like those produced by strawberries). From them, stems stretch over 6 feet tall. As the sharp green stems grow, they block most of the sunlight that normally reaches the mud surface. Native cordgrass rarely grows taller than 4 feet and creates relatively little shade.
Belowground, out of sight but potentially more troubling for the small invertebrates that are an important source of food for shorebirds and fish, hybrid Spartina produces a lattice of incredibly dense roots that trap sediment washing in on the tide. Over time, hybrid cordgrass creates islands that rise over a foot above the surrounding mudflats. This spread reduces feeding habitat for the hundreds of thousands of shorebirds that rely on mudflats in San Francisco Bay and nearby estuaries during migration and winter. An endangered bird, the California clapper rail, needs open channels for feeding and cannot use the dense thickets that form in channels invaded by hybrid Spartina.
My own research shows that strong differences exist between the way California cordgrass affects species higher up in the food web and hybrid Spartina’s impact on organisms. Previous studies in other marshes show that the sediment around plants often contains more invertebrates than do nearby mudflats, probably because stems and roots slow the water movement, prevent erosion of sediment and reduce soil temperatures by shading. If the simple presence of vegetation helped invertebrates, you’d expect to see more of them within the hybrid. However, my results don’t show this. California cordgrass contained more invertebrates than did mudflats, but the hybrid contained about the same number or fewer organisms than did unvegetated areas. This result is significant because it indicates that hybrid Spartina may be so dense that even the smallest organisms cannot find space to live within it.
This invasion is not just a problem for the creatures that fly or crawl in the bay. Some of the channels invaded by hybrid Spartina carry rainwater from storm drains to the bay. If they are filled by cordgrass, they can carry less water. Navigation channels could be similarly clogged. Public access to the shoreline may be affected. People who enjoy walking along the mudflats, fishing from shore or digging for clams will find their way blocked by a wall of 7-foot-tall grass. Blocked bayfront views, which are valuable partly because residents enjoy watching wildlife in the bay, could lead to decreased property values. Hybrid Spartina will eventually spread to the enormous restoration project planned at the South San Francisco Bay Salt Ponds, threatening that project’s ability to recreate native wetland habitats. And because the seeds of hybrid Spartina float on tidal currents, it can spread to other ecologically sensitive areas.
To address these concerns, a team of professors, research associates, graduate students, technicians and undergraduate interns are using tools that range from low-tech (digging worms from the mud) to high-tech (satellite global-positioning technology) to describe the biology and impacts of hybrid Spartina in order to focus control efforts.
The UC Davis team joined international researchers at a conference on invasive Spartina in San Francisco this past November to report the results of four years of research. Research associate Debra Ayres and graduate student Christina Sloop, using DNA techniques to study the process of hybridization, have found that most of the spread of Spartina in San Francisco Bay can be attributed to “super hybrids,” plants genetically programmed to grow fast, self-
Professor Don Strong and his students have calculated the growth rate of hybrid Spartina, finding that it is growing so fast that it doubles the area covered in less than five years.
Ted Grosholz, Cooperative Extension associate specialist, and Lisa Levin of Scripps Institute of Oceanography study benthic invertebrates (“benthic” refers to organisms living at the surface of the sediment). By comparing the chemical composition of organisms living in the marsh to the composition of hybrid Spartina and other possible food sources, they learned that the hybrid Spartina is eaten by only a few organisms—mostly species that do not serve as the main food source for shorebirds or fish. Instead, the marsh food web is supported by a thin layer of algae growing on the mudflats and in gaps between the stems of native cordgrass. Postdoctoral scholar Christy Tyler found that this algae is more productive within the native cordgrass than within hybrid Spartina.
Meanwhile, undergraduate Nicole Christiansen completed an internship in the Grosholz lab and found that more invertebrates are in the mud closer to the shore, the areas most vulnerable to Spartina invasion. Taken together, these results show that as hybrid Spartina spreads, the base of the food web will suffer.
While the data paint a frightening picture of this invasion, some of the UC Davis team’s research can be applied toward determining the best way to control the hybrid’s spread. Graduate student Caz Taylor used computer simulations of the cordgrass invasion in Willapa Bay to determine the most cost-effective schedule for removing Spartina. She found that where reproduction is fast, as in San Francisco Bay, it is most cost-effective to first remove the isolated islands that are scattered across the mudflat, to prevent more islands from forming, before moving on to the meadows that stretch along the shoreline.
Cordgrass control efforts in the San Francisco Bay area are coordinated by the Coastal Conservancy’s Invasive Spartina Project. Efforts began in 2003, but the work has been constrained by the need to avoid harming California clapper rails during the spring and summer nesting season, while still removing vegetation before the plants can set seed in mid-fall. While mechanical methods, such as digging out the plants or covering them with black fabric to block sunlight, can work for small areas of Spartina, these methods are labor-intensive and cannot work fast enough to outrun the spread of the invasion. Removing the more extensive infestations requires the use of herbicide. Last fall, 16 sites covering 435 acres around San Francisco Bay were treated.
The consensus among researchers at the conference was that the problem in California is manageable but must be acted upon quickly before San Francisco Bay looks like Willapa Bay, where 100 years of cordgrass invasion have caused millions of dollars in losses to shellfish and salmon habitat. There, islands of Spartina look like “bacteria in a petri dish” coalescing into meadows, says UC Davis graduate student Janie Civille. A researcher from England who has studied a similar invasive cordgrass in that country was asked whether the increase in elevation could actually protect San Francisco Bay marshes from the rise in sea levels caused by global warming. He replied that shoreline areas in England already experiencing rising sea levels have seen no benefits from the presence of cordgrass.
Graduate students often struggle to explain to their families why anyone should care about their esoteric research project. Digging in the mud at Point Reyes may not seem like an obvious way to solve an ecological disaster in San Francisco Bay, but my work forms one piece of a much larger puzzle. These types of collaborative projects provide learning opportunities for ecologists at all levels, undergraduates through grad students and professors, but more importantly, they allow us to assemble a plan to protect California’s ecosystems.
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Elizabeth Brusati ’95 finished her Ph.D. in ecology in fall 2004. She currently works for the California Invasive Plant Council in Berkeley. Photos by Elizabeth Brusati and Ted Grosholz.
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