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Globetrotting Grass—Chronicles of a North American Invasive Species
Plants & Gardens News | Volume 21, Number 3 | Fall 2006/Winter 2007
by Niall Dunne
Spartina alterniflora invading along the shoreline of San Francisco Bay (Photo by Jenica Allen).
There's a lot of noise in the media these days about exotic invasive plants (many of them of horticultural origin) degrading and destroying North America's natural habitats—and how these invaders need to be stopped, chopped, and quickly phased out. This is a welcome development.
What we don't hear much about, however, at least not in this country, are the North American plants causing problems in exotic ecosystems. They are legion. For instance, Monterey pine (Pinus radiata) is disrupting moist eucalypt forests in Australia. Late goldenrod (Solidago altissima) and giant goldenrod (S. gigantea) are fouling up meadows and conservation areas the length and breadth of continental Europe. Woodbine (Parthenocissus inserta) is running amok in the forests of Ukraine. Surely these plants have lessons to teach us about the complex, global phenomenon of human-facilitated ecological invasion.
This is the story of one North American invader—perhaps the "godfather" of them all—which is garnering a lot of attention from invasion biologists at home and abroad: smooth cordgrass, or more formally, Spartina alterniflora. This salt-marsh grass from the East Coast is one of the most intensely studied invasives around (entire conferences have been organized to find ways of controlling it). Researchers have found that the plant is not only capable of radically altering the estuarine ecosystems it colonizes, it also has been undergoing rapid and sometimes dramatic evolutionary changes in response to its newly adopted environments.
Portrait of an Accidental Villain
Spartina alterniflora is native to the Atlantic and Gulf coasts of North America, where it's the dominant plant of the intertidal zone. It's not a bad-looking grass: erect, smooth-stemmed, dark green, four feet high, with multiple foot-long late-summer flower spikes. But you don't really see it in horticulture—probably because not too many gardens are equipped with functioning salt marshes. In its home range, this deciduous perennial is a kind of one-stop deluxe salad bar, anchoring the estuarine food web by producing enormous amounts of tasty detritus for bottom dwellers to munch on. It also provides important structural habitat for invertebrates, fish, waterfowl, and wetland mammals.
Smooth cordgrass has a very high stem density that allows it to trap large amounts of sediment, and this has made it a popular choice for estuarine reclamation and erosion control, as well as for stabilizing mudflats to keep shipping lanes open. The species has been planted all around the world for these purposes, in places far away and exotic as China, New Zealand—and California. (It also has a long history of accidental introductions.)
Okay, so what's the catch?
Well, because of its adaptability, high growth rate, large seed production, and capacity for vegetative reproduction (it spreads by underground rhizomes)—and maybe half a dozen other reasons—smooth cordgrass has become highly invasive in many of the areas where we've planted it. Its tale is yet another sad riff on that ancient theme of humans trying to get one over on nature but nature having other ideas.
The negative impacts of invading Spartina alterniflora include the displacement of native flora and fauna and increased coastal flooding due to sediment buildup and reduced tidal flow. Smooth cordgrass can outcompete many other marsh plants; in addition, it can invade lower down the shore due to its high tolerance for tidal submersion. As with marriage customs and lavatory systems in different countries, not all estuaries around the world work the same way: Many are algal-based and consist primarily of bare mudflats (often containing diverse populations of invertebrates). Invading Spartina can colonize these mudflats with ease and bury them under dense swathes of grass and sediment.
The most detailed studies of this phenomenon have actually been carried out in San Francisco Bay, where smooth cordgrass was introduced in the 1970s to stabilize levees. (Okay, so San Francisco isn't exactly "abroad," but it is far enough away from the East Coast for Spartina alterniflora to be considered exotic there.) The alien grass is taking over the bay's highly productive mudflats and thus threatening native invertebrate communities—and the fish, migratory shorebirds, and waterfowl that depend on them.
In a study published earlier this year, researchers from the Scripps Institution of Oceanography (using fancy isotopic labeling techniques) traced the flow of carbon and nitrogen through Spartina-infested tidal flats—sampling the animals they found and determining what they were eating for breakfast. The team concluded that the entire trophic structure of the mudflats (the hierarchy of it food web) is being altered from the bottom up. The microalgae and the surface animals that eat them (mainly bivalves and small crustaceans) are getting pushed out by the invading marsh grass and its associated subsurface feeders.
This is a classic example of what serious ecological invaders do: rewire ecosystems and generally leave them more impoverished in the bargain. How do they do it? Well, theories abound about how plants become invasive. A history of repeat introductions seems important. Finding a habitat that's vulnerable to invasion—perhaps because of recent disturbances—is often another key factor. It certainly seems to help if the invaders have certain life-history traits that make them good colonizers and competitors—adaptability, fast growth rates, and so on. Release from the attentions of natural enemies may also be a contributing factor. But there's something else, too: Species may develop invasive tendencies due to rapid evolutionary change. Think about it: When an exotic plant arrives in a novel environment, the selection pressure must be intense. It's do or die.
Survival of the Freakiest
Smooth cordgrass has undergone rapid, genetic-level changes in its adopted environments—sometimes subtle, sometimes not so subtle. Most dramatically, the plant has hybridized with other Spartina species to create brand-new species, two of them pernicious invaders in their own right.
It began in Poole Harbor, in the south of England, around 1870. Smooth cordgrass was unintentionally introduced there via shipping ballast, whereupon it quickly crossed with native Spartina maritima to form the hybrid S. ×townsendii, which had an uneven complement of chromosomes and was thus unable to set seed. However, subsequent chromosome doubling created a stable, fully fertile, and super-vigorous new species, S. anglica, which rapidly invaded tidal mudflats around Britain (as well as upper-intertidal marshes occupied by S. maritima) and has gone on to wreak even more havoc around the world than S. alterniflora. (Sometimes hybrid progeny acquire the best traits of their parents and end up being better competitors than both.)
What's that? The natural evolution of a new species within the last 150 years, with no evidence of divine intervention? That's quite a blow to the idea of Intelligent Design, but in and of itself not a very remarkable achievement for a plant: Hybridization has long provided the plant kingdom with a steady source of new species. The truly perverse part of the story is that Spartina anglica—the freakish offspring of our native marsh grass—was introduced to the Pacific Northwest in the 1960s as a forage plant for cattle and is now invading all the way up and down Puget Sound.
But that's not the all of it. In San Franscisco Bay, Spartina alterniflora has recently crossed with the native upper-intertidal species Spartina foliosa to create another fertile, vigorous hybrid that's invaded around 2,000 acres of marsh and tidal-flat habitat (the Scripps study mentioned earlier was actually conducted on this new hybrid). Scientists have found that S. alterniflora produces 21 times the amount of pollen produced by S. foliosa and is actually better at fertilizing the native's flowers than the native is itself. This adds insult to injury because, in the long term, S. alterniflora threatens S. foliosa with genetic extinction.
Smaller-scale evolutionary changes have also been detected in invading smooth cordgrass. In San Franscisco Bay, a dwarf form of the plant has appeared and is spreading rapidly. Though smaller than the regular form or "wild type" of the species, it has a much higher tiller (lateral, ground-level shoot) density, and this apparently gives it a competitive advantage in the upper intertidal zone of the marsh. DNA tests have confirmed that the changes are not due simply to variations in the environment.
A population of smooth cordgrass invading Willapa Bay, Washington, seems to have evolved a higher shoot density than normal. The population has been living without any natural enemies since its introduction in the 1870s. Recent greenhouse studies have shown that the invading plants have lost their resistance to insect herbivores. This may have occurred due to inbreeding in the population (as the researchers conclude). However, it's also possible that the loss of resistance may have been the result of an evolutionary trade-off reallocating resources to more pressing fitness needs.
These kinds of rapid evolutionary changes are widespread among invading species (even American ones), and it all seems like just a whole lot more bad news for the native species trying to cope with the unfriendly exotics. However, it needn't be a completely doom-and-gloom scenario. Native populations are in the same evolutionary boat as the invaders and under extreme selective pressure (after all, they're being attacked!). It's safe to say (and we're starting to see) that at least some of them are undergoing rapid change themselves to cope with exotic predators or competitors—they may even start to fight back.
Niall Dunne is the editor of Plants & Gardens News.