Spartina alterniflora (smooth cordgrass) is the Jekyll and Hyde of North American aquatic plants. On its native east coast, it is just another helpful plant. It prevents salt marshes from being eroded, snow geese love to nibble at its roots, crabs and fish feast on the detritus it produces, and in Virginia and several other states it helps to define the high-water mark. If this 鈥渟partina line鈥� moves, it will affect the amount of salt marsh owned by the state, and a person can have their property reassessed and their taxes raised or lowered.
The contrast with the spartina in the Pacific north-west couldn鈥檛 be more stark. Introduced a century ago, the plant has become the guest who takes over the house, a relentless invader threatening animals and humans alike. It produces so much biomass that it can turn shallow water into marshy land within a decade and it now threatens to destroy oyster beds and an important bird sanctuary.
Why does it behave so differently on the west coast? Ecologist Donald Strong of the University of California, Davis, and colleagues have established that the species hybridised with native Californian cordgrass, S. foliosa, to produce taller plants that spread more aggressively and grow lower down towards the low-water mark. Worse, the latest research suggests that the Californian hybrid spartina is evolving. Unlike Spartina alterniflora on its native Atlantic coast, the Californian variety can self-pollinate, which accelerates its spread.
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How did it get to the west coast in the first place? It鈥檚 thought that spartina seeds from the east hitched a ride to California on transcontinental trains that carried gold prospectors and oysters 100 years ago. The oysters were loaded onto trains from New York and New Jersey and dispatched westwards to replenish the Pacific stocks that had begun to dwindle 鈥� victims of pollution and sediment from gold mining in the Sierra Nevada. This ultimately proved to be insufficient and the San Francisco Bay oyster industry eventually moved north to Willapa Bay, Washington, around 1940. The spartina probably moved north with it. But it found its way back to San Francisco in the 1970s thanks to the US Army Corps of Engineers, which deliberately introduced the plant as part of a plan to reclaim salt marshes. The consequences were dire.
鈥淚n a decade, spartina turns shallow water into marshy land鈥�
The urgent question is: how best can it be controlled? A variety of methods have been tried, including the common weedkiller glyphosate. But now the authorities have hit on a new plan: biocontrol. They hope that introducing an aphid-like insect called Prokelisia marginata will kill the plant.
It remains to be seen if the plan will work. What is certain is that one man鈥檚 weed is another鈥檚 high-tide mark; a conserver of ecosystems in the east has become an unwanted invader in the west.
