Anything can be an alien: seaweeds…
Do you dream, perhaps, of a Mediterranean vacation, floating silently over the teeming life of warm, shallow sea grass beds, trolling slowly for your dinner? When you go, you may catch more seaweed than fish on your line. Along the northern coast of this inland sea, the "killer" green alga called Caulerpa taxifolia is rapidly taking over. Divers first noticed Caulerpa in a small patch outside the Monaco aquarium in 1984. Recognized as a tropical species commonly used in aquaria, no one thought it would survive the cool water temperatures of winter. But this seaweed menace proved more robust than expected, and spread quickly along the coast into France and Spain, and as far east as Turkey.
Today, Caulerpa's feathery green fronds and snaking root systems cover over 3,000 hectares (7,000 acres) of shallow seabed. And cover they do: rocks, sand, mud, seagrass beds; nothing is safe from its encroachment. Although Caulerpa is not toxic to humans, fish and sea urchins avoid it and seek more familiar ground-no easy task, when Caulerpa covers up to 100% of the ground, down to a depth of 35 m (114'). So much for catching dinner.

...plants...
Perhaps you are eschewing a sun-drenched holiday to explore the misty rainforest estuaries of the Pacific Northwest?
Beware the invading smooth cordgrass, Spartina alterniflora. A beloved native of the east coast's signature salt marshes, this grass is now a scourge of west-coast estuaries, where its characteristic circular patches expand across the tidal flat and displace native mud-dwellers.
Smooth cordgrass in the Pacific was first discovered in Willapa Bay, Washington, in the late 1800s. Its precise origin remains a mystery, but it was probably introduced accidentally, mixed in either with oyster shipments or with the rock and dirt ballast of wooden sailing ships. Spartina was later planted in other Northwest bays to control erosion and create duck (and duck-hunter) habitat.
Simply by growing along the water's edge, smooth cordgrass drastically alters the shoreline. Its tall, stiff stems and long green leaves slow the flow of water, causing sediments to build up so high that the growing marsh is raised above the tideline. The semi-terrestrial meadow is no longer a suitable habitat for certain intertidal clams, for clam diggers (there goes dinner, again), or for the thousands of migratory and resident birds that depend on these estuaries for food. Boat access is affected too: with the ocean now farther away, you can no longer simply slide into your kayak from your backyard.
Invasive tendencies seem to run in the family: a second species of Atlantic cordgrass is also invading the Pacific, a South American cordgrass is invading North America, and worst of all, an English cordgrass planted widely in Europe, China, and Tasmania, is spreading rapidly around the world.

(photo:Spatina alterniflora in Willapa Bay, Washington)