Outside magazine, March 1998
Imagine if suddenly at every party there was a boorish crasher, hoarding the hors d'oeuvres, draining the punch bowl, snarling at the guests. Picture your neighborhood swarmed by odd-looking ne'er-do-wells who loiter menacingly on every corner. First Tabby disappears. Then Spot. And then, good Lord, the Schwartzes next door have vanished and a cadre of green crabs has taken their place. Welcome to the reality of invasive species.
In the United States, almost half of the country's threatened or endangered species are in peril not because humans are out there shooting them or paving them over, but because they're being pushed aside by stronger, wilier alien species. By ecological thugs, actually: a jumble of escaped pets and more pathological pests that, after habitat loss, pose the most critical threat to biodiversity. Of course, human-aided invasion has been taking place for as long as people have been moving around the globe, bringing paying passengers, such as cows and wheat, and stowaways, such as weeds, rats, and plague, along with them. And some pests migrate under their own steam. (Still others don't need importation: They can be normally mild-mannered critters turned to monsters on their home turf by a sudden transformation in the landscape — as when forests are cleared for agriculture or housing — or by a disruption in the food chain.) But given the right conditions — when, say, a tasty niche becomes available to some ecological time bomb that is aggressive and adaptable; an unfinicky eater possessed of hardiness, a short reproductive cycle, and high genetic variability; and that can claim a cozy, symbiotic relationship with human beings — well, nastiness will surely follow.
And these days, neighborhoods are going to ruin more quickly than Darwin ever dreamed. Of course, some scientists argue that the process is only natural. (After all, humans are part of nature, and nature works by constant change.) But evolution speciates as well as snuffs out, isolates creatures behind borders, in pockets, over long stretches of time, to make new ones through measured labor. Today, the balance between borders and time is badly out of kilter. In Hawaii, immigrant species are arriving two million times faster than the slow, historic pace that allowed the archipelago's glorious diversity to flourish. At the current frenzied rate, evolution is a reductionist game akin to an all-out barroom brawl in which only a few tattooed toughs are left standing. So peer closely at the perps in the following rogues' gallery. They or one of their pushy relatives may not skitter out from under your morning paper as soon as tomorrow or throw a shadow across your campsite by next weekend. But then again, they might.
THE GREEN CRAB
When the first set of long-stemmed green crab eyeballs poked its way up out of San Francisco Bay in 1989, the collective response from biologists and fishermen was concise: Uh-oh. Since the dollar-pancake-size European native (previous page) eats every living thing it can get its formidable claws on — with the lone exception of the unpalatable sea urchin — it can forage and destroy a much larger zone than native crabs, which it also eats. A green crab can even survive out of water for up to a week, in contrast to the average crab's several hours, a creepy ability that presents the image of several million iridescent crabs laying long siege to Fisherman's Wharf, hunting up some taffy and cotton candy.
On the Eastern Seaboard, where it arrived in the early 1800s, the green crab has not only dined deeply into shellfish harvests, but may actually have provoked evolutionary change in the shapes of marine snail shells (they're thicker now). Drastic poisoning, fencing, and trapping seem to have merely amused the invertebrate. And eight years after invading the Bay Area — the crab likely traveled to San Francisco in seaweed-packed shipments of bait worms from Maine — it's already hitched 520 miles northward to Coos Bay, Oregon; the Dungeness crabs in that region's rich fishery are now poised to be eaten by their green cousins, as is the $20 billion annual shellfish haul in Puget Sound, just a few hundred miles upcurrent.
Back in San Francisco, though, it's quite possible that residents may soon forget about these particular crabs. Not that they're going away. But the green crab is merely the current headliner in the marine freak show presently wowing the West Coast — an act with a closed-ended gig. To wit: Fully 90 percent of San Francisco Bay's bottom-dwelling biomass is alien, and a new, potentially more ill-natured species arrives, on average, every 12 weeks.
THE MELELEUCA TREE
That lovely little sapling you planted last spring? It wants to kill you. Or so it seems in south Florida, where in 1906 developers introduced the northeastern Australian paperbark tree in the hope it would drain swamps, thus creating land more profitably occupied by tract homes and miniature golf courses. Cheered by the melaleuca's subsequent success, one confused Johnny Appleseed airdropped millions of seeds over the Everglades in 1936. (He wanted to "restore" trees to the naturally grass dominated ecology.) Because it sucks up five times as much water as native grasses, melaleuca is accelerating the already disastrous drying of the Everglades. Impenetrable thickets of the stuff strangle 60 to 100 percent of native wetland plants, and today melaleuca covers nearly half a million acres and can spread at a Jumanji-like rate of 50 acres per day. Efforts to control it — including armies of machete-wielding migrant workers — have encountered a spectacular self-defense mechanism: Detecting external stress, the melaleuca ejects millions of tiny seeds in clouds of grainy dust. Hack at it, poison it, set it alight — you're just speeding up the invasion process. Most insidious of all, this pyrophyte has managed to change Florida's normal fire ecology — one of low-intensity groundfires — to one more closely resembling its native Australia's. The "paper" bark wicks flames into the canopy, where oils in the leaves and wood burn at 1,500 degrees, hot enough to explode surrounding native hardwoods like popcorn. The soil, often burned to a different chemical composition, can be left coated with an oily residue, a sort of vegetative napalm resistant to virtually all growth. Except, naturally, more melaleuca.
THE BROWN TREE SNAKE
This poster-reptile for the invasive species movement has reduced Guam, where it arrived shortly after World War II in the belly of a military transport, to a credible version of hell. The island's lush forests, once snake-free save one tiny, blind species, now swarm with all manner of insects, the result of nine of 11 native bird species having been wiped out by the snake. Certain parts of Guam — parts you really don't want to visit — now have one of the highest snake-population densities in the world: 12,000 per square mile, about ten times greater than total snake density in the Amazon. An unremarkable predator in its native and snake-rich Australia and New Guinea, the expatriate version can grow to ten feet in length. Brown tree snakes terrorize the dark, dripping from trees and biting people in their beds and babies in their cribs. And as if to purposefully spread more darkness, they slither across power lines, causing blackouts every three days on average.
Boiga irregularis adapts in ways that make it an ideal surburban invader. It eats hamburgers, polystyrene foam, live poultry, even puppies, which it subdues with a combination of constriction and mild venom before swallowing whole. Resourceful hitchhikers, brown tree snakes have been caught trying to depart Guam coiled inside pallets of bombs and power steering units. Once en route, they have a good chance of arriving alive — and because a female can store sperm for several years, starting a family when settled. An estimated seven brown tree snakes have made the seven-hour, subfreezing plane trip to virtually snakeless Hawaii, where their advent would likely mean the end of the state's few remaining native bird species. Four snakes survived the trip (they were later killed). For now, slumbering infants seem safe. But the potential damage to the Aloha State — including its tourist-based economy — is so immense that President Clinton has authorized the exploration of chemical and biological warfare against the serpent.
THE ZEBRA MUSSEL
The Germans tagged the innocent-looking zebra mussel with the name Wandermuschel for a reason: Its enthusiasm for reproduction and its designs on world domination verge on the terrifying. As an eighteenth-century freeloader attached to the hulls of barges plying Europe's canal networks, the zebra mussel spread steadily westward from its home in the Caspian and Black Seas, hopping the English Channel by the early nineteenth century (perhaps in a shipment of fish) and, a little over a decade ago, besting the Atlantic. Ah, America, land of opportunity. Unchecked by Old World predators, the zebra mussel immediately began its conquest — first the Great Lakes, then the Mississippi, some 20 states and counting. It adheres en masse to any surface, destroying boat engines, jamming locks, sinking buoys, and clogging water lines of all kinds — including cooling intakes at nuclear power plants (they've been measured at a density of 700,000 per square meter). The female can produce up to a million eggs per year, which fan out like sinister, waterborne glue, ready in turn to begin expanding and replicating, replicating, replicating. Undiscriminating pests, zebra mussels will also attach themselves to other animals, such as crawfish and native mussels, sealing them shut; 150 mussel species in the Mississippi drainage, the world's most diverse concentration, are now in peril. Such spawning has birthed a widespread antizebra offensive, expected to cost as much as $5 billion by 2002. Desperate tactics include blasting with fire hoses and sound waves, gassing, chlorination, and hammering with electromagnetic pulses — all to little effect. And more bad news: They're still restless. Zebra mussels seem poised, inevitably, to leap the Continental Divide and into western rivers and lakes. Already, individuals have been intercepted trying to smuggle themselves into California aboard trailered pleasure boats.
THE FERAL PIG
Voyaging sailors long ago learned that turning a few pigs loose on shore is the easiest form of animal husbandry. Clever and resourceful (go figure), pigs can live almost anywhere, on almost anything. Drop some off on an island or a remote coast, and there will almost assuredly be pork on the hoof awaiting the return voyage. Not the Babe-licious kind, though: Left to its own devices even for a couple of generations, the juicy pink domesticated swine quickly reverts to a more free-range entrëe — an ornery 300-pound beast with stiff bristles, sharp tusks, and a whole bunch of abandonment issues. Called razorback hogs in the Southeast, wild boars in the Southwest, and just plain pigs in Hawaii, feral pigs are distinctly more eager to turn the dining tables than their barnyard cousins. They will gut unwary dogs and charge incautious humans without hesitation. In especially piggy woodlands, it's not wise to walk unarmed at dusk. For forests, the unpleasant truth is that their diet is extremely unchoosy — including fungi, insects, eggs, plants, and unlucky small animals — and their manners are worse yet: They root up soil so effectively that erosion and pig-polluted runoff choke streams and coral reefs. Disease-carrying mosquitoes breed in their wallows. And because they often favor nonnative foods and range widely, pigs tend to accelerate the spread of other exotics, sowing the seeds in their manure as they go. Despite this litany of porcine offense, a public weaned on a certain E. B. White character gets uncharacteristically squealish when wildlife officials try to trap or shoot feral pigs mucking up sensitive environments. They probably forgot all about McRibs.
Photographs by Liittschwager/Middleton; James Balog; Ed Reschke/Peter Arnold; Tui de Roy/Minden Pictures