For a long time, veteran environmental journalist Fred Pearce thought in stark terms about invasive species: they were evil interlopers spoiling pristine 'natural' ecosystems. Most conservationists and environmentalists share this view. But what if the traditional view of ecology is wrong--what if true environmentalists should be applauding the invaders?
Fred Pearce acknowledges that there are horror stories about alien species disrupting ecosystems but most of the time the tens of thousands of introduced species usually swiftly die out or settle down and become model eco-citizens. The case for keeping out alien species looks increasingly flawed he reports in his recent book,
The New Wild: Why Invasive Species Will Be Nature's Salvation.
It is time to stand back and look at the evidence when we come to judge and respond to 'invasive' species writes Pearce. At this point, pretty much the majority of species is invasive rather than endemic.
Some interesting examples include water hyacinth in Africa, Zebra mussels in Lake Erie, and the estuary of the San Francisco Bay.
Water Hyacinth
This is one of the fastest growing and most aggressive plants on Earth. It usually spreads by sending out short runner stems that produce daughter plants, but it also produces seeds that can survive in soils or lake sediments for up to fifteen years. Theoretically, a single plant can produce 140 million offspring a year. An 'infestation' typically doubles in size within a week. The spectacularly fecund colonist has spread to more than fifty countries.
In 1998, the weed had covered four-fifths of the Ugandan shore of Lake Victoria, and ships took five hours to push through the weed to dock at the Kenyan lake port of Kisumu. In spite of all attempts to eliminate the weed it seemed hopeless. And then everything changed. Less than a year later, the weed was in rapid retreat, Kisumu's port was open, cargo boats moved with ease, and fishers were back in business. What happened? Some claimed it was a success for the South American weevils which were introduced to eat the weed's leaves. They must have helped, but what had changed dramatically in the intervening months was the quality of the lake's water. It had been flushed clean, and with that the weed had departed.
This lends credence for a theory that the real problem in Lake Victoria was never so much the weed itself as the choking pollution. Over the years during which the weed had taken hold, the lake and rivers that delivered water to it had filled with sewage from cities such as Kampala and Kisumu. Effluent from sugar factories, paper mills, tanneries, and breweries proliferated across the lake's catchment, and silt washed into the lake as forests were chopped down and soils eroded. This pollution had turned a once-clear, well-oxygenated lake into a muddy body with no oxygen in its bottom layers. Water hyacinth, raised in the fetid swamps of the Amazon loved it. But then things abruptly changed.
In 1998 there was a major El Nino event in the Pacific Ocean. It caused unusual weather around the world, including massive and unseasonable rains in East Africa. Rivers brought torrents of fresh water that cleansed the lake of its accumulated pollution. The weed was suddenly starved of the rich soup of nutrients on which it depended.
As Pearce reports, the story fits, as do subsequent events. There has been no big El Nino since 1998 and no exceptional rains to flush the lake clean. The pollution is building up again, and in 2013 hyacinth was advancing once more, the bays and beaches covered in weed.
The conclusion--it appears that blaming the alien weed--and seeking solutions to the lake's problems by trying to remove it has been the wrong diagnosis, leading to the wrong treatment. Lake Victoria remains sick. It may never be freed of weed until its wider ecological problems are solved.
Zebra Mussels
Zebra mussels in Lake Erie don't look obviously tough. They are tiny and largely immobile. But they spread easily, attach to any surface, and can attain extraordinary concentrations. Most importantly, they didn't mind the lake's pollution. As water filter feeders, they ran vast amounts of the lake's vile water through their bodies in order to extract the plankton, their main food. This was good news for the lake. Zebra mussels turned out to be the best janitors Erie every had. They were almost the only filter feeders able to swallow the gunk and survive. Each of the tiny animals was gulping down, filtering, and excreting as much as a quart of water a day. Billions of the creatures can filter a lot of water that way. During the 1990s, there were probably enough of them to process the lake's entire volume in a week. Along with the plankton, they ate up most of the pollutants suspended in the water, excreting it onto the lake floor. Along with human cleanup efforts inspired by the Clean Water Act, the effect on the lake water has been transformative. You can see down for thirty feet in some areas, compared to less than six inches half a century ago. As light has penetrated the lake, some aquatic plants have revived. They, in turn, have become nurseries for fish such as the yellow perch.
Clams, and shrimp-like creatures have declined because the lake's plankton are being consumed by the zebra mussels. And there are fouling problems on the lake floor from their excretion. Yet, despite the negative aspects, a new ecological environment is rapidly emerging in the lake, with its own checks and balances. As Pearce says, "We cannot tell quite how it will work out. The revival of the ancient sturgeon which has almost been hunted to extinction may limit and perhaps even reverse the proliferation of zebra mussels. So the battle of supremacy between the two species might now get interesting. But surely all this is better than a 'dead' lake."
San Francisco Bay
San Francisco Bay is the most invaded estuary in the world. Some alien species are crucial and must stay. A third of California's native butterflies would be in trouble without the state's alien eucalyptus trees, and the bees need eucalyptus nectar during winter.
The bay is dynamic, constantly being purged by freshwater floods. It is after those events that introduced species make their move, but if they didn't something else would.
It is this dynamism that makes the bay what it is. Those conservationists who bemoan every change in the bay, every arriving alien species, and every disappearance are missing the point. The constant change is the reason why, despite more than a century of bombardment by alien species, San Francisco Bay is in such robust health.
Pearce concludes, "Much of what ecologists have previously regarded as natural, pristine and permanent now seems to be artificial, accidental, and recent, with different species constantly moving in and out. Notions of being 'native' and 'alien' have little meaning. Dynamism and change are the norm in nature.
