杏吧原创

ISOLATION has profound effects. It can pull a group of people together or
drive an individual over the brink. In the natural world, it helps to create new
species. Indeed, without physical barriers to separate organisms the Earth would
probably have only half of its present biodiversity. So, for a professional
botanist like Wendy Strahm, a visit to a new island should be as enticing as the
presents under the tree on Christmas morning鈥攆ull of encounters with
surprising plants that have evolved in isolation from the rest of the world. But
it鈥檚 usually more like just another day at the office.

鈥淚t鈥檚 depressing. I go to any island and I already know the flora,鈥� says
Strahm, who has clocked up years of fieldwork on the tropical isles of the
Indian Ocean and is now chief botanist for the Species Survival Commission of
the IUCN, the World Conservation Union, in Gland, Switzerland. 鈥淭he same trees
have been planted and the same plants have been introduced. We鈥檙e turning the
world into something identical everywhere.鈥� This effect has been dubbed the
McDonaldisation of the biosphere. The Big Macs and Coca-Colas of the natural
world, plants such as lantana and strawberry guava and animals like rats,
sparrows and cats, are steadily invading the Earth鈥檚 ecosystems and ousting the
natural inhabitants.

Slowly, conservation biologists and ecologists are realising that
homogenisation of the world鈥檚 species is as big a threat as global warming,
deforestation or desertification鈥攁nd just as deserving of policy makers鈥�
attention. 鈥淏iological invasion is really a bigger impact than a lot of the
horrible things we hear about all the time, like global warming,鈥� says David
Burney, a palaeoecologist at Fordham University in New York City. 鈥淢any of the
impacts of global warming could in time be reversed, but once you homogenise the
biodiversity of the world, there鈥檚 really no going back from that.鈥�

Even today, relatively few biologists鈥攁nd still fewer
nonscientists鈥攈ave any inkling of the scale of the problem. Even those
studying alien species tend to get engrossed in trying to understand and manage
local populations rather than considering the big picture. 鈥淚t鈥檚 very easy for
someone to see what is happening around them, but it鈥檚 not easy to see over the
horizon鈥攖hat it鈥檚 just a local example of something that鈥檚 happening all
over,鈥� says Peter Vitousek, an ecologist at Stanford University in Palo Alto,
California.

Alien invasion

Vitousek first came across the idea that species invasions were a global
problem in The Ecology of Invasions of Animals and Plants, written in
1958 by the British ecologist Charles Elton. 鈥淚 read the book in an
undergraduate class in 1969, and it persuaded me not to be a political economist
but to be an ecologist,鈥� he recalls. It was not until the mid-1990s, however,
that he and a group of colleagues fully appreciated Elton鈥檚 insight. 鈥淚 can鈥檛
say that one night after many pitchers of margaritas we all of a sudden decided
that this is it,鈥� says Vitousek, 鈥渂ut we were all working on invasions and on
other aspects of global change, and over a period of years the feeling got
stronger that we were missing an important point by not talking about invasion
as global change.鈥�

Global homogenisation could have a huge impact on biodiversity. Ecologists
believe that isolation has been a major force in generating the rich variety of
life on Earth. Where there are physical barriers, different species can evolve
to make their living in much the same way without treading on too many toes. But
remove the barriers and extinctions are likely as these species fight for
resources, while other animals and plants encounter organisms they are not
adapted to cope with. 鈥淚f isolation is required to have as many species as we
have, then we鈥檙e in trouble,鈥� says Randy Westbrooks of the US Department of
Agriculture鈥檚 Animal and Plant Health Inspection Service in Whiteville, North
Carolina. 鈥淲e鈥檙e giving every species of plant and animal a chance to hitch a
ride all around the world.鈥�

A quick calculation shows just how important isolation has been in creating
biodiversity. Westbrooks, Vitousek and their colleagues recently plotted the
number of mammal species on each continent against the logarithm of the
continent鈥檚 area. The result was a straight line. In other words, the land area
of a continent predicts almost perfectly how many mammal species live there. But
if you extend that line out to the total land area of the Earth鈥攊n effect
pretending that the world is just one supercontinent鈥攖he total comes to
only about 2000 mammal species, or fewer than half the 4200 that actually exist
(see Diagram). And you get a similar set of results for calculations on
other groups of organisms.

So the breaking up of the world鈥檚 landmass into continents and islands has
approximately doubled terrestrial biodiversity. Ocean currents and land barriers
probably have the same effect in the seas. And history shows that when isolation
breaks down, it can lead to extinctions. When the Panamanian land bridge first
joined North and South America just 3 million years ago, for example, the more
robust North American mammals almost completely wiped out South America鈥檚
marsupial fauna. 鈥淚t鈥檚 clear that regional mixing does cause loss of
biodiversity,鈥� says Vitousek.

All mixed up

Of course, when humans introduce alien species they are unlikely to generate
mixing on the scale made possible by the Panamanian land bridge. But there are
already examples where the McDonaldisation of island habitats is well under way.
Mauritius in the Indian Ocean still contains 685 of its original 765 species of
land plants, but is now also home to 730 introduced species that have
established naturalised populations in the wild. The Seychelles Islands, around
1500 kilometres to the northwest, support 300 native land plants (down from the
original 325) and 350 naturalised introductions鈥攎any of which are the same
as on Mauritius. 鈥淒iversity is becoming more and more homogenised between the
two,鈥� says Strahm. Adding insult to injury, many of the lost native species owe
their extinction to introduced ones. Likewise, Mauritius has 9 native and 15
introduced land birds, while the Seychelles have 12 native and 11
aliens鈥攂ut only 3 of the 11 are different from those introduced to
Mauritius.

鈥淚f you go to New Zealand, you鈥檒l feel like you鈥檙e in England,鈥� she
continues. 鈥淵ou鈥檒l see blackbirds and chaffinches.鈥� Indeed, 36 of New Zealand鈥檚
113 species of land birds have been introduced by humans, notes Strahm鈥檚
colleague Mick Clout, a New Zealander who heads the IUCN鈥檚 Invasive Species
Specialist Group. 鈥淭he biodiversity of birds in New Zealand is as high as it鈥檚
ever been,鈥� he adds. 鈥淏ut we鈥檝e lost the moa [a large, flightless relative of
the kiwi], we鈥檝e lost the biggest eagle the world鈥檚 ever seen, and we鈥檝e gained
the sparrow and the starling.鈥�

Oceanic islands such as New Zealand and Mauritius have sustained the worst
ecological damage from invading species so far, but continental areas have not
escaped unscathed. Nearly 2000 species of introduced plants have become
naturalised in Australia, for example. Between 2000 and 4000 naturalised plant
species鈥攄epending on who you ask鈥攊nfest the continental US. In
Britain, 43 per cent of wild plant species have come from elsewhere, according
to data compiled by Vitousek and his colleagues.

What鈥檚 more, the rate of species introductions appears to be increasing. In
San Francisco Bay between 1850 and 1960, for example, one new invader took hold
every 55 weeks, on average. From 1960 to 1990, the average was one new species
every 14 weeks, says Andrew Cohen, a marine biologist with the San Francisco
Estuary Institute in Richmond, California. The main reason for the rise, Cohen
thinks, is the boom in international trade which brings more ships鈥攁nd
more accidental stowaways鈥攆rom a wider range of foreign ports into San
Francisco.

Invasions are also becoming more frequent in terrestrial ecosystems, and for
similar reasons. Many of the new players in expanding world trade are countries
in Asia and South America, where biodiversity is highest and programmes for
screening potential invaders are underdeveloped, notes William Gregg, from the
US Geological Survey鈥檚 Biological Resources Division in Reston, Virginia.

But sheer numbers of exotic species don鈥檛 mean much. We need to distinguish
between catastrophic invaders and minor annoyances, notes Harold Mooney, an
ecologist at Stanford University who heads an invasive-species task force for
SCOPE, the Scientific Committee on Problems of the Environment. As a general
rule of thumb, many ecologists reckon that about 10 per cent of species
introductions actually 鈥渢ake鈥� and establish naturalised populations, and about
10 per cent of these become a nuisance. But predicting which 10 per cent has
proved to be a challenge. 鈥淚t hasn鈥檛 been all that successful, quite honestly,鈥�
says Mooney. One hallmark of a problem invader is that it often creates an
ecological role for itself that didn鈥檛 exist before in the ecosystem it has
infiltrated. A good example is the devastation cats, rats and pigs have caused
to ground-nesting birds on islands with no native land-based predators. And on
Hawaii, molasses grass (Melinis minutiflora) introduced from Africa
produces much more dead foliage than native grasses, causing more frequent and
widespread fires that drive out the native plants, says Carla D鈥橝ntonio, a plant
ecologist at the University of California at Berkeley.

To really put invasive species into the context of other threats to
biodiversity, however, means moving beyond case studies like these to look at
broader trends. An as-yet unpublished study by two environmental groups in the
US does just that. For a quarter of a century, land managers from The Nature
Conservancy have painstakingly compiled a list of some 6500 species in North
America that are under threat of extinction, along with the reasons why each
species is in trouble. When researchers from the Conservancy and the
Environmental Defense Fund sifted through this database, they found that alien
species were the second most common reason for the problem, affecting 49 per
cent of the species. Only habitat alteration, which affected 85 per cent, was a
bigger threat. Experts believe this ranking will hold good elsewhere in the
world as well, though there are as yet no good studies outside the US.

Under attack

Another study awaiting publication gives us something new to worry about.
Thomas Stohlgren and his colleagues at Colorado State University in Fort Collins
surveyed plant communities in the Colorado Rockies and the central grasslands of
Colorado, Wyoming, South Dakota, and Minnesota. They expected to find fewer
exotic species on sites with vigorous, diverse natural vegetation. 鈥淭here would
be no room at the inn, so to speak,鈥� says Stohlgren. Instead, they found that
the most heavily invaded regions were those with the highest diversity of native
plants. 鈥淭he weeds are invading our hotspots of native diversity. There is room
at the inn,鈥� he says. 鈥淭he high-diversity areas are under attack.鈥�

The best way鈥攊ndeed, the only practical way鈥攖o combat the threat
of introduced species is to keep them out in the first place. Ideally, countries
would simply block the movement of species likely to create problems鈥攂ut
to do that, scientists need a better understanding of which species these are.
Mooney鈥檚 task force has just launched a three-year programme to try to predict
problem species more accurately and develop better ways of keeping them in
check.

But if a potentially dangerous species does take hold in a new country the
priority should be to eradicate it before the trouble starts. 鈥淪o many of these
invasives problems have not been nipped in the bud,鈥� says Strahm.

So far, this call for preventive action has caused barely a ripple in most
policy makers鈥� ponds. The US government鈥檚 noxious weeds programme, for example,
which Westbrooks coordinates, has a budget of just $450 000 for detecting
and eradicating early invasions throughout the US. 鈥淚t doesn鈥檛 take but a few
projects to suck that up,鈥� he says. With a few exceptions鈥攎ost notably New
Zealand鈥攆unding is just as miserly elsewhere in the world. And meanwhile,
the global-trade juggernaut rolls on.

All this often leaves the ecologists feeling like David battling Goliath.
They can only hope that public perception of invasive species as a major form of
global change will grow, giving them just the sling they need for the fight.