Welcome weeds: How alien invasion could save the Earth
20 January 2011 by Garry Hamilton
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Far from ravaging threatened ecosystems, non-native species could be powerful
allies in the fight to save them
WHEN Ariel Lugo takes visitors to the rainforests of Puerto Rico, he likes
to play a little trick. First the veteran forest ecologist shows off the
beautiful surroundings: the diversity of plant life on the forest floor;
the densely packed trees merging into a canopy high overhead; the birds whose
calls fill the lush habitat with sound. Only when his audience is suitably
impressed does he reveal that they are actually in the midst of what many
conservationists would dismiss as weeds - a ragtag collection of non-native
species growing uncontrolled on land once used for agriculture.
His guests are almost always taken aback, and who wouldn't be? For years
we have been told that invasive alien species are driving native ones to
extinction and eroding the integrity of ancient ecosystems. The post-invasion
world is supposed to be a bleak, biologically impoverished wasteland, not
something you could mistake for untouched wilderness.
Lugo is one of a small but growing number of researchers who think much of
what we have been told about non-native species is wrong. Aliens, they argue,
are rarely as monstrous a threat as they have been painted. In fact, in a
world that has been dramatically altered by human activity, many could be
important allies in rebuilding healthy ecosystems. Given the chance, alien
species may just save us from the worst consequences of our own destructive
Many conservationists cringe at such talk. They view non-native species as
ecological tumours, spreading uncontrollably at the expense of natives. To
them the high rate of accidental introductions - hundreds of alien species
are now well established in ecosystems from the Mediterranean Sea to Hawaii
- is one of the biggest threats facing life on Earth. Mass extinction of
native species is one fear. Another is the loss of what many regard as the
keys to environmental health: the networks of relationships that exist between
native species thanks to thousands or even millions of years of co-evolution.
Innocent as charged
Such concerns have fuelled an all-out war. Vast sums are being spent on campaigns
to eradicate or control the spread of highly invasive exotics. Conservation
groups enlist teams of volunteers to uproot garlic mustard from local parks.
Government agencies fill waterways with poisonous chemicals to halt the advance
of Asian carp. Most governments have no choice but to join the fight: under
the terms of the Convention on Biological Diversity, signatory nations are
required to do everything they can to eradicate or control the spread of
threatening alien invaders.
Advocates for non-native species do not deny that they can sometimes create
major problems, particularly in cases where disease-causing microbes are
introduced into a new host population. But they argue that often the threat
is overblown. For one thing, many species are not nearly as problematic as
they are made out to be.
Take purple loosestrife, a Eurasian marshland plant frequently listed among
the world's worst weeds and the target of multimillion-dollar eradication
campaigns. It stands accused of destroying wetlands across North America,
where it arrived more than 150 years ago, but there is as yet no documented
evidence of any serious damage it has caused. Similarly, the notorious cane
toad, introduced into Australia in the 1930s to control pests of the sugar-cane
crop, is considered a major threat to the continent's unique fauna. Its highly
toxic skin has long been seen as a death sentence for unsuspecting native
predators, while its rapid spread is thought to have occurred at the expense
of other amphibians. Yet the first serious impact study on cane toads recently
concluded that they may in fact be innocent of all charges (New Scientist,
11 September 2010, p 18).
Even more surprising is the mounting evidence that many invaders are, in
fact, good citizens in their new environments. Salt cedar - a group of Old
World shrubs and trees belonging to the same family as the tamarisk - has
been shown to provide valuable nesting habitats for birds in the arid American
southwest. One of the beneficiaries is the south-western willow flycatcher,
an endangered species at the centre of a 30-year, $127 million recovery project.
Yet a costly programme to eradicate salt cedar is under way on the basis
that it is using up valuable groundwater, though there is no proof that
eliminating it will replenish water supplies.
In California, Australian eucalyptus trees provide a vital winter habitat
for monarch butterflies, a species that has been in dramatic decline for
decades due to deforestation in traditional overwintering grounds in places
such as central Mexico. The widely loathed purple loosestrife, meanwhile,
is favoured by bees, butterflies and waterfowl.
These are not isolated examples. In 2006, Laura Rodriguez at the University
of California, Davis, published a study of the impact of non-native species.
She found that they help natives in many environments and in a variety of
ways: by providing new habitats and sources of food, by acting as hosts for
organisms to live on and in, and by providing services such as pollination
(Restoration Ecology, vol 17, p 177). Art Shapiro, also at UC Davis, has
found that 40 per cent of native butterflies in Davis depend exclusively
on non-native plants for their survival (BioScience, vol 54, p 182). In the
marshlands of southern Spain, red swamp crayfish from the US have become
a major food source for birds, otters, turtles and fish, including threatened
species that breed and overwinter in the region (Conservation Biology, vol
25, p 1230).
There are other less conspicuous benefits. Only once conservationists had
eliminated feral cats from Macquarie Island in the south-west Pacific did
they realise that these non-native predators had become a vital link in the
local food web. Since the last cat was killed in 2000, exploding rabbit
populations have eaten much of the island's unique flora bare.
Anthony Ricciardi, a biologist at McGill University in Montreal, Canada,
is not convinced by these examples. "The current rate of invasion has absolutely
no analogue in the geological past. It's a massive experiment that's under
way," he says. He argues that it is impossible to tell which invaders will
be beneficial and which will be the next Nile perch, which is blamed for
wiping out some 200 cichlid fish species in Lake Victoria in east Africa.
"We still don't know what the full negative impact of invasive species is
because most invasions aren't studied."
For this reason, he and others conclude that all non-natives should be presumed
guilty until proved innocent, with no expense spared to limit their spread.
Even those who think such a view is too extreme admit that non-native species
can cause major headaches. "There's no doubt there are massive problems
associated with some invaders," says Andrew MacDougall, a plant ecologist
at the University of Guelph in Canada.
Employing the precautionary principle may sound sensible, but if Lugo and
other revisionists are correct, the indiscriminate eradication of aliens
is not only unwarranted but could even have detrimental effects. In our
fast-changing world, non-native species may be vital in maintaining ecosystem
health. "A lot of the reason we've been afraid of exotics is because they
are so well adapted for a lot of human-modified conditions and because they
have been able to spread so rapidly," says Dov Sax at Brown University in
Providence, Rhode Island. "But these are the same reasons why these species
might provide benefits to humans in the future."
What happened in Puerto Rico offers a glimpse of how enemies may turn into
allies. Once almost completely deforested, this Caribbean island has been
the site of a large-scale, unplanned ecological experiment which began in
the middle of the last century, when people began abandoning rural landholdings
and moved to the cities. At first the results resembled a classic invasion
storyline: much of the island was overrun by non-native weeds. However, the
dominance of the invaders didn't last. As the decades passed, more and more
species began taking root in the understorey. Some were aliens, but many
- to the astonishment of observers, Lugo included - were scarce native species.
That's not all. When non-native plants were excluded from abandoned pastures,
seeds of native pioneers - those that would normally be the first to colonise
clearings - did not germinate. The problem was the harsh conditions of the
altered habitat, including compacted soil, increased soil temperatures and
reduced humidity in ground exposed to the sun, and the presence of ants with
an appetite for seeds. Alien species such as the African tulip tree may have
ameliorated the situation, improving soil quality as they grew. The non-natives
would also have attracted birds and bats whose droppings would have contained
viable seeds, including those of native plants. Similarly, the success of
nitrogen-fixing exotic trees like white leadtree and white siris may have
helped in places where nutrients had been depleted by human activity. "What's
happened," says Lugo, "is the introduced species have somehow restored soil
and canopy conditions."
Today, most of Puerto Rico's new forests support more tree species than its
traditional forests, some by as much as 30 per cent. A long list of non-natives
- including former plantation crops such as mango, grapefruit, banana, coffee
and avocado - have become established in the wild, increasing the island's
total tree species count from 547 to 770. What's more, the new forests seem
as ecologically sound as the old ones. "We're starting to study nutrient
cycling, water-use efficiencies, nutrient-use efficiencies and carbon
sequestration, and we don't find much difference," says Lugo. The new forests
"function beautifully", he adds.
At the very least the experience of Puerto Rico should reassure those who
see all aliens as inevitably damaging. "This is not radioactive waste that's
coming in," says Mark Davis, a plant ecologist at Macalester College in Saint
Paul, Minnesota. "They're just species. If they're able to establish, then
they'll have ecological functions just as native species will." It also suggests
that invasion is part of a natural process of adaptation and reorganisation
that is an unavoidable side effect of expanding human activity. "We've created
completely new environments," says forest ecologist Jack Ewel at the University
of Florida in Gainesville. "When you create a new physical environment, it's
no surprise that a new suite of organisms proves to be better adapted to
it than what was there before."
So how can we use these insights? Some conservationists are tentatively
considering using non-native species to fill ecological niches left vacant
by extinct natives. However, the complexity and uniqueness of ecosystems
means such tinkering is a risky business.
Sax and Ewel acknowledge that the introduction of new species can backfire.
But they want an end to an all-out war on alien species that increasingly
seems misguided. For one thing, many campaigns against non-natives can themselves
cause unwanted ecological surprises, not to mention a huge drain on resources.
Fighting invasions with bulldozers and pesticides might also create just
the right conditions for more invasions. And focusing on non-native species
may divert attention away from more constructive conservation strategies,
especially preserving native habitat conditions.
"There's been a big emphasis on looking at the negative side of the ledger
when it comes to exotic species," says Sax. "But there are exotics that do
a better job than natives of providing particular ecosystem services, and
we need to start thinking more about the positive side if we're going to
make informed decisions."
"In many cases, we're cutting ourselves off from a valuable ally," says Ewel.
Lugo agrees. "Many of these invasions will probably have a pay-off in the
Invasive species: myth versus reality
|They enjoy an unfair advantage
Several studies indicate that non-native and native species suffer
comparable levels of predation and disease. Many highly invasive species
only begin proliferating decades after they are introduced.
They establish permanent dominance
Alien species make headlines when they are at their worst, during
the early stages of an invasion. What is rarely reported is that most of
these population explosions are soon followed by equally steep
They are a leading cause of extinction
There is no evidence of any plant species having died out as a
result of non-native species arriving and the few such extinctions known
to have occurred among animals have mostly involved new predators consuming
prey with a limited range and nowhere to run.
They are a major threat to biodiversity
Ecosystems have been surprisingly adept at absorbing new species.
Almost everywhere researchers have looked, overall species richness has risen
due to non-native arrivals.
They are a huge drain on the economy
A widely quoted study concludes that damage from invasive species
costs the US $137 billion each year (BioScience, vol 50, p 53). The study
has been roundly criticised for ignoring major economic benefits and for
including the cost of controlling species that may not need controlling,
as well as factoring in events of questionable relevance, such as bird deaths
caused by domestic cats.
Garry Hamilton is a freelance writer based in Seattle and the author
of Super Species: The creatures that will dominate the planet (Firefly,