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Subject: Friendly Invaders<br>
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NY Times<br>
September 9, 2008<br><br>
<br><br>
<h1><b>Friendly Invaders </b></h1><br><br>
<br><br>
By
<a href="http://query.nytimes.com/search/query?ppds=bylL&v1=CARL ZIMMER&fdq=19960101&td=sysdate&sort=newest&ac=CARL ZIMMER&inline=nyt-per">
CARL ZIMMER</a><br><br>
New Zealand is home to 2,065 native plants found nowhere else on Earth.
They range from magnificent towering kauri trees to tiny flowers that
form tightly packed mounds called vegetable sheep. <br><br>
When Europeans began arriving in New Zealand, they brought with them
alien plants crops, garden plants and stowaway weeds. Today, 22,000
non-native plants grow in New Zealand. Most of them can survive only with
the loving care of gardeners and farmers. But 2,069 have become
naturalized: they have spread out across the islands on their own. There
are more naturalized invasive plant species in New Zealand than native
species.<br><br>
It sounds like the makings of an ecological disaster: an epidemic of
invasive species that wipes out the delicate native species in its path.
But in a paper published in August in The
<a href="http://topics.nytimes.com/top/reference/timestopics/organizations/p/proceedings_of_the_national_academy_of_sciences/index.html?inline=nyt-org">
Proceedings of the National Academy of Sciences</a>, Dov Sax, an
ecologist at
<a href="http://topics.nytimes.com/top/reference/timestopics/organizations/b/brown_university/index.html?inline=nyt-org">
Brown University</a>, and Steven D. Gaines, a marine biologist at the
University of California, Santa Barbara, point out that the invasion has
not led to a mass extinction of native plants. The number of documented
extinctions of native New Zealand plant species is a grand total of
three.<br><br>
Exotic species receive lots of attention and create lots of worry. Some
scientists consider biological invasions among the top two or three
forces driving species into extinction. But Dr. Sax, Dr. Gaines and
several other researchers argue that attitudes about exotic species are
too simplistic. While some invasions are indeed devastating, they often
do not set off extinctions. They can even spur the evolution of new
diversity.<br><br>
“I hate the ‘exotics are evil’ bit, because it’s so unscientific,” Dr.
Sax said.<br><br>
Dr. Sax and his colleagues are at odds with many other experts on
invasive species. Their critics argue that the speed with which species
are being moved around the planet, combined with other kinds of stress on
the environment, is having a major impact. <br><br>
There is little doubt that some invasive species have driven native
species extinct. But Dr. Sax argues that they are far more likely to be
predators than competitors.<br><br>
In their new paper, Dr. Sax and Dr. Gaines analyze all of the documented
extinctions of vertebrates that have been linked to invasive species.
Four-fifths of those extinctions were because of introduced predators
like foxes, cats and rats. The Nile perch was introduced into Lake
Victoria in 1954 for food. It then began wiping out native fish by eating
them.<br><br>
“If you can eat something, you can eat it everywhere it lives,” Dr. Sax
said. <br><br>
But Dr. Sax and Dr. Gaines argue that competition from exotic species
shows little sign of causing extinctions. This finding is at odds with
traditional concepts of ecology, Dr. Sax said. Ecosystems have often been
seen as having a certain number of niches that species can occupy. Once
an ecosystem’s niches are full, new species can take them over only if
old species become extinct.<br><br>
But as real ecosystems take on exotic species, they do not show any sign
of being saturated, Dr. Sax said. In their paper, Dr. Sax and Dr. Gaines
analyze the rise of exotic species on six islands and island chains.
Invasive plants have become naturalized at a steady pace over the last
two centuries, with no sign of slowing down. In fact, the total diversity
of these islands has doubled. <br><br>
Fish also show this pattern, said James Brown of the
<a href="http://topics.nytimes.com/top/reference/timestopics/organizations/u/university_of_new_mexico/index.html?inline=nyt-org">
University of New Mexico</a>. He said that whenever he visits a river
where exotic fish have been introduced, “I ask, ‘Have you seen any
extinctions of the natives?’ ” “The first response you get is, ‘Not yet,’
as if the extinction of the natives is an inevitable consequence. There’s
this article of faith that the net effect is negative.” <br><br>
Dr. Brown does not think that faith is warranted. In Hawaii, for example,
40 new species of freshwater fish have become established, and the 5
native species are still present. Dr. Brown and his colleagues
acknowledge that invasive species can push native species out of much of
their original habitat. But they argue that native species are not
becoming extinct, because they compete better than the invasive species
in certain refuges.<br><br>
These scientists also point out that exotics can actually spur the
evolution of new diversity. A North American plant called saltmarsh
cordgrass was introduced into England in the 19th century, where it
interbred with the native small cordgrass. Their hybrid offspring could
not reproduce with either original species, producing a new species
called common cordgrass. <br><br>
Long before humans moved plants around, many plants hybridized into new
species by this process. “Something like a third of the plant species you
see around you formed that way,” Dr. Sax said.<br><br>
Biological invasions also set off bursts of natural selection. House
sparrows, for example, have moved to North America from Europe and have
spread across the whole continent. “Natural selection will start to
change them,” Dr. Sax said. “If you give that process enough time, they
will become new species.” <br><br>
“The natives themselves are also likely to adapt,” Dr. Sax added. Some of
the fastest rates of evolution ever documented have taken place in native
species adapting to exotics. Some populations of soapberry bugs in
Florida, for example, have shifted from feeding on a native plant, the
balloon vine, to the goldenrain tree, introduced from Asia by landscapers
in the 1950s. In five decades, the smaller goldenrain seeds have driven
the evolution of smaller mouthparts in the bugs, along with a host of
other changes.<br><br>
In Australia, the introduction of cane toads in the 1930s has also
spurred evolution in native animals. “Now that you have cane toads in
Australia, there’s a strong advantage for snakes that can eat them,” said
Mark Vellend, of the University of British Columbia. Cane toads are
protected by powerful toxins in their skin that can kill predators that
try to eat them. But in parts of the country where the toads now live,
black snakes are resistant to the toxins in their skin. In the parts
where the toad has yet to reach, the snakes are still
vulnerable.<br><br>
Dr. Brown argues that huge negative effects of invasions are not
documented in the fossil record, either. “You see over and over and over
again that this is never the case,” he said. Species have invaded new
habitats when passageways between oceans have opened up or when
continents have collided.<br><br>
“The overall pattern almost always is that there’s some net increase in
diversity,” Dr. Brown said. “That seems to be because these communities
of species don’t completely fill all the niches. The exotics can fit in
there.”<br><br>
In a recent paper in the journal Science, Peter Roopnarine of the
California Academy of Sciences and Geerat Vermeij of the University of
California, Davis, looked at the history of invasions among species of
mollusks, a group that includes mussels, clams and whelks. About 3.5
million years ago, the mollusks of the North Pacific staged a major
invasion of the North Atlantic. Before then, the Arctic Ocean had created
a barrier, because the mussels could not survive in the dark,
nutrient-poor water under the ice. <br><br>
A period of
<a href="http://topics.nytimes.com/top/news/science/topics/globalwarming/index.html?inline=nyt-classifier">
global warming</a> made the Arctic less forbidding. Yet the migration did
not lead to a significant drop in the diversity of the Atlantic native
mussels. Instead, the Atlantic’s diversity rose. Along with the extra
exotic species, new species may have arisen through
hybridization.<br><br>
The Arctic Ocean is now warming again, this time because of human
activity. Computer projections indicate it will become ice-free at least
part of the year by 2050. Dr. Roopnarine and Dr. Vermeij predicted that
today’s mollusks would make the same transoceanic journey they did 3.5
million years ago. They also expect the invasion to increase, rather than
decrease, diversity.<br><br>
But critics, including Anthony Ricciardi of
<a href="http://topics.nytimes.com/top/reference/timestopics/organizations/m/mcgill_university/index.html?inline=nyt-org">
McGill University</a> in Montreal, argue that today’s biological
invasions are fundamentally different from those of the past. <br><br>
“What’s happening now is a major form of global change,” Dr. Ricciardi
said. “Invasions and extinctions have always been around, but under human
influence species are being transported faster than ever before and to
remote areas they could never reach. You couldn’t get 35 European mammals
in New Zealand by natural mechanisms. They couldn’t jump from one end of
the world to another by themselves.”<br><br>
It is estimated that humans move 7,000 species a day. In the process,
species are being thrown together in combinations that have never been
seen before. “We’re seeing the assembly of new food webs,” said Phil
Cassey of the University of Birmingham in England. Those new combinations
may allow biological invasions to drive species extinct in unexpected
ways.<br><br>
<a href="http://health.nytimes.com/health/guides/disease/botulism/overview.html?inline=nyt-classifier">
Botulism</a>, for example, is killing tens of thousands of birds around
the Great Lakes. Studies indicate that two invasive species triggered the
outbreak. The quagga mussel, introduced from Ukraine, filters the water
for food, making it clearer. The sunlight that penetrates the lakes
allows algae to bloom, and dead algae trigger an explosion of
oxygen-consuming bacteria. As the oxygen level drops, the
botulism-causing bacteria can multiply. The quagga mussels take up the
bacteria, and they in turn are eaten by another invasive species: a fish
known as the round goby. When birds eat round gobies, they become
infected and die.<br><br>
“If you pour on more species, you don’t just increase the probability
that one is going to arrive that’s going to have a high impact,” Dr.
Ricciardi said. “You also get the possibility of some species that
triggers a change in the rules of existence.”<br><br>
Dr. Ricciardi argues that biological invasions are different today for
another reason: they are occurring as humans are putting other kinds of
stress on ecosystems. “Invasions will interact with climate change and
habitat loss,” he said. “. We’re going to see some unanticipated
synergies.”<br><br>
Both sides agree, however, that decisions about invasive species should
be based on more than just a tally of positive and negative effects on
diversity. Invasive weeds can make it harder to raise crops and graze
livestock, for example. The Asian long-horned beetle is infesting forests
across the United States and is expected to harm millions of acres of
hardwood trees. Zebra mussels have clogged water supply systems in the
Midwestern United States. Exotic species can also harm humans’ health.
“<a href="http://health.nytimes.com/health/guides/disease/west-nile-virus/overview.html?inline=nyt-classifier">
West Nile virus</a>,
<a href="http://health.nytimes.com/health/guides/disease/the-flu/overview.html?inline=nyt-classifier">
influenza</a> these things are invasions,” Dr. Ricciardi said.<br><br>
On the other hand, some invasive species are quite important. In the
United States, many crops are pollinated by honeybees originally
introduced from Europe. <br><br>
“It’s not that this is all good or all bad, and I’m not sure science
should be the arbiter,” Dr. Brown said. “Placing values on these things
is the job of society as a whole.”</blockquote><br>
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<br>
Gary R. Goff<br>
104 Fernow Hall, Cornell University, Ithaca, NY 14853. <br>
ph. 607/255-2824; fax 607/255-2815; e-mail:
grg3@cornell.edu<br>
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