permaculture

["Saor Stetler" <sstetler@earthlink.net>]

>From NYT:


http://www.nytimes.com/2008/09/09/science/09inva.html?_r=1
<http://www.nytimes.com/2008/09/09/science/09inva.html?_r=1&8dpc&oref=slogin
> &8dpc&oref=slogin

September 9, 2008


Friendly Invaders 


By CARL ZIMMER
<http://query.nytimes.com/search/query?ppds=bylL&v1=CARL%20ZIMMER&fdq=199601
01&td=sysdate&sort=newest&ac=CARL%20ZIMMER&inline=nyt-per> 

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. 

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.

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 Proceedings
<http://topics.nytimes.com/top/reference/timestopics/organizations/p/proceed
ings_of_the_national_academy_of_sciences/index.html?inline=nyt-org>  of the
National Academy of Sciences, Dov Sax, an ecologist at Brown University
<http://topics.nytimes.com/top/reference/timestopics/organizations/b/brown_u
niversity/index.html?inline=nyt-org> , 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.

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.

"I hate the 'exotics are evil' bit, because it's so unscientific," Dr. Sax
said.

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. 

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.

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.

"If you can eat something, you can eat it everywhere it lives," Dr. Sax
said. 

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.

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. 

Fish also show this pattern, said James Brown of the University of New
Mexico
<http://topics.nytimes.com/top/reference/timestopics/organizations/u/univers
ity_of_new_mexico/index.html?inline=nyt-org> . 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." 

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.

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. 

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.

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." 

"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.

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.

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.

"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."

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. 

A period of global warming
<http://topics.nytimes.com/top/news/science/topics/globalwarming/index.html?
inline=nyt-classifier>  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.

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.

But critics, including Anthony Ricciardi of McGill University
<http://topics.nytimes.com/top/reference/timestopics/organizations/m/mcgill_
university/index.html?inline=nyt-org>  in Montreal, argue that today's
biological invasions are fundamentally different from those of the past. 

"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."

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.

Botulism
<http://health.nytimes.com/health/guides/disease/botulism/overview.html?inli
ne=nyt-classifier> , 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.

"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."

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."

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. "West
Nile
<http://health.nytimes.com/health/guides/disease/west-nile-virus/overview.ht
ml?inline=nyt-classifier>  virus, influenza
<http://health.nytimes.com/health/guides/disease/the-flu/overview.html?inlin
e=nyt-classifier>  - these things are invasions," Dr. Ricciardi said.

On the other hand, some invasive species are quite important. In the United
States, many crops are pollinated by honeybees originally introduced from
Europe. 

"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."