permaculture

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

Toby Hemenway (and others) have long-argued that nature does not need humans
to micro-manage her when it comes to introduced non-native species but this
is a sacred cow that may cause continued distress to some…


http://e360.yale.edu/feature/invasive_species_reconsidered_finding_a_value_i
n_non-natives/2373/


24 Feb 2011: Report


Invasive Species Reconsidered:
Finding a Value in Non-Natives


One of the tenets of conservation management holds that alien species are
ecologically harmful. But a new study is pointing to research that
demonstrates that some non-native plants and animals can have beneficial
impacts.


by carl zimmer

The tale of the honeybee is a sadly familiar one: a once-thriving species is
on the ropes. After brutal bouts with mites and fungi, honeybees are now
facing their most dangerous threat yet: a mysterious disease called colony
collapse disorder. In the winter of 2010 alone, U.S. beekeepers reported
losing 34 percent of their hives to CCD, which may be caused by viruses,
pesticides, or some diabolical combination of factors. Researchers are
working hard to figure out exactly why the honeybees are dying and how to
save them because of their ecological importance. Honeybees pollinate many
of the country’s fruit and vegetable crops, and they also carry out the same
service for many species of wild plants. In Brazil, honeybees help keep
isolated rain forest fragments from dying out by moving their pollen from
tree to tree.

Amidst all the concern for the honeybees, it’s easy to forget an important
fact about them. They’re not native to the New World.

The earliest records of honeybees in this hemisphere come from English
settlers who arrived with hives aboard their ships in the early 1600s. They
brought the bees to make honey they could eat and wax they could burn. Over
the past four centuries, new stocks of honeybees have arrived at least eight
times, from Europe, the Near East, and Africa. 

Honeybee

Allen Donikowski/Flickr

Honeybees were introduced into the Western Hemisphere in the 1600s and have
become an ecologically important species.

Introduced species can, in some cases, become dangerous invaders, wreaking
havoc on their new homes. They may gobble up native species, outcompete
them, or just infect them to cause new diseases. Much of modern conservation
management is organized around keeping alien species out and killing off the
ones that made it in. And yet there are no loud voices calling for the alien
honeybees to be wiped out in the New World. 

“It’s almost like everyone politely ignores that they’re not native,” says
Dov Sax, a conservation biologist at Brown University. 

Sax and some of his fellow biologists think that it’s time to give some
serious consideration to this paradox. In a paper published in
<http://dx.doi.org/10.1111/j.1523-1739.2010.01646.x> Conservation Biology,
Sax and two colleagues argue for recognizing the ecological value of some
introduced species. “We predict the proportion of non-native species that
are viewed as benign or even desirable will slowly increase over time,” they
write.

The fact that a journal like Conservation Biology would publish such a
statement is a testament to how seriously some researchers are taking the
idea. “It’s considered edgy, but it’s not considered nuts,” says Sax. Not
nuts, perhaps — but certainly not innocuous. The new paper is eliciting
strong responses from other conservation biologists — ranging from hearty
endorsements to fierce protests.

Sax and his co-authors — Julian Olden of the University of Washington and
Martin Schlaepfer of State University of New York in Syracuse and INRA in
Rennes, France — point to a number of studies that have documented the
benefits of introduced species. Some non-native species provide habitat and
food for native animals and plants, for example. In the southwestern United
States, tamarisk trees have been aggressively spreading along rivers. The
trees started drawing up so much water that conservation biologists feared
it might not leave enough for the native plants. If the native plants
disappeared, so might the animals that depended on them, such as the
endangered southwestern willow flycatcher. The government spent millions of
dollars to stop the tamarisk, using bulldozers, herbicides, and even
tree-munching beetles. Yet in recent years some researchers have concluded
the initial worries about the threat of tamarisks to the water table were
exaggerated. At the same time, conservation biologists have found the
southwestern willow flycatcher nesting in tamarisk trees. Their fledglings
do as well in the introduced trees as they did in the native ones. Getting
rid of the tamarisk would mean getting rid of the habitat of an endangered
bird.

Introduced species can also help restore native ecosystems on degraded land.
In Puerto Rico, for example, much of the native forest was destroyed for
farming, and in recent decades conservation biologists have been trying to
nurture them back on abandoned farmland. Native trees do a poor job of
pioneering this degraded landscape. Alien trees, such as African tulip trees
and rose apple, have colonized them instead. These new forests remain
dominated by alien trees for their first three or four decades. But the
forests are also a habitat in which native trees can begin to thrive again.
After 60 to 80 years of growth, Puerto Rican forests become mixes of both
alien and native trees. 

Introduced species can promote diversity by acting like ecosystem engineers,
reworking their new habitat. Off the coast of Chile, for example, a
gelatinous invertebrate called Pyura praeputialis forms massive mats,
providing nooks and crannies in which other species can thrive. Juan Carlos
Castilla of Pontificia Universidad Católica de Chile and his colleagues have
found 116 species of invertebrates and algae living in these alien
ecosystems, while nearby intertidal rocky shores were home to just 66
species.

Honeybees demonstrate another benefit that introduced species can offer.
Other introduced species can pollinate plants as well, while some animals
help native plants in other ways. In Hawaii, a bird called the Japanese
white eye spreads the seeds of a native vine. These new partnerships between
native and non-native species show that they aren’t precisely linked like a
lock and key. “In reality, the world is a lot messier,” says Schlaepfer. 

Schlaepfer and his colleagues stress that they’re not calling for every
introduced species to be protected, or even tolerated. Instead, they hope
the conservation community will at least consider the possibility that
introduced species can do some good. “It’s going to cause one more round of
reflection,” says Schlaepfer.

The new paper has attracted strong criticism, some directed at its big
message and some at the evidence the authors marshall to build it. Thomas
Gavin, a Cornell conservation biologist, sharply disagrees with the way
Schlaepfer and his colleagues envision the way ecosystems work —
particularly with the idea of analyzing introductions of species “as if the
system were simply a collection of interchangeable parts, like some
human-devised machine. I reject this notion.”

Other conservation biologists take issue with the examples that Schlaepfer
and his colleagues select. To suggest that the Japanese white eye has a
positive role because it pollinates a native vine is “totally ludicrous,”
says David Richardson of the University of Cape Town in South Africa. “This
bird is the main disperser of many non-native plants into native
ecosystems,” he points out. “The net outcome of this species just has to be
negative.”

Yet Richardson agrees that it’s time to think differently about invasive
species. He thinks conservation biologists have to set aside purist ideas
about restoring ecosystems to some pre-human state. In cases where habitats
have been radically altered, he says, “removing alien species just because
they are alien is futile.” 

A particular passionate endorsement comes from Scott Carroll, a conservation
biologist at the University of California, Davis. “My admittedly ludicrous
response to the paper is ‘Yes! Go! May your sensible perspective sprout
wings!’” he says. 

Carroll makes a similar argument in the latest issue of the journal
Evolutionary Applications, where he calls for a new kind of science he calls
<http://onlinelibrary.wiley.com/doi/10.1111/j.1752-4571.2010.00180.x/abstrac
t> “conciliation biology.” Simply trying to eradicate all exotic species can
be costly, he argues, and can even harm native species. Introduced cats were
eradicated from Maquarie Island off the coast of Australia, after having
driven two of the island’s bird species extinct. But with the cats gone, an
introduced population of rabbits exploded, devouring the native plants.

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The failure on Maquarie Island, Carroll argues, stems from a failure to
appreciate that species are constantly evolving. In the mid-1900s,
scientists introduced a virus into Australia and its surrounding islands to
control rabbits. Conservation biologists assumed that they could get rid of
cats and the virus would still keep the rabbits at low levels. But the
virus, meanwhile, had evolved to become less deadly, which enabled it to
spread more efficiently but made it ineffective in controlling the rabbits. 

Rather than try to restore ecosystems to their pre-industrial states,
Carroll argues, conservation biologists should manage the evolution of
species to make ecosystems resilient. If a vine starts to spread across a
new habitat, for example, conservation biologists can help native insects to
evolve mouthparts that allow them to devour the vines more quickly. Rather
than signaling defeat, Carroll sees conciliation biology as a way to reach
more sustainable outcomes in a human-dominated world.

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