Title: Getting the Drop on Hawaiian Invasives , By: Levy,
Database: Academic Search Premier
Section: Feature
Getting the Drop on Hawaiian Invasives
Contents
Lessons from down under
Will it protect the birds paradise?
For more than 20 years, Jim Denny has trekked into the
Denny speaks of his hope that rat poison will soon rain dawn
on this comer of
The problem of rats and other introduced predators laying
waste to native species is an old one, one that is most severe in island
ecosystems.
After European contact, she problem intensified wish she
arrival of two additional species of rat, cats that quickly went feral and
spread throughout she islands, and the mongoose. (The mongoose, a native of
India, was introduced to control rats in she sugarcane fields. But rats ate
nocturnal and mongooses are not, so the mongoose flourishes by preying on
ground-nesting birds.)
These predators are only part of she wave of drastic change
that has devastated native animals and plants in Hawaii. The introduction of
goats and cattle, and the logging of Hawaiian forests, led to major changes in
habitat. One of she worst blows to birds was the accidental introduction of
mosquitoes, which carry avian pox and malaria. At the turn of the Last century,
whole communities of native birds suddenly vanished from pristine forests,
wiped out by mosquito-borne disease. These days, native forest birds are found
only at higher elevations, where imported mosquitoes are few or absent.
In 1893, the islands were home to 68 species of native
birds. Today, 29 of those species are extinct, and 17 more are endangered. More
than half the birds on she US list of threatened and endangered species are
Hawaiian.
Native forest birds now survive in a handful of relatively
intact, high-elevation habitats, such as Hawaii Volcanoes National Park and the
Hakalau Forest National Wildlife Refuge on the big island of Hawaii, the Hanawi
Natural Area Reserve on Maul and the Alakai Swamp on Kauai. These places are
the focus of intense efforts to control rats. Since 1994, biologists have been
luring rats using baits laced wish diphacinone. The drug, originally developed
to treat human cardiac patients, is particularly lethal to rats.
The baits are placed in tubes that keep most nontarget
animals out, making this one of she safest ways to kill off unwanted rodents.
But running these bait stations is a labor-intensive process, and the remnants
of Hawaii's native forests stand in rugged, remote terrain--some of it
impossible to access except by helicopter. The high cost and logistical
difficulties of using bait stations have put rat control out of reach in some
of Hawaii's crucial habitats.
Now many biologists are supporting a proposal to have helicopters
drop rat baits dosed with diphacinone on critical habitat areas in Hawaii. Some
Hawaiians object to this idea. Hunters who follow she ancient tradition of
stalking wild pigs fear that the poison will harm their dogs or make she pigs'
meat too toxic to eat.
Diphacinone is widely used to control rats on sugarcane and
macadamia nut plantations in Hawaii, though it's never been dropped there by
air. Wildlife managers in New Zealand, on the other hand, have lots of
experience wish aerial drops of rat poison. They've used she technique to do
away with introduced rats on many offshore islands, which harbor threatened
populations of native seabirds and lizards. "New Zealand has achieved
spectacular results," says Katie Swift, a biologist with the US Fish and
Wildlife Service (FWS) in Honolulu." And in Hawaii we feel that time is
really running out for many of our native species,"
Lessons from down under
Small islands off the mainland are New Zealand's prime
storehouses of native biodiversity, home to rare skinks, geckos, songbirds, and
nesting seabirds. The arrival of rats devastated populations of these species,
a problem that was dramatized when rats reached Big South Cape Island in the
1960s. The rats did in the last of the island's bush wrens and short-tailed bats,
two creatures that conservationists were actively trying to save.
By the late 1970s, biologists had begun experimenting wish
various poisons to control rats on these islands. They were surprised to frond
that in many cases, rats could be completely eradicated. If she island was far
enough from any rat-infested Piece of land and rarely visited by boats that
might carry stowaway rodents, it would remain rat free.
David Towns, a biologist wish New Zealand's Department of
Conservation, explains that she most strongly affected native species have life
histories that make them vulnerable to rat predation: They produce few
offspring and are slow to grow to maturity. So their response to freedom from
rats can be slow but dramatic. "Some shoreline skinks and geckos showed
orders-of-magnitude increases in population following removal of rats,"
Towns says. "On such islands the general impression is of an arid and
rasher dead environment when rats are present. But within 5 to 10 years of rat
eradication, you'll see waves of lizards diving for cover as you land and walk
across the rocky coasts." Seabirds that nest in island burrows, such as
petrels, shearwaters, and prions, are heavily targeted by rats and may take 15
years or mote to return once rats have extirpated them from an island. But when
they do come back, they seem to thrive.
Rat poison campaigns have also been used to rescue
threatened populations of forest birds in mainland New Zealand. The numbers of
kokako, a blue-gray songbird known for its haunting voice, had been slowly
dwindling. After baits were put out to control rats and introduced opossums,
the kokako rebounded. The number of nesting pairs to fledge healthy young went
up from 13 percent to 42 percent. After eight years of pest control in one area,
the population of kokako had tripled.
The most commonly used rat poison in New Zealand has been
brodifacoum, a toxicant that, like diphacinone, works by inhibiting the
production of blood dotting factors, causing animals to die of internal
bleeding. Brodifacoum works faster than diphacinone and is lethal to more
species and at lower doses.
Aerial brodifacoum campaigns on offshore islands have all
succeeded in eradicating rats on the first attempt. There have been problems,
however, with the poison hitting nontarget animals. On Tawhitinui Island, the
entire population of western weka, a native species of rail, was wiped out by
poison bait intended for ship rats. (The Department of Conservation now
captures weka before aerial poisoning and releases them after the bait is
gone.) Native predators and scavengers, such as hawks and owls, can succumb to
secondary poisoning from eating the bodies of deed or dying rats. Some other
sensitive species, inducting the endangered little spotted kiwi, seem to
weather brodifacoum broadcasts with little ill effect.
Broadscale use of brodifacoum is now under review in New
Zealand, and the chemical is being used only on remote, uninhabited islands. In
addition to risks to nontarget wildlife, there is the possibility that in mainland
areas, where people might hunt pigs or deer that have accumulated brodifacoum
residues, humans could be poisoned from eating contaminated meat.
Brodifacoum is a second-generation anticoagulant, developed
in the 1970s after some rat populations began to show a resistance to the
milder first-generation chemicals, like diphacinone. It takes several feedings
of diphacinone to kill a rat, but with brodifacoum it takes only one dose. So
in terms of blitzing the rat population, brodifacoum is the best bet: There's
little risk that a few rats will get only a partial dose and survive to form a
new founding population.
Gregg Howald, a biologist with the Island Conservation and
Ecology Group, has used the techniques pioneered in New Zealand to wife out
rats on small islands off the coasts of California and British Columbia. On
Anacapa Island, a crucial seabird nesting habitat in California's Channel
Islands, he and his colleagues used aerial drops of brodifacoum. The risk to
nontarget species made the job complex. Before they dropped any poison, they
had to capture about 1000 Anacapa deer mice, a native rodent that might
otherwise have been wiped out. They also trapped as many birds of prey as they
could. The mice and the predators were released hack to the island after the
danger of poisoning had passed.
"Rat eradication is a pretty tall order," explains
Howald. "Rats are notoriously difficult to control at a population
level." This is why he and others have seen the risks of brodifacoum as
worthwhile. But Howald acknowledges that widespread use of the chemical may be
a case of overkill In a recent experiment he tested diphacinone and brodifacoum
baits on separate islets in the Gulf of California, in northwestern Mexico. He
found that both poisons succeeded in completely eradicating rats.
Nobody can hope to eliminate rats from islands as big, and
as populated, as Kauai or Maui. The high-elevation forests where most native
Hawaiian birds and plants survive will need king-term, continuing efforts to
limit rat numbers; there will always be rats outside the treated areas, ready
to move hack in. In that situation, using diphacinone makes a lot of sense. The
risks, says Howald, are a lot lower.
"Diphacinone is selectively more toxic to rats and a
few other species like mongooses," says Swill "it takes huge amounts
of it before birds start to show any symptoms." For example, in laboratory
tests brodifacoum killed mallard ducks at a dose of 0.26 milligrams (nag) per
kilogram (kg), but a lethal dose of diphacinone is orders of magnitude larger:
3158 mg per kg. The chemical is also metabolized by rats much more quickly than
brodifacoum, so there's less risk of nontarget predators, such as hawks or
hunting dogs, being affected by eating a poisoned rat.
Because diphacinone was used for years as a blood thinner
for human cardiac patients, a lot is known about how it affects people. And
people can absorb moderate doses without ill effects, as can cattle. In the
1970s, ranchers in Latin America injected the drug into their cows to poison
vampire bats that parasitized the herds. Researchers took samples of milk and
tissue from these cows and determined that the amount of diphacinone present
was nowhere near the levels used in heart medication.
The concentration of diphacinone used is so low that it
would be impossible for a pig to accumulate dangerous amounts in its tissues by
eating rat bait, says Swift. "The rodenticides people buy in the hardware
store are much more tonic to humans and pets than diphacinone," she adds.
"If your dog got into a box of rat bait from the hardware [store], it
would be much more likely to die than a hunter's dog that got into some
diphacinone."
Swift and her colleagues have spent years studying ways to
use diphacinone safely in Hawaiian forests. They've found that it breaks down
quickly on the forest floor. It's not water soluble, so it shouldn't leach into
streams or groundwater. They set up an array of baits with cameras rigged to
photograph the animals attracted to the poison pellets: 99 percent of them were
rats or house mice, another introduced rodent. Of the few birds that showed up
to take the halt, none was a native species.
David Foote, of the US Geological Survey's Pacific Island
Ecosystems Research Center, has been running trials of aerial diphacinone
broadcast in the forests of Hawaii Volcanoes National Park. The process works
well to control rats, eliminating 99 percent of the population for several
months, until rats from outside the treated areas have time to make their way
in.
In Foote's study area, rats threaten not only native birds
but rare plants like Clermontia, a flowering shrub endemic to Hawaii.
"Rats can really ravage the seedlings and young saplings, particularly
during dry periods, when they tend to chew through the bark looking for a
source of moisture."
Foote is confident that diphacinone won't harm native
insects or snails; the poison affects the vitamin K-dependent production of
clotting factors, a process that does not exist in invertebrate physiology. To
make sure, he studied native picture-wing Drosophila flies that fed on rat bait
and found no ill effects. But the fact that invertebrates seem impervious to
diphacinone may also complicate matters.
"One of the greatest concerns has to do with a large
species of introduced slug, which also feed on Clermontia," Foote says.
"We're concerned that the baits of compressed cereal grain are acting as a
food source for the slugs, and that one of the future challenges of doing rat
control work will be trying to deal with elevated numbers of slugs as a
consequence."
Park managers have been chipping away at the problem of
invasive species for decades now. Hawaii Volcanoes now maintains large tracts
of forest protected by fences and free of fatal cattle, goats" and pigs.
They've attached aggressive introduced weeds like banana poka and strawberry
guava. Going after rats, says Foote, is the next logical step.
"Many people see Hawaii as a basket case with regard to
conservation work, because of the sheer numbers of endangered species," he
says. "But if you could see the forests here, you'd realize that simple
measures in terms of exclusion or suppression of invasive species [have] a
dramatic impact for recovery of native communities. There is a tremendous
amount of hope for the future of Hawaii's native biodiversity."
Will it protect the birds paradise?
Will knocking down rat populations in Hawaiian forests make
a real difference for sensitive birds? It's difficult to answer this question
with field experiments. Juvenile birds tend to move away from their home
territories once they fledge. So large areas of forest must be studied to
produce valid data on differences in nest success and fledgling survival
between areas with and without ram Like ground-based rodent control, this kind
of study is labor intensive and costly, so few researchers have been able to
address this issue directly.
Elepaio, perky native songbirds that often perch with their
long tails cocked, still thrive in high-elevation forests on Kauai and Hawaii.
But on Oahu the elepaio is endangered. Eric VanderWerf, a biologist with the
FWS, has found evidence that rats are a major problem for Oahu elepaio.
Because it is difficult to find and monitor a large number
of elepaio nests in the wild, VanderWerf planted a series of artificial nests,
filled with quail eggs, and observed their fates before and after a rat-control
program in prime Oahu elepaio habitat. Cameras wired to the artificial nests
snapped mug shots of predators. In every case the culprit was a black rat,
believed to be foe greatest threat to native birds because they are acrobatic
climbers, unlike Polynesian and Norway rats. After a halting program had
knocked down the rat population, the survival rate of artificial nests shot up
from less than 40 percent to 80 percent--and the survival rate of wild elepaio
nests in the study area reached 82 percent. The number of successful fledglings
per pair of nesting elepaio nearly doubled.
More evidence comes from foe Alakai Swamp on Kauai, where
foe world's only population of puaiohi hangs on. Jim Denny remembers long days
spent slogging through the mud searching for puaiohi during the early 1990s,
when some feared foe species was extinct. One day, as he rested during a steep
climb, one of foe rare thrushes found him, suddenly appearing in a nearby shrub
just long enough for Denny to shoot an identifiable photo. This encounter
helped lead biologists to foe remote drainage where she last of foe puaiohi--a
community of about 200 birds--were making their stand. The population is now
getting a boost from captive breeding efforts sponsored by foe Peregrine Fund.
A few eggs were collected from wild nests and carefully incubated in captivity.
The first of the captive-raised birds were released in she Alakai in 1999, and
they hit foe ground tanning.
Six of eight females released in late winter had settled
down to nest by July, and several mated with wild males--an encouraging sign
that captive-raised birds get along well with their wild brethren. Puaiohi will
renest if they lose their offspring early in the season. In 1999, the six
released females built 21 nests and fledged seven young.
"By far she single largest cause of nest failure was
rat predation," says Erik Tweed of fire US Geological Survey. Biologists
monitored the nests daily, and in many cases only foe remains of chicks or
whole families were found, when she birds had been healthy the day before. The
bones of one mother puaiohi were found in a black rat's nest; broken eggs and
she car-casses of many nestlings showed characteristic rat tooth marks and
sometimes were found in rat burrows. "If we were able to control rats
effectively," Tweed says, "I don't see any reason why puaiohi nests
wouldn't be 80 to 90 percent successful."
The Alakai is so rugged and remote that controlling rats
with bait stations is very difficult. Snap traps, a partial solution used in
some areas, are not an option here because puaiohi spend a lot of time on the
ground, and biologists don't want to risk killing one of the precious birds in
a rat trap. It's a classic example of a situation that argues for aerial
broadcast of rat bait.
Tweed has his reservations, however. "If you do
anything on such a large scale, you don't know what the consequences are going
to be" he says. As he hikes through the Alakai, Tweed has noted that the
scats left behind by feral cats often contain rat bones and fur. If the rat
population is knocked down by aerial broadcast of bait, he fears that cats
might target endangered birds as prey instead.
Trent Richard Malcolm, of the Maul Forest Bird Recovery
project, faces similar problems--but he and his colleagues are struggling to
protect one of the rarest of Hawaiian birds, the po'ouli, with a total known
population of three individuals. Hanawi, like the Alakai, is in rough,
inaccessible territory, mad efforts at rat control using bait stations and
traps have not been as successful as Malcolm and other land managers had hoped.
"We are in the process of attempting to capture all three birds to move
them into captivity for propagation," says Malcolm. "If we lose just
one bird to predation, the species may be doomed to extinction."
"I'm no different from any other environmentally
conscious person who's ever read Silent Spring," says Malcolm.
"Initially, I cringed at the idea of dropping toxicants from the sky. But
it's important to remember that not all toxicants behave the same in the
environment. If diphacinone is proven to be environmentally safe, the benefits
of its aerial broadcast in Hawaii's native forest could be enormous."
PHOTO (COLOR): The apapane, pictured above, is Hawaii's most
common native forest bird; it has survived the onslaught of introduced
predators and diseases better than many other endemic species. Photograph: © Jack
Jeffrey Photography.
PHOTO (COLOR): A black rat perches on an ohia branch, The
black rat is the introduced predator now considered the greatest threat to
native forest birds in Hawaii. Photograph: © Jack Jeffrey Photography.
PHOTO (COLOR): US Geological Survey researcher Erik Tweed
checks a bait station set to lure rats in the Alakai Swamp on Kauai.
Photograph: © Jack Jeffrey Photography.
PHOTO (COLOR): US Geological Survey researcher Erik Tweed
checks a bait station set to lure rats in the Alakai Swamp on Kauai,
Photograph: © Jack Jeffrey Photography.
PHOTO (COLOR): US Geological Survey researcher Erik Tweed
holds the remains of a puaiohi nestling that was killed by a rat. Photograph: ©
Jack Jeffrey Photography.
PHOTO (COLOR): A small population of puaiohi survives in a
remote valley in Kauai's Alakai Swamp. Biologists are working to increase the
population through captive propagation, but birds nesting in the wild suffer
from intense rat predation. Photograph: © Jack Jeffrey Photography.
PHOTO (COLOR): The 'i'iwi's flamboyant bill is designed to
feed on nectar from the tubular flowers of native Hawaiian plants. The bird
remains fairly common in high-elevation forests an Kauai, Hawaii, and Maui but
is extinct on Lanai and nearly so on Oahu and Molokai. Photograph: © Jack
Jeffrey Photography.
PHOTO (COLOR): The elepaio is a species of flycatcher
endemic to Hawaii. This photo was taken an the island of Hawaii; separate
subspecies live an Kauai and Oahu, The birds are in decline on Oahu because of
nest predation by rats and mosquito-borne disease. Photograph: © Jack Jeffrey
Photography.
~~~~~~~~
By Sharon Levy
Sharon Levy (e-mail: levyscan@humboldtl.com) is a freelance
writer based in