For Want of a Pollinator, a Flower May Be Lost--or a Forest
The extinction of bird species in New Zealand--and elsewhere--may be making
it more difficult for plants to propagate
By David Biello | February 4, 2011 | 6
Earth's last passenger pigeon-Martha-died in 1914 at the Cincinnati Zoo, the
final remnant of flocks that once darkened the sky. What's unknown, even
nearly a century after this extinction, is how many of the plants in the
eastern woodlands of the U.S. might have suffered as a result-or even gone
extinct themselves. Now, new findings from halfway around the world suggest
plants may indeed suffer in the absence of the animals they have relied on
for pollination or dispersing seeds.
The work was done by a team of biologists, fresh from showing that New
Zealand's mistletoe species are struggling due to a lack of pollinating
birds, who set out to determine if other plants shared this problem. After
all, the country has lost nearly half of its native land bird species since
They homed in on one native shrub-Rhabdothamnus solandri, or New Zealand
gloxinia, renowned for its brilliant orange flowers-which relies on several
species of birds for pollination, and is found nowhere else in the world.
The tui (Prosthemadera novaeseelandiae), bellbird (Anothornis melanura) and
stitchbird (Notiomystis cincta) all have beaks and long tongues perfectly
suited for the shrub's 10-millimeter-long orange flowers. But bellbirds and
stitchbirds are gone on the New Zealand's North Island thanks to bird-eating
animals brought along by European colonizers, such as house cats (Felis
catus), ship rats (Rattus rattus), stoats (Mustela erminea) and brushtail
possums (Trichosurus vulpecula).
Fortunately, those bird species survive on some small islands off the
coast-Little Barrier, Tiritiri Matangi and Lady Alice-too small to have had
much contact with humans or the invaders they often bring along. And in
many cases mammal pests, such as the feral cats that roamed Little Barrier
Island until 1980, have been eradicated.
So the biologists painstakingly catalogued the instances of the flowering
shrub in plots on both the bird-friendly islands and the mainland. As a
result of the missing birds, the flowering shrubs on the mainland produce
smaller fruit and only 37 seeds per flower, compared with 232 seeds per
flower for shrubs on the bird-friendly islands. Nearly 80 percent of flowers
on the small islands showed evidence that birds had visited, whereas only 25
percent of mainland flowers did so. And although fully grown shrubs persist
on the mainland in roughly similar numbers, there are less than half as many
young plants sprouting up to replace them.
This pollination failure has likely been going on for a long time.
"Pollination probably failed about 1870, which is when bellbirds and
stitchbird densities on the North Island plummeted," says biologist Dave
Kelly of the University of Canterbury in Christchurch, one of the scientists
involved in the research.
Other possible pollinators, such as insects, apparently are not up to the
task in the case of R. solandri, unlike the case for endemic New Zealand
mistletoes or trees that have also lost bird pollinators, according to
Kelly. Flowers encased in chicken wire mesh cages on the mainland and the
offshore islands to keep out birds also failed to produce many seeds,
"showing that birds are essential for pollination," the biologists wrote in
the paper detailing their findings, published online in Science on February
3. Kelly adds: "Usually when a bird-adapted flower is not well pollinated by
insects it's because the flower is too big for insects to touch the right
parts [for pollination] as they visit it."
Of course, humans could take over for the missing birds, pollinating by hand
and sowing the seeds. When the biologists did so, R. solandri immediately
boosted its numbers. As for why the decline of the brilliantly colored
flowering shrub had gone unnoticed, Kelly offers the idea of "shifting
baselines"-people (and scientists) tend to ignore reports of abundance that
existed before their own lifetimes-and wrote in the Science paper, "This
decline could very easily have escaped notice, because it is so gradual.. It
may be that similar slow plant declines as a result of failing ecological
interactions have begun elsewhere."
After all, many regions of the world host plants that rely on specialized
birds for pollination, from the tropics of Central and South America to
South Africa and Southeast Asia. "I would bet that there are other plants
declining in other parts of the world for this same reason, but it's not
been measured yet," Kelly says. "In New Zealand we want to check other
bird-pollinated plants to see if they too are declining. We know that a
majority of them have [a] reduced seed set."
At the same time, R. solandri is still not considered in danger of
extinction for two reasons: First, it still manages to produce some seed
even without effective pollinators. Second, the shrub can apparently live
for years. "We don't know exactly how long-lived the plant is as it does not
seem to have growth rings," Kelly admits.
As for 19th-century North America, the passenger pigeon was a big consumer
of the nuts of hickories, beeches and chestnuts in the vast eastern
deciduous forest of North America. That forest has largely disappeared-and
chestnuts were almost entirely wiped out by disease-but it is possible that
the trees are also missing their one-time avian agent of dispersal. That
could very easily be a problem for plants the world over, given that at
least 190 species of birds have gone extinct since 1500 and more than a
thousand are currently at risk of disappearing.
"The interactions among species are important for keeping ecosystems
functioning properly," Kelly notes. "If we can get the birds right, the
plant conservation will come as a secondary benefit."
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