December 11, 2007

Birds Glad Cats Eat Rats

A rare
burrowing bird known as a
Cook's petrel
seems to have dug a real hole for itself: it lives on a small
island crawling with hungry rats and cats.

Although the pests eat
both eggs and chicks, new research suggests that the old adage "my enemy's
enemy is my friend" holds true for the petrel, native to New Zealand's Little Barrier Island.

we found was that this little seabird did better when both cats and rats were
on the island, rather than just rats," said Matt Rayner, a conservation
ecologist at the University of Auckland. When the non-native cats were voted
off the island with traps and poison, three times as many chicks perished than
with both predators around.

really shows the not-so-pretty, unforeseen consequences of trying to restore an
ecosystem to the way it was," Rayner said.

Rayner and
his colleagues' study of the island's delicate food web is detailed in today's
early edition of the journal
Proceedings of the National Academy of Sciences.

A three-species

Little Barrier Island, situated off of New Zealand's northern coast, is about half of the size of New York City's Manhattan Island.
threatened Cook's petrel, a seagull-like bird, burrows small tunnels in the volcanic
there and
lays just one egg each mating season.

little guys evolved without any sort of predation, so their clutch size is as
small as it gets," Rayner said. He explained that English settlers likely
dropped off domestic
on the island in the 18th century, while small Pacific rats likely
have lived there
for close to 500 years.

documented the survival of chicks on the island from 2004 through 2007, combining
the data with other
information collected over the years.

Starting in 1970, when both cats and
rats roamed the island, scientists took note of petrel chick survival. About one in three petrel chicks
made it to adulthood,
but eradication of the cats in 1980 caused chick survival to
plummet to less than one in 10—a finding that initially seemed strange.

think the cats had policed
the unruly rat population, lightening the predatory load on the petrels because
rats served as a tasty alternative. In turn, the numbers of enemy
were kept in check. Eradication of the rodents in 2004, however
(effectively booting both of the petrel's new enemies off the island) boosted
chick survival to roughly three in five.


The results
seemed to lose ground at lower, subtropical altitudes where lizards, owls and
other predators join the hunt for Cook's petrels.

"It is
… puzzling that Pacific rats had such a small impact on Cook’s petrel eggs and
chicks at low altitudes, yet were so devastating at higher altitudes," the
authors said in their study. Rayner noted that this finding really spells out
the sensitivity of island ecosystems—especially as they get larger.

"Islands like these are natural storehouses of diversity we need to protect," he said.
"The larger the island, though, the tougher it is to control. You have so
many different habitats to consider when attempting to manage a threatened

Rayner said
Little Barrier Island isn't the only island out there with threatened
that need rescue from mankind's follies, but cautioned that each
needs to be intensely studied before a repair to the ecosystem can be effected.

need to develop ways to account for the incredible number of variables on an
island ecosystem before we start tampering with it," he said. "It's
not easy, but it is absolutely necessary."