The voracious rodents attack bird nesting colonies, eating eggs, chicks, and sometimes even adult birds.
Now 102 of 328 recognized seabird species are considered threatened or endangered by the World Conservation Union, with predation by invasive species ranking among the top dangers.
"Seabirds are important ecological actors in the oceans and on islands, but 30 percent of all seabirds are at risk of extinction," said study co-author Bernie Tershy of the University of California, Santa Cruz.
"Invasive rats are likely the single largest threat to seabirds," said Tershy, also a former grantee of the National Geographic Conservation Trust. (The National Geographic Society owns National Geographic News.)
Smaller seabird species and those that nest in burrows or rock crevices are particularly at risk, the study said.
That group includes storm-petrels, auklets, murrelets, and shearwaters, according to lead author Holly Jones of Yale University in New Haven, Connecticut.
"Rats can have a larger impact on entire seabird populations in species with adults that are small enough to prey on," Jones said.
"Burrow- and crevice-nesting seabirds share the same underground habitat [as rats], which makes a predation encounter more likely."
Traveling with humans as ship stowaways, three rats species native to Europe and Asia have become established on about 90 percent of the world's major islands and island chains, experts say. In many cases the original invasions occurred centuries ago.
The Pacific rat first reached New Zealand, Hawaii, and many South Pacific islands on the canoes of early Polynesian voyagers, and Roman ships helped spread the black rat and brown rat throughout the Mediterranean.
Many seabird species considered vulnerable to rats have been showing local or global declines for decades and have now reached perilously low numbers, according to the researchers.
On rodent-infested islands, seabird breeding colonies often persist only on isolated rock outcrops that remain inaccessible to rats.
The researchers present their findings in the February edition of the journal Conservation Biology.
Even some large species, such as the Laysan albatross, have proven vulnerable to the invaders.
"Because most seabirds evolved in the absence of any [land-based] predators, many have no evolutionary adaptation to avoid predation by rats," Yale University's Jones said.
The birds also have a hard time shifting their breeding locations to different islands to escape danger.
"Many seabirds are [genetically] programmed to return to breed where they were born," Jones noted.
Alan Saunders directs the Cooperative Islands Initiative at the University of Auckland in New Zealand. He was not part of the new study.
"No species is safe when rats arrive," Saunders said.
"Island ecosystems are especially vulnerable to induced change, and more extinctions have taken place on islands than anywhere else."
In addition to seabirds, he noted, invasive rats have caused extinctions of many native land birds, reptiles, frogs, and even plants.
"By consuming fruit, seeds, and flowers, rats can change the structure and composition of forests, and alter the entire ecology of islands."
Good Riddance to Rats
On Anacapa Island, part of California's Channel Islands National Park, breeding populations of the rare Xantus's murrelet and ashy storm-petrel had been decimated by black rats that were introduced prior to 1940.
But a successful restoration initiative between 2001 and 2002, which focused removing all non-native rodents using biodegradable poisons, has turned things around.
"Since rat eradication, [seabird] numbers seem to be increasing," Jones said.
Effective rat-removal techniques have also been pioneered in New Zealand, where more than a hundred previously infested islands are now rat-free.
While eradication projects such have been opposed by some animal rights activists, conservationists say rat removal is the only way some seabirds and other island species can survive.
"The eradication of invasive species from islands, especially rodents, has heralded a new era in conservation management globally," said the University of Auckland's Saunders.
"Spectacular ecological responses have been measured following rat eradications."
(Related news: "Canada Province Rat-Free for 50 Years" [March 31, 2003].)
Experts agree the new study should help conservation managers in different parts of the world prioritize islands for rat eradication and protect the seabird species most at risk.