Solving the Mystery of Christmas Island’s Disappearing Rats

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In these blog parts, we’re used to tales of direct, Darwinian competition between native rodents and intruders, but new research suggests the story of the Indian Ocean’s Christmas Island is a bit more complicated.

Although native rats became extinct on the previously uninhabited island less than a decade after Eurasian black rats jumped from ship to shore, around the turn of the 20th century, researchers have concluded that a pathogen — and not merely competition for food and other scarce resources — helped exterminate two endemic species of rat. This study is the first to demonstrate extinction in a mammal because of disease, specifically “hyperdisease conditions,” which can cause unusually rapid mortality from which a species never recovers.

“This study puts into play pathogenic organisms as mediators of extinction,” said Alex D. Greenwood of the Biological Sciences Department at Old Dominion University in Norfolk, Va., and the Division of Vertebrate Zoology at the American Museum of Natural History, in a press release. “Our study is the first to correlate a pathogen with an extinction event in mammals, although we know about disease-associated extinction in snails and disease-associated population declines in amphibians.”

In 1899, the S.S. Hindustan pulled into port at Christmas Island, bringing the black rats with it (see the timeline from www.plosone.org below). Just a few years later, a parasitologist noticed that the fleas on these rats harbored a pathogenic protozoan related to the same organism that causes sleeping sickness in people. Although the black rats were clearly adapted to the protozoan, the two species of native rats were soon observed staggering around the island’s footpaths, stricken with illness, and were extinct by 1908. This is not an unusual phenomenon: The researchers report more than 80 percent of mammals that have gone extinct in the past 500 years have lived on islands.

Indeed, at least one mammalian species, the Tasmanian devil, is currently displaying symptoms of a very severe population collapse due to disease — in this case, from an apparently infectious type of cancer. In the past decade, at least a quarter of the total population has died out, and some biologists predict the Tasmanian devil’s extinction within a few years if the cancer continues to spread. Another famous Australian mammal, the Tasmanian tiger, or thylacine, is also thought to have suffered severe decline around 1910 because of disease, although it was also hunted for sport.

Yet, despite occasional references in the literature, scientists have never confirmed that disease was responsible for the extinction of rats on Christmas Island. So Greenwood and his colleagues used ancient DNA procedures to determine whether a rat-specific protozoan could be detected in museum samples; the team collected samples from 21 specimens (almost all that exist) to see if the infectious agent showed up in the population before and after contact with the black rats. None of the three pre-contact samples was infected with the protozoan, but six of the 18 post-contact samples were infected, suggesting a very high rate of infection.

“This is not a case of humans overhunting — I don’t think anyone was that hungry,” says Ross MacPhee, a curator of vertebrate zoology at the American Museum of Natural History who proposed “hyperdisease conditions” as a mediator of extinction in 1997. “Within nine years of contact, these abundant, endemic species were evidently completely knocked out by an introduced disease — nothing else was around at the time that could have done the job. This study puts something else on the table as a reason for extinction.”

The results of this study contrast with the conventional view of the effects that pathogens have on species. Most pathogens are self-limiting because the number of new hosts reduces or because disease-resistant individuals increase proportionally as more susceptible individuals die out.

“This study should get people to think about the spread of pathogen pollution,” Greenwood said. “Pathogen pollution is the introduction of animal or plant diseases into a new environment. This pollution could affect many species that are in decline or in small numbers, ranging from accidental to active introduction, like the building of Pleistocene Park in Russia or the repopulation of species for conservation purposes.”

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