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Islands comprise just 5.3 percent of the Earth’s dry surface, but a whopping 75 percent of bird, amphibian, mammal, and reptile extinctions have occurred on them. As a result of their isolation, island ecosystems are extremely delicate, and their residents are easily disturbed by outside forces. Case in point: more than eight out of 10 recorded extinctions on islands have been linked to invasive species, according to the US-based nonprofit Island Conservation.
Many conservation biologists work to reverse this shocking statistic by trying to eradicate invasive species from islands. But not Jamie MacKay. The University of Illinois graduate student and his colleagues intentionally released mice onto New Zealand’s Saddle Island so they could study, in intricate detail, how invasions take place. In 2009, the scientists released 15 mice onto an otherwise rodent-free island and tracked their behavior as they explored their new home. The island’s mice had been eradicated the year before, but no one was expecting them to stay gone from the tiny 60,000-square-meter speck of dry land.
“Basically the [New Zealand] Department of Conservation has given up on this island,” says Mark Hauber, a University of Illinois biologist who oversaw the study and acts as advisor to MacKay. Because Saddle Island is just one kilometer from New Zealand’s North Island, rodent eradications don’t last long. Mice are skilled swimmers, able to move from island to island or to simply jump off a boat and paddle to shore.
So rather than trying to keep it invader-free, researchers are using Saddle Island as a living laboratory for invasive species research. (As if to prove the point, a lone ship rat suddenly showed up on the island during the experiment, which had to be paused until the rat could be caught and removed.)
MacKay, Hauber, and their colleagues studied a particular problem: how do a few invasive mice give rise to a flourishing population? Classic ecological theory holds that individual animals can have a harder time surviving and reproducing when they’re part of a small population, a phenomenon called the Allee effect. Large groups give individual animals a better shot at survival by increasing the chances of finding a good mate, and lowering the odds of being gobbled up by a predator.
To see how mice might overcome the Allee effect, MacKay’s team released the radio-collared mice onto Saddle Island, one pair at a time, over the course of a year. Each mouse in the pair was released at the same time but in a different spot, so researchers could see how—and if—they found each other. (The researchers retrapped and removed each pair of mice after data collection concluded.)
By comparing the introduced rodents’ behaviors to those of the island’s pre-eradication population, as well as those in the mainland population, the researchers were looking to discern whether mouse behavior is flexible enough to overcome the Allee effect. If so, that could help explain why they are such skilled invaders.
Using the data collected by the radio collars, the scientists saw that the newly introduced mice moved more than twice as far at night than their pre-eradication counterparts did when the island was fully populated. On the sparsely populated island, the introduced mice also had home ranges that were roughly 10 times larger than those that occupied the island before the eradication. When mice of the opposite sex found each other, they tended to remain close. In one instance in which two males were released simultaneously, they found each other, but then kept their distance. To Hauber, this suggests that the drive to find mating opportunities was behind the increased nightly movements.
“[The study] makes it clear why islands keep getting invaded so successfully,” says Hillary S. Young, a University of California, Santa Barbara biologist who was not involved in the research. “If you have one male and one female on an island, they’re going to find each other,” she says.
“It highlights the challenges of mouse eradications,” she adds, because successful reintroductions can occur so swiftly and efficiently.