Embracing invasive species: A murder-mystery tale

Here’s a story of species introduction, invasion and trophic cascades to boggle the mind of even the most seasoned community ecologist.

Macquarie Island is a 60-mile square piece of paradise in the southwest Pacific Ocean, south of Australia and New Zealand where one can find four species of seals and five of penguins.
In the early 19th century, feral cats were introduced to the island by whalers, and the cats proceeded to devastate the island’s native seabird populations. Sixty years later, rabbits were introduced to the island by seal hunters. The rabbits flourished and eventually became the cats’ main prey. But the rabbits still maintained large enough numbers to cause catastrophic damage to the island’s vegetation.

And the plot thickens.

In 1968, in an effort to control the rabbits, the Australian government introduced a European flea that spreads a rabbit virus. As a result, rabbit numbers fell from a peak of 130,000 in 1978 to less than 20,000 in the 1980s, and the vegetation recovered to near its original density.

Good balance, right?  Of course not – that’d be too simple.  With fewer rabbits as food, the cats began to eat the island’s native burrowing birds, inciting a cat eradication program in 1985. The last cat was killed in 2000. But with no more cats around to eat them, and the rabbit virus unable to keep up with the rising populations, the rabbits again decimated large swaths of vegetation. In six years the rabbits substantially altered the vegetation cover on the island, which no doubt is leading to large changes in the density of other terrestrial species.

The authors of this study, which appears today in the British Ecological Society’s Journal of Applied Ecology, point out that eradicating invasive species is never as simple as it seems, and that in relatively short time periods ecosystems can shift to accommodate such invaders to the point where the new species supports the food web. They say that despite the difficulty of predicting indirect effects of invasive species, they should play a major role in management decisions.

It’s difficult enough to predict complex species interactions in intact ecosystems, let alone in disturbed ones. But these results at least warn that not only can invasive species can assimilate with speed into non-native habitats, but that these habitats assimilate quickly in response.