No, we’re not talking about something dreamed up by the X-Files. Terrestrial aliens are species that have been removed from their native habitat and introduced to a previously uncolonized area. Sometimes these arrivals are deliberate, like the planting of attractive plants in gardens or the spread of domestic horses. Other times, the new species travels unnoticed, like zebra mussels in ship bilge water or weed seeds in soil.
Some of the most problematic introductions have arrived with people as they move around the globe. First rats and mice were spread as early ships explored and traded, then domestic cats were brought to keep down rodent populations. Particularly on islands, cats have been blamed for declining endemic bird, mammal, and lizard populations. Rigorous eradication programs have been underway for decades, under the assumption that removing cats would allow native species to rebound. However, the results have been more complicated.
On Macquarie Island, cats were finally eradicated in 2000. A study in the Journal of Applied Ecology has found that, once cats were removed, the rabbit population decimated the local vegetation. The cost of remediation could run as high as $15 million (US). Using satellite imagery to measure the damage, ecologists estimate that about 40% of vegetation has been affected. Nearly 20% of the island shows moderate to severe changes.
Officially discovered in 1810, Macquarie Island lies approximately halfway between Australia and Antarctica. A variety of grassland and fern ecosystems cover the island, along with cliffs that support large seabird colonies. It was used as a base for sealing expeditions, with seals and penguins killed for fur and blubber.
Why are all those rabbits on a remote subantarctic island anyway? The population dates back to the 1870s, when sealers introduced the species to provide a local food source while on-land. Luckily, rabbits are a favourite prey of domestic cats – already resident on the island thanks to previous visitors. Despite this fact, the rabbit population boomed to problem proportions – nibbling all the vegetation and prompting control measures in the 1960s. As in Australia, myxomatosis was introduced to reduce rabbit numbers. This viral disease is spread by fleas and causes lesions, eye infections, and rapid death.
The initial controls were seen as successful, with the rabbit population plummeting from 130,000 in 1978 to 20,000 in the 1980s. Local vegetation recovered. But the abundance of rabbits had prompted a large cat population, and they were hungry without rabbits. Native ground-nesting birds were next on the list of cat prey – and an intensive control program was started in 1985. After 15 years, the last cat was removed.
But without predation pressure the rabbits started to thrive. Myxomatosis alone has been insufficient to keep the numbers down and vegetation damage has accelerated. Removing cats has not addressed the related rabbit issue, with some possible input from a growing rat population. Ecologists refer to the phenomenon as a trophic cascade – when species are interconnected by predation-prey relationships, and changing one population has knock-on effects.
The lesson is timely, with many invasive species being targeted with control measures. It also demonstrates the challenge of rolling back human impacts on complex ecosystems. As we continue to colonize and modify habitats across the globe, more thought and deliberation should precede our march of progress.
Photo credit: Sebastian Nebel