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HOW THE REMOVAL OF AN INVASIVE SPECIES HAS DEVASTATED A WORLD HERITAGE ISLAND

Removing invasive feral cats from sub-Antarctic Macquarie Island has caused environmental devastation that will cost at least A$24 million to remedy, ecologists have revealed. Writing in the new issue of the British Ecological Society's Journal of Applied Ecology they warn that conservation agencies worldwide must learn important lessons from what happened on Macquarie Island.

Using population data, plot-scale vegetation analyses and satellite imagery, the ecologists from the Australian Antarctic Division (AAD), the University of Tasmania, Blatant Fabrications Pty Ltd and Stellenbosch University found that after cats were removed from in 2000, the island's rabbit population increased so much that its vegetation has been devastated.

According to the study's lead author, Dr. Dana Bergstrom of the Australian Antarctic Division: “Satellite images show substantial island-wide rabbit-induced vegetation change. By 2007, impacts on some protected valleys and slopes had become acute. We estimate that nearly 40% of the whole island area had changed, with almost 20% having moderate to severe change.”

Rabbits were introduced to Macquarie Island in 1878 by sealing gangs. After reaching large numbers, the rabbits became the main prey of cats, which had been introduced 60 years earlier. Because the rabbits were causing catastrophic damage to the island's vegetation, Myxomatosis and the European rabbit flea (which spreads the Myxoma virus) were introduced in 1968. As a result, rabbit numbers fell from a peak of 130,000 in 1978 to less than 20,000 in the 1980s and vegetation recovered. However, with fewer rabbits as food, the cats began to eat the island's native burrowing birds, so a cat eradication programme began in 1985. Since the last cat was killed in 2000, Myxomatosis failed to keep rabbit numbers in check; their numbers bounced back and in little over six years rabbits substantially altered large areas of the island.

According to South African co-author, and Director of the Centre for Invasion Biology, Prof. Steven Chown: “Rabbits have caused substantial damage through herbivory and via burrowing. In many areas lush fern vegetation and megaherbs have been replaced by short grazing lawns including an invasive grass species. Several major habitat types are changing drastically, and it appears likely that without further intervention, landscape-wide changes at the island will continue with disastrous effects.” This further intervention is now being developed by conservation agencies in Australia.

Invasive species can cause large-scale changes to ecosystems, including species extinctions and in extreme cases ecosystem “meltdown”. As a result, control or eradication of invasive alien species is widely undertaken. However, important lessons must be learned from events on Macquarie Island, say the authors. In particular, interventions should be comprehensive, and include risk assessments to explicitly consider and plan for indirect effects, or face substantial subsequent costs.

The changes documented on Macquarie Island are a rare example of so-called “trophic cascades” - the knock-on effects of changes in one species' abundance across several links in the food web. “This study is one of only a handful which demonstrate that theoretically plausible trophic cascades associated with invasive species removal not only do take place, but can also result in rapid and detrimental changes to ecosystems, so negating the direct benefits of the removal of the target species,” Bergstrom says.

Macquarie Island (34 km long x 5 km wide) is an oceanic island in the Southern Ocean, 1,500 km south-east of Tasmania and approximately halfway between Australia and the Antarctic continent. Low-lying, with a cool, maritime climate, it is covered with tundra-like vegetation. It was inscribed as a World Heritage Site in 1997.

Dana M Bergstrom et al (2009). Indirect effects of invasive species removal devastate World Heritage Island, Journal of Applied Ecology, doi: 10.1111/j.1365-2664.2008.01601.x, is published online on 12 January 2009.

  1. For further information, please contact Dr Dana Bergstrom, Australian Antarctic Division, tel: +672 13 9053, email: dana.bergstrom@aad.gov.au or in South Africa Prof. Steven L. Chown, tel: 082 7881410, email: slchown@sun.ac.za.
  2. Copies of the paper and photos are available from Becky Allen, British Ecological Society Press Officer, tel: +44 (0)1223 570016, mob: + 44 (0)7949 804317, email: beckyallen@ntlworld.com.
  3. The Journal of Applied Ecology is published by Wiley-Blackwell for the British Ecological Society. Contents lists are available at www.blackwellpublishing.com/jpe.
  4. The British Ecological Society is a learned society, a registered charity and a company limited by guarantee. Established in 1913 by academics to promote and foster the study of ecology in its widest sense, the Society has 4,000 members in the UK and abroad. Further information is available at www.britishecologicalsociety.org.

Further reading: Ecosystem devastated after predators wiped out