The word ‘alien’ conjures up images of little green figures in spaceships, but it is also applicable to animals, plants and microbes that have been introduced by humans to a place where they don’t belong. Wherever humans have travelled they have not travelled alone.
Invasion biology in Antarctic regions including subantarctic islands has historically focused on the direct impacts of the bigger aliens, such as cats, rabbits and rats on Macquarie Island. However, recent research of less obvious introductions of plants and invertebrates has shown that these organisms too can expand rapidly and considerably alter population and ecosystem processes. For example, a European grass, Agrostis stolonifera, has dramatically invaded drainage ecosystems on Marion Island over 50 years, excluding native plants and significantly decreasing biodiversity at invaded sites. On South Georgia an alien carnivorous beetle has indirectly caused a change in adult body size in populations of its major prey species, another beetle. Beetles that weren’t eaten suddenly found that there was a lot more food around and grew larger.
The isolation and harsh environment of Antarctica and the subantarctic islands have resulted in limited floral and faunal environments in which many niches are unfilled. Here, many plant and animal groups found on warmer continents are missing, such as flowering plants in continental Antarctica, ferns and sedges on Heard Island and weevils on Macquarie Island. Species from groups not present in these ecosystems are the greatest invasive threat, as they can fill in the gaps. Antarctica’s harsh climate and ice cover protect against alien colonisation, but on milder subantarctic islands — getting warmer in recent decades — there are increasingly favourable conditions for alien species to colonise and spread.
Recently, South Africa’s Professor Steven Chown and colleagues provided statistical evidence for the strong relationship between the number of alien species on subantarctic islands and the number of human visitors over the last 200 years or so. Increasing world travel, including the recent development of Antarctic ecotourism, has dramatically increased the invasion threat to both subantarctic islands and Antarctica itself. For instance, Antarctic visitors have often recently travelled to the Arctic, whose animals, plants and microbes are biologically adapted to cold climates and could successfully establish in Antarctic environments, which previously would have been beyond their reach. Antarctic travellers are now potential carriers of propagules, such as seeds, egg cases and spores of these species, to Antarctic ecosystems.
Dana Bergstrom1 and Jennie Whinam2
1. Australian Antarctic Division
2. Tasmanian Department of Primary Industries, Water and Environment