Antarctica's ice sheets could totally collapse if the world's fossil fuels are burnt off, according to a recent climate change simulation. While we are unlikely to see such a dramatic event any time soon, we are already observing big changes and it's worth considering what the worst case scenario might look like for the continent's ecosystems. How long before Antarctica turns into grassy tundra?
For now, life thrives mostly at the very edge of the continent — it's driven by the plankton-rich Southern Ocean and clustered around seasonally ice-free areas of coastal land. The interior might be sparsely inhabited, but the continent is not as barren as many think. There are around 110 native species of moss and two flowering plants, the Antarctic hairgrass and pearlwort. These plants have flourished along the relatively mild Antarctic Peninsula in recent decades. However they can't go much further — they already occur at almost the most southern suitable ice-free ground.
With ice-caps and glaciers receding already in the Peninsula region, native land plants and animals are benefiting from more easily available liquid water. Already we are starting to see increased populations, greater areas occupied and faster growth rates, consequences only expected to increase — everything is currently limited by the extreme physical environment.
It may eventually prove too warm for some native species, but the bigger issue in upcoming decades and centuries will be whether new and currently "non-native" species will arrive that are stronger competitors than the native organisms.
Native polar species are inherently weak competitors, as they have evolved in an environment where surviving the cold, dry conditions is the overriding selective pressure rather than competition from other biological sources. If humans (or other wildlife expanding their range southwards) bring new competitors and diseases to Antarctica, that may pose a very grave risk to the existing biodiversity. Some native species would likely be pushed into the remaining more extreme regions where they can avoid competition and continue to rely on their inherent stress tolerance abilities.
We usually split the process of natural colonisation — which applies even today in Antarctica — and that of movement of "alien" species by human agency. The best available data for the Antarctic region come from some sub-Antarctic islands, where it appears humans have been responsible for many more successful colonisations than nature. In fact, over the recent centuries of human contact with the region we have introduced 200-300 species compared to just two or three known natural colonisations.
Penguins, seals and flying seabirds move between islands and the Antarctic Peninsula, so there is potential for some natural colonisation. Vagrant birds are regularly observed across the sub-Antarctic and even along the Peninsula, some of which have colonised successfully (such as the starlings, redpolls and mallard ducks on Macquarie Island).
Migrants such as skuas and gulls, which spend time on land at both ends of their migration, could be important natural vectors of transfer for invertebrates, plant seeds and spores, and microbes into an ice-free Antarctica. Importantly, bird colonies also fertilise surrounding rock and soil with faeces, eggshells and carcasses. Plant and animal life flourishes near seabird colonies, encouraged by this enrichment.
However it can be tough to predict what Antarctic melt would mean for individual species, never mind entire ecosystems. Take penguins, for instance — they have already survived previous inter-glacial retreats, but at reduced population sizes. This time round it is likely that Adélie and emperor penguins who are more dependent upon sea ice would decline, while less ice-dependent species such as gentoos and chinstraps might benefit. Indeed, there is already some evidence that emperors are struggling (although also that they may be adapting and learning to emigrate).
However the fact fish-eating gentoo penguins are increasing on the Peninsula while Adélies and chinstraps (both krill eaters) aren't doing so well suggests prey availability can be more to blame than ice cover. Figuring out the impact of large-scale environmental change at ecosystem or food-web level is hard — it's a complex process that will no doubt throw up some unexpected results.
The sub-Antarctic islands are full of examples of such unexpected impacts. Pigs, dogs, cats, sheep, reindeer and rabbits have all been intentionally introduced in the past, with often devastating effects. Rats and mice were introduced to South Georgia and other islands accidentally by sealers and whalers, for instance, and have decimated seabird populations. A recent eradication campaign appears to have been successful and pipits, ducks and small seabirds are showing some immediate signs of recovery.
The removal of non-native cats from Macquarie and Marion Islands has similarly helped native burrowing seabirds, although responses in such ecosystems can be far more complex and unpredictable — the removal of cats from Macquarie also led to increase in the introduced rabbit population, and considerably increased damage to sensitive native vegetation.
Antarctic biodiversity is far more complex than widely assumed, with up to 15 distinct biogeographic regions that have been evolutionarily isolated for many millions of years. Humans present the greatest threat, not only of introducing new species, but also of moving "native" species between regions within Antarctica. This could be even more damaging, as these native species would already be pre-adapted to polar life.
Visitors to Antarctica are subject to increasingly strict biosecurity measures but accidental introductions continue to occur, often through food shipments for scientists. Changes in sea and land ice affect access to new areas, so we can only expect plant and invertebrate invasions to increase unless biosecurity becomes more effective.
While cost issues may be raised, it is worth remembering that prevention will always be better — and cheaper — than subsequent control and eradication, even if such action is possible.
Image by Pier Nirandara under Creative Commons licence.