The subantarctic islands are unique in that some are truly oceanic in origin and have never been in contact with continental landmasses. So how did the lush vegetation that is such a feature of these green oases in the Southern Ocean get there?
Australian Antarctic Division terrestrial ecologist, Dr Dana Bergstrom, told delegates at The Third International Forum on the Sub-Antarctic that there were three main ways plants could be dispersed from the southern hemisphere continents to the subantarctic: water, wind and animals (birds, seals and humans). Some plants have developed adaptations that favour different dispersal methods.
On Macquarie Island, for example, there are species of Epilobium (tiny herbs), bryophytes (mosses and liverworts) and lichens, which are well adapted for dispersal by wind. These adaptations include fine, dust-like seeds or spores, which can survive extreme conditions.
‘Seeds or spores that disperse in the jet stream are exposed to very dry, cold conditions and high UV, while those that disperse in low passage storms have to survive wet and freezing conditions,’ Dr Bergstrom said.
Studies have shown that subantarctic islands share more species in common if they are connected by winds, than if they are not, even if they’re physically close.
With water dispersal, seeds or plant parts would need to survive in saline water for days to years. Similarly, if seeds are attached to animal fur or feathers, they may need to tolerate immersion in sea water.
Birds are excellent dispersal agents because they move between water, air and land and can carry plant material in their guts, beaks or stuck to their feathers.
‘A bird carrying plant material in its beak could be picked up by a strong wind and deposited elsewhere in a few days. In fact a courier pigeon released in Australia did get caught in a storm and was dropped on Macquarie Island three days later,’ Dr Bergstrom said.
Birds also regurgitate seeds, which can remain viable in their guts for weeks.
Humans have become a significant dispersal mechanism more recently in the subantarctic islands’ history. During the International Polar Year in 2007–09, Dr Bergstrom coordinated an international project, Aliens in Antarctica, which found that clothing and equipment carried by expeditioners (such as field biologists) were a significant source of ‘propagules’ (seeds, spores and eggs). This work led to stricter quarantine protocols to prevent alien introductions. It also led to the development of ‘decision trees’ for environmental management.
‘If a plant is native then our action is to conserve and protect it. If a plant is introduced, we should control or eradicate it. But what if there’s doubt? What is the impact of the wrong decision?’ Dr Bergstrom said.
‘If the plant is introduced and we leave it there, we’ve permanently altered the natural evolution of the island’s ecosystem and possibly degraded its heritage values.
‘The subantarctic is giving us these sorts of arguments about the complexity of the interactions between humans and some of the most pristine islands on the planet.’
Dr Bergstrom said the subantarctic is offering us a unique opportunity to build our knowledge – teasing out such issues in the subantarctic, may help us address more complex problems in continental ecosystems.
