Ice‐Bound Antarctica: Biotic Consequences of the Shift from a Temperate to a Polar Climate

The Antarctica of today is a forbidding place: isolated from the other landmasses of the planet, with more than 99.6% of its surface covered in permanent ice, which is on average more than 2 km thick. Organisms that live there face chronic and extreme environmental stresses, and generally belong to cryptic and microscopic groups that the casual observer often fails even to notice. No wonder the early human explorers a little over a century ago considered it a barren and desolate place! It has not always been like this, and Antarctica’s fossil record tells us that it once hosted lush temperate and even subtropical forests, dinosaurs, early mammals and diverse groups of other biota. The main Antarctic landmass has been located at high southern paleolatitudes for around 130 My. However, while transient alpine (mountaintop) glaciations are likely to have been a feature of high Antarctic mountains even during the “greenhouse Earth” periods of the geological past, the first ice sheets did not form until at least 34 Ma, in a period of declining global atmospheric CO2 concentrations and global cooling. Antarctica originally formed part of the southern supercontinent called Gondwana. As that broke up, the other southern continents drifted northwards, finally leaving the isolated Antarctica that we see today. The opening of the Drake Passage and Tasman Gateway led to the eventual physical isolation of the continent and – once this was surrounded by deep ocean water – allowed the formation of the Antarctic Circumpolar Current in the Southern Ocean at 25–20 Ma. While this accelerated the formation of continent‐wide icesheets, tundra ecosystems and cool temperate forests are thought to have persisted until at least 14 Ma on parts of East Antarctica, and possibly as late as 5 Ma on the Antarctic Peninsula. Montane systems are a fundamental element of Antarctica’s architecture, with different parts of the continent undergoing several episodes of intense volcanism and mountain building over geological time. Vast mountain ranges – the Transantarctic Mountains, the Ellsworth Mountains, the Antarctic Peninsula, the enigmatic ice‐buried Gamburtsev Mountains and other ranges across the continent – characterize the Antarctica of today. Indeed, arguably most of Antarctica’s contemporary biology is found in what would elsewhere be characterized as montane or alpine ecosystems, from the coasts to the high mountain summits. The contemporary biology of Antarctica, despite its apparently low diversity and cryptic nature, has revealed remarkable and important insights over the last decade or so, particularly as the rapidly developing tools of molecular biology have been brought to bear on it. These show that, across all areas of the continent, the life that is present today is not the result of sporadic and opportunistic recent colonization processes following complete glacial wipe‐out, but rather in large part represents the last vestiges of the ancient fauna, flora and microbiota that Antarctica inherited as it became free from Gondwana. Over the millions and tens of millions of years of its isolation, this biota has evolved and radiated, and we are only now appreciating the complex signal of regional endemism and dispersal contained in its contemporary biodiversity, and the implications this has for understanding the geological and glaciological history of the continent.