Sentinels to climate change. The need for monitoring at South Africa’s Subantarctic laboratory

Subantarctic islands form ideal sentinels to climate change. These islands support terrestrial and marine ecosystems that are relatively simple and extremely sensitive to perturbations. They provide an ideal natural laboratory for studying how ecosystems respond to a changing climate in the Southern Ocean. Initial studies and subsequent analyses at the Prince Edward Islands in the Indian sector of the Southern Ocean have all shown that there has been a climatological rise of >1°C in the sea surface temperature since 1949. Mirrored to this rise is a decrease in rain, an increase in extreme events, an increase in winds from the warmer sector in the northwest, and annual sunshine hours have risen by 3.3 h since the 1950s. It has been proposed that climate changes reported at the Prince Edward Islands correspond in time to a southward shift of the Antarctic Circumpolar Current (ACC) and in particular its frontal systems the Subantarctic Front (SAF) and the Antarctic Polar Front (APF), between which the islands lie (Figure 1). The Prince Edward Islands, like many other oceanic islands within the Southern Ocean, are seasonally characterised by vast populations of marine organisms and a diversity and abundance of seabirds that use the islands as breeding grounds. It is estimated that the islands support over 5 million breeding pairs of top predators including seabirds, penguins and seals during the peak breeding season. The energy necessary to sustain these top predators is derived from the close interaction between the oceanic environment and the islands themselves. Changes in the intensity and geographic position within these frontal systems are likely to coincide with dramatic changes in the distribution of species and total productivity within the Southern Ocean. Long-term research in both the offshore and near-shore environments of the Prince Edward Islands is critical if the mechanisms needed to sustain high concentrations of marine life in a changing environment are to be better understood and conserved. If ecosystems are pushed beyond certain thresholds or tipping points, there is a high risk of dramatic biodiversity loss and accompanying degradation of a broad range of ecosystem services. Furthermore, a real threat brought on by a warmer climate will be the increase in ease in which pristine Subantarctic islands such as the Prince Edward Islands can be invaded by alien species.