Strategies of survival and resource exploitation in the Antarctic fellfield ecosystem.

Antarctic fellfields present organisms with a heterogeneous habitat characterised by a wide variety of environmental stresses. These include low temperatures, limited moisture availability, frequent and often rapid freeze-thaw and hydration-dehydration cycles, exposure to high photosynthetic photon flux density and ultraviolet (uv) irradiance, seasonal snow cover, high winds, cryoturbation and, depending on location south of the Antarctic Circle, considerable daylight in summer. Most of these factors vary both predictably and unpredictably in spatial and temporal planes. In response to this adverse environment, fellfield organisms have developed a variety of strategies to overcome physiological stress and to exploit the limited resources available during the short austral growing season. A high degree of synchronisation exists, so that investment in non-essential activity and adaptations is minimised. Here, we review the combined suites of co-adapted traits used by different fellfield taxa to achieve energy acquisition, growth and reproduction under adverse levels of two principal limiting factors: low temperatures and the scarcity of water. To this end, a detailed characterisation of the Antarctic fellfield microenvironment is followed by a synthesis of available data on the morphology, physiology, life history and behaviour of successful Antarctic flora and fauna. Tolerance of low temperatures by fellfield organisms is achieved by elevation of standard metabolism, production and accumulation of cryoprotectants, supercooling, melanic pigmentation, behavioural avoidance, compact growth forms and synchronised reproduction and extended life cycles. Low moisture conditions are overcome by dehydration resistance, anhydrobiosis, development of resting stages and by behavioural avoidance of desiccating conditions. Occupancy of the Antarctic fellfield habitat is considered to require the ability to respond rapidly to ephemeral resources and to tolerate severe environmental stresses. During summer, organisms rely on opportunism to maintain a positive energy balance. During winter, resistance adaptations are used to withstand the potentially lethal climate, especially in habitats not protected by snow cover. This deterministic framework has led to the selection of species that are genetically and physiologically pre-adapted for resource acquisition yet sufficiently robust to withstand cold and desiccation stresses. Non-adapted taxa fail to become established. Despite the environmental selection pressures, available evidence suggests that colonisation of the fellfield habitat has not required the evolution of any adaptations, only the refinement of those already possessed to an extent by some temperate forms. This has led to the convergence of survival strategies. It is hypothesised that, in the short term, the majority of Antarctic fellfield biota are able to absorb the predicted effects of a changing climate by their high levels of physiological tolerance and life-cycle flexibility.