Life history traits

From our anthropocentric point of view, Antarctic terrestrial habitats are potentially stressful in many respects, facing a range of 'extreme' conditions, both in terms of chronic exposure and extreme or acute events. It would be more accurate to say that the conditions experienced in the Antarctic, with respect to many variables, lie at the extreme end of the range or continuum of conditions found worldwide (Peck et al. 2006, Chown and Convey 2006a). In an evolutionary context it can, therefore, be argued that selection imposed on organisms by the Antarctic environment may be expected to be strong, leading to more clearly identifiable consequences with fewer confounding factors than elsewhere. This chapter provides an overview of the evolutionary consequences of environmental stress as seen in the life history strategies present, and assesses the likely consequences and vulnerabilities of current trends of environmental change for biota with this suite of contemporary strategies. Over the year, both Antarctic and Arctic terrestrial habitats experience similar extreme low temperatures during the winter months. However, the Antarctic also experiences chronically low summer temperatures, even in comparison with those of the Arctic (Convey 1996a, Danks 1999). This means that low thermal energy input is a constraint faced by most Antarctic terrestrial biota. Clearly, the temperature (and hence thermal energy) available to an organism is most accurately described by its microenvironment, rather than the longer term averages of air temperature that are the basis of these generalisations but, in the absence of many robust and extended microclimatic datasets, the latter provide at least a reasonable baseline. It is not only the long term temperature average, but also the scales and patterns of temperature variation that are of significance in terms of potential impacts on biology. Here, such variables as the upper and lower extremes experienced, diurnal and annual ranges, short-term means and rates of change, and the predictability of changes, will also be influential (for general discussion see Gaines and Denny 1993, Kingsolver and Huey 1998, Sinclair 2001, Vasseur and Yodzis 2004). The chronically low temperatures of the Antarctic are likely to be near minimum threshold temperatures for many physiological processes, even during the short summer season of the continent or Antarctic Peninsula, and will spend long periods of the year below these thresholds. Even in the subantarctic, where the normal seasonal pattern of temperature variation tends to be broken down by the over- riding influence of thermal damping from the surrounding ocean, the resulting temperatures are chronically low (Chown and Crafford 1992, Convey 1996a, Smith 2002). This leads to a particular prediction in the context of climate change - a small temperature increment experienced by an organism in an environment chronically near to its operational threshold will have a relatively greater biological impact than the same increment experienced in a less extreme environment (Convey 2001), or one that is more variable. Despite the more obvious factor of chronically low temperature, the significance of the lack of liquid water in many habitats and periods of the year is regarded as being at least as important in understanding the biology of Antarctic terrestrial biota (Kennedy 1993a, Sømme 1995, Block 1996). Water availability in most terrestrial habitats across the globe is governed by precipitation patterns (ie rainfall). However, other than in the subantarctic and, increasingly, in the maritime Antarctic during summer, precipitation as rain is either unusual or unknown in the Antarctic. Therefore, water availability in these terrestrial habitats is more accurately governed by patterns of thaw, both directly from thawing of snowfall, and indirectly through the melt of long term ice and glaciers. Thus it is normal for free water availability to be separated temporally from the timing of precipitation and it is often the case that it is separated spatially. Associated with desiccation stress is the impact of wind, which acts both as a stressor and a disturbance factor (Bergstrom and Selkirk 2000). The final major environmental variable to be considered in the context of environmental selection pressures is that of solar radiation. Here, two elements may be significant in the context of climate change. First, the pattern of direct insolation experienced is affected by variables including cloud and snow cover, both of which are expected to or already seen to change as part of the overall climate change processes seen in the Antarctic (Convey 2006). Changes in irradiance received obviously have implications for autotrophic primary production, the basis of ecosystem processes. Second, much attention has been given to the potentially deleterious consequences of changes in exposure to shorter wavelength ultra-violet (UV-B) radiation that may be experienced as a consequence of the formation of the spring ozone hole over Antarctica. This is a recent anthropogenic phenomenon (Farman et al. 1985). Although it is separate from the processes that have led to climate warming and associated changes in most regions, some interactions have been suggested. For example, recent work has shown that increases in temperature and decreases in precipitation at Marion Island, and possibly at other South Indian Ocean Province islands, are related to phase changes in the semi-annual oscillation, which in turn are linked to Antarctic stratospheric ozone depletion (Rouault et al. 2005). In the context of selection on the biota, these three major environmental variables are often unlikely to act in isolation. Rather, their interactions and predictability will be important (Convey 1996b).