Terrestrial biology in the instrumental period
- This page is part of the topic Antarctic climate and environment change in the instrumental period
Contemporary terrestrial and freshwater ecosystems within Antarctica occupy only 0.34% of the continental area (British Antarctic Survey, 2004), the remainder being permanent ice and snow. The combined land area of the isolated sub-Antarctic islands is likewise small. Individual areas of terrestrial habitat are typically ‘islands’, whether in the true sense of the word, being surrounded by ocean, or in the sense of being surrounded and isolated by terrain inhospitable to terrestrial biota in the form of ice (Bergstrom and Chown, 1999). While the most biologically developed and most studies of terrestrial exposures are found in coastal regions of the continent, particularly along the Antarctic Peninsula and in Victoria Land, terrestrial habitats exist in all sectors of the continent, and both in coastal and inland locations.
Terrestrial biological research within Antarctica has, however, been much more spatially limited, with major areas of activity restricted to the South Orkney and South Shetland Islands, Anvers Island, the Argentine Islands and Marguerite Bay along the Antarctic Peninsula/Scotia Arc, and the Dry Valleys and certain coastal locations in Victoria Land. Terrestrial and freshwater research along the continental Antarctic coastline has largely been limited to areas in the vicinity of the Schirmacher Oasis, Windmill Islands and Davis Station, Casey Station, and mountain ranges in Dronning Maud Land. Sporadic biological records exist from more widely dispersed locations, but in most cases these relate to single short field visits to these locations, often by non-biologists or non-specialists. Indeed, there remain many instances where the only biological records available, or only species descriptions that exist, derive from the original exploring expeditions of the ‘heroic era’. Even where terrestrial biological research is undertaken within a region or by a national operator, both taxonomic and process-based research coverage is extremely uneven across different regions or operators.
All Antarctic terrestrial ecosystems are simple in global terms, lacking or with low diversity in specific taxonomic or biological functional groups (Block, 1984; Smith, 1984; Convey, 2001). It is therefore likely that they lack the functional redundancy that is typical of more diverse ecosystems, raising the possibility of new colonists (arriving by both natural and, more recently, human-assisted means) occupying vacant ecological niches. Such colonists could include new trophic functions or levels, threatening the structure and function of existing trophic webs (Frenot et al., 2005, 2008; Convey, 2008). Responses of indigenous biota will be constrained by their typically ‘adversity-selected’ life history strategies, which have evolved in an environment where abiotic environmental stresses and selection pressures (i.e. properties of the physical environment) far outweigh in importance biotic stresses and pressures (i.e. competition, predation, etc.) (Convey, 1996a).
The growth and life cycle patterns of many invertebrates and plants are fundamentally dependent on regional temperature regimes and their linkage with patterns of water availability (Convey et al., 2006a). In detail, the interaction between regional macroclimate and smaller scale ecosystem features and topography define the microclimate within which an organism must live and function. There has to date been remarkably little effort to identify connections between macro and microclimatic scales, or to probe the application of large-scale macroclimatic trends and predictions at microclimatic scale. Distinct patterns in sexual reproduction are evident across the Antarctic flora and are most likely a function of temperature variation - indeed recent increase in the frequency of successful seed production in the two maritime Antarctic flowering plants (Convey, 1996b) is proposed to be a function of warming in this region. In addition, phenology of flowering plants is cued to seasonality in the light regime. In regions supporting flowering plants, wind is assumed to play a major role in pollination ecology of grasses and sedges resulting in cross-pollination. The lack of specialist pollinators in the native fauna, combined with high reproductive outputs in non-wind pollinated species implies a high reliance on self-fertilisation.
The Antarctic biota shows high development of ecophysiological adaptations relating to cold and desiccation tolerance, and displays an array of traits to facilitate survival under environmental stress (Hennion et al., 2006). While patterns in absolute low temperatures are clearly important in determining survival, perhaps more influential is the pattern of the freeze-thaw regime, with repeated freeze-thaw events being more damaging than a sustained freeze event (Brown et al., 2004; Sinclair and Chown, 2005). How these patterns change in the future will be an area of major importance.
The response of Antarctic plants to increased UV-B radiation (280-315 nm) associated with the ozone hole, provides an illustration of another suite of ecophysiological/biochemical strategies. Reported responses vary widely between studies, ranging from negative effects on chlorophyll concentrations in tissues, on growth, and evidence of DNA damage in some species (e.g. Ruhland and Day, 2000; Xiong and Day, 2001; Robinson et al., 2005; Turnbull and Robinson, 2008), through little change (e.g. Björn, 1999; Lud et al., 2002; Boelen et al., 2006) to consistent positive effects, such as increased concentrations of UV-B screening pigments (Newsham et al., 2002). Previously it has been suggested that higher plants and bryophytes could differ in their abilities to synthesize UV-B screening pigments (Gwynn-Jones et al., 1999), but most recent data from Antarctic studies do not support this proposition (e.g. Newsham et al., 2002; Newsham, 2003; Newsham et al., 2005; Dunn and Robinson, 2006; Clarke and Robinson, 2008). The majority of Antarctic bryophytes studied have potential UV screening compounds inside their cells and/or attached to their cell walls (Clarke and Robinson, 2008), suggesting widespread UV screening potential in these species. A recent study has estimated that the cost of synthesising new protective pigment molecules on exposure to UV-B represents approximately 2% of the carbon fixated by a common Antarctic liverwort, analogous to estimates of 1-10% of biomass invested in cryoprotectants or desiccation protectants by many Antarctic terrestrial invertebrates and microbes (Snell et al., 2009).
Table 4.2 The occurrence of alien non-indigenous terrestrial species across Antarctic biogeographical zones (extracted from Frenot et al. (2005); see also Greenslade (2006) for a detailed description of established and transient alien species, and species recorded only synanthropically, from sub-Antarctic Macquarie Island).
A meta-analysis of the response of polar vegetation to UV-B radiation concludes that Antarctic bryophytes and vascular plants respond in a similar fashion to vegetation from other regions, with UV-B exposure leading to decreased above-ground biomass and height and increased DNA damage (Newsham and Robinson, 2009). Plants also appear able to protect themselves from elevated UV-B radiation through the induction of UV screening pigments (Newsham and Robinson, 2009), although this likely comes at a cost to biomass production (Snell et al., 2009). However this meta-analysis does suggest that the method by which UV-B radiation is applied to plants plays an important part in determining the strength of plant response to UV-B.
The final suite of life history traits includes elements relating to competition and predation. Their potential significance is illustrated by reference to ecosystem changes caused through the introduction of new predatory invertebrates to certain sub-Antarctic islands (e.g. Table 4.2). The introduction of carabid beetles to parts of South Georgia and Îles Kerguelen, where such predators were previously absent, is leading to extensive changes to local community structure, which threatens the continued existence of some indigenous and/or endemic invertebrates (Ernsting et al., 1995; Frenot et al., 2005, 2008). Regional warming has also been predicted to rapidly increase the impact of certain indigenous predators (Arnold and Convey, 1998). Providing an analogous impact within the decomposition cycle, detailed studies on Marion Island indicate that indigenous terrestrial detritivores are unable to overcome a bottleneck in the decomposition cycle, hence illustrating a further ecosystem service likely to be strongly influenced by recently introduced non-indigenous species (Slabber and Chown, 2002).
The lack of attention to these traits to date is unfortunate, particularly with respect to the understanding of alien species’ impacts. It is already well known that Antarctic terrestrial biota possess very effective stress tolerance strategies, in addition to considerable response flexibility. The exceptionally wide degree of environmental variability experienced in many Antarctic terrestrial habitats, on a range of timescales between hours and years, means that predicted levels of change in environmental variables (particularly temperature and water availability) are often small relative to the range already experienced. However, as illustrated above with biochemical responses to UV-B exposure, any change in the balance of use of specific strategies carries a quantifiable cost, and carries implications for changes in the allocation of resources within the organism.
Given the absence of more effective competitors, predicted and observed levels of climate change may be expected to generate positive responses from resident biota of the maritime and continental Antarctic, and this is confirmed in general terms both by observational reports of changes in maritime Antarctic terrestrial ecosystems, and the results of manipulation experiments mimicking the predictions of climate change (Convey, 2003, 2007). Over most of the remainder of the continent, biological changes are yet to be reported, as might be expected given the weakness or lack of evidence for clear climate trends over the instrumental period. Potentially sensitive indicators of change have been identified amongst the biota of this region (e.g. Wasley et al., 2006), particularly in the context of sensitivity to changes in desiccation stress (Robinson et al., 2003). More local scale and short-term trends of cooling over recent decades in the Dry Valleys of Victoria Land have been associated with reductions in abundance of the soil fauna (Doran et al., 2002). The picture is likely to be far more complex on the different sub-Antarctic islands as, in addition to various different trends being reported in a range of biologically important variables, many also already host (different) alien invasive taxa, some of which already have considerable impacts on native biota (Frenot et al., 2005; Convey, 2007, Table 4.2).
The best-known and frequently reported example of terrestrial organisms interpreted to be responding to climate change in the Antarctic is that of the two native Antarctic flowering plants (Deschampsia antarctica and Colobanthus quitensis) (Figure 4.61) in the maritime Antarctic (Fowbert and Smith, 1994; Smith, 1994; Grobe et al., 1997; Gerighausen et al., 2003). At the Argentine Islands numbers of plants increased by two orders of magnitude between the mid 1960s and 1990 (Fowbert and Smith, 1994), although it is often overlooked that these increases have not involved any change in the species’ overall geographic ranges, limited in practice by extensive ice cover south of the current distribution. These increases are thought to be due to increased temperature encouraging growth and vegetative spreading of established plants, in addition to increasing the probability of establishment of germinating seedlings. Additionally, warming is proposed to underlie a greater frequency of mature seed production (Convey, 1996b), and stimulate growth of seeds that have remained dormant in soil propagule banks (McGraw and Day, 1997). However, since 1990, there has been no further increase in the Argentine Islands populations, while there has also been no significant warming trend in either the annual or seasonal air temperature data record at this location over the period 1990-2008, which might suggest the link between environmental conditions and plant responses is even closer than initially thought (Parnikoza et al., in press).
Changes in both temperature and precipitation have already had detectable effects on limnetic ecosystems through the alteration of the surrounding landscape and of the time, depth and extent of surface ice cover, water body volume and lake chemistry (with increased solute transport from the land in areas of increased melt) (Quesada et al., 2006; Lyons et al., 2006; Quayle et al., 2002, 2003. The latter authors highlight that some maritime Antarctic lake environmental changes actually magnify those seen in the atmospheric climate, highlighting the value of these locations as model systems to give ‘early warning’ of potential changes to be seen at lower latitudes. Predicted impacts of such changes will be varied. In shallow lakes, lack of surface ice cover will lead to increased wind–induced mixing and evaporation and increases in the diversity at all levels of the ecosystem. If more melt water is available, input of freshwater into the mixolimna of deeper lakes will increase stability and this, associated with increased primary production, will lead to higher organic carbon flux. Such a change will have flow–on effects including potential anoxia, shifts in overall biogeochemical cycles and alterations in the biological structure and diversity of ecosystems (Lyons et al., 2006).
Alien microbes, fungi, plants and animals, introduced directly through human activity over approximately the last two centuries, already occur on most of the sub-Antarctic islands and some parts of the Antarctic continent (Frenot et al., 2005, 2008; Greenslade, 2006; Convey, 2008, Table 4.2). The level of detail varies widely between locations and taxonomic groups (although at the microbial level, knowledge is virtually non-existent across the entire continent). On sub-Antarctic Marion Island and South Atlantic Gough Island it is estimated that rates of establishment through anthropogenic introduction outweigh those from natural colonization processes by two orders of magnitude or more. Introduction routes have varied, but are largely associated with movement of people and cargo in connection with industrial, national scientific programme and tourist operations. Although it is rare to have a record available of a specific introduction event, and there are undoubtedly instances of natural colonization processes resulting in new establishment, the impact of undoubted human-assisted introductions to some sub-Antarctic islands (particularly South Georgia, Kerguelen, Marion, Macquarie) is substantial and probably irreversible. Thus a range of introduced vertebrates and plants have led to large shifts in ecosystem structure and function, while in terms of overall diversity some islands now host a greater number of non-indigenous than indigenous species of plant. The large majority of aliens are European in origin.
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