10 1 Polar Deserts Ian D. Hogg1 and Diana H. Wall2 Department of Biological Sciences, University of Waikato, New Zealand; 2 Department of Biology, and Natural Resource Ecology Laboratory, Colorado State University, USA 10.1 Introduction: a Cold, Dry Environment As extreme environments go, polar deserts may be among the most extreme and include some of the coldest, driest habitats on the planet. They are commonly defined as having less than 250 mm of annual precipitation and with maximum temperatures of below 10°C (Fountain et al., 1999; Doran et al., 2002a). Using these criteria, habitats include those found in both the high Arctic as well as on continental Antarctica. However, few extreme environments match the Dry Valleys of the Ross Sea region in Antarctica (Fig. 10.1). There, cold, dry air from the polar plateau is gravity-fed – the katabatic winds – on to an area of roughly 4800 km2 of mountains and exposed valley floors along the southern Victoria Land coast of the Ross Sea region (Campbell and Claridge, 2006). Captain Scott, on discovering the Dry Valleys in 1903, commented on their extreme nature by observing that ‘We have seen no living thing, not even a moss or a lichen’. However, despite these initial observations and the harsh, dry conditions, a range of microbes, non-vascular plants and animals manage to eek out an existence here. These include mosses, lichens, microscopic invertebrates such as nematode worms, tardigrades and microarthropods (all <1 mm). Indeed, 176 the largest year-round animals are the 1.3 mm springtails (Fig. 10.2), which survive on the remedial autotrophic (algal) and heterotrophic (bacterial, fungal) organisms available. Even for these residents, activity is limited to a few months a year and usually only during the austral ‘summer’ period (i.e. November to March). We use the Dry Valleys as an example of an ‘extreme’ polar desert in order to describe life in these habitats. To avoid overlap with other chapters, we restrict our focus to the terrestrial realm of polar desert life. A more general overview of polar terrestrial environments can be found in Convey (Chapter 5, this volume), and Laybourn-Parry and Bell (Chapter 6, this volume) provide a focus on polar freshwaters. Here, we continue with a more detailed look at the physical environment of the Dry Valleys, before turning our attention to the biotic components, their survival mechanisms, trophic interactions as well as food webs and conclude with a section on the consequences of environmental changes for polar desert habitats. 10.2 The Physical Realm The Dry Valleys of Southern Victoria Land shown in Fig. 10.1, are part of the © CAB International 2012. Life at Extremes: Environments, Organisms and Strategies for Survival (ed. E.M. Bell) Polar Deserts 177 Fig. 10.1. Satellite image of the Dry Valleys and the, predominantly ice covered, surrounding region. Cartography by Brad Herried. Transantarctic mountains and occur over a latitude of approximately 77−78°30′S. They comprise the largest ice-free area on the Antarctic continent (Ugolini and Bockheim, 2008). Within the main area (about 77°S), three large valley systems are found (Taylor, Wright and Victoria Valleys). There are also numerous smaller valleys such as the McKelvey, Balham and Barwick Valleys in the vicinity of these larger systems. In addition, another series of small valleys are immediately to the south of the main area (approx. 78°30′), collectively known as the ‘Southern Dry Valleys’ these include Garwood, Marshall, Miers and Hidden Valleys (see Fig. 10.1). All areas have been the subject of geological and biological studies beginning with the Scott expeditions of the early 1900s. Mountains comprising the Dry Valleys have elevations ranging from sea level to 800 m and are surrounded by mountains reaching 178 I.D. Hogg and D.H. Wall nearly 2000 m. The sub-zero temperatures (average air temperature ~−17°C), low snowfall (ranging from ~3 mm water equivalent year−1 inland to ~50 mm year−1 water equivalent near the coast), high ablation rates and high winds contribute to an extreme ecosystem of low biological diversity and activity. The Dry Valleys have remained relatively stable for the past few million years compared to recent anthropogenic modifications of landscapes in other temperate deserts (Brown et al., 1991; Fountain et al., 1999; Virginia and Wall, 1999), providing an opportunity for understanding Fig. 10.2. The springtail Gomphiocephalus hodgsoni (Collembola) is the largest year-round inhabitant of the Dry Valleys (actual size 1.3 mm). Note the considerably reduced furcula (spring) on the underside of the abdomen. © Barry O’Brien. the physical controls on biological activity and diversity in an extreme ecosystem. The Dry Valley landscapes are composed of barren soils, glaciers, perennially ice-covered lakes and ephemeral meltstreams (Fig. 10.3) (Fountain et al., 1999). Arid soils in these valleys are considered the oldest, driest and coldest on earth (Campbell et al., 1998), and comprise the largest land feature of the Dry Valleys, occupying about 95% of glacier-ice-free surfaces below 1000 m (Burkins et al., 2000). The dominant soil features are soil polygons (Fig. 10.4) that are formed by the freezing and thawing of soils and rock (Campbell and Claridge, 1987). Soils are derived from bedrock and tills composed of granites, sandstones, dolerites and meta-sedimentary rocks that range from Holocene to Miocene in age (Denton et al., 1989; Bockheim, 1997; Hall and Denton, 2000). These desert soils have high salinity that generally relates to soil surface age (Campbell and Claridge, 1987; Bockheim, 2002), a neutral to high pH, in general a coarse texture (95–99% sand), extremely low organic matter content (0.01–0.03% organic carbon by weight; Campbell and Claridge, 1987; Burkins et al., Fig. 10.3. Typical Dry Valley landscape: the lower Garwood Valley looking up-valley towards the Garwood Glacier. The Joyce Glacier is visible in the background. © Philip Ross. Polar Deserts 179 Fig. 10.4. Patterned ground (soil polygons) visible along the eastern face of the Canada Glacier in Taylor Valley. © Leo Sancho. 2000) and permafrost at 10–30 cm depth (Pastor and Bockheim, 1980; Bockheim, 1997; Ugolini and Bockheim, 2008). 10.3 The Diversity of Life in the Dry Valleys Polar Desert The extreme environment limits life forms within the Dry Valleys. Seasonal visitors to the Dry Valleys include tourists, researchers, occasional Adélie penguins (Pygoscelis adeliae) and skuas (Catharacta maccormicki) flying overhead. The mummified remains of crabeater (Lobodon carcinophagus), Weddell (Leptonychotes weddellii) and leopard seals (Hydrurga leptonyx), as well as the odd penguin (Fig. 10.5), suggest that not all of these individuals are able to leave the Dry Valleys. Resident Dry Valley life is most commonly found where there is at least some access to available water. Examples include, near the feet of glaciers, snow patches, meltwater streams or areas of permafrost thaw, or for aquatic soil animals (nematodes, rotifers and tardigrades) where soil moisture or soil relative humidity is high enough to maintain activity. No vascular plants occur in the Dry Valleys, but algae, bryophytes (mosses) and colourful lichens (Fig. 10.6) are found where there is adequate moisture, such as at higher altitudes receiving snow, or near glacial meltstreams or flush zones. All Antarctic terrestrial diversity (e.g. number of species or taxa) is lower than that found in temperate and tropical ecosystems. However, the terrestrial diversity of the Dry Valleys is considered low, even relative to local coastal habitats, such as near the Mackay Glacier where, for example, lichen and moss diversity is noticeably higher (Seppelt et al., 2010). Overall, soil biodiversity is also low relative to non-Dry Valley, Antarctic sites both further north (Northern Victoria Land, 74°S; Barrett et al., 2006) and further south (Queen Maud Mountains, 83°S; Green et al., 2011). Most invertebrate species appear to be endemic, although some taxa (e.g. algae) 180 I.D. Hogg and D.H. Wall Fig. 10.5. Hidden Valley, one of the ‘Southern Dry Valleys’, looking towards the Adams Glacier (visible in the background). A dead penguin is visible in the foreground. Penguins and seals occasionally wander into the Dry Valleys, but either due to starvation or disorientation seldom make it back out again. This provides one of the few sources of external nutrients to these ecosystems. © Ian Hogg. Fig. 10.6. Lichens (Acarospora) growing on the surface of a rock (pencil provided for scale). © Ian Hogg. Polar Deserts are a mixture of endemic and cosmopolitan species (Esposito et al., 2006). The distribution of taxa across the Dry Valleys is irregular and patchy as some soils are not suitable habitats for organisms (Virginia and Wall, 1999). Soils at small spatial scales (<1 m2) have no visible algae or mosses, nor invertebrates due to high soil salinity, low carbon, and the high heterogeneity in other chemical and physical properties (Virginia and Wall, 1999; Courtright et al., 2001; Elberling et al., 2006). Biotic communities of the Dry Valleys generally have high numbers of a single invertebrate species in dry barren soils (e.g. 10–50 individuals g−1 dry weight soil), but more species and increased complexity of communities (more than two taxa) near meltstreams (Barrett et al., 2004). For example, soils of ephemeral meltstreams are generally richer in carbon and nitrogen compared to dry soils, and support greater abundances and higher taxonomic diversities (e.g. two to three species each of nematodes and 181 arthropods as well as tardigrades, rotifers and several species of mosses and algae). To understand the functioning of the Dry Valley ecosystem, it is helpful to know something of the key life forms and their survival mechanisms. Below, we describe these biotic components of the Dry Valleys along with an indication of their diversity and biogeography as well as notes on their natural history. 10.3.1 Plant and fungal life Mosses Mosses or bryophytes are among the more noticeable of life forms in the Dry Valleys where they occasionally form lush cushions in areas of high soil moisture, such as in glacial flush zones (Fig. 10.7). On the northern margin of the Dry Valleys (e.g. Granite Harbour near the Mackay Glacier), there are at least ten species of moss (Seppelt et al., 2010). However, within Fig. 10.7. Flush area in front of the Canada Glacier in Taylor Valley. Extensive moss beds are found in the darker areas visible in the middle of the photograph. © Ian Hogg. 182 I.D. Hogg and D.H. Wall the Dry Valleys proper (e.g. Canada Glacier in Taylor Valley), there are only three (Seppelt et al., 2010). This can be partly attributed to the harsher conditions within the Dry Valleys, but also the greater access to nutrients (e.g. skua guano) and moisture at coastal sites (e.g. Granite Harbour). Mosses can provide substrate for lichens, as well as maintaining a moist habitat for a range of invertebrates (e.g. rotifers, springtails, nematodes; Simmons et al., 2009a). Dead and decomposing mosses also provide valuable nutrients to developing ‘soil’ communities. Lichens Lichens are symbiotic organisms consisting of an algal and fungal component that are widely distributed in temperate desert and other ecosystems, but their range in the Dry Valleys is much more limited. In temperate regions, the relationship between the symbionts is usually one-to-one (i.e. a specific algal species matched with a specific fungal species). However, in Antarctica where access to the required or even preferred symbiont may be limited, it has recently been discovered that there may be some flexibility in this relationship (Wirtz et al., 2003). Similar to the mosses, there are far fewer species found within the Dry Valleys than at nearby coastal sites (e.g. Granite Harbour) with only two species easily visible on the Dry Valley floors and restricted to small <1 m2 patches of moist rock within glacial flushes (Green et al., 1992). In total, there are roughly 15 species of lichen found in the Dry Valleys with most species more prevalent on valley rock sides and at higher altitude (T.G.A. Green, University of Waikato, 2010, personal communication). However, although lichen occurrences within the valleys are small and very sporadic (e.g. Fig. 10.6), they are much more common and visible on the mountains between the valleys (Fig. 10.8). This is due in part to the increased stability, larger substrate size and corresponding habitat heterogeneity, as well as the increased moisture levels at higher altitudes (e.g. cloud effect, snow retention). Fig. 10.8. Buellia frigida, a lichen species commonly found growing on rocks in higher elevation areas throughout the Dry Valleys. © Rod Seppelt. Polar Deserts Algae Approximately 80 species of algae are found within the Dry Valleys, the majority being diatoms found in the lake and stream ecosystems (Broady, 2005; Esposito et al., 2006, 2008). However, a few species inhabit soils and these are often observed as hypoliths (growing under rocks) and endoliths (growing within rocks). Taxa are predominantly ‘blue-green algae’ (cyanobacteria) and usually represented by a single species, Oscillatoriales sp. (Cowan et al., 2010). Large mats of cyanobacteria are also frequently observed around the margins of lakes, small ponds and streams, where they can become exposed (and part of the terrestrial realm) when water levels decline. The depth of water covering the algal mats in stream channels may only be a few centimetres, which is sufficient for growth of the desiccation-tolerant filamentous cyanobacteria and associated diatoms which comprise the mats (Alger et al., 1997). Even in cold summers, soil temperatures will rise above freezing for at least a few hours per day over several weeks (Barrett et al., 2008), although overall biological activity is constrained by low water availability rather than temperature (Ball et al., 2009; Treonis et al., 2000). Fungi The globally ubiquitous fungi are a critical component of the Dry Valley ecosystems. They feed off the limited nutrients present in these habitats and are important in the development of ‘living’ soils. They can frequently be observed as hypoliths occurring under and around the margins of rocks – particularly translucent quartz rocks, which moderate the local thermal microenvironment and reduce ambient light levels (Cowan et al., 2010). Cowan et al. (2010) have questioned whether the fungal presence in these habitats is parallel with, or successional to, the algal hypoliths (i.e. taking advantage of their initial productivity). Nevertheless, they serve to stabilize the soil surface and subsurface and also provide a valuable food source to many of the animal components described below. 183 10.3.2 Animals Protozoa Although no longer included as part of the animal kingdom, protozoans are microscopic single-celled organisms found in a range of aquatic and soil habitats. Very little study has been undertaken on the protozoans of the Dry Valleys, although representatives include flagellates, small amoebae and ciliates (Bamforth et al., 2005). However, they are thought to be widespread and Bamforth et al. (2005) found individuals in 92% of soil samples collected from the Dry Valleys. Protozoans are also likely to contribute to the diets of multicellular animals such as rotifers, tardigrades and nematodes (Bardgett, 2005). Tardigrades Tardigrades or ‘water bears’ are common inhabitants of soil and aquatic habitats throughout Antarctica. Within the Dry Valleys, roughly four species have been identified (Adams et al., 2006). However, there has been much confusion and ‘synonymization’ of previous species designations. Ongoing morphological and molecular studies will help to resolve this issue further as molecular studies have shown there are at least seven species of tardigrades in the Taylor Valley (B.J. Adams, Brigham Young University, 2010, personal communication). Rotifers Rotifers are minute animals (<0.01 mm), most commonly known as a component of the plankton in freshwater lakes but also moist soils. Even in the Dry Valley region, at least 15 species are associated with lakes and ponds (Adams et al., 2006). At least three species are associated with moist soils and with mosses and lichens where they can access the higher levels of relative humidity which they require to remain in an active state (Schwarz et al., 1992). In the absence of moisture, rotifers can produce drought-resistant eggs, allowing them to survive prolonged dry periods and also to exploit episodic melting of 184 I.D. Hogg and D.H. Wall snow and glaciers. Little is known of their diets in the Dry Valleys. However, elsewhere, they are known to consume bacteria, algae and protozoans, all of which are present in the Dry Valleys. Nematodes Nematodes are clearly among the most widespread of multicellular animals on the planet. As a consequence, they can exist almost anywhere and different species exhibit various lifestyles from fully free-living to obligate parasitic. The Dry Valley soils are home to at least four genera of free-living nematodes: Scottnema, Plectus, Geomonhystera, all consumers of bacteria and Eudorylaimus, an algal consumer (Wall, 2005; Adams et al., 2006). Most are represented by only a single species. However, ongoing molecular analyses may revise these totals slightly, particularly for Plectus (B.J. Adams et al., 2011, unpublished results). By far, the most commonly encountered species of nematode in the Dry Valleys is Scottnema lindsayae (Fig. 10.9), which occurs in a range of different soil types and chemistries (Wall Freckman and Virginia, 1998). Collembola Collembola or springtails (e.g. Fig. 10.2) are small, primitive, insect-like animals, usually <2 mm in body length, although some, such as the ‘giant’ springtails of New Zealand Fig. 10.9. Scottnema lindsayae, a widespread Antarctic nematode and the most common soil invertebrate found in the Dry Valleys. © Manuel Mundo. are >5 mm. Springtails were once classed as insects but are now considered sufficiently different to have been placed in their own class (Ellipura). Springtails are primarily decomposers found in a variety of soil and aquatic detrital habitats. Globally, they are the most widely distributed hexapods with representatives found from the tropics to within a few hundred kilometres of both poles. They are also the most diverse hexapods in both polar regions. As their common name suggests, many species are known for their propulsive, spring-like ‘tails’ (furcula), which are used for dispersal and predator avoidance. However, such devices are usually confined to surface-dwelling species, and those species found lower in the soil profile have reduced or absent furcula as well as shortened legs and antennae. Few of the Antarctic species have fully developed furcula and the single species common in the Dry Valleys (Gomphiocephalus hodgsoni) has a considerably reduced furcula (see Fig. 10.2). The diet of G. hodgsoni consists predominantly of fungal, bacterial and algal material (Davidson and Broady, 1996). Mites Mites (Acari) are one of the more recognizable and common inhabitants of the Dry Valleys and, alongside the springtails, are the only other free-living terrestrial arthropods. Where present, they are usually found on the underside of small, dark rocks or occasionally scurrying about on the surface. They move relatively quickly for their small size and this usually alerts the observer to their presence. They may have greater dispersal abilities relative to springtails, and as a consequence, tend to be found in a wider range of locations (Stevens and Hogg, 2002, 2006). They also seem to be able to tolerate slightly wetter habitats than those of the springtails and are often found within glacial flush zones (Sinclair and Stevens, 2006). Within the Dry Valleys, two species are encountered, Stereotydeus mollis and Nanorchestes antarcticus (Strandtmann, 1967). They are both small (<1 mm), and are presumed to have very similar diets to G. hodgsoni (above). Polar Deserts 10.3.3 Microbes Bacteria are perhaps the most widespread and numerous of life forms on the planet. They perform a number of roles within ecosystems including nutrient cycling (e.g. decomposition) as well as occasionally serving as vectors of disease. Despite this, comparatively little work has been undertaken on the microbiology of the Dry Valleys terrestrial environments. Initial work had suggested that the microbial diversity of Dry Valley soils was limited and that most of the taxa were psychrotrophic (cold-adapted) varieties unique to Antarctica (Smith et al., 2006). However, more recent work using advanced molecular techniques has suggested the opposite. Indeed, the diversity of microbes in Dry Valley soils may actually rival that of more temperate habitats (Cary et al., 2010). The other surprising finding has been that instead of having a ‘typical’ microbial flora throughout the Antarctic, there are actually regional and local differences (Barrett et al., 2006). In addition to their important role in the breakdown of available nutrients, microbes also provide a valuable component of the diets for several of the invertebrates (e.g. nematodes) in the Dry Valleys. 10.4 Mechanisms for Survival in Polar Deserts Some soil animals of the Dry Valleys appear to be specifically adapted to exist in the cold dry environment. For example, the most common nematode of the Dry Valleys, Scottnema lindsayae, survives and reproduces better at 10°C than it does at 15°C (Overhoff et al., 1993). However, in order to endure the cold, dry conditions, most animals must possess at least some survival mechanisms to respond to the extreme and potentially rapid (minute-by-minute scale) fluctuations in moisture and temperature. 10.4.1 Desiccation and anhydrobiosis Nematodes, rotifers and tardigrades are often active in water films around soil particles. 185 When adverse environmental conditions occur, such as low soil relative humidity or rapid sublimation rates, the animals can desiccate and enter what is termed anhydrobiosis (an ametabolic state entered during extreme desiccation; Wharton and Barclay, 1993). They achieve this by altering their morphology to reduce surface area and become ametabolic (Wharton, 2003). However, within minutes to hours of a return to favourable environmental conditions, anhydrobiotic invertebrates can activate (Treonis and Wall, 2005). Algae, mosses and lichens also tolerate desiccation and can survive prolonged periods without access to water, yet within minutes can return to an active metabolic state when favourable conditions occur (Pannewitz et al., 2003; Schlensog et al., 2004; McKnight et al., 2007). In many cases, these ametabolic states can last for months or years until favourable environmental conditions return (Browne et al., 2002; Treonis and Wall, 2005). For example, a species of nematode, Panagrolaimus, collected from Armenia has been reported to have survived more than 8 years in an anhydrobiotic state (Aroian et al., 1993). 10.4.2 Freeze tolerance, freeze avoidance and ‘supercooling’ Prior to entering an anhydrobiotic state, species such as nematodes can also change their biochemistry to produce antifreeze compounds (e.g. trehalose, inositol; Crowe and Madin, 1974). All of these responses appear to be under genetic control (Adhikari et al., 2009), and can allow animals to tolerate temperatures much lower than would be possible in the ‘normal’ active state. Remarkably, for one species of Antarctic nematode (Panagrolaimus davidi), this can allow them to survive temperatures as low as −80°C (Wharton and Brown, 1991). The triggering of cold-tolerant genes and corresponding responses can also allow nematodes to cope with more minor fluctuations in temperature and moisture (Adhikari et al., 2010). In contrast, arthropod species, such as springtails (Collembola) and mites, lack the 186 I.D. Hogg and D.H. Wall ability to enter an anhydrobiotic state and for them, freezing is always lethal (Sømme, 1981). Instead, they rely on freeze-avoidance techniques such as the minimizing of icenucleating compounds (e.g. food) within their bodies at times when temperatures are likely to be decreasing (e.g. onset of winter; Sinclair and Sjursen, 2001). Springtails are also capable of lowering their freezing points through the production of antifreeze proteins such as glycol. For the common Dry Valley springtail (G. hodgsoni), such adaptations allow individual animals to ‘supercool’ to as low as −35°C before freezing occurs (Sinclair and Sjursen, 2001). 10.5 Trophic Structure and Food Webs Trophic structure in Dry Valley ecosystems is very simple. Indeed, they are often viewed as one of the few examples of an ecosystem that may be driven entirely by abiotic factors (Hogg et al., 2006). However, this may also be the result of research focused at inappropriate spatial or trophic levels, and the generality of this view has been questioned (e.g. Caruso et al., 2007). Regardless, they are certainly among the simplest of ecosystems on the planet. One of the major factors limiting life in the Dry Valleys is access to nutrients. Along the coastlines and on offshore islands (e.g. Beaufort, Ross Islands), skua colonies and/ or penguin rookeries as well as other marine sources provide an ample (and often overwhelming) supply of nutrients. However, within the Dry Valleys proper, which can be a considerable distance from the sea, these sources decrease with distance from the coast (Burkins et al., 2000) and include both autochthonous (produced within) or allochthonous (external) sources (Barrett et al., 2010). Hopkins et al. (2006) provided a theoretical analysis of the sources of carbon available in Dry Valley ecosystems and a simplified representation of these sources is provided in Fig. 10.10. Allochthonous sources are primarily marine in origin and include nutrients that have been blown in (e.g. marine sediments), walked in (e.g. dead and dying seals and penguins; see Fig. 10.5), or are legacies of past events (e.g. seawater Sources of Carbon Allochthonous Sea Springtail Windborne Dead seals, penguins Bacteria Fungi Legacy Autochthonous Cyanobacteria Mosses, Lichens Hypoliths, Endoliths Fig. 10.10. Sources of carbon in the Dry Valleys. © Ian Hogg and Diana Wall. Mite Nematode Polar Deserts intrusions). Autochthonous sources include the primary producers, primarily cyanobacteria and other algal taxa found in lakes, ponds and soils. Species found in lakes often produce large ‘mats’ that can be exposed when water levels drop. These then dry out and can be blown about the Valleys spreading the nutrients over a much wider area. There are also legacy effects in areas where lakes and ponds once existed. These old lake beds are repositories of nutrients produced when the lake was active. Controls over soil carbon balance are also influenced by events occurring over glacial time scales (e.g. lake inundation), which is in turn influenced by temperature (Burkins et al., 2000). Most taxa are thought to be predominantly involved in the decomposition/ detritus pathway as there are no vascular plants for herbivores such nematodes, mites and springtails, and there are no known predators (Wall, 2005). Nevertheless, recent observations have suggested that a 187 previously unidentified species of tardigrade may be a facultative predator on other tardigrade species (U. Nielsen, Colorado State University, 2010, personal communication). Further work, which is ongoing, is required to confirm this. However, in the interim, Dry Valley food webs can be thought to have primarily three trophic levels: producers; consumers; and decomposers. A simplified view of a theoretical food web is provided in Fig. 10.11. Cyanobacteria (blue-green algae), as well as eukaryotic algae (e.g. diatoms), are a major food source for virtually all of the consumers (Wall, 2005). There is no evidence that lichens or mosses are directly consumed, and instead are more often used as suitable habitats where access to other food sources (e.g. algae, fungi) is facilitated (Simmons et al., 2009b). However, once dead, all components of the food web (including mosses and lichens) would be processed by the decomposers. In turn, both bacterial and fungal Producers Eukaryotic Algae Cyanobacteria Lichen Moss Consumers Nematode (Wet) Rotifer Tardigrade Mite Springtail Nematode (Dry) Decomposers Bacteria Fungi Fig. 10.11. A simplified version of the Dry Valley food web showing primary producer, consumer and decomposer trophic levels. All components feed into the decomposers; however, these pathways have not been added for simplicity. © Ian Hogg and Diana Wall. 188 I.D. Hogg and D.H. Wall components provide a major food source to the consumers – bacterial components being more important for the smaller consumers, such as protozoa and nematodes. these increases have been closer to a staggering 0.1°C year−1. 10.6.2 10.6 Environmental Changes and Biotic Response 10.6.1 Climate changes Global atmospheric changes, such as global warming, are providing some of the most serious challenges facing the Earth’s biota. Mean global air temperatures are expected to increase in the range of 1.4−6.4°C within the next 50–100 years (Solomon et al., 2007). In both polar regions, temperature increases may be double the projected global means, due to the melting of snow/ ice cover and the resulting decrease in albedo (reflecting of the sun’s rays by snow). Furthermore, the rates at which increases occur are very likely to exceed any rates previously experienced by the biota. In addition to predicted increases in the mean temperatures and the rates of increase, the increased incidence of extreme-weather events such as droughts, floods and storm events is likely to provide additional challenges (see also Psenner and Sattler, Chapter 23, and Glover and Neal, Chapter 24, this volume). In contrast to the Antarctic Peninsula where warming has occurred more rapidly in the past 50 years than any location on the planet (~3°C increase from 1950 to 2000, Doran et al., 2002b; Montes-Hugo et al., 2009; Turner et al., 2009), the Dry Valley region has cooled within the past 20 years (Doran et al., 2002b; Thompson and Solomon, 2002). This cooling climate has more recently been punctuated by annual warming events (Doran et al., 2008). In contrast, and on a broader scale, detailed climate data collected by NASA over the last 50 years and covering the vicinity of the Dry Valleys show a noticeable warming trend of around 0.05°C year−1 for the area of West Antarctica (Fig. 10.12). Indeed, for areas of the Western Antarctic Ice Sheet (WAIS), Biotic responses to change One of the surprising aspects of the Dry Valley desert system is the mosaic of diversity that occurs within and among valleys. Throughout the Antarctic, ‘higher’ level taxa such as springtails and mites often show remarkable genetic differences even over distances as small as a few kilometres (e.g. Fanciulli et al., 2001; McGaughran et al., 2008; Hawes et al., 2010). This suggests that instead of being dictated solely by climatic variables, the distribution of organisms is determined by historical events and chance survival of organisms within particular habitats. These past influences can even be observed on relatively small spatial scales. For example, in Taylor Valley, two different maternal lineages (mitochondrial DNA haplotypes) of the springtail, G. hodgsoni, were found to trace an ancient, glacial lake shoreline in Taylor Valley from when temperatures were warmer (Nolan et al., 2006). The current explanation is that ancestral individuals would have been separated during glacial advances and then genetically diverged in isolation. One haplotype represented the resident haplotype of Taylor Valley while the other represented a neighbouring area. Towards the end of the last glacial cycle (roughly 20,000 years ago), warmer temperatures and melting of the glacial lake may have facilitated dispersal of individuals via ‘rafting’ (sensu Hawes et al., 2008) on meltwater ‘moats’ around the edge of the lake. Here, the two lineages were reunited and have co-existed to the present. What remains unknown is whether this co-existence is benign, or whether there is any local, competitive advantage (or disadvantage). Specifically, although the ‘neighbouring’ haplotype traces the shoreline, it does not occur at other, up-valley sites. An alternative explanation is, simply, that their absence ‘up valley’ reflects the limited dispersal abilities of springtails (e.g. Stevens and Hogg, 2003). Polar Deserts 189 1000 km Temperature trend (˚K) –0.1 –0.05 0 0.05 0.1 Fig. 10.12. A heat map of Antarctica showing surface temperature changes based on temperature records collected over the previous 50 years (1957–2007). Of particular note are the ‘red’ areas over West Antarctica. © NASA 2007, used with permission. Although these local genetic differences are perhaps not surprising for arthropods with restricted dispersal abilities, the same may hold true for other taxa such as lichens (Green et al., 2011) and even microbes (e.g. Pointing et al., 2009). Lichens that can disperse themselves by small spores can be easily be blown about by the wind. So, one would expect these organisms to disperse very easily. In fact, the opposite seems to be true. Genetic analyses of a common lichen species in the Dry Valleys have shown that isolated populations are in fact genetically distinct from each other (Ruprecht et al., 2010). Microbial communities of the Dry Valleys also show a remarkably high level of diversity, sometimes rivaling that of more temperate zones (Cary et al., 2010). There is also tremendous habitat complexity reflecting a legacy of geological history and physical conditions that create soil heterogeneity. Consequently, even for the microbes, different valleys are likely to house different communities that have developed in isolation over hundreds of thousands of years (Pointing et al., 2009). This suggests that dispersal or colonization is not as easy or common as previously thought, nor is it solely responsible for the 190 I.D. Hogg and D.H. Wall distribution of microbes. Instead, the Dry Valley landscape has been tiled with a mosaic of genetically distinctive populations reflecting past and present conditions at these sites. Some of the likely consequences of global climate changes will be shifts in wind patterns, retreating glaciers, increased stream flow from melting glaciers and the melting of icesheets exposing ‘new’ ground. All of these events have the potential to increase the dispersal capacities of organisms. On one hand, the inherent genetic diversity of Antarctic organisms could be advantageous as this variability may provide some potential to respond to environmental changes. Most Dry Valley life should presumably have little trouble exploiting new, suitable habitat as they are by their very nature, early successional taxa. However, not all soil habitats are, or will be, capable of supporting life (Courtright et al., 2001; Wall, 2005). Alternatively, and of more certainty, is that the mixing up of local populations has the potential to destroy possibly millions of years of history and local adaptation. The decreases in air temperatures recorded in the Dry Valleys in the late 1990s resulted in many interconnected effects on the ecosystem including decreased stream flow, increased ice thickness on lakes, and a decline in nematode populations (Doran et al., 2002b; Barrett et al., 2008), as well as changing the composition of diatom species in streams (Esposito et al., 2006). Manipulative experiments and natural climate variation showed that warming alters soil chemistry and hydrology increasing soil moisture and modifying soil habitats resulting in a decline in the dominant nematode species (Simmons et al., 2009b). Warming events also affected biota and chemistry of streams and lakes (Foreman et al., 2004; Lyons et al., 2005). Collectively, these results indicate that the Dry Valley ecosystems and their biota are sensitive to even small changes in seasonal air temperatures. 10.7 The Human Connection Aside from the detrimental impacts imposed on Dry Valley ecosystems by climate change, humans can also have more direct influences resulting from their activities and presence in Antarctica. These include physical disturbances (e.g. hiking, construction activities), chemical contamination (e.g. fuel spills) and the introduction of alien species. However, humans also have the opportunity and responsibility to ensure that these unique extreme habitats are protected for future generations. Given the potentially negative consequences of environmental changes, formulating appropriate strategies to mitigate these changes is one of the many challenges facing those responsible for protecting Antarctica’s polar desert habitats. All activities in Antarctica are dictated by the Antarctic Treaty and it is the responsibility of all nations working in Antarctica to ensure that obligations under the treaty system are met. The Scientific Committee on Antarctic Research (SCAR) provides scientific advice on these issues. In the case of the Dry Valleys this has been addressed through the formation of an ‘Antarctic Specially Managed Area’ (ASMA). Ongoing research supported by the various national programmes and funding agencies is a key component of this decision-making process. Perhaps one of the more serious threats to Antarctic biodiversity is the introduction and establishment of alien species. Many of these organisms are brought in on clothing, footwear, fresh foods and transportation vectors (aircraft, ships). Current and projected temperature increases as well as an increase in the potential numbers of visitors to the Dry Valleys (e.g. tourists, researchers) all increase the probability of alien species being introduced. A concerted effort through several of the national programmes (e.g. United States Antarctic Program; Antarctica New Zealand) is now underway to ensure that all visitors adhere to the strictest of standards to minimize these risks. Of further concern is the longer distance movement and introduction of species already within Antarctica, or indeed from outside of Antarctica, to the Dry Valleys. The competitive response of Dry Valley organisms when confronted with such new arrivals is entirely uncertain. Experience from the Subantarctic Polar Deserts South Georgia Island is not reassuring. For example, Convey et al. (1999) suggested that introduced species of springtail have indeed displaced the local resident species. Even walking to and from study sites can have a negative effect. Ayres et al. (2008) found that minimal trampling disturbance resulted in measureable differences in the ratios of living to dead nematodes in underlying soils. In cases of well-used walking trails, mortality rates of nematodes were as high as 76%. As the numbers of visitors to the Antarctic Dry Valleys increase (e.g. through research activities, tourism), these effects will be exacerbated. Guidelines outlining proper procedures when working and visiting the Dry Valleys are now required reading for all visitors to the Dry Valleys. Strategies include avoiding particularly sensitive areas (e.g. moss beds), and using previously established trails. Other, often localized, effects include chemical contamination, particularly through the use of hydrocarbon-based products such as gasoline and oil-based lubricants (Aislabie et al., 2004). Most contaminated sites are the result of spills during construction or transportation events. However, some are the result of earlier scientific activities such as the Dry Valley Drilling Project where diesel fuel was used as a drilling fluid at Lake Vida in the Victoria Valley (Aislabie et al., 2004). Such activities are now strictly monitored and all chemicals used in the Dry Valleys must pass a Preliminary Environmental Evaluation (PEE) before the project is even allowed to proceed. However, any number of guidelines and restrictions are still not always 191 enough to protect fragile extreme ecosystems. In these cases, education is often the best strategy. The greater the awareness of the value of extreme environments as a functional and integral component of global ecosystems, the more likely they will be used in an appropriate manner. In summary, we have used the Dry Valleys of Antarctica as an example of an extreme polar desert. Accumulated knowledge continues to lead to a greater understanding of the organisms, their interactions and their survival capacity. There is a wealth of evidence building that these ecosystems are also experiencing rapid rates of environmental changes that could significantly affect the responses of biota and functioning of the ecosystems. As Antarctica has many unexplored terrestrial areas and the fewest species of any continent, there is a need to move forward on research priorities such as determining the composition and functioning of present ecosystems prior to changes resulting from global warming and invasive species. An enhanced understanding of the response of these relatively simple polar desert ecosystems to global environmental changes will provide critical insights for predicting the consequences for more complex ecosystems elsewhere – the so-called global barometer. 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