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Keywords:

  • Antarctic;
  • Arctic;
  • belowground;
  • climate change;
  • polar regions;
  • precipitation;
  • soil fauna;
  • warming

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information

The polar regions are experiencing rapid climate change with implications for terrestrial ecosystems. Here, despite limited knowledge, we make some early predictions on soil invertebrate community responses to predicted twenty-first century climate change. Geographic and environmental differences suggest that climate change responses will differ between the Arctic and Antarctic. We predict significant, but different, belowground community changes in both regions. This change will be driven mainly by vegetation type changes in the Arctic, while communities in Antarctica will respond to climate amelioration directly and indirectly through changes in microbial community composition and activity, and the development of, and/or changes in, plant communities. Climate amelioration is likely to allow a greater influx of non-native species into both the Arctic and Antarctic promoting landscape scale biodiversity change. Non-native competitive species could, however, have negative effects on local biodiversity particularly in the Arctic where the communities are already species rich. Species ranges will shift in both areas as the climate changes potentially posing a problem for endemic species in the Arctic where options for northward migration are limited. Greater soil biotic activity may move the Arctic towards a trajectory of being a substantial carbon source, while Antarctica could become a carbon sink.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information

Arctic and Antarctic ecosystems are responding rapidly to recent climate changes (e.g. Post et al. 2009; Turner et al. 2009a). For example, local warming in the Arctic has enhanced plant growth, caused a shift in vegetation composition towards greater cover of shrubs, and a northward migration of the tree line (Serreze et al. 2000; Callaghan et al. 2011). Increased temperatures across maritime Antarctica have similarly led to local population expansion of the two native vascular plant species (Fowbert & Smith 1994), but also decreased survival rates of Adélie penguins with confounding impacts on both terrestrial and marine food webs (Ducklow et al. 2007). Global circulation models predict that twenty-first century climate changes will be greater, both in terms of magnitude and speed of response, in the polar regions (ACIA 2005; Turner et al. 2009a). As polar region ecosystems appear to be highly sensitive to climate changes (Wall 2007) the impacts may be particularly pronounced in the Arctic and Antarctic.

Terrestrial ecosystems in the polar regions are strongly influenced by historical events, such as glaciations during ice ages, as well as past and present environmental constraints. Ecosystem processes, including primary productivity and nutrient cycling, and species diversity are limited by low temperatures and/or limited water availability (Millennium Ecosystem Assessment 2005), and the biota have adapted to the harsh conditions. For instance, most soil invertebrates rely on life history traits that allow them to remain dormant throughout most of the year whilst taking advantage of short-term favorable conditions (Convey 1996). Such survival strategies enhance the performance of the native biota under current climate conditions, but associated metabolic costs may impair growth rates and the organisms' competitive ability under more favorable conditions. Many native species are therefore at risk of being outcompeted by more competitive species if the environment changes (Bergstrom & Chown 1999).

Understanding climate change response of belowground communities in the polar regions is of high importance. Soil organisms play a vital role in ecosystem functioning (Nielsen et al. 2011a), and climate changes could cause a change in communities with potential impacts on ecosystem processes (e.g. Wardle 2002). For instance, while soil invertebrates may not play a large role in decomposition of organic material under current environmental constraints (Wall et al. 2008) their role may increase should the climate become more suitable. One of the prominent concerns of climate change is the potential loss of carbon (C) from permafrost. The northern circumpolar permafrost alone store about 1672 pg C or roughly twice the amount of C found in the atmosphere (Tarnocai et al. 2009). An increase in temperature could increase the efflux of C from Arctic soils significantly, which would further accelerate climate change locally and globally (Schuur et al. 2009). Arctic climate change responses could thus have far reaching impacts. Furthermore, both the Arctic and Antarctic host a range of unique ecosystem types that support many endemic invertebrate species above and below ground. Understanding climate change responses in the polar regions may also help us predict future impacts in other ecosystem types, such as those of increasing temperature and altered precipitation regimes around the globe, for example by allowing us to identify which mechanisms trigger climate change responses (Wall 2007; Nielsen et al. 2012).

In this review we give an overview of the current knowledge of climate change impacts on soil invertebrate communities in the polar regions. Our goal is not to provide an exhaustive report of previous findings but rather to highlight key information that may help us predict future belowground climate change responses in the Arctic and Antarctic. We aim not just to draw parallels in expected belowground climate change responses between the two regions but also emphasize why belowground communities may respond differently to climate changes based on geography and current environmental conditions. To accomplish this, we first provide a brief description of the Arctic and the Antarctic as habitats including key differences that may contribute to different responses of their respective soil invertebrate communities to predicted climate changes. We then outline observed and predicted climate changes for the twenty-first century, and discuss how these changes may directly and indirectly influence soil communities based on experimental and observational data as well as potential implications of climate change impacts on C dynamics. We conclude the article with some predictions on the future state of soil communities in the polar regions and priorities for future research.

Characteristics of terrestrial ecosystems in the polar regions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information

The Arctic is largely defined by a change in vegetation type from forest to shrub and tundra, and we will follow the Millennium Ecosystem Assessment (2005) definition of the Arctic: ‘all non-forested land north of 55° N′ for simplicity. The transition between forest and tundra, the main ecosystem type of the Arctic, generally coincides with average June temperatures of around 10 °C but varies widely in latitude between continents due to differences in geography and local weather patterns. The Arctic spans several continents as well as a number of larger and smaller islands, and covers approximately 12 million km2 of which roughly 2.5 million km2 is ice covered. The remaining area is defined as either barren (< 50% vegetation cover) or tundra (> 50% vegetation cover; often referred to as the world's largest wetland although not all is strictly wetland), which cover about 3 and 5 million km2, respectively (Millennium Ecosystem Assessment 2005). The Arctic supports well developed and diverse vascular plant communities (Callaghan et al. 2004), and is home to about 2000 species of soil invertebrates (Chernov 1995). Moreover, the Arctic is home to a range of migratory birds and large land mammals, including humans, which have a substantial impact on local ecosystems and contribute to the dispersal of flora and fauna (ACIA 2005).

By contrast, Antarctica is a mostly ice covered, isolated continent and a few islands located south of 60° S. Of Antarctica's 14 million km2 surface area only about 0.32% remains seasonally ice and snow free (Ugolini & Bockheim 2008), and most of this is considered polar desert with no visible vegetation. Antarctica is colder and drier than comparable latitudes in the Arctic (Convey 1996). The terrestrial ecosystems within Antarctica are spatially isolated and fragmented making it difficult for species to disperse between microhabitats (Huiskes et al. 2006), and the Southern Ocean functions as a massive colonization barrier for terrestrial organisms (Convey et al. 2008). With limited dispersal opportunities, the main vectors for dispersal of flora and fauna, including potential invasive species, to and within Antarctica are scientists and tourists (Chown et al. 2012). Antarctica's plant communities are limited to two species of vascular plants, approximately 111 mosses and 273 lichens (Ochyra et al. 2008; Øvstedal & Smith 2009). Areas of maritime Antarctica have well-developed plant communities in terms of ground cover but in general primary productivity is very low.

These differences indicate that the Antarctic biodiversity is more constrained by present climatic conditions, dispersal limitations and habitable land surface area than the Arctic (the key characteristics of the Arctic and Antarctic are summarised in Table 1). An amelioration of any of these factors may therefore have disproportionally large impacts on terrestrial ecosystems in Antarctica.

Table 1. Key geographic and environmental characteristics of the Antarctic and Arctic
 The AntarcticThe Arctic
  1. a

    Ugolini & Bockheim (2008).

  2. b

    Millennium Ecosystem Assessment (2005).

  3. c

    Ochyra et al. (2008).

  4. d

    Øvstedal and Smith.

  5. e

    Callaghan et al. (2004).

  6. f

    Adams et al. (2006).

  7. g

    Convey (2008).

  8. h

    Danks (1990).

  9. i

    Chernov (1995).

  10. j

    Turner et al. (2009ab).

  11. k

    Kennedy (1995).

  12. l

    Convey (1996).

Landscape~14 million km2 of which only ca. 0.32% is seasonally ice and snow freea~12 million km2 of which 2.5 million km2 is ice coveredb
Continental Antarctica is a polar desert with no to sparse vegetation. Some areas in maritime Antarctica more vegetatedb~3 million km2 is polar desert or sparsely vegetated (< 50% vegetation cover) while the remaining ~5.1 million km2 is tundra (> 50% vegetation cover)b
VegetationTwo native vascular plant species, both restricted to maritime Antarctica. Approximately 111 species of moss and 273 lichensc,dSome 5900 species including 1800 vascular plant speciese
Soil invertebratesSome 520 terrestrial invertebrates of which about 170 are endemicf,g, but likely underestimatedOver 2000 species of soil invertebrates incl. some 700 mites, 400 springtails, 500 nematodes and 70 oligochaetesh,i
Other characteristicsGeographically isolated and the Southern ocean function as a large-scale dispersal barrierLess isolated but some dispersal limitation between continents and islands
Habitable areas very fragmented within the continentHabitable areas larger, widespread and more connected
Ozone-hole currently limits warming in interior continental Antarcticaj and high UV radiation may be damaging to flora and faunakClimate gentler than comparable latitudes in the Antarcticl

Observed and predicted climate changes in the polar regions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information

Here we give a brief overview of the climate changes observed in the polar regions since modern records began and scenarios for the twenty-first century because that is the scale at which most climate models work. Fine-scale climate changes are inherently difficult to model and beyond the scope of this paper; hence, we will focus on larger scale patterns.

As noted earlier it is well documented that the climate in the polar regions has changed since modern records began (e.g. ACIA 2005; Millennium Ecosystem Assessment 2005; Turner et al. 2009a), and that some of the largest increases in temperature measured on Earth have been recorded in the Arctic and Antarctic. Two well known examples illustrate this particularly well: temperatures at the Faraday/Vernadsky Station in maritime Antarctica increased by up to 0.56 °C per decade over the last half of the twentieth century (Turner et al. 2005) and temperature increases of up to 1.19 °C per decade between 1982 and 2005 were recorded in Greenland (Comiso 2006). Climate changes are however somewhat less conspicuous at larger scales. For instance, rapid climate warming is mostly limited to maritime Antarctica (e.g. along the Antarctic Peninsula and Scotia Arc; Turner et al. 2009a). There is little evidence for any significant long term increases in temperature over continental Antarctica (Steig et al. 2009) although short-term (decadal) cooling trends were recorded in areas such as the McMurdo Dry Valleys (MDVs) (Doran et al. 2002; Turner et al. 2005). However, more recently there is evidence for the occurrence of extreme warming events in the McMurdo Dry Valley Region of continental Antarctica (e.g. Barrett et al. 2008a) as well as in the Arctic (Vincent et al. 2009), which may pose particular problems for the soil invertebrates.

The Arctic has experienced a general increase in mean annual temperature and precipitation, with trends being most pronounced over the last few decades (Serreze et al. 2000; Comiso 2006). Satellite data indicate that the average temperature north of the Arctic Circle increased by 0.72 ± 0.1 °C per decade between 1982 and 2005 (Comiso 2006). However, while Greenland experienced an increase of 1.19 ± 0.2 °C dec−1, the increase over northern Eurasia was 0.13 ± 0.16 °C dec−1 in the same period (Comiso 2006). Some regions even showed a cooling trend in the early part of last century, but it now appears that all of the Arctic is warming (Millennium Ecosystem Assessment 2005). Similarly, both local increases and decreases in temperature and precipitation have been observed in Antarctica (Turner et al. 2009a). Across the continent annual surface temperature increased by approximately 0.12 °C dec−1 between 1957 and 2006, but warming was more pronounced over West Antarctica (~0.17 °C dec−1) than East Antarctica (~0.10 °C dec−1; Steig et al. 2009). Warming was particularly prominent on the Antarctic Peninsula (maritime Antarctica) over the last half of the twentieth century, in particular due to a great increase in winter temperatures (Turner et al. 2005). Precipitation patterns in Antarctica are not well documented and general patterns are thus not well known (Turner et al. 2009a).

Climate models predict a more directional warming, and increased precipitation, throughout the polar regions over the next century, although the magnitude will show considerable regional variation. In the Arctic, it is expected that the mean annual temperature will increase by 4–7 °C and precipitation will increase 5–10% on average (but up to 35% in some high arctic regions; ACIA 2005). For Antarctica, it is expected that precipitation rates will increase by 2.9 mm a−1 dec−1 and lead to an increase in snowfall of about 20% over the twenty-first century though most of this will fall during winter (Bracegirdle et al. 2008). Moreover, surface temperatures will on average increase by about 0.34 °C dec−1 or about 3 °C in total across the continent with the greatest temperature increases occurring in the continental high altitude interior of East Antarctica (Bracegirdle et al. 2008). The timescale of this increase is however questionable in that the rate will depend on the amelioration of the ozone hole that currently develops over Antarctica each spring. It appears that this phenomenon has been causing local cooling over interior continental Antarctica, and large increases in temperature are unlikely here until the ozone hole phenomenon dissipates (Turner et al. 2009b).

Finally, but perhaps more importantly, most models predict an increase in the frequency of extreme events, i.e. heat waves, large precipitation events, drought etc. in both the Arctic and Antarctic (Tebaldi et al. 2006; Krinner et al. 2007; Steig et al. 2009). Despite their short timeframe, extreme events can have disproportionally large impacts by exposing biota, vegetation and ecosystems to unusual climatic conditions. Such events may therefore be of greater importance than long-term gradual changes in climate (Fig. 1; Jentsch & Beierkuhnlein 2008; Bokhorst et al. 2012; Nielsen et al. 2012). However, local changes in the frequency of such events are inherently difficult to model and predict.

image

Figure 1. Conceptual diagram showing potential ecosystem impacts related to climate change. Ecosystems experiencing no climate change are expected to show limited short-term changes. Under gradual climate change biota will adapt to the new conditions while new species may colonize. This may cause changes in biotic interactions and lead to a slow change in ecosystems over time. By contrast, an extreme event may cause highly altered ecosystem properties and significant mortality of organisms above and below ground with potential trophic cascades and resulting in a highly altered ecosystem. The insert shows a hypothetical relationship between extreme climatic events and ecological responses. The threshold indicates that extreme ecological responses occur when events deviate significantly from ‘normal’ conditions and/or last for a long time.

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Observed belowground community responses to climate changes

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information

In this section we bring forward key findings that outline how climate change may impact on soil invertebrate communities of the polar regions, information that is useful for understanding potential future states. However, as will be apparent shortly, there is a dearth of in-depth knowledge across taxa and many groups have received little attention. We will focus on broad-scale responses but recognize that fine-scale responses may differ from those observed at larger scales. For instance, local soil moisture availability will depend on topography and vegetation type in addition to the interaction between changed precipitation and temperature (e.g. Peck et al. 2006).

Climate related impacts on soil invertebrates

Soil invertebrates of the polar regions are well adapted to variation in climatic conditions at daily to annual time scales, and it has therefore been suggested that climate changes may not have a substantial direct impact on these organisms (e.g. Peck et al. 2006). There is however mounting evidence that climate change can impact soil invertebrate communities in the polar regions. In this section we discuss how warming and increased precipitation as well as other climate change related factors, such as freeze-thaw cycles (FTCs), depth of active layer and UV-B radiation, might influence soil invertebrate communities. Impacts more closely associated with microbial and vegetation responses to climate change will be considered in the next section.

A review of the current literature revealed a handful of studies that report on long term trends of soil invertebrates associated with observed climate changes and a limited number of studies that report on the impacts of climate change manipulations. After excluding articles that presented results covered elsewhere, we were left with seven studies with 58 observations (Kennedy 1994; Sinclair 2002; Convey et al. 2002; Convey 2003; Bokhorst et al. 2008; Day et al. 2009; Simmons et al. 2009) and five studies with 15 observations (Coulson et al. 1996; Reuss et al. 1999; Sjursen et al. 2005; Dollery et al. 2006; Tsyganov et al. 2011) that report on soil invertebrate responses to experimental warming and/or water addition in Antarctic and the Arctic, respectively. None of these studies report on the response of soil invertebrates to water addition in the Arctic, while three studies report on the impact of water addition in the Antarctic with two of them further reporting on the interactive effect of warming and water addition. The observed responses (positive, neutral, negative) to experimental warming and/or water additions of all soil invertebrate groups investigated are summarized in Fig. 2 (mean effect sizes presented as ln response ratios of dominant groups are presented in Table S1). Care should be taken in interpreting observed responses to climate change simulations as such manipulations are often poor proxies for natural climate change (Bokhorst et al. 2011), and high variability in soil invertebrate communities within ecosystems may obscure significant patterns under low replication. However, a general pattern emerges in the climate change responses reported to date (Fig. 2). Across groups of soil invertebrates and ecosystem types the response to climate warming is highly variable with negative, positive and no responses observed, while water additions generally have no impact or a positive influence on soil invertebrate communities. Closer examination of the reported impacts indicate that most negative warming responses are displayed by invertebrate groups sensitive to low soil moistures (e.g. collembolans; Day et al. 2009) in ecosystems where soil moisture availability may already be a limiting factor (e.g. Coulsen et al. 1996; Simmons et al. 2009). Similarly, in both of the studies that reported on the interaction between warming and water addition, negative impacts of warming on nematodes (Simmons et al. 2009) and collembolans (Day et al. 2009) were alleviated by water addition.

image

Figure 2. Summary of observed soil fauna group responses to warming and/or water addition in (a) the Antarctic and (b) the Arctic. Only studies that experimentally increased mean temperature (i.e. not extreme events) and/or added water to plots are included. An observation is defined as the response of a distinct soil fauna group (including Protista, Rotifera, Tardigrada, Nematoda, Collembola, Acari, Diptera) to climate change within a single site or habitat type, i.e. one study can have multiple observations if it reports on responses of several soil fauna groups, ecosystems and/or sites (soil fauna responses were considered significant if P < 0.05). When multiple studies were found to report results from the same experiment, only the most recent, longer-term, data were included.

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Combining experimental data and long term records of soil invertebrate community responses to observed climate change allows us to draw some conclusions on potential belowground impacts. Experimental warming has been shown to increase nematode densities in the Arctic and maritime Antarctic (Convey 2003), while a local cooling trend observed between 1986 and 2000 (Doran et al. 2002) had a negative impact on densities of the dominant nematode Scottnema lindsayae in the MDVs (Barrett et al. 2008b). These results suggest that warming will have a largely positive impact on local nematode densities but may change community composition (e.g. Nielsen et al. 2011b) although local responses may vary depending on local soil characteristics (e.g. Simmons et al. 2009). Warming has also been shown to increase mite densities in Polar ecosystems (Kennedy 1994; Sjursen et al. 2005; Dollery et al. 2006), although other studies found no response (Coulson et al. 1996; Webb et al. 1998) suggesting that the response is both habitat and group specific. For instance, Sjursen et al. (2005) found an increase in oribatid mites and a decrease in mesostigmatid mites under increased temperatures, respectively, although the pattern was only significant in a glade and a fellfield and not in a heath. By contrast, collembolans often respond negatively to experimental warming in the Arctic (Sjursen et al. 2005; Dollery et al. 2006) while both positive (Kennedy 1994; Day et al. 2009) and negative responses have been observed in maritime Antarctica (Bokhorst et al. 2008). The idiosyncratic responses of mites and collembolans to warming are supported by laboratory studies, which show that the mites are generally more tolerant to high temperatures than collembolans (Block et al. 1994; Hodkinson et al. 1996). Moreover, it should be mentioned that there is often a change in community composition under changed climate even if there were no overall response in density of the group (e.g. Webb et al. 1998; Sjursen et al. 2005; Dollery et al. 2006). There is very little information on the response of other taxa for either the Arctic or the Antarctic. It is however established that water stress is likely to impact groups that live in moist to wet soils or on water films on soil aggregates (Convey et al. 2003), such as nematodes, rotifers, tardigrades, enchytraeids and Diptera larvae (Hodkinson et al. 1999), and soft bodied animals such as collembolans and prostigmatid mites as well as nymphs of other mites (e.g. Day et al. 2009; Bokhorst et al. 2012). In short, increased temperatures are likely to increase the abundance of soil invertebrates in both the Arctic and the Antarctic as long as it is not associated with a significant decrease in soil moisture availability, and it is furthermore likely to lead to a shift in the composition of the belowground communities as soil organisms differ in their ability to cope with climatic changes.

As mentioned earlier a ‘novelty’ of predicted climate change scenarios is the potential increase in the frequency and magnitude of extreme events (ACIA 2005; Tebaldi et al. 2006; Krinner et al. 2007), and there is evidence that such events may have a disproportionally large impact on soil communities in the polar regions. For example, a particularly warm summer increased soil moisture availability throughout Taylor Valley (MDVs, Antarctica), which was evident for several years, and caused a change in nematode community structure and an increase in the abundance of a nematode species associated with more moist soils (Barrett et al. 2008a). However, life in the polar deserts of the MDVs is generally limited by water due to low temperatures, and increased temperatures may have opposite impacts elsewhere (Nielsen et al. 2012). Accordingly, an unusually hot and dry Arctic summer caused high mortality of collembolans in an area with relatively low precipitation (Coulson et al. 1996), indicating that drought during hot summers may have negative impacts on belowground community composition in the Arctic. Similarly, the Ward Hunt Island region in the Canadian high arctic experienced a hot summer in 2008 (air temperatures up to 20.5 °C), and high temperatures had a significant influence on the landscape scale distribution of key soil microhabitats with potential large-scale impacts on soil communities (Vincent et al. 2009). Such extreme events may therefore lead to alternate ecosystem states with long lasting effects. However, very few studies have experimentally assessed the impacts of extreme events in the polar regions and we could learn a lot from such manipulations. For instance, recent work has shown that extreme winter warming has a strong negative impact on microarthropod communities (e.g. Bokhorst et al. 2012). This work highlights potential negative implications of increased winter temperatures or through decreased snow cover which buffers temperature variability in the soil.

Other climate change related factors that may influence on belowground communities include the frequency of FTCs, the development and duration of ice layers, and increased UV radiation. It is likely that increased physiological stress associated with FTCs could have a negative impact on soil invertebrates (Turner et al. 2009a) but the impact will depend on the severity of the FTCs. A study conducted in the sub-arctic found that the frequency of FTCs had a positive influence on the abundance of collembolans and oribatid mites at the site where the increase in FTCs were most pronounced (Konestabo et al. 2007). As the authors hypothesize it is likely that an increase in time where the soil is thawed in the plots with increased FTCs compared with continuously frozen may outweigh the negative effects of the FTCs themselves. In particular, it needs to be considered that most polar region biota are well equipped to survive and even metabolize at temperatures well below the point of freezing, and only severe frost may have any significant impact on their survival. Moreover, the impact of increased variation in air temperature on soil temperature depends strongly on local characteristics, and it has for instance been shown that the temperature of Antarctic dry soils are much more responsive to changes in air temperature than moist or wet soils due to the buffering capacity of water (Lewis Smith 1999). By contrast, the aforementioned study by Bokhorst et al. (2012) indicated that an increase in FTCs negatively impacted mite and collembolan densities. The authors ascribe the contrasting results to a higher frequency of FTCs than that imposed by Konestabo et al. (2007). Hence, the impact of FTCs on belowground communities will depend on frequency and how low the realized soil temperatures are. Moreover, the susceptibility to FTCs is group specific with laboratory studies suggesting that collembolans are less tolerant to FTCs than oribatid mites (Coulson et al. 2000). Changes in the frequency of FTCs may thus cause a shift in soil communities.

Also altered depth and extent of snow cover, permafrost melt and recession of glaciers may impact belowground communities. Snow packs provide insulation against climate extremes, and soils underneath packs are typically warmer and show lesser variation in temperature (Walker et al. 1999). Snow packs also influence soil moisture availability and soil chemistry, and through this have a large impact on the distribution of soil biota (Gooseff et al. 2003) but impacts may be both positive and negative depending on local conditions and more knowledge is needed to make good predictions of future impacts. Warming is expected to increase the depth of the active layer by 30–40% as permafrost melts in the Northern Hemisphere by the end of the twenty-first century (Stendel & Christensen 2002). This may increase the area of high biological activity in the soils and enhance soil biodiversity. However, the melt of permafrost may also have severe impacts on soil communities through the formation of thermokarsts, a problem very evident in the Arctic (e.g. Osterkamp et al. 2000; Vogel et al. 2009). Also the large-scale retreat and melt of glaciers (Cook et al. 2005) will influence belowground communities. For example, the water released from glaciers in arid regions, such as the polar deserts of Antarctica, may enhance water availability and thereby promote soil organisms, and glacial recessions may provide new areas to colonize (Kennedy 1995; Kaufmann 2002).

Finally, it has been suggested that the increased UV radiation associated with the formation of the ozone hole over Antarctica could limit soil communities (Kennedy 1995). We find it questionable that this should have a large direct impact on growth and survival rates of soil biota although there is some evidence that UV exposure can influence belowground invertebrate communities (e.g. Rinnan et al. 2005; Tosi et al. 2005). However, there may be significant indirect effects through impacts on soil microbes and plant communities. When plants are subjected to high UV irradiation they produce UV-B absorbing compounds which increase their metabolic cost and contribute to a potential limitation of aboveground biomass and plant height (Newsham & Robinson 2009). Moreover, incubation studies suggest that UV-B can inhibit the growth of several Antarctic fungi in the surface layers of the soil (Hughes et al. 2003). This could indirectly influence belowground communities by altering resource quality (i.e. plant palatability) and quantity, and through this nutrient cycling (Convey et al. 2002).

Vegetation and microbial driven climate change effects

There is a strong link between the plant community composition aboveground and belowground communities (Wardle 2002), and climate change is likely to have strong effects on soil invertebrate communities through changes in vegetation type. While the strength of the link between above and below ground communities has not been explicitly investigated in the polar regions there is some evidence that support such a link. For instance, as discussed in Nielsen et al. (2011ab) Antarctic soils with vegetation harbor substantially greater abundances of soil fauna and support different communities than does bare ground. It is also apparent that soil fauna communities differ between vegetation types in the Arctic (e.g. Coulson et al. 2003).

Vegetation composition responses to recent climate changes are widespread in the Arctic (Callaghan et al. 2011) and the Antarctic (e.g. Fowbert & Smith 1994; Parnikoza et al. 2009). For example, in the Arctic, soil temperature directs changes in the functional composition of the vegetation (Brooker & van der Wal 2003) and increasing temperatures generally enhance net and gross primary production (Marchand et al. 2004). In maritime Antarctica, the observed local range expansion of vascular plants as well as bryophytes (Fowbert & Smith 1994) appears to be tightly linked to local temperature trends (Parnikoza et al. 2009). Climate warming is also expected to have a positive influence on net carbon assimilation in Antarctica, as the temperature is generally lower than the temperature required for optimum carbon assimilation shown by native plants (Kennedy 1995). Moreover, a warmer and wetter climate is likely to enhance successful colonization events as well as germination of plants residing in the local soil seed bank (Lewis Smith & Ochyra 2006). Hence, considering the current low biomass, productivity and diversity of plants in Antarctica, warming and increased precipitation is very likely to lead to the development of more favorable habitats for soil organisms thus enhancing the complexity of the soil food webs in Antarctica.

Similarly, climate change responses of the microbial communities could have cascading impacts on the soil invertebrate communities, particularly in the polar deserts dominated by vast expanses of bare ground where the soil invertebrates rely almost solely on the microbial communities for resources (e.g. Nielsen et al. 2011b). There is some evidence that microbial communities respond to imposed long-term climate changes (Rinnan et al. 2007; Timling & Taylor 2012), but whether directional responses exist is still unknown. However, several studies have found that warming tends to increase nutrient availability and enhance decomposition rates (e.g. Schmidt et al. 2002; Hill & Henry 2011), which is likely to positively impact invertebrate communities. For example, Rinnan et al. (2007) speculates that limited microbial biomass responses to climate change manipulations could be due to increased grazing by soil invertebrates. This is supported by the large increase in nematode populations observed in the plots (Reuss et al. 1999). Hence, greater microbial activity may contribute to trophic cascades. By contrast, it seems that a warmer and wetter climate will have a more consistent impact on microbial communities in Antarctic ecosystems. A recent study used open top chambers to investigate the impacts of warming on microbial communities across a latitudinal gradient in Antarctica (Yergeau et al. 2012). The authors found that the soil microbial communities show consistent responses to short-term climate warming with an increase in both bacterial (under vegetation only) and fungal abundances, and changes in the bacterial community composition likely reflecting greater nutrient availability (Yergeau et al. 2012). Increased microbial biomass would likely be able to support more soil invertebrates, while changes in the microbial community composition may impact on the species composition of soil invertebrates.

Implications of belowground community responses to climate change for C dynamics

The climate change impacts on soil biota in the polar regions may have implications for global C dynamics through enhanced rates of decomposition and faster turnover of nutrients. While this is mainly driven by enhanced microbial activity, it appears likely that the role of soil invertebrates in decomposition processes, which is currently limited mainly due to climatic constrains of biological activity (e.g. Wall et al. 2008), will increase as the climate becomes more favorable. This is a concern considering the potential positive feedback on future climate changes (Chapin et al. 2005). Already it appears that permafrost melt and greater biological activity associated with climate warming during the late 20th century have caused the Arctic as a whole to switch from being a C sink to a C source (e.g. Schuur et al. 2009; Jahn et al. 2010). However, there is still large uncertainty about the scale of C efflux as warming has been found to both increase (Biasi et al. 2008), decrease (Sjögersten et al. 2008) or have no impact on soil C efflux locally (Lamb et al. 2011). These idiosyncratic responses appear to be related to differences in soil moisture. For example, a study explored soil respiration rates to warming in the Arctic, and found that dry tundra is more responsive to climate warming than moist and wet tundra (Oberbauer et al. 2007). The authors hypothesize that the response of respiration rates in wet and moist tundra to climate warming are dampened due to higher water tables and soil moisture contents (Oberbauer et al. 2007). Similarly, a recent study suggests that Arctic wetland ecosystems generally act as C sinks rather than C sources (Lund et al. 2010). Hence, it is likely that warming will have the most pronounced impact on C stocks in drier habitats initially and may only later, if at all, impact C stocks in moist to wet habitats. Moreover, a study of old C release from Arctic soils showed that the release of old C increased with time since permafrost thaw because initially the increase in C efflux is offset by increased uptake of C by plants (Schuur et al. 2009). This suggests that short-term measurements of soil respiration are unlikely to capture the true extent of C release, and we might therefore have underestimated the potential impacts of Arctic climate warming. By contrast, the minimal organic content of Antarctic soils suggests that these soils are likely to become a C sink under a warmer and wetter climate. Indeed, a recent study found that warming had a positive effect on aboveground biomass as well as C stocks of soils dominated by vascular plants in maritime Antarctica (Day et al. 2008).

The future state of belowground communities in the polar regions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information

Based on current knowledge described above we have outlined some broad predictions of future climate change responses of belowground communities, but there are two important aspects of the observed and predicted climate changes that need consideration before we proceed. First, the rates of change, both that observed and those predicted for the twenty-first century, are high compared to most known climate changes in Earth's geological history. It has however been suggested that very rapid increases in temperature occurred at the end of the last glacial period (up to 10 °C at mid- to high-latitudes over as little as 3–5 years around 14 700 years BP and similar changes over roughly 60 years around 11 600 years BP; Steffensen et al. 2008). These abrupt temperature increases led to a complete turnover of plant species locally within Europe and North America. Despite the shift in plant community composition the event appeared to have no significant impact on overall biodiversity, i.e. species ranges shifted but the climate change did not cause large-scale species extinctions (Willis et al. 2010). One major difference between these past climate change events and those predicted for the twenty-first century in the polar regions suggests that current climate changes could have a different outcome. While the retreat of the ice sheet after the last glaciation would allow northward migration and range expansion of ecosystems and associated species, and therefore not cause large-scale extinctions, this is not possible today at least for some Arctic ecosystems. In particular, a northward shift in range of the Arctic tundra may be geographically restricted (i.e. there is no new land to colonize; Fig. 2). Thus, temperature increases are likely to decrease the total area of tundra and likely the number of species found therein. In contrast, this is much less likely in Antarctica where large landmasses are currently snow or ice covered and new land may become available as glaciers and snow packs retract, retreat southward or to higher elevation (Fig. 3). Secondly, the predicted climate changes include an increase in frequency of extreme events (Tebaldi et al. 2006; Krinner et al. 2007; Turner et al. 2009a). Such events may have disproportionally large impacts on ecosystems by subjecting them to conditions beyond their climatic thresholds and result in alternate ecosystem states (e.g. Jentsch & Beierkuhnlein 2008; Nielsen et al. 2012). In short, the predicted climate changes may have greater biotic impacts than those observed in the past.

image

Figure 3. Conceptual diagram of polar region landscape scale responses to a warmer and wetter climate (not to scale; see text for details). Arctic tundra, barrens and polar desert will shift northward and may decrease in area due to encroachment of woodlands. Antarctic vegetation will expand southward as glaciers and snowpacks retreat. In the Arctic, higher temperatures will likely increase decomposition rates, potentially causing increased efflux of C. However, the net release of C will depend on the balance between increased decomposition and increased uptake by vegetation particularly as the cover of shrubs and trees increase. More C is likely to be incorporated into the vegetation and soil organic matter pools in Antarctica, but this will be limited by climatic constraints.

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Community composition

We predict a general change in the composition of soil invertebrate communities throughout both the Arctic and Antarctic. However, we expect that belowground invertebrate communities will respond most strongly to changes in the microbial community composition and to vegetation type changes caused by climate changes in the Arctic. In Antarctica, the belowground invertebrate communities will respond positively to increased temperatures and water availability through stress release, which will allow greater growth rates and lower induced mortality. Furthermore, we expect to see an overall increase in the abundance and biomass of soil invertebrates as microbial communities become more active and plant communities expand, become more complex and show greater primary productivity. This is also likely to promote the diversity and complexity of Antarctic soil food webs and create opportunities for stronger biotic interactions. By contrast, climate changes may not have a significant impact on overall biomass in the Arctic where the communities are more complex, although we do predict a shift in the soil food web structure as organisms such as mites and collembolans differ in their capacity to tolerate increased temperature and associated greater or lower soil moisture availability.

Finally, while it is difficult to foresee the impact of a potential increase in the frequency of extreme events, such events are likely to have disproportionally large impacts on the distribution of species and local communities and the effects may be positive or negative depending on local conditions. For example, in the Antarctic MDVs, heat waves tend to increase soil moisture availability and through this could promote the development of (by local standards) diverse soil communities and provide dispersal opportunities for soil organisms through the development of streams (e.g. Nielsen et al. 2012). Hence, the positive impacts may outweigh the negative impacts here. Still, extreme events are likely to have a negative impact on soil invertebrates on more broad scales. For instance, the development of thermokarsts and increased wild fires as observed recently in the Arctic (Mack et al. 2011) could have substantial impacts on soil communities beyond the initial disturbance impacts through loss of habitat and nutrients.

Diversity

While the species diversity of soil organisms is unlikely to increase significantly in the Arctic ecosystems that already display rather high diversity (e.g. Wu et al. 2011), a warmer and wetter Antarctica would be favorable for range expansion of the existing species and promote successful colonization events by new species (e.g. Turner et al. 2009a; Convey 2011; Nielsen et al. 2011b). Furthermore, glacial recession and increased soil formation processes are likely to provide new habitable areas (Fig. 3). Hence, the diversity of soil invertebrates is likely to increase in Antarctica under the predicted climate changes. However, increasing temperatures may also promote the colonization of invasive species, both in the Arctic and the Antarctic. Considering the low diversity of Antarctic ecosystems this may have substantial impacts on the communities (i.e. the native species may be eliminated from the communities through competition but they will be substituted by other new species), but the effect at large scales is a net increase in species diversity. By contrast, the connectivity and greater diversity currently observed in Arctic ecosystems suggests that competitive invasive species could have a negative impact on species diversity locally, and potentially even at larger scales. Moreover, limited northward range shift of ecosystems may have substantial impacts on overall species diversity of Arctic communities, which is not of great concern for Antarctic biota. Hence, we predict an overall increase in species diversity in Antarctic ecosystems both at fine and broad scales although some native species such as the nematode S. lindsayae may experience a reduction in their distribution. It seems likely that climate change will have little impact on species diversity in the Arctic in the short term, but that displacement of native ecosystems through encroachment by other ecosystem types (i.e. woodland and forests etc.; Fig. 2) could lead to species extinctions in the longer term. Ultimately, the impact will depend on the balance between species introduction and extinction.

Soil processes

Turnover rates and primary productivity are likely to go up in both the Arctic and the Antarctic as climate conditions become more favorable. A warmer and wetter climate would promote greater activity of soil invertebrates and the microbial community. The role of soil invertebrates in nutrient cycling would increase through their participation in decomposition processes and increased microbial grazing (e.g. Wall et al. 2008) with potential ecosystem effects. In particular, the accelerated rate of decomposition is likely to lead to a net efflux of C in the Arctic, which could become a substantial source of C with potential implications for future climate changes through positive feedbacks (Fig. 3). However, we stress that more data are needed to resolve this question. In contrast, we expect that the development of more extensive microbial, soil invertebrate and plant biomass will lead to an accumulation of organic matter in soils throughout most of Antarctica (Fig. 3). Antarctic ecosystems may thus become a C sink during the expansion and following succession of the above- and belowground communities, although this is unlikely to offset the increase in C release from the Arctic.

Research priorities

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information

It is evident that climate change is likely to impact significantly on belowground communities and ecosystem functioning in the Arctic and Antarctic, although responses will depend on the current state of the ecosystems. However, our literature review reveals a scarcity of information on the impact of climate change on soil invertebrates in the polar regions. Furthermore, as highlighted by Convey (2011), predicting climate change responses is limited by our lack of baseline knowledge of polar region biota. An increased use of molecular approaches in conjunction with more ‘old-fashioned’ methods would enable us to make considerable progress in cataloguing the diversity and distribution of soil organisms, and their role in ecosystem functioning.

Our poor understanding of climate change impacts on belowground communities in the polar region could with relative ease be addressed by upgrading existing climate manipulation facilities to incorporate both warming and precipitation manipulations and then supplement these with sites so as to cover an environmental gradient. As noted previously by Wall et al. (2011) it is essential that such experimental facilities take a more holistic approach to responses induced by climate change as well as other human induced changes and document both changes in processes and communities of key organisms above- and belowground across ecosystem types to fully understand the mechanisms that underpin observed responses. These climate change facilities should be complemented by long term monitoring stations that record natural variation in communities over time and facilitate greater data sharing (e.g. Wall et al. 2011). This would not only increase our knowledge on polar region ecosystems considerably but also help substantiate fundamental ecological principles.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Characteristics of terrestrial ecosystems in the polar regions
  5. Observed and predicted climate changes in the polar regions
  6. Observed belowground community responses to climate changes
  7. The future state of belowground communities in the polar regions
  8. Research priorities
  9. Acknowledgements
  10. Statement of authorship
  11. References
  12. Supporting Information
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