Ecological Biogeography of the Terrestrial Nematodes of Victoria Land, Antarctica

Abstract The terrestrial ecosystems of Victoria Land, Antarctica are characteristically simple in terms of biological diversity and ecological functioning. Nematodes are the most commonly encountered and abundant metazoans of Victoria Land soils, yet little is known of their diversity and distribution. Herein we present a summary of the geographic distribution, habitats and ecology of the terrestrial nematodes of Victoria Land from published and unpublished sources. All Victoria Land nematodes are endemic to Antarctica, and many are common and widely distributed at landscape scales. However, at smaller spatial scales, populations can have patchy distributions, with the presence or absence of each species strongly influenced by specific habitat requirements. As the frequency of nematode introductions to Antarctica increases, and soil habitats are altered in response to climate change, our current understanding of the environmental parameters associated with the biogeography of Antarctic nematofauna will be crucial to monitoring and possibly mitigating changes to these unique soil ecosystems.


Introduction
Understanding the global distribution of biodiversity is critical for studying the evolution, ecology and dynamics of ecosystems and to address how global scale changes in climate, invasive species, and land use will affect ecosystems, ecosystem services, and subsequently, people. Antarctic terrestrial ecosystems might seem less sensitive to global change because this polar desert has low species diversity distributed across a limited area of biologically active ice-free land, comprising less than 0.32% of the continent's 14 million km 2 (Chown and Convey 2007). However, terrestrial ecosystems of Antarctica are not immune to global changes Chown et al. 2012b). Small changes in polar climate are amplified through biophysical feedbacks leading to biologically significant alterations in soil habitats and their communities (Doran et al. 2002;Nielsen et al. 2011a). The low species diversity of Antarctic soils makes them uniquely suited for studying the relationships between soil biodiversity and ecosystem functioning, and identifying how global changes may affect species level changes in biodiversity, community composition and distribution Simmons et al. 2009). Measures to conserve, manage and sustain ecosystem functioning in Antarctic and Earth's other low diversity terrestrial environments will rely on knowledge of species diversity, distributions, and their role in ecosystem processes (Adams et al. 2006; Barrett et al. 2008;Wall 2004).
Aboveground, the diversity and biogeography of terrestrial flora (mosses, lichens and liverworts) has been recently assessed and used to further refine the geographic floral regions of Antarctica (Peat et al. 2007). It is well known that the warmer maritime and subantarctic ecosystems have higher precipitation, organic soils, a more diverse and abundant vegetation (Bölter et al. 2002;Maslen 1979;Nielsen et al. 2011b;Peat et al. 2007) and greater soil faunal diversity (including earthworms and beetles) than continental Antarctica (Block and Christensen 1985;Chown and Van Drimmelen 1992). For example, the northern maritime Antarctic has 100-115 moss and c. 350 lichen species compared to continental Antarctica's 20-30 moss and c. 90 lichen species (Peat et al. 2007). Throughout Victoria Land vascular plants are absent and fauna are reduced to only a few soil groups and are represented by a patchy spatial distribution of protozoans, nematodes, rotifers, tardigrades, springtails (Collembola), and mites (Acarina) (Adams et al. 2006;Bamforth et al. 2005;Frati et al. 1997;Moorhead et al. 1999;Stevens and Hogg 2002;Virginia and Wall 1999).
Nematoda are a major component of soil food webs in all terrestrial ecosystems including the exposed lands of Antarctica, though their spatial distribution and abundance are highly heterogeneous. In more productive ecosystems, they typically have much higher diversity (Wall Freckman and Virginia 1998) than the Antarctic (Boag and Yeates 1998;Bunt 1954;Maslen 1981). For example, 431 nematode species were recorded from a Cameroon tropical forest ecosystem, with a maximum of 89 species found in 200 individuals enumerated in a soil core (Bloemers et al. 1997). In contrast, the diversity of nematodes in all of Antarctica, including the continental, maritime, and Sub-Antarctic is 54 nematode species, of which only c. 22 species, all endemic, occur on the ice-free terrestrial areas of the continent (Andrássy 1998;Andrássy 2008).
In Antarctica, soil nematodes have been studied primarily in localized and easily accessible areas largely centered around research bases and concentrated on the Antarctic peninsula and islands of the maritime Antarctic and further south in ice-free areas. As a consequence there is relatively little known of their regional biogeography or of the habitats that are suitable for functioning communities. Additionally, there are many remote inland ice-free areas which have yet to be sampled (Convey 1996;Wall 2005), adding to questions on how widespread species are, and whether species rich communities and habitats exist in the more extreme climate zones of the continent.
Regional to continental-scale descriptions of the Antarctic nematofauna have pointed to a paucity of distributional records for much of the continent (Andrássy 1998;Velasco-Castrillón and Stevens 2014). Amongst all regions of Antarctica, Victoria Land is arguably the most intensively studied (Adams et al. 2006). Victoria Land is "that part of Antarctica which fronts on the western side of the Ross Sea, extending southward from about 70°30'S to 78°00'S, and westward from the Ross Sea to the edge of the polar plateau" (USGS 2003). Here, we synthesize information on the nematode biodiversity, geographic distribution and soil and sediment habitats of the terrestrial nematodes in Victoria Land, Antarctica. Much of this information comes from a series of studies to assess nematode diversity and distribution begun in austral summer [1989][1990] by Wall (formerly Freckman) and Virginia and extending to the present as part of the McMurdo Dry Valley Long Term Ecological Research program funded by the US National Science Foundation (www.mcmlter.org). We report on findings of these studies through 2004 which captures most of the biodiversity information gathered by this research group, whereas more recent research has focused on nematode species response to climate change and soil resource manipulations (Ayres et al. 2010;Doran et al. 2002;Simmons et al. 2009). For purposes of our synthesis, we define two areas, Northern Victoria Land -the area from about 70°30'S to about 76°S, encompassing Terra Nova Bay, Edmonson Point and Cape Hallett (Figure 1); and Southern Victoria Land -the area from about 76°S to about 78°S including all of the McMurdo Dry Valleys and nearby coastal regions (Adams et al. 2006) (Figure 2).
The McMurdo Dry Valleys (76°5'to 78°5'S, 160°0' to 164°0'E) are located along the TransAntarctic Mountains in Southern Victoria Land and comprise about 4,800 km 2 of ice-free land and have different geo/ecological legacies and climatic conditions (Lyons et al. 2000;Moorhead et al. 1999). They are the oldest, driest and coldest deserts on earth (Beyer et al. 1999;Campbell et al. 1998;Fountain et al. 1999). Annual precipitation is less than 10 cm water equivalent, most of which sublimates before it melts (Doran et al. 2002;Fountain et al. 1999). Mean annual air temperature is -20°C ) and surface soil temperature ranges from -59°C in winter to 26°C for short periods during summer (Doran et al. 2002). No vertebrate animals or vascular plants are present and mosses and lichens are rare and mostly confined to ephemeral meltponds, streams and lake moats (Cameron et al. 1970;Horowitz et al. 1972;Kappen 1993). Across the region soils are poorly developed, coarse textured (95 to 99% sand by weight) (Bockheim 1997), low in organic carbon (<1%) (Burkins et al. 2000), saline, and have low biological activity compared to warmer ecosystems (Ball et al. 2009;Barrett et al. 2006a;Parsons et al. 2004). Nematodes are the dominant soil invertebrate, but many soils (~35%) lack extractable soil invertebrates and approximately 50% of McMurdo Dry Valleys soils that contain invertebrates have only one invertebrate species (Freckman and Virginia 1997;Wall Freckman and Virginia 1998).
The distributions of the Dry Valley metazoan species are associated with specific sites and correlate to soil habitat differences in organic matter content, moisture and salinity, and microclimate differences encountered over environmental gradients of coastal to interior sites, latitude, and soil chronosequences and differences in glacial tills .
Coastal areas of Victoria Land are a moister environment than the Dry Valleys and are habitat for birds and marine mammals (e.g. skua gulls, penguins, and seals). Penguin rookeries are associated with ornithogenic soils with significant inputs of carbon and nitrogen transferred from the marine environment to the soil (Bargagli et al. 1997). Ornithogenic soils are the only soils south of the Antarctic Circle containing high concentrations (14-21%) of organic matter (Campbell and Claridge 1966;Heine and Speir 1989). However, even with high C and N availability these soils often have lower nematode diversity than soils of the Dry Valleys, probably owing to very high concentrations of salts and soil compaction and cementing (Porazinska et al. 2002a;Sinclair 2001).
Each of the unique soil ecosystems of Victoria Land imposes considerable physiological constraints on nematode life history traits, requiring adaptive responses to freeze/thaw cycling, osmotic and desiccation stress, and a short growing season (Convey 1996). Nematode responses include cryoprotective dehydration via anhydrobiosis (Adhikari et al. 2009;Adhikari and Adams 2011;Crowe et al. 1992), as well as tolerance to inter and intracellular freezing (Adhikari et al. 2010;Wharton 2003Wharton , 2010 and multiyear lifecycles (de Tomasel et al. 2013;Overhoff et al. 1993;Yeates et al. 2009). In addition to stress survival, anhydrobiosis also facilitates long-distance aeolian dispersal , an important mechanism implicated in explanations of their geographic distributions and population genetic structure (Adams et al. 2006;Courtright et al. 2000). All of the nematodes of Victoria Land are inferred to be microbivores with the exception of Eudorylaimus, which is omnivorous (Yeates et al. 1993) (but see Wall 2007).
Nematodes were first collected in Victoria Land by the British 'Discovery' expedition of 1901-1903, from Discovery Bay, South Victoria Land and described by Steiner (1916) as Dorylaimus antarcticus (syn. Eudorylaimus antarcticus (Yeates 1970)). The nematodes of Victoria Land then remained largely unstudied for over half a century, until the work of Yeates (1970) and Timm (1971). Between them, these two papers described or redescribed all Victoria Land genera of the time and laid the foundation for future taxonomic work. Yeates (1970) recorded Plectus from southern coastal Victoria Land and synonymized Dorylaimus antarcticus and Antholaimus antarcticus with Eudorylaimus antarcticus. However, subsequent studies have described further Eudorylaimus species from continental Antarctica: E. glacialis (Andrássy 1998), E. nudicaudatus (Heyns 1993) and E. shirasei (Kito et al. 1996), E. quintus (Andrássy 2008) and E. sextus (Andrássy 2008). Due to the taxonomic uncertainty of early accounts (Adams et al. 2006), we will henceforth use Eudorylaimus sp. in reference to all previous reports of distribution. Timm (1971) synonymized Plectus murrayi with P. antarcticus (de Man 1904) and studied parts of southern and northern coastal Victoria Land and the McMurdo Dry Valleys. He also re-described three known species: E. antarcticus (Steiner 1916), Monhystera villosa (Bütschli 1873) and Plectus frigophilus (Kirjanova 1958), and described two new species, Scottnema lindsayae and Panagrolaimus davidi. Monhystera villosa was later synonymized with Geomonhystera antarcticola (Andrássy 1998). These early studies focused exclusively on the identification and description of nematode species and not their ecologies.
In the McMurdo Dry Valley Region, most nematological studies have investigated the diversity, ecology and distribution patterns of up to three nematode genera; Eudorylaimus, Plectus, Scottnema (Adams et al. 2006), while the coastal areas of Victoria Land remain less well known (Adams et al. 2006;Bargagli et al. 1997;Barrett et al. 2006a;Porazinska et al. 2002a;Raymond et al. 2013a;Sinclair and Sjursen 2001;this paper;Timm 1971;Vinciguerra 1994). Our effort here is a synthesis of the biogeographic distribution of nematodes in Victoria Land and a consideration of the soil habitats that are associated with nematode distribution, diversity and abundance.

Materials and methods
Based on published and unpublished data, we summarized biogeographic information on the species represented within each nematode genus described in Victoria Land. In addition to published papers, we present information obtained from data on soil, and lake and stream sediment samples collected throughout Victoria Land, by the authors and team members during the austral summers between and including 1990 and 2004. Data referred to as "this study (year)" were derived from nematode soil extraction procedures optimized for Antarctic soils and all nematodes were identified to species . Frozen soils from these samples are archived at the Wall lab in the Department of Biology at Colorado State University, Fort Collins, CO, USA. Formalin-preserved extracted specimens from these soils are archived in the meiofauna collection of the Monte L. Bean Life Science Museum at Brigham Young University, Provo, UT, USA. Non-occurrences are not reported but can be extrapolated from Tables 1-5. A brief summary of published information on the ecology of each genus is also provided ( Table 6).

Scottnema (Rhabditida: Cephalobidae)
Scottnema is an exclusively Antarctic genus comprised of only one species, S. lindsayae (Timm 1971). Scottnema lindsayae (synonymous with S. lindsayi) is thought to have evolved from a common ancestor of the genus Acrobeles (Shishida and Ohyama 1986), with a recent phylogenetic analysis placing the genus Stegelletina as its closest relative (Boström et al. 2011). S. lindsayae is the most southerly known occurring nematode in the world, found as far south as Mt Harcourt (83°08.99'S, 163°21.81'E) near the base of the Beardmore Glacier .
Biogeographic distribution. Scottnema lindsayae is the dominant nematode of Victoria Land (Table 1) based on abundance and widespread distribution in numerous samples from the McMurdo Dry Valleys (Courtright et al. 2001;Freckman and Virginia 1990;, 1997Moorhead et al. 1999;Porazinska et al. 2002b;Powers et al. 1995b;Powers et al. 1998;Treonis et al. 1999Treonis et al. , 2000. S. lindsayae was first described in Victoria Land in samples from Wright Valley and the southern coastal region (Marble Point, Strand Moraines) (Timm 1971) and has since been recorded in the northern coastal region occurring as far north as Luther Cirque (72°22.20'S, 169°53.10'E) ( Table 1).
Habitat. S. lindsayae survives in a wide range of terrestrial habitats (Table 1). In Victoria Land S. lindsayae occurs most commonly in dry, bare and sandy or rocky soils and has been found at 30-40 cm soil depth near south shore of Lake Hoare (Powers et al. 1995b). Less frequently, S. lindsayae occurs in the moister habitats such as: snow covered soil (subnivian); near streams and in lake sediments (this paper; Treonis et al. 1999;Vinciguerra 1994); and, under mosses (e.g. Bryum antarcticum) (Timm 1971;Vinciguerra 1994). S. lindsayae has also been found associated with an algal mat (Timm 1971) but whether the algal mat was from soil, a lake or a stream is unknown.
In comparison with other nematodes of Victoria Land, S. lindsayae occurs most frequently and at greater abundances in soil habitats with lower moisture, higher pH, higher EC, and higher inorganic C (Courtright et al. 2001;Freckman and Virginia 1997;Moorhead et al. 1999;Porazinska et al. 2002b;Powers et al. 1998;Treonis et al. 1999). In these habitat types, S. lindsayae may comprise >99% of invertebrates present (Treonis et al. 1999(Treonis et al. , 2002, and may be the only invertebrate present. Treonis et al. (2000) found that S. lindsayae becomes anhydrobiotic in coarse textured Dry Valley soils at a gravimetric soil moisture threshold of ~2%. In a study of 32 samples from one site on King George Island (62°05.51'S, 58°28.21'W), Mouratov et al. (2001) suggested soil moisture content may be one of the main factors determining the distribution of S. lindsayae and found that the species has a preference for soil moisture of 2-5%. Many studies in the McMurdo Dry Valleys (Barrett et al. 2006c; , collected in 1993, 1994, 1995, 1996, 1997 and 2001 North side 77°38.00' 162°53.00' soil (0-2.5, 2.5-5, 5-10, 10-20 cm High (Powers et al. 1994a;1995a) South shore  Porazinska et al. 2002b;Powers et al. 1998) have identified a relationship between greater abundance of S. lindsayae and low soil moisture. S. lindsayae tolerates a wide range of soil moistures, but is typically absent from flowing meltstreams and saturated soils. Interactions between soil moisture and salinity are complex and create changing osmotic conditions in soils. In a comparative study of dry soil and moist soil under snowpacks no correlation was found between S. lindsayae and soil moisture (Gooseff et al. 2003), which could be attributed to changing osmotic potential and salinity. Soil salinity factors (EC and pH) have a significant influence on the distribution of S. lindsayae in the Dry Valleys (Freckman and Virginia 1997;Poage et al. 2008;Porazinska et al. 2002b). For example, S. lindsayae are found predominantly in soils with an EC<700 mS cm -1 (Courtright et al. 2001;Nkem et al. 2006a;Poage et al. 2008), and appear unable to tolerate salinity over 4100 mS cm -1 ). S. lindsayae is recorded at a range of elevations, from the McMurdo Dry Valley floors to about 600 and 1300 m above sea level (at Mt. Suess and Battleship Promentory, respectively) in Victoria Land (Moorhead et al. 2003;Porazinska et al. 2002b;Powers et al. 1998; this paper) and 800 m above sea level outside of Victoria Land (Adams et al. 2006). On Ross Island, S. lindsayae occurs in soils located away from penguin rookeries and in soils with ornithogenic inputs (Sinclair and Sjursen 2001), but is absent within rookeries (Porazinska et al. 2002a;Sinclair 2001;Yeates et al. 2009). Similar observations are not recorded for Victoria Land. Other studies recording the presence of S. lindsayae outside of Victoria Land have found the nematode amongst mosses (e.g. Saniona uncinata) and at King George Island, associated with a perennial plant (Deschampsia antarctica) (Mouratov et al. 2001;Shishida and Ohyama 1986;Vinciguerra 1994;Wharton and Brown 1989).

Reference
Edge of Lake Canopus *77°33.00' 161°31.00' algal growth at the edge of the lake spp.
Present (Wharton and Brown 1989) Between Lake Vanda and Lake Bull NP NP dry algae around the edge of small ponds spp.
Present (Wharton and Brown 1989) Between Lake Vanda and Lake Bull NP NP wet algae in meltwater and around the edge of small ponds spp.
Present (Wharton and Brown 1989) Bull Pass  (Wharton and Brown 1989) from Edmonson Point and Vinciguerra et al. (1994) found P. antarcticus, P. frigophilus and P. acuminatus at Terra Nova Bay. In the McMurdo Dry Valleys, only P. murrayi and P. frigophilus occur, with P. murrayi the most abundant and widespread (Table 2). P. murrayi and P. frigophilus (Kito et al. 1991;Shishida and Ohyama 1986) are endemic to the Antarctic, but not solely to Victoria Land. Close to Victoria Land, P. murrayi and P. frigophilus have been recorded frequently from Ross Island (e.g. Cape Royds, Cape Evans, Cape Crozier, McMurdo Station and Rocky Point) (Dougherty et al. 1960;Murray 1910;Porazinska et al. 2002a;Sinclair 2001;Wharton and Brown 1989) and P. frigophilus has been recorded on Dunlop Island (Timm 1971;USGS 2003). P. antarcticus occurs primarily in the maritime, and thus most of the recordings of P. antarcticus on the continent are assumed to be P. murrayi (Andrássy 1998).
Habitat. All Plectus spp. of Victoria Land occupy similar habitats. They are present in soils and sediments (Ayres et al. 2007) and are frequently associated with moist environments and areas supporting algae (e.g. Nostoc commune) and moss (e.g. Bryum antarcticum) ( Table 2). This is consistent with the habitats in which Plectus spp. are found in other regions of Antarctica (Andrássy 1998;Andrássy and Gibson 2007;Timm 1971;Wharton and Brown 1989;Yeates 1970).
Soil moisture is a critical factor determining the suitability of habitats for Plectus spp. Mouratov et al. (2001) studying Plectus spp. in the maritime Antarctic found that they had a preference for soil water content of 7-10%. In the McMurdo Dry Valleys, Courtright et al. (2001) similarly observed P. murrayi was more likely to occur in habitats with higher moisture contents. This moisture requirement may explain other distributional trends in the occurrence of Plectus. In the maritime Antarctic, Mouratov et al. (2001) found Plectus spp. abundance to be highest in the deepest soil layer they studied and under the moss, Saniona uncinata. In these environments soil moisture is likely to be higher at depth in the soil profile and also under mosses than in bare surface soil habitats. Courtright et al. (2001) also noted that P. murrayi were more frequently found in soils with higher NH 4 -N, NO 3 -N, organic C, and organic C/organic N ratios than other nematode genera (e.g. Scottnema). Plectus spp. seem to be sensitive to variation in soil salinity and only occur in soils with low EC (<100 mS cm -1 ), which typically are moist environments where salts have been leached from the soil or sediment. Shishida and Ohyama (1986) noted that P. frigophilus seems to prefer habitats of fresh water algae to those of mosses.
Habitat. E. antarcticus in Victoria Land occurs at varying elevation and most commonly in soils and in lake sediments. The genus has also frequently been associated with algal mats, both dry and moist found in meltwater, streambeds and lakes. E. antarcticus has been reported less frequently in areas of moss and from soils. In contrast, outside Victoria Land (e.g. Ross Island) the occurrence of E. antarcticus in a moss habitat (e.g. Bryum argenteum) is common, but it does not occur in penguin rookeries (on Ross Island or in Victoria Land). In soils of the McMurdo Dry Valleys E. antarcticus tends to be found in soils with higher moisture, NH 4 -N, NO 3 -N, organic C, and organic C/organic N ratios, and only occurs in soils with low salinity (EC <100 mS cm -1 ) (Courtright et al. 2001).

Panagrolaimus (Panagrolaimida: Panagrolaimidae)
Biogeographic distribution. The Antarctic Panagrolaimus consists of two species, P. magnivulvatus and P. davidi (but see Raymond et al. 2013b). Both are endemic (Andrássy 1998). P. davidi is the only species recorded from Victoria Land and its occurrence is rare (see Table 4). Until the present study, the only record of P. davidi in Victoria Land was from Marble Point (Timm 1971). The current study shows that P. davidi is also present in the northern coastal region of Victoria Land, at Edmonson Point and Cape Hallett and in Miers Valley, one of the McMurdo Dry Valleys. Thus, P. davidi occurs most frequently in coastal regions but is not necessarily restricted to them.
P. davidi has been recorded from Ross Island (e.g. Freckman and Virginia 1993;Porazinska et al. 2002a;Sinclair 2001;Sinclair and Sjursen 2001;Timm 1971;Wharton and Brown 1989). Panagrolaimus spp. have also been reported from several of the maritime islands (summarized in Andrássy 1998 and references therein, see also Raymond et al. 2013b).
Habitat. Penguin rookeries and moss-covered soils appear to be the most favorable habitats for P. davidi in Victoria Land and are consistent with the habitats where P. davidi has been found in other Antarctic ice-free areas (Porazinska et al. 2002a;Sin- Wharton and Brown 1989). Evidence indicates P. davidi occurs in habitats of high primary productivity and soil organic matter (as does P. magnivulvatus) regardless of its source of origin (e.g. mosses or penguin guano) though it is primarily associated with penguin rookeries (Porazinska et al. 2002a;Sinclair and Sjursen 2001). The presence of P. davidi is strongly correlated with organic carbon, organic nitrogen, chlorophyll a (a measure of primary productivity) and ammonium (Porazinska et al. 2002a;Sinclair and Sjursen 2001). The species is also more abundant in the highly productive areas of moss and algae along snow melt streams than in adjacent soils (Sinclair and Sjursen 2001).

Geomonhystera (Monhysterida: Monhysteridae)
Several nematode species originally described as Monhystera were redescribed by Andrássy in 1981 as Geomonhystera. Among these was Monhystera villosa from the Antarctic (Timm 1971), which Andrássy subsequently redescribed as a new species, Geomonhystera antarcticola (Andrássy 1998). It is the only known species of Geomonhystera on the continent, thus, we report all published observations of the genus from Victoria Land as G. antarcticola. Biogeographic distribution. G. antarcticola are generally rare, and along with P. davidi are the least abundant and most patchily distributed of all nematodes in Victoria Land. Other species of Geomonhystera occur in the islands of the maritime Antarctic (Signy, Coronation, Elephant, Intercurrence and Galindez) where G. antarcticola is one of the most common nematode species (Maslen 1981;Newsham et al. 2004;Spaull 1973a, b, c). They were originally recorded as Monhysterid genus A. and renamed as Monhystera villosa by Maslen (1979). Newsham et al. (2004) identified specimens from Signy Island as G. villosa. Sohlenius et al. recorded Monhystera from the Nunataks of Dronning Maud Land, East Antarctica (Sohlenius et al. 1995(Sohlenius et al. , 1996, and they have also been recovered from Macquarie Island of the Sub-Antarctic (Bunt 1954) and Signy Island of the maritime Antarctic (Caldwell 1981;Maslen 1981;Spaull 1973a, b, c;Wharton and Block 1993) but only identified as Monhystera spp., so it is unknown whether these nematodes could also be Geomonhystera. Some previously recorded Monhystera of the subantarctic (M. vulgaris, and M. filiformis) (Bunt 1954) are not Geomonhystera but more likely Eumonhystera  or Halomonhystera (Andrássy 2006).

Discussion
Nematode diversity in Victoria Land is low compared to the Antarctic Peninsula, but the presence of a few cryptic species is likely (Barrett et al. 2006c;Raymond et al. 2013b). Extensive sampling across broader geographic scales, combined with molecular techniques will likely recover additional species from both locations. With the exception of Panagrolaimus davidii and Geomonhystera spp., all species are widely distributed throughout Victoria Land, from the south coast and the most southern McMurdo Dry Valleys to the northern coastal region. This distribution suggests that their dispersal is ubiquitous and primarily by wind while in anhydrobiois (Nkem et al. 2006b), and it is the suitability of the soil habitat that determines the likelihood of population and community establishment and functioning . Our knowledge of nematode biodiversity, distribution, and function in Victoria Land is based on clusters of studies from a few distinct regions, such as the McMurdo Dry Valleys, and far northern coastal Victoria Land, which are accessible from established research stations. The rest of Victoria Land (including other inland ice-free areas) has been largely inaccessible. Studies throughout the McMurdo Dry Valleys are also patchy with some valleys being studied heavily (e.g. Taylor Valley) whilst others (e.g. Barwick Valley) have barely been investigated. More undescribed nematodes may occur in these less studied regions.

Conclusions
Habitat suitability for each nematode species is determined primarily by variations in soil factors such as quantities and types of organic material, moisture and salinity Virginia and Wall 1999). Scottnema lindsayae is the most abundant and widespread nematode and has a unique tolerance for a wide range of extreme soil habitats, and it is also the most tolerant to low soil moisture and high salinity of all the nematode species studied. These conditions define the most common soil habitats throughout the cold desert ecosystems of Victoria Land and explain the high abundance and broad distribution of S. lindsayae throughout the region. There are less extensive suitable habitats available in Victoria Land for Plectus spp. and Eudorylaimus antarcticus as their distributions are limited to habitats with higher moisture, greater organic material and lower salinity. P. davidii has a very limited biogeographic distribution, almost entirely restricted to coastal Victoria Land. This species is found in habitats with high primary productivity, of which there are few. Factors defining suitable habitats and the biogeographic distribution of Geomonhystera spp. in Victoria Land are the least understood, largely due to very low abundance and limited occurrence, although they have been recovered from sites across Victoria Land. There appears to be an association with algae but little else is known of their habitat requirements.
We have made considerable progress in understanding the basic relationships between soil properties and the distribution of the key nematode taxa throughout Victoria Land. Suitable habitats can be defined by moisture, salinity, organic matter and nutrient content, and the interactions between these factors. Manipulations of soil moisture and field observations of environmental change during pulse warming events show that nematode community composition can respond on time scales of seasons to decades (Ayres et al. 2010;Doran et al. 2002). The climate of Victoria Land is expected to change with warmer conditions Jones et al. 1998;Salby et al. 2011;Solomon et al. 2007;Steig et al. 2009;Thompson and Solomon 2002) leading to increasing soil moisture, redistribution of salts, and potentially higher productivity (Gooseff et al. 2011;Nielsen et al. 2012). These changes may alter the spatial distributions of suitable habitats for individual nematode species and/or alter population size and community diversity (Nielsen et al. 2011b). Studies have shown the important role of nematodes in carbon cycling, suggesting that changes in nematode biogeography will be linked with changes in ecosystem functioning in Antarctic soils .
The nematofauna of Victoria Land are capable of long distance dispersal by wind (Nkem et al. 2006b) but the Antarctic continent is effectively isolated from source populations elsewhere in the southern hemisphere (Convey et al. 2008;Convey and Stevens 2007). This leaves anthropogenic dispersal by way of tourists and scientists as the primary mechanism for the movement of alien species to Antarctica (Chown et al. 2012a). From a field sample collected in Wright Valley in the 2011-2012 field season, we recovered an individual living female Cuticularia fermata, a nematode heretofore known only from South Orkney Island (subantarctic island). Whether this specimen was transported to the site on clothing or equipment used by scientists or if there are established, low-density, isolated populations in the area is unknown. It is highly likely that the frequency of nematode introductions to Victoria Land will increase as tourism and scientific research increases (Chown et al. 2012a). There is a growing international consensus that action is needed to reduce the potential introductions of invasive soil species to continental Antarctica and the Peninsula and maritime regions (Chown et al. 2012b). A greater knowledge of nematode biogeography will be essential in understanding how to protect special soil habitats to preserve existing biodiversity and to prevent the introduction of non-native species and the potential harm they cause to the unique soil ecosystems of Antarctica.
Wynn-Williams for help collecting and processing soil samples. Ethan Adams, Eric Sokol and Ian Hogg helped process the Cuticularia fermata sample, which was collected as part of the NZ TABS project (http://nztabs.ictar.aq). This work could not have been completed without the dedicated, expert helicopter support provided by personnel of the US Coast Guard, US Navy VXE-6, Petroleum Helicopters Inc., and logistic and science support by ITT, Antarctic Support Associates, and Raytheon Polar Services. Brad Herried and Paul Morin of the Polar Geospatial Center (http://www.pgc.umn.edu) helped generate Figure 1. We very much appreciate the thoughtful, constructive criticisms of three anonymous reviewers. This research was supported by National Science Foundation Grants DPP 88-18049 and DPP 89-14655, OPP 9120123, OPP 9421025, and the McMurdo Long Term Ecological Research program (OPP 9211773, OPP 9810219).