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Cave Ecology pp 415-434 | Cite as

Research in Calcretes and Other Deep Subterranean Habitats Outside Caves

  • Stuart HalseEmail author
Chapter
Part of the Ecological Studies book series (ECOLSTUD, volume 235)

Abstract

The outstanding difference between traditional subterranean fauna studies and those carried out recently in Australia is the Australian emphasis on the fauna that occurs deep underground, but outside caves, across large parts of the landscape. This work has shown that the Australian arid zone, particularly in the western half of the continent, is rich in subterranean fauna, with the geologies supporting most species being calcrete and alluvium in the case of stygofauna and iron-rich rocks in the case of troglofauna. It is likely that, altogether, as many as 4500 species of stygofauna and troglofauna occur in the two most species-rich regions of Western Australia — the Pilbara and Yilgarn. Striking characteristics of the stygofauna communities in these regions include little overlap in species composition of communities of the hyporheic zone and deeper groundwater, very high levels of endemism in individual calcrete bodies, and the existence of extensive radiations of candonid ostracods in the Pilbara and copepods in calcretes of the Yilgarn. Characteristics of the troglofauna communities include extremely small ranges of many species, with linear ranges of 1–2 km apparently being common, and extensive radiation of schizomids and some other invertebrate groups in iron formations of the Pilbara.

20.1 Introduction

Box 20.1

The outstanding difference between traditional subterranean fauna studies and those carried out recently in Australia is the emphasis in Australia on the fauna outside caves within the network of small cavities that occur deep underground across large parts of the landscape. Much of the arid zone, especially in the western half of Australia, is rich in subterranean fauna. Despite the ancient age of the land mass in which many species occur, it appears that most of the species (or at least the lineages from which they have evolved) moved underground during the past 15 million years seeking moisture as the Australian continent moved north and became increasingly arid (Byrne et al. 2008).

This chapter deals with the subterranean fauna occurring outside caves in the Australian arid zone (Fig. 20.1). It focuses on the characteristics of the habitats in which stygofauna and troglofauna occur, as well as the taxonomic structure of these subterranean fauna communities and some of the more general characteristics of subterranean fauna outside caves. Two other important texts on Australian subterranean fauna are Humphreys (2016), which focuses on biogeography and the origin of the fauna, and Hose et al. (2015), which provides more information about eastern Australia.
Fig. 20.1

Locations of the Pilbara and Yilgarn regions (or cratons) and other places mentioned in text

Despite the large areas of karstic habitats in Australia, the continent has very few large caves. This is especially so in the arid zone, where there are arguably only two large cave systems. One is on the Nullarbor Plain (Webb and James 2006) and the other is around Camooweal, north-west of Mount Isa (Grimes 1988; Eberhard 2003) (Fig. 20.1). Despite their large size, both systems have relatively depauperate subterranean faunas (Richards 1971; Eberhard 2003). The same is true of the Judbarra/Gregory karst area on the edge of the arid zone in the Northern Territory (Moulds and Bannink 2012) (Fig. 20.1).

In very general terms, stygofauna in the Australian arid zone occurs mostly in unconfined, surficial regional aquifers that extend across the landscape. These aquifers mostly lie within alluvium (sometimes containing calcrete bodies), other detrital geologies, or in fractured or weathered rock. Troglofauna species mostly occur in the vadose (or unsaturated) zone, but there is no commonly used terminology to define their habitat, which extends downward from a meter or more below-ground surface to the water table. Troglofauna are found mainly in mineralized or weathered rock, calcrete, and detritals (consisting of scree, alluvium, or colluvium). The formations in the vadose zone containing troglofauna in Australia share some features with the milieu souterrain superficial (MSS) or shallow superficial habitats (SSH) of the northern hemisphere (Mammola et al. 2016), although the overall scale and geological setting of the Australian formations is quite different.

20.2 Subterranean Habitat Other than Caves

Box 20.2

While the fauna of caves can be collected relatively easily by skilled cavers and these cavers can see where the animals are and what they are doing at the time of collection, sampling areas outside caves is a blind process. Either hauls nets or traps are used to collect stygofauna and troglofauna. The operator is usually remote from the sampling device, and it is rarely possible to collect information about the exact habitat occupied by the animals. Information on their behavior is never available. As a result, the ecological preferences of stygofauna and troglofauna occupying the broader landscape are very poorly understood even at the level of the types of spaces used by the animals and the degree of interconnectedness of these spaces.

The main feature of aquifer and vadose zone habitats used by both stygofauna and troglofauna is that the available subterranean spaces are relatively small and mostly comprise what Howarth (1983) termed microcaverns (<5 mm in width) and mesocaverns (5–500 mm) (Fig. 20.2; see also Chap.  3). Mesocaverns occur mostly in calcrete and in the upper layers of extensively weathered rock formations. Most of the calcrete bodies that have been investigated are of groundwater origin, with the production of calcrete being principally the result of a shallow depth to the water table combined with a climatic regime that comprises low annual rainfall with occasional very heavy rain events and high rate of evaporation (Mann and Horwitz 1979). As groundwater levels rise following recharge from the heavy rain events, calcium and carbonate are transported in groundwater into areas of high evaporation where calcium carbonates are precipitated. As a consequence of changing baseline groundwater levels over time because of long-term climatic variation, there is re-working and re-forming of calcrete during wet and dry periods, which creates mesocaverns and smaller spaces both above and below the water table in the calcrete. Spaces tend to be largest around the water table; deeper sections of calcrete are often quite compact and may lack any spaces. It is also common for areas of calcrete to contain substantial pods of clay and silt that lack spaces, so that overall the habitat structure within a calcrete body can be quite heterogeneous. Figure 20.2 shows a 30 cm length of calcrete core from just below the water table; it is composed of porcelain-like clasts of calcrete bound together by cementing carbonate and partially infilled with cream-colored clay and fine sand. The resulting unit has a discontinuous, vuggy texture.
Fig. 20.2

Habitats in which subterranean fauna species occur. (a) Palaeochannel containing extensive subterranean calcrete in the Yilgarn; (b) drill core through saturated calcrete; (c) schematic illustration of subterranean habitat where calcrete is present; (d) iron ore range in the Pilbara, with deep gullies and an exposed face of hardcap containing mesocaverns; (e) drill core through mineralized iron ore formation; (f) schematic illustration of subterranean habitat in iron ore range with a deep water table (not illustrated)

The amount of weathering in rock formations in the Pilbara and Yilgarn is relatively high because of the very old age of these two cratons (Johnson 2009; see also Fig. 20.1). Weathering breaks down rocks and may lead to formation of spaces within the rock (vugginess). The surface of many rock formations in the Pilbara consists of a ferricrete duricrust (commonly called hardcap; Fig. 20.2d) that has resulted from weathering of the exposed host rock and may extend as deep as 60 m. Especially in its upper layers, the hardcap is frequently vuggy and may also contain mesocaverns and even small caves. Occasionally, the hardcap has been folded as a result of tectonic activity and so may occur at depth below unmodified rock formations. The other process that leads to vugginess in rock formations is mineralization, whereby various substances are leached from fresh rock with the consequent enrichment of iron or other mineral elements (Morris 1983). Mineralization can be substantially deeper than weathering because many of the processes leading to mineralization occur at depth during rock formation (Evans et al. 2013).

As already mentioned, the vadose zone habitats in Australia (or at least the upper strata) have some similarities to the MSS zone described by Juberthie (1983) and Ortuño et al. (2013). This is particularly the case in and around low ranges containing iron ore formations, especially where detritals form an important component of the landscape profile (Morris and Ramanaidou 2007), and in palaeovalleys filled with alluvium/colluvium and calcretes (Morgan 1993). There are also analogies between the hardcap in Australia and the canga of Brazilian iron formations (see also Chap.  21). However, a large proportion of the subterranean habitat in most iron ore formations occurs in what is most appropriately regarded as vuggy bedrock.

The relationship between high numbers of stygofauna and the occurrence of calcrete in palaeochannels in the Yilgarn region of Western Australia is well documented (Humphreys 2001, 2008; Guzik et al. 2010), with up to 75 species recorded from an individual calcrete body or cluster of calcrete deposits (EPA 2016). Another relatively well-studied relationship is the occurrence of high numbers of troglofauna species in mineralized banded iron formations and channel iron deposits farther north in the Pilbara region (EPA 2007, 2011, 2012), with more than 100 species having been recorded from sections of the banded iron formation of the Hamersley Range and about 25 species per mesa (flat-topped hill) from individual small mesas in the Robe Valley (unpublished data).

It should also be noted that it is not uncommon for a site to yield high numbers of stygofauna and low numbers of troglofauna, or vice versa. This is sometimes the result of different geologies occurring in the vadose zone and in the underlying groundwater aquifer; in other situations, the flow of water may have kept spaces open in the aquifer, whereas they have been filled by fine sediment in the vadose zone.

Box 20.3

The sampling methods used to collect stygofauna and troglofauna from subterranean habitats across the landscape have been described by Eberhard et al. (2009) and Halse and Pearson (2014). Stygofauna are sampled using groundwater monitoring bores to access the water table and underlying aquifer(s). The bores usually have a slotted PVC casing. This casing prevents the bores collapsing below the water table, while the vertical slots (usually 1–3 mm wide and extending the full depth of the bore below water table) allow stygofauna to migrate into the bore void from the surrounding aquifer. Stygofauna are collected by dropping a weighted haul net made of very fine mesh to the bottom of the bore, agitating the sediments at the bottom, and then slowly retrieving the net back through the water column. Troglofauna are usually sampled in holes drilled for mineral exploration. These holes are uncased and may be open at the ground surface, although sometimes they have a short PVC collar to reduce the likelihood of the hole collapsing. Troglofauna are collected either by trapping or scraping. Traps consist of short PVC cylinders that have slightly smaller diameter than the hole and moderate-sized perforations along their length to allow entry of troglofauna. The traps are baited with leaf material, lowered to the desired depth on a piece of cord and left in place for 6–8 weeks before being retrieved. Scraping consists of lowering a weighted, reinforced haul net to the bottom of the hole (or just into the water table) and then pulling it back to the surface along the wall of the hole, thus scraping troglofauna from the wall.

20.3 Subterranean Fauna Outside Caves

Arid and semiarid regions of Western Australia contain very high richness of stygofauna and troglofauna. Eberhard et al. (2009) estimated, based on a regional sampling program, that 500–550 stygofauna species occur in the Pilbara region alone, while Guzik et al. (2010) used expert opinion to estimate that 4140 subterranean fauna species, comprising 2680 stygofauna and 1460 troglofauna species, occur in the western half of Australia, mostly in the arid zone. More recently, Halse (2016) proposed, based on a combination of sampling results and extrapolation of the pattern of increasing richness from further sampling (mostly for environmental impact assessment), that nearly 3000 species of subterranean fauna occur in the Pilbara (Table 20.1).
Table 20.1

Number of subterranean fauna species in the Pilbara, as collected by Bennelongia Environmental Consultants (BEC) or estimated to be present based on extrapolation of the collecting results to date by BEC and other environmental consultants

Faunal group

No. of species

Collected by BEC

Estimated

Stygofauna

Crustacea

  Amphipoda

106

200

  Isopoda

31

75

  Syncarida

70

300

  Copepoda

130

250

  Ostracoda

194

300

  Other

4

4

Hydracarina

23

40

Annelida

74

150

Mollusca

5

10

Total

637

1329

Troglofauna

Isopoda

81

200

Pseudoscorpiones

66

150

Schizomida

59

120

Araneae

53

130

Palpigradi

18

40

Diplopoda

24

50

Chilopoda

51

120

Symphyla

38

80

Pauropoda

27

60

Diplura

90

200

Thysanura

47

100

Blattodea

27

40

Hemiptera

23

50

Coleoptera

69

150

Other

7+diptera

21

Total

680

1511

Other environmental impact assessment sampling suggests that the Yilgarn has stygofauna richness similar to the Pilbara but fewer troglofauna species. On this basis, it is considered likely that more than 4500 stygofauna and troglofauna species occur in the Pilbara and Yilgarn. This figure more-or-less matches that proposed independently by Guzik et al. (2010) for the larger western half of Australia, but undoubtedly most of the richness is on the Western Shield where the Pilbara and Yilgarn occur (Fig. 20.1). As is a regular feature of subterranean fauna species with their localized distributions, nearly all stygofauna and troglofauna species in the Pilbara and Yilgarn are endemic to the region in which they occur (Humphreys et al. 2008; Halse and Pearson 2014; Halse et al. 2014).

20.3.1 Stygofauna

Box 20.4

Most stygofauna species are crustaceans. The aquifers they use in the Australian arid zone occupy a variety of geologies that provide suitable spaces for animals. Alluvial and colluvial aquifers are important habitat because of their widespread occurrence and frequent high species richness, while areas of saturated calcrete (usually within alluvial or colluvial aquifers) are important habitat because of high species richness and very fine-scale endemism. Some aquifers in iron formations, especially channel iron deposit, may also support moderate numbers of stygofauna species. However, depth to the water table is a constraint on stygofauna occurrence, with assemblages usually being sparse where depth to the water table is much more than 30 m.

A detailed account of the stygofauna of the Pilbara is given by Halse et al. (2014). Although information on the Yilgarn is extensive, it is less consolidated and the first moderately comprehensive overview is provided here. General information about the communities in Yilgarn calcretes is provided by Humphreys (2001) and Humphreys et al. (2008).

A significant feature of both the Pilbara and Yilgarn is the relatively small overlap in species composition of the hyporheic fauna of streams and the stygofauna communities of deeper groundwater aquifers (Halse et al. 2002). While some species typical of the hyporheos are found in regional aquifers, such as darwinulid ostracods, the candonid ostracod Candonocypris tenuis, many cyclopoid copepods, and possibly phreatoicid isopods (Knott and Halse 1999; Pinder et al. 2010; Schön et al. 2010), the reverse rarely occurs. The low overlap is probably partly a result of the water associated with the alluvium of the ephemeral rivers and creeks being poorly connected to regional groundwater (Dogramaci et al. 2012), but the absence of deeper groundwater species in the hyporheos also suggests that the ecological and life history characteristics of these stygobitic groundwater species make them unsuited to hyporheic conditions.

Overall, the higher level taxonomic composition of stygofauna assemblages in the Pilbara and Yilgarn is similar, despite some differences in the proportions of major taxonomic groups (Fig. 20.3). Six groups are considered here in more detail. Copepods dominate the fauna of both areas, comprising approximately 60% of the animals in the Yilgarn and 40% in the Pilbara. However, individual species are often represented by large numbers of animals and copepods comprise only 44% and 20% of species in the Yilgarn and Pilbara, respectively (Halse et al. 2014; unpublished data), which is similar to the representation of 20–40% of species in European communities (Galassi et al. 2009). Perhaps of most interest, there appears to have been explosive speciation of harpacticoid copepods in some Yilgarn calcretes where copepod species may represent almost half the fauna (Karanovic and Cooper 2011, 2012).
Fig. 20.3

Proportions of stygofauna in the Pilbara and Yilgarn belonging to different taxonomic groups. (a) Pilbara species, (b) Pilbara abundance, (c) Yilgarn species, and (d) Yilgarn abundance. Based on collecting results of Bennelongia Environmental Consultants

Ostracods represent 24% and 13%, respectively, of the animals in the Pilbara and Yilgarn (Fig. 20.3) and 30% and 10% of the species. The greater contribution of ostracods to the fauna of the Pilbara reflects the enormous radiation of candonid ostracods in this region, consisting of 11 described endemic genera and more than 108 collected species (Karanovic 2007; Reeves et al. 2007, unpublished data). By global standards, where ostracods typically constitute about 3% of all species (Eberhard et al. 2005), both the Pilbara and Yilgarn are rich in ostracods, but the Pilbara has exceptional diversity.

Another group showing high species richness is dytiscid beetles in Yilgarn calcretes and some other parts of the arid zone (Watts and Humphreys 2009; Eberhard et al. 2016). Strangely, only one dytiscid species has been recorded from the Pilbara (Watts and McRae 2013). The large number of stygofauna dytiscid beetles collected to date from the western half of Australia (approx. 100), despite single calcretes almost never containing more than three species, is a consequence of the high species turnover between calcretes. Beetles are estimated to represent 2.2% of the animals in the Yilgarn and 4.5% of the species.

Amphipod species have much the same pattern of occurrence in the Yilgarn as dytiscids, but they are more speciose and occur in higher abundance. They are also abundant in the Pilbara and represent 16% and 7%, respectively, of the animals in the Pilbara and Yilgarn (Fig. 20.3) and 17% and 20% of the species (Halse et al. 2014; unpublished data). This is similar to the overall representation of amphipod species in stygofaunal assemblages globally (19%, Eberhard et al. 2005). Much of the stygofaunal research in the Yilgarn and Pilbara has been on amphipods, with species in the Yilgarn considered to be confined to single calcretes (although these may sometimes be more accurately described as a cluster of adjacent calcrete bodies), while species in the Pilbara mostly have ranges confined to the catchments of individual tributaries of major rivers (Finston et al. 2004, 2007; Cooper et al. 2007; Bradford et al. 2010, 2013; King et al. 2012).

Based on limited taxonomic and genetic work to define species units (e.g., Guzik et al. 2008), syncarids comprise 3.1% and 4.1%, respectively, of the animals in the Pilbara and Yilgarn and 11% of the species in both regions. This is a substantially higher proportion of species than recorded globally (Eberhard et al. 2005). Limited surveys have shown that syncarids are also ubiquitous in alluvial aquifers of better watered coastal areas of Australia (Cho et al. 2005; Camacho and Hancock 2012; Cook et al. 2012), and it is likely that more survey will show the Australian fauna is at least as rich as that of Europe (see Camacho and Valdecasas 2008) and with perhaps less of an arid zone focus than most groups of Australian stygofauna.

Oligochaetes represent 9% and 8%, respectively, of the animals in the Pilbara and Yilgarn (Fig. 20.3) and 11% and 8% of the species compared with a global average of 2% of species (Eberhard et al. 2005). As in other parts of the world (Creuzé des Châtelliers et al. 2009), many oligochaetes in the Pilbara and Yilgarn are quite widespread and also have surface occurrences, so that they should be treated as stygophiles (and sometimes possibly stygoxenes). The greater number of stygal species in the Pilbara and Yilgarn is principally attributable to the collection of relatively large numbers of enchytraeid species during sampling (31% of Pilbara and 50% of Yilgarn species compared with 11% of European species). It is also of interest that phreodrilids are quite common as stygofauna in the arid Pilbara and Yilgarn (Pinder 2008; Brown et al. 2015).

20.3.2 Troglofauna

Information on the occurrence of troglofauna outside caves comes almost entirely from environmental impact assessment surveys associated with mining proposals and so data are strongly biased toward the sampling of hard rock geologies. Areas of calcrete have usually been sampled at low intensity, if at all, because of the difficulty maintaining open holes for sampling in soft substrata. Halse and Pearson (2014) have provided a description of the taxonomic composition of troglofauna in the Pilbara, but the first account of the overall composition of Yilgarn troglofaunal assemblages is presented here.

Box 20.5

Troglofauna in the Australian arid zone are taxonomically much more diverse than stygofauna. Sampling has been strongly biased toward iron formations where exploration drill holes (for mining) provide access to subterranean habitat. Nevertheless, it is likely that iron formations provide some of the most important troglofaunal habitat. Areas of unsaturated calcrete also seem to provide important habitat, provided the water table is not too shallow and soil salinity is not too high. While species ranges are still being documented, one of the outstanding characteristics of Australian arid zone troglofauna species is that they appear to have very small ranges, which may sometimes be <1 km2.

One of the peculiarities of the information on troglofauna in Pilbara and Yilgarn is that there has been no attempt to assess the occurrence of troglofaunal mites and collembolans. Both groups comprise significant components of the fauna in other parts of the world (Ducarme et al. 2004; Kováč et al. 2016), and troglofauna species belonging to these groups have been observed frequently in Pilbara and Yilgarn samples (Greenslade 2002).

Several of the groups that are prominent in troglofaunal assemblages of the Pilbara (cockroaches, schizomids, dipterans) are absent, or nearly so, from the Yilgarn (Fig. 20.4). Isopods are the dominant group in assemblages of the Yilgarn. Based on the number of animals collected, they represent 6% and 43% of the fauna in the Pilbara and Yilgarn, respectively, and 12% and 30% of species. Isopod occurrence is globally variable with 12% of species in the Balkan Peninsula (Sket et al. 2004) and 26% of species in Portugal (Reboleira et al. 2013), and the variation between regions in Western Australia reflects this.
Fig. 20.4

Proportions of troglofauna in the Pilbara and Yilgarn belonging to different taxonomic groups. (a) Pilbara species, (b) Pilbara abundance, (c) Yilgarn species, and (d) Yilbara abundance. Based on collecting results of Bennelongia Environmental Consultants

Hemipterans (mostly Meenoplidae) appear to have variable ranges (Fig. 20.5). The group is relatively abundant, representing 23% and 10% of animals in the Pilbara and Yilgarn, respectively, but little more than 3% of species in each region (Fig. 20.4). Some troglophilic species appear to have ranges extending over hundreds of kilometers, while other potentially troglobitic species appear to have small ranges (JM McRae, unpublished data). Culver and Pipan (2008) considered troglobitic hemipterans to be more common in shallow subterranean habitats than caves. Records from the Pilbara, in particular, suggest that hemipterans may occur at considerable depths (Halse and Pearson 2014; unpublished data).
Fig. 20.5

Degrees of troglomorphy in meenoplid hemipterans and nocticolid cockroaches. (a) Troglobitic meenoplid, (b) troglophilic meenoplid, (c) troglobitic nocticolid, and (d) troglophilic nocticolid with eyespot

Cockroaches, mostly belonging to the family Nocticolidae, are also abundant in the Pilbara, where they represent 19% of animals but only 4% of species, although cockroaches are one of the many groups in which the use of genetic species concepts is likely to substantially increase the number of species recognized (Trotter et al. 2017; Fig. 20.5). A single cockroach has been collected from the Yilgarn. While comparative data are difficult to obtain, the diversity of troglofaunal cockroaches in the Pilbara appears to be unusually high (Roth 1991; Moulds and Bannink 2012).

In contrast to the abundance of hemipterans and cockroaches, the proportions of beetles in the troglofauna assemblages of the Pilbara and Yilgarn are surprisingly low, especially when the Yilgarn is comparatively rich in stygofaunal beetles. Only 6% and 3% of troglofaunal animals and 10% and 9% of troglofaunal species are beetles in the Pilbara and Yilgarn, respectively. Typically, beetles comprise more than a third of the species in troglofaunal communities elsewhere in the world (Culver and Sket 2000; Sket et al. 2004; Niemiller and Zigler 2013). While more taxonomic investigation is likely to substantially increase the number of beetles known from the Pilbara and Yilgarn (e.g., Baehr and Main 2016; Table 20.1), their proportion of the known fauna is not expected to change greatly.

Millipedes represent 9% and 6% of animals in the Pilbara and Yilgarn, respectively (Fig. 20.4), as a result of the widespread occurrence of the circumtropical troglophilic Lophoturus madecassus (see Car et al. 2013). The group comprises only 3.5%and 1.7% of species known from these regions compared with 10% of the fauna in the Balkan Peninsula (Sket et al. 2004).

Perhaps the most iconic troglofaunal group in the Australian arid zone is the minor arachnid order Schizomida. Its occurrence is indicative of a taxonomically rich troglofauna community, and collection of schizomid species in mesas of the Robe Valley in the Pilbara led to the first troglofauna-based recommendations against mine approval by the Environmental Protection Authority in Western Australia (EPA 2007). Schizomids occur moderately often in the vadose zone and in caves across northern Australia (e.g., Harvey 2001), as well as in humid surface habitats of the tropics more generally (Monjaraz-Ruedas 2013). They have been collected in high abundance in the iron formation ranges of the central Pilbara and comprise 7% of animals and 9% of species in troglofauna assemblages of the Pilbara as a whole (Fig. 20.4). Some of the diversity of schizomids in the Robe Valley of the Pilbara has been documented in detail by Harvey et al. (2008) and Harms et al. (2018).

Diplurans usually comprise a small to moderate proportion of troglofauna assemblages (1.1% in the Balkan Peninsula, Sket et al. 2004; 1.4–6% in superficial subterranean habitats, Culver and Pipan 2008; 7% in Portugal, Reboleira et al. 2013). In contrast, they comprise 13% and 7% of species in the Pilbara and Yilgarn, respectively, despite accounting for only 3% of the animals in each region. Some of these species are certainly troglophiles, and determining the proportion of troglobites is likely to require detailed taxonomic investigations and, ideally, life history studies to understand species ranges. However, the estimated median range of 16 km2 for Pilbara species (Halse and Pearson 2014) suggests the proportion of troglobites may be quite high.

For pauropods, symphylans, and, to a lesser extent, palpigrads, it is difficult to distinguish troglofaunal from epigean species, because all animals of these three groups lack eyes and pigment. Furthermore, collection from subterranean habitats does not necessarily mean a species is troglofauna because most animals collected in drill holes are clearly identifiable as epigean species that have “fallen” into the drill hole. Many holes lack collars and are open at the surface with nothing to prevent surface species falling in. Even when holes are collared with PVC pipe, there is often subsidence around the collar and space for surface species to enter the hole. Bearing in mind the uncertainties associated with interpreting captures of the three groups in the Western Australian context, palpigrads and symphylans that are clearly troglobitic are regularly recorded in other parts of the world (e.g., Sket et al. 2004) and the described palpigradid Eukoenenia guzikae from the Yilgarn is considered to be troglobitic (Barranco and Harvey 2008). Halse and Pearson (2014) suggested that at least some of the pauropod species collected from the Pilbara are also likely to be troglobites because of their small ranges and, more particularly, the hostile surface soil conditions in the arid Pilbara. Currently, pauropods, symphylans, and palpigrads are considered to comprise 4.9%, 5.6%, and 2.6% of animals and 6.3%, 13.6%, and 2.3% of species in the Pilbara and Yilgarn, respectively (Fig. 20.4).

20.4 Species Distributions

As a group, subterranean fauna species are characterized by small ranges. This is especially so for troglofauna species (Halse and Pearson 2014), which in the Pilbara appear to have ranges that are mostly at least an order of magnitude smaller than those of stygofauna species (Eberhard et al. 2009; Halse et al. 2014). Linear ranges of <1 km sometimes occur and ranges of 1–2 km are probably quite common among arid zone troglofauna in the Pilbara (Table 20.2). In contrast only about 5% of Pilbara stygofauna species are likely to have linear ranges of <30 km (Halse et al. 2014). There is probably less difference between ranges of stygofauna and troglofauna in calcretes in the Yilgarn where a habitat feature (i.e., the calcrete body) is often the factor limiting ranges rather than characteristics of the species themselves or the distribution of subtle habitat differences within the calcrete.
Table 20.2

Median linear ranges (recalculated from Halse and Pearson 2014) of different groups of troglofauna species in the Pilbara and the main geologies from which the groups are known in the Pilbara and Yilgarn

Troglofauna group

Median linear range (km)

Major habitats

Pseudoscorpiones

5.3

Mineralized rock, detritals (incl. calcrete)

Palpigradida

21

Mineralized rock, detritals (incl. calcrete)

Schizomida

2.6

Mineralized rock

Araneae

2.2

Mineralized rock (incl. calcrete)

Chilopoda

6.2

Mineralized rock, detritals (incl. calcrete)

Diplopoda

4.5

Mineralized rock, detritals (incl. calcrete)

Pauropoda

6.6

Detritals, mineralized rock (incl. calcrete)

Symphyla

3.2

Detritals, mineralized rock (incl. calcrete)

Isopoda

1.8

Mineralized rock, detritals (incl. calcrete)

Diplura

4.5

Mineralized rock, detritals (incl. calcrete)

Thysanura

3.7

Mineralized rock, detritals (incl. calcrete)

Blattodea

6.1

Mineralized rock

Hemiptera

68

Mineralized rock, detritals (incl. calcrete)

Coleoptera

8.7

Mineralized rock, detritals

Diptera

159

Mineralized rock

The pattern of subterranean fauna species being restricted to single calcretes or calcrete clusters in the Yilgarn led to Steven Cooper and others proposing the calcrete island hypothesis in relation to stygofauna (Cooper et al. 2002, 2007). It also seems to apply to troglofauna species (Javidkar et al. 2016). Under this hypothesis, most species in calcretes of the Yilgarn region are expected to be restricted to individual calcrete bodies that may have linear ranges of only tens of kilometers at most. The areas between calcrete bodies, which include intervening sections of the palaeochannel valleys hosting the calcretes, are unsuitable for stygofauna and troglofauna because of high salinity (Humphreys et al. 2008), lack of suitable voids and spaces, or otherwise inhospitable habitat. A series of papers by Tomas Karanovic on the copepods of the Yeelirrie calcrete illustrate the extreme levels of geographic replacement and local endemism that may occur within calcretes, with some stygofauna species appearing to have linear ranges of <5 km (Karanovic and Cooper 2011, 2012; Karanovic et al. 2015).

Another generalization is that weathered and mineralized iron ore deposits provide rich troglofauna habitat. The occurrence of rich troglofauna communities in iron ore ranges in Australia is analogous to the occurrence of troglofauna in iron ore mining areas of Brazil (Silva et al. 2011; see Chap.  21), although in Brazil the animals have mostly been collected from caves rather than from microcaverns within areas of vuggy iron ore (Fig. 20.2). That said, the factors affecting the importance of different types of iron ore deposits for troglofauna in Western Australia are still being studied. For reasons still to be explained, banded iron formations and channel iron deposits in the Pilbara support greater numbers of troglofauna species than banded iron formations in the Yilgarn, with Pilbara communities being more complex and, as already mentioned, supporting groups such as schizomids and cockroaches that are absent (or very nearly so) from the Yilgarn.

The vuggy habitats found in banded iron and other rock formations, especially if the available spaces are mostly microcaverns, probably provide few pathways for significant lateral underground dispersal (Fig. 20.2). Therefore, the troglobitic species in rock habitats would be expected to have smaller ranges than species inhabiting various types of detritals (scree and alluvium/colluvium) where the potential for dispersal through the matrix is likely to be greater. Despite this, and based on the current very limited understanding of the habitats that species occupy, it appears that the ranges of most troglofauna species in the Pilbara (Table 20.2) are small and determined by factors other than the broad type of geology in which the species occurs. For example, species occurring in mineralized rock do not consistently have smaller ranges than species in detritals. Probably the most important factor affecting range is whether species are troglophilic and have a surface dispersal phase, rather than relying on below-ground dispersal as troblobites do, but other intrinsic biological differences between groups may also affect species’ ranges. It should be emphasized that groups for which median ranges are relatively large because they include some widespread troglophiles, such as palpigrads and hemipterans, also contain some presumed troglobitic species with very small ranges.

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Authors and Affiliations

  1. 1.Bennelongia Environmental ConsultantsJolimontAustralia

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