Transactions of the Royal Society of South Australia (2014), 138(2): 147–160
COMMUNITY ECOLOGY OF AQUATIC INVERTEBRATES IN GNAMMAS (ROCK-HOLES) OF NORTH-WESTERN EYRE PENINSULA, SOUTH AUSTRALIA Brian V. Timms
Centre for Ecosystem science, school of Biology, Earth and Environmental sciences, University of new south Wales, Kensington nsW 2052. Email:
[email protected]
Abstract
Gnammas are small hollows, usually on granite outcrops, that contain water intermittently. They are of two basic types: pans are shallow (50 cm) and form by weathering on a hemispherical front in solid granite, but often aided by vertical joints. much is known about those in southern Western australia, but similar gnammas on granite rocks of north-western Eyre Peninsula (EP), south australia are unknown. This study shows such pools are less speciose than in the west, with fewer large branchiopods, cladocerans, ostracods, and copepods, no snails and overall hardly any endemics. This is largely explained by isolation effected by the large dry nullarbor Plain between southern Wa and EP; this isolation also affects the fauna within the EP region because highly isolated rocks lack some species. as in Wa, EP pits have a largely eurytopic fauna of insects but with a few numerous and characteristic crustaceans whereas pans are dominated by crustaceans, though by different species than in Wa. Community structure in pits on the EP resembles those in Wa. However, EP pans are different, largely due to the influence of endemics, and consequently, the fauna of pits and pans on the EP differ less than in Wa. although the physical environment is similar in the two areas, biogeographical differences lead to distinctive communities in each. almost none of these gnammas are in the conservation estate and many of the pit gnammas are severely sedimented. KEy Words: pan gnammas, pit gnammas, Lynceus, Cladocera, ostracoda, momentary species richness, multivariate analysis.
Introduction
The north-western Eyre Peninsula (hereafter EP) is part of the Gawler Craton and is underlain by granitic rocks (Parker et al. 1985). in places, this granite is exposed as distinct inselbergs such as mt Wudinna and Pildappa rock, both just north of the Eyre Highway and near the towns of Wudinna and minnipa respectively (Fig 1). The geomorphology of these inselbergs is well known (e.g. Twidale & Corbin 1963; Twidale 1971; Twidale & Vidal romani 2005), and their iconic gnammas have often been mentioned and illustrated, particularly the hemispherical pit gnamma on Pildappa rock (Twidale & Foale 1969; Timms, 2008; Bayly 2011). Part of the surface of Pildappa rock has even been coarsely mapped to show the extent and types of gnammas (Twidale & Corbin, 1963). However, their biology is virtually unknown, though some support a vulnerable plant Limosella granitica and many of the deeper ones often contain the pea shrimp Lynceus magdaleanae (Timms, 2013a), both gnamma endemics.
Gnammas are of many types, often divided into deeper pit gnammas and shallower pan gnammas (Twidale & Corbin, 1964; Twidale &Vidal romani, 2005). in general terms, their formation is understood to be due to the initial development of a water-holding hollow, followed by weathering of the granite and then removal of the waste products. in the pans, weathering is mainly horizontal along superficial lamination planes, whereas in the pits, weathering may occur across a hemispherical front into solid granite, often focused along a vertical joint (Twidale &Vidal romani, 2005). subsequent variation and sometimes other modes of origin mean that pit gnammas vary in origin and shape with up to 10 types being recognised (Timms, 2013b). There is a third main type of gnamma, the pipe gnamma, usually found in laterite in desert environments (Bayly et al., 2011), but there is none on EP. Pan gnammas usually occur on the flattish summits of inselbergs and are far more common than pit gnammas which can occur anywhere on the inselberg (Timms, 2013b). 147
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at least in southern Western australia, pan gnammas support an unique assemblage of aquatic invertebrates while pit gnammas are less speciose momentarily and tend to have a eurytopic (i.e. widespread and tolerant) fauna (Pinder et al., 2000; Timms 2014). The crustacean Branchiopoda, ostracoda, and Copepoda dominate numerically in both, though there are relatively more insect species in pits and they are far more eurytopic. There are many endemic species in pans, but few in pits (Timms, 2014). The prevalence of these crustaceans is related to their preadaptation of having resistant eggs and speciation in rock pool refuges in past climatic fluctuations (Pinder et al., 2000), whereas insects are advantaged by easy active dispersal (Jocqué et al. 2010).
To date, almost all knowledge on the ecology of gnammas is from studies on granitic inselbergs in Western australia (Bayly 1982, 1997; Pinder et al. 2000; Jocqué et al. 2007, Timms, 2012a,b, 2013b, 2014). The sole study from eastern australia (Bishop 1974) suggests a quite different scenario on pans in sandstone, with few species (both crustaceans and insects) and hardly any endemics. a study on gnammas of the Gawler ranges, just north of the EP (White et al. 2011) listed a few invertebrates, identified to various levels, but enough to indicate a depauperate fauna with some affinity to that in eastern Wa gnammas (Bayly et al. 2011; Timms 2013b, 2014). The gnammas on north-western EP present a series of pools, both pans and pits, on granitic inselbergs of broadly similar age to those of southwest australia, but isolated by the extensive intervening nullarbor Plain of limestone. at least two of the iconic diplostracans (clam shrimps) of western gnammas also occur on EP (Timms, 2006, 2013a), but it is yet to be determined whether the speciose ostracods and cladocerans of Wa also occur further east. They certainly do not occur in the Great Victoria desert, but these sites are extremely isolated and hence limited by poor dispersal of invertebrates (Bayly et al. 2011).
setting my work in context by describing the general limnology of the gnammas of north-western Eyre Peninsula, i test three hypotheses about the invertebrate fauna of EP gnammas in this paper. The first is that pan and pit gnammas on the EP support substantially different invertebrate communities from those in southern Western australia. The second is that the EP gnamma fauna of pits and pans are not as diverse as in Western australia, probably owing to the barrier to dispersal presented by the arid nullarbor Plain and lack of speciation opportunities on EP. Finally, i hypothesise that the faunal difference between pans and pits observed in Wa applies also to the EP gnammas. i also make some comparisons with gnammas elsewhere in the world. Methods
although pan gnammas are common on most granitic inselbergs on the north-western EP centred on minnipa and Wudinna (Fig. 1), i sampled the three rocks where pan gnammas are most numerous: Pildappa, Polda and Little Wudinna rocks. Two other rocks (Parla and Peela, Fig. 1) were added on one occasion in a wider comparative assessment. in each case, 10 pan gnammas were chosen for study, with the proviso that each pool was at least 100 cm in diameter and more than 8 cm deep when full. By contrast, pit gnammas are uncommon in the immediate area, and of the 21 known for northwest EP (author, unpublished data), 10 larger ones were chosen for study, but with a possible disadvantage that six were on one outcrop, Peela rocks. most study pools were visited five times across three years: July 2011, July 2012, august 2012, July 2013 and september 2013. However, the extra Parla and Peela pans were studied only in september 2013. Visits were also made to many pit gnammas in october 2011, march 2013 and october 2013 but some were dry as were all pans on these occasions.
on each visit to each pool, a water sample was taken to determine conductivity in μs/cm with an adWa332 conductivity meter and turbidity was measured in a secchi disc tube calibrated in nephelometric Turbidity Units (nTU). This tube is not accurate at very low turbidities as values less than five nTU are noted as such rather than a lower figure. depth was determined with a stout tape measure and, when a pool was deemed full, its length and width measured and volume was calculated. as pans were assumed to be saucer-shaped, the formula V=4d2z/3 was used, whereas pits were assumed to be hemispherical in shape and the formula V = d2zπ/8 was used where V = volume, d = average diameter and z = depth .
To sample the meiofauna in both types of gnammas, a small plankton net (opening 10 cm by 8 cm, length 50 cm, net mesh 159 µm) was used for one minute in areas in which the bottom sediment was stirred in order to catch epibenthic species as well as planktonic species. in the pans, macroinvertebrates were caught with a 12-cm diameter household sieve of 1-mm mesh whereas in pits, i used a pond net of 1-mm mesh and 148
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Figure 1. map of the Wudunna – minnipa area showing location of the inselbergs mentioned in the text.
with an opening 25 cm wide, 20 cm high and 30 cm deep supported on a d frame and with a handle 1.8 m long was used. Both nets were used for two minutes. Experimental macroinvertebrate sweeps for shorter and longer periods suggested that 80–90% of species present were caught within two minutes. macroinvertebrates were sorted alive in a white tray, and representatives of all species caught were preserved in alcohol for later identification, and the remainder returned alive to the pools, together with all tadpoles caught. The whole meiofauna collection was preserved in alcohol for later study. abundances were estimated on a log scale.
To test the hypotheses about faunal relationships between these groups of gnammas, i used multivariate analyses calculated using PrimEr (v6) (Clarke & Gorley 2006). The data sets included the five sets of pan gnammas and the pit gnammas sampled in september 2013, the three-year data sets on both the gnamma types of Pildappa, Polda and Little Wudinna rocks and of the pit gnammas on various rocks. i also compared the fauna of the pans and pits of the Western australian Wheat belt (Timms, 2012, 2013b, 2014) with those of the pans and pits of EP. Field and laboratory counts were further log-transformed (log10(x +1)) and a similarity matrix constructed using the Bray—Curtis similarity coefficient. non-metric multidimensional scaling (nmds) ordination was then performed to visualise patterns in assemblage composition among the data sets. Where appropriate, oneway analyses of similarity (anosims) were used to test for differences among groups followed by simPEr (similarity percentages) analysis of the species contributing to group similarities and differences. The correlation coefficient was used to test a possible relationship between gnamma size and momentary species richness (msr) and a student’s t-test to look for significant differences between msr means. Physicochemical features
Results
Pans varied in volume and depth from averages of 160 litres and 8.2 cm (maximum depth) on Parla rock to 2900 litres and 18.1 cm on Polda rock (Table 1). Pans on Parla and Pildappa rocks were much more incised into the rock and sometimes with considerable undercut edges (see Twidale & Corbin, 1963) than those on Peela and Little Wudinna rocks. nevertheless, all had the general form of a saucer-shaped depression with largely flat floors and edges of variable degrees of slope. some, particularly on Pildappa rock, were pans within pans, meaning that there were one or two deeper saucer-shaped areas within the bigger ‘saucer.’ Geomorphologically, the pits were quite different, being much deeper and generally hemispherical in cross149
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section with larger volumes (Table 2). The pits were of a number of forms: Pildappa West and East and Pygery are of the hemispherical type, Poondarra is an armchair pit, Peela pits 1–4 are canoe gnammas, and Peela pits 5 and 6 are plunge pools, although the latter is on a joint so has characteristics of a canoe gnamma as well (Timms, 2013b). Table 1. Comparisons of physicochemical features of the pans.
Parameter* Parla Pildappa Polda L. Wudinna Maximum Depth (cms) 8.2 ± 1.2 15.6 ± 1.3 18.1 ± 1.6 17.7 ± 1.9 Volume (litres) 160 ± 78 540 ± 99 1900 ± 652 1130 ± 270 328 ± 458 522 ± 41 734 ± 53 436 ± 115 Conductivity (!S/cm) Turbidity (NTU) 5.0 ± 0.1 11.8 ± 2.3 5.9 ± 0.7 12.4 ± 2.7 *means SE. Parla and Peela Rocks based on one visit, others on five visits.
Peela 10.8 ± 1.8 660 ± 338 206 ± 60 11.9 ± 5.0
Table 2. Pit gnammas physicochemical features. Pit Gnamma
Latitude oS
Longitude oE
Pildappa West Pildappa East Poondarra Pygery Peela 1 Peela 2 Peela 3 Peela 4 Peela 5 Peela 6
32o 45’ 05.0” 32o 45’ 04.7” 32o 49’ 01” 32o 59’ 09.8” 33o 00’ 08.9” 33o 00’ 09.2” 33o 00’ 09.6” 33o 00’ 09.9” 33o 00’ 08.0” 33o 00’ 12.0”
135o 13’ 53.2” 135o 13’ 51.0” 135o 22’ 48” 135o 28’ 14.3” 135o 43’ 28.7” 135o 43’ 28.1” 135o 43’ 28.7” 135o 43’ 28.0” 135o 43’ 36.4” 135o 43’ 32.1”
Volume (litres) 2149 2556 1367 1364 637+ 286 2705+ 1696+ 119 305
Max depth (cms) 60 58 40 35 49 22 42 49 70 37
Conductivity (!S/cm SE) 366 ± 72 502 ± 51 368 ± 182 388 ± 192 286 ± 135 118 ± 18 84 ± 13 45 ± 34 733 ± 368 417 ± 52
Turbidity (NTU SE) 38 ± 22 10 ± 3 380 ± 191 32 ± 19 38 ± 22 16 ± 5 6 ±1 24 ± 11 160 ± 108 35 ± 7
Generally, conductivities ranged from 300–700 µs/cm, with some of those (pans and pits) on Peela rock being much lower, and conductivities of the pans of Polda rock often higher (Tables 1 & 2). The lowest values were from Peela pits 2–4, each with considerable through-flow in wet periods (Table 2). Highest conductivities were recorded in large Polda pans, with 8 of the 50 records >1000 µs/cm and one reaching 1900 µs/cm. Pans generally held clear waters whereas pits were generally slightly more turbid. Poondarra and Peela 5 pits usually had cyanobacterial (‘blue-green algal’) blooms when visited, so their average turbidities were higher. This study did not include pH measurements, but in late winter 1991, Bayly (pers. comm., 5 February 2014) noted a range of 6.0 to 7.6 (mean 6.85) in 16 pan gnammas and 7.3 to 8.7 in the two pit gnammas on Pildappa rock. From general observations over 2008–2013, the average hydroperiod is about 4.5–5.5 months, with pools filling in may/ June and usually drying in early october. irregular storms may add water in summer, but it persists for only days to 1–3 weeks.
Species richness in pits and pans
overall, 54 taxa were encountered. The actual species richness is probably a little higher as some taxa represented more than one species (e.g. nematodes, ‘other chironomini,’ unidentified red mites, and the hydrophilid beetle Berosus spp). The separate totals for pans and pits were 41 and 45 taxa respectively. although many species occur in both (appendix 1), notable exceptions include the ostracod Ilyocypris n. sp., and the cladocerans Armatalona imitatoria, Leberis n.sp., Plurispina n.sp. and Rak n.sp. in pans only and pea shrimp Lynceus magdaleanae, the mayfly Cloeon sp. and the five odonates in pits only. some others are far more likely to be found in pans, including the cladoceran Neothrix armata, the hemipterans, the dytiscid beetles Allodessus bistrigatus and Sternopriscus multimaculatus and the biting midge Dasyhelea sp. although larval midges (Chironomus spp.) occur in both types of gnammas, they are far more common in pits than in pans; indeed, most of the records in pans are from deeper pools, indicating importance of pool depth or some factor related to this. 150
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mean momentary species richness (msr) varied from 11.0 to 20.6 in the pan gnammas, with a grand mean of 18.8 for the three well-studied series of pans (n = 150)(Table 3). For the pit gnammas (n = 57), individual means ranged from 7.2 to 10.6 and overall averaged 9.1 (Table 3). The grand mean of the first five pits (Pildappa W to Peela 1) was 8.84 and was significantly lower than the mean for the five pan gnammas (t = 4.42, p = 0.0058). in both pans and pits, there was no significant relationship of msr with gnamma size (pans r2 = 0.4135; pits r2 = 0.0148). However, the small pans of Parla rock had much lower msr whereas the large pans of Polda rock had the highest msr (Tables 1 and 3). (df4)
Table 3. Comparative cumulative and momentary species richness in Eyre Peninsula pan and pit gnammas. Pans Cumulative SR crustaceans insects Momentary SR No. of assessments Pits Cumulative SR crustaceans insects Momentary SR No. of assessments
Parla Rock 11 7 2 11.0 1 Pildappa West 28 8 19 9.7 7
Pildappa Rock 29 13 14 19.4 5 Pildappa East 22 4 18 7.4 7
Polda Rock 33 13 16 20.6 5 Poondarra
L. Wudinna Rock 32 11 18 16.6 5 Pygery
Peela Rock 18 9 7 18.0 1 Peela 1
Peela 2
Peela 3
Peela 4
Peela 5
Peela 6
21 7 13 9.8 5
13 5 7 7.2 5
28 7 18 10.1 7
21 5 17 9.8 6
23 4 18 10.6 5
26 5 17 9.8 5
20 5 14 8.2 5
19 5 11 8.2 5
not surprisingly, values for cumulative species richness were higher, ranging from 29–33 with a mean of 31.3 for the well-studied pans, and from 13 to 28 with a mean of 22.1 for the pits. The ratio of crustaceans to insects averaged 0.78 in the pans (Parla and Peela rocks excluded because only one sampling) but 0.39 in the pits (Table 3, appendix 1). Invertebrate community composition of pits and pans
The nmds for the 10 pans each on the five rocks and the 10 pit gnammas (Fig. 2) suggest a separation of pits from pans, with some further division between some pan groups. although all p statistics were 0.05%) for gnammas in the same size range and unaffected by unusual geomorphology (Timms, 2014). again crustaceans dominate by numbers, but insects are more speciose in pit gnammas. The ratio for each EP site averages at 0.39 crustaceans: insects and for the each Wa site, the average ratio is 0.36. among the crustaceans in both sets, the pea shrimp Lynceus magdaleanae often dominates, as do a range of ostracods, with ostracods more speciose in Wa (Timms, 2013b). Cladocerans are virtually absent in both sets, whereas molluscs occur only in the Wa set. insects, active dispersers in both sets, are all widespread species (Jocqué et al. 2010) and are generally represented by small and changing populations (< 10 individuals) in each gnamma. There was no significant difference in community composition in time and space among the ten EP pit gnammas (Fig. 3), and there was little difference from the 50 pits studied across the wide expanse of southern Wa (Timms, 2013b) (Fig. 5). 156
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Conclusions
The data presented in this paper supported hypotheses (i) and (ii). despite pan and pit gnammas occurring on similar granitic rocks and with most physicochemical features similar, there is a distinct difference in msr and community structure of aquatic invertebrates between pans and pits, both on EP and in Wa. This difference is explained in terms of the shorter and seasonal hydroperiods of pans and their generally shallower depth which provides a more specialised habitat than the longer-lasting, less seasonal hydroperiods of pits with their similarity to small ordinary pools (Pinder et al. 2000; Timms 2014). as a consequence, pan gnammas have a specialised fauna whereas pit gnammas have a largely eurytopic one (Jocqué et al. 2010; Timms 2014). Community composition of the pan gnammas of EP and Wa differ because of the large component of endemic ostracods, cladocerans and large branchiopods in Wa and the few endemic cladocerans in EP pans. The lower diversity of EP pan gnammas is probably explained largely by isolation for passive dispersers inflicted by the extensive arid nullarbor Plain. Pit gnammas on the EP share similar diversity, according to their size range, to the Wa pit gnammas, with the cumulative species list only a little longer in Wa for similar group numbers of pits. This similarity is influenced by widespread actively dispersing insect and the few dominant shared crustaceans such as Lynceus magdaleanae and, to a lesser extent, the ostracod Heterocypris sp. and the cladoceran Daphnia carinata, plus a general lack of other passively dispersing crustaceans.
Hypothesis (iii) was only partly valid for EP gnammas. it is best explained by reference to Fig. 5 and Table 6. Community structure in pit gnammas only marginally differs between EP and Wa gnammas. Their positions on the nmds are extremely close with dissimilarities the lowest of any pairing of Wa vs EP and pans vs pits. Both contain a largely eurytopic fauna. dissimilarities are understandably highest between Wa pits and EP pans and between Wa pans and EP pits. The difference between Wa pans and EP pans is high but the difference between pits and pans is less in EP than in Wa (Table 6). This is probably explained by the scarcity of pan endemics in EP compared to their abundance in Wa. species contributing to all these similarities and differences are almost exclusively crustaceans, not insects (Tables 4,5,6).
This study shows that each group of gnammas are unique; there are basic similarities imposed largely by nature of the habitat but biogeographical processes ensure differences in community structure. on a world scale, the gnammas (pans and pits) of southern Wa are by far the most diverse with 83 genera (and ca 220 species), followed by EP (49 genera), north america (46 genera), southern africa (24 genera), Europe (9 genera) and south america (6+ genera)(Jocqué et al. 2010; Timms 2014, this study). These figures suffer from lack of studies particularly in south america, but certainly show the special case of southern Wa with EP in its shadow. Given diversity and species differences between southern Wa and EP, the gnammas on EP need to be considered separately for protection and conservation. Presently many granitic inselbergs of EP with gnammas are partly protected in water reserves, but only one (Corrobinnae rock) is in the conservation estate. This does not protect them from natural sedimentation and most are partially filled with sediment. This situation has arisen because of neglect; in the past many pit gnammas were valuable water resources so they were well maintained by the aboriginal inhabitants of the area (Bayly, 1999). The effect of volume reduction on their fauna is unknown, but most of the examples studied here were all at least 35 cm deep and so retained water for many months, long enough for all inhabitants to complete their life cycles. Acknowledgements
i am grateful for field assistance rendered on various trips by Peri Coleman, nathan Gazsik, Wendy Timms and John Vosper, to various taxonomists listed in Timms (2012b, 2013b) and to Craig Williams (mosquitoes), Chris madden (chironomids), russ shiel (cylopoids, cladocerans), alexey Kotov and artim sinev (both for cladocerans) and stuart Halse and Koen martens (both for ostracods), and to Jason morton for help with statistics. i am grateful to ian Bayly and andrew Boulton for their helpful comments on earlier drafts. 157
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Appendix 1 – Species encountered in Eyre Peninsula gnammas 9.,: B6'&8/8($/8C#1(.D(1#,.1%"#$%&$' /$&%#$GH#%("56$61&6$(I%61:I /$&%#$GH#%("56$61&6$(I71##$I /$&%#$GH#%($#860.%#!"#$%&'()*+,*-%*#*% .&-/)#*,/*($1(,*0-/ 1*20#/*($*3/#*4* 5*$36403/7(-"* 8%6403/7(*3)*4* 9-6#*(-"* 93)*4*-6#*(/)/4*463/* !%:%3/'($(-"* !%",/+/*($(-"* ;-&3/'2/#*($(-"*
