Research Article |
Corresponding author: Richard Mazebedi ( mazebedir@biust.ac.bw ) Academic editor: Oana Teodora Moldovan
© 2020 Richard Mazebedi, Thomas Hesselberg.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Mazebedi R, Hesselberg T (2020) A preliminary survey of the abundance, diversity and distribution of terrestrial macroinvertebrates of Gcwihaba cave, northwest Botswana. Subterranean Biology 35: 49-63. https://doi.org/10.3897/subtbiol.35.51445
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Inventories of cave species and in-depth understanding of cave ecosystems are essential for informing conservation approaches for the unique and vulnerable cave fauna. Gcwihaba cave is the largest cave in Botswana but its ecology is poorly understood. This study set out to provide the first quantitative survey of the cave’s terrestrial macroinvertebrates. Macroinvertebrates were collected from sample sites at 10 m intervals into the cave from the cave entrance. At each site, macroinvertebrates on the cave floor were collected by quadrat sampling while macroinvertebrate from cave walls were collected by visual opportunistic searches. Moisture content, pH and electrical conductivity of the cave floor substrate were measured at each site to examine the influence of the floor properties on the distribution of macroinvertebrates on the cave floor. Twelve species in 10 families and 8 orders of terrestrial macroinvertebrates were collected. The occurrence of taxa varied across the sites, with most taxa occurring in the light and twilight sectors of the cave (within 30 m), whereas the dark sector (beyond 30 m) was dominated by cave cockroaches (Gyna sp.). The abundance of the cave cockroaches, darkling beetles (Tenebrionidae, Tenebrio sp.) and cave wasps (Sphecidae) positively correlated with floor substrate of high moisture content and high electrical conductivity, which became increasingly common with distance into the cave. The abundance of other taxa from the cave floor positively correlated with a floor substrate of high pH and low moisture, which was common near the cave entrance.
Bat guano cave, beetles, crickets, cockroaches, environmental variables, quadrat sampling, spiders, visual survey
Ecological studies of cave ecosystems are essential for understanding, conserving and managing subterranean ecosystems (
Macroinvertebrates are an important component of the cave biota because of their relatively high diversity compared to the vertebrate biota, and because of their significant roles in cave ecosystem functions (
Gcwihaba cave, in the remote area of northern Botswana, is the largest in the country but ecologically poorly understood. Few studies at the caves include a non-comprehensive survey of its bat species (
The current study surveyed terrestrial macroinvertebrates of Gcwihaba cave and examined their distribution against distance into the cave to identify environmental drivers of their community structure and dynamics. The distance from cave entrance into the cave is associated with gradients of environmental variables such as light, temperature and humidity, which can potentially influence the distribution of macroinvertebrates within the cave (Lungi et al. 2015;
Gcwihaba cave (also known as Drotsky’s cave) is in the north west part (Ngamiland district) of Botswana, along the border with Namibia. It is located under one of the five low-lying dolomite hills in the Gcwihaba Valley, the Gcwihaba hills (20°01'30.2"S, 21°21'27.5"E and 20°1'26.4"S, 21°21'14.4"E) (Figure
The cave is mainly horizontal and has two entrances in the steep and rocky western slope of the Gcwihaba hills (Figure
Terrestrial macroinvertebrates were collected from the cave between 10am and 3pm, from the 6th to the 10th of July 2019 (4 days), by a team comprising two cave guides, the principal investigator and a research assistant. The samples were collected from the cave floor and walls at approximately 10 m distance intervals (sample sites) into the north east chamber, starting from the South entrance (Figure
To measure cave floor cave properties, namely pH, electrical conductivity (EC) and moisture content, triplicate 20g samples of cave floor substrate were collected into labelled plastic bags at each survey site. Each sample was collected from where the quadrat was placed for macroinvertebrate collections. All the substrate samples were kept in a cool dark container prior to analyses.The cave floor substrate was mainly soil at the cave entrance but at other sample sites, it was mixture of soil and bat guano with the proportion of bat guano generally increasing with distance into the cave. The cave floor properties were measured as described in
Abundance frequencies of each taxa at the sample sites were compared using Chi Square (χ2) tests. Simpson diversity index was computed for samples from each survey site and used to compare macroinvertebrate diversity across the sample sites. Relationships between the measured substrate variables were examined using the Pearson Correlation test. Canonical correspondence analysis (CCA) was used to examine the associations between the measured variables and macroinvertebrate taxa collected from the cave floor. CCA requires that species data should be unimodal and there was evidence from preliminary detrended correspondence analyses (DCA) that the macroinvertebrate data exhibited unimodal responses (length of first axis was 4.46;
A total of 12 macroinvertebrate taxa including 8 orders and 10 families were collected, amongst which 10 taxa were identified to genus level. The common macroinvertebrate taxa in Gcwihaba cave (those collected from four sample sites or more) were cave cockroaches (Gyna sp.), cave crickets (likely Spelaeiacris sp.), darkling beetles (Eurychora sp.) and violin spiders (Loxosceles sp.) (Figure
Common macroinvertebrate taxa found in Gcwihaba cave (photos by R. Mazebedi). A Cave cockroach (Gyna sp.) in family Blaberidae B cave cricket (likely Spelaeiacris sp.) in the family Rhaphidophoridae C darkling beetle (Eurychora sp.) in the family Tenebrionidae D violin spider (Loxosceles sp.) in the family Sicariidae.
All the cockroaches (Blaberidae) were collected from the cave floor. Other taxa mostly found on the cave floor include pseudoscorpions (Withiidae), darkling beetles (Tenebrionidae.), assassin bugs and sphechid wasps (likely Sphex sp.). Violin spiders (Sicariidae), thread-legged bugs (Emesinae), moth larvae (Actiidae), flat spiders (Selenopidae.) and cellar spiders (Pholcidae.) were found more on the cave wall than on the cave floor. The number of cave crickets (Rhaphidophoridae) and darkling beetles (Tenebrionidae) collected from the cave floor was similar to that collected from the cave wall (Table
Taxa collected from Gcwihaba cave, Botswana. For each taxon, the overall number of individuals collected (Total count), the number of individuals collected from the cave floor (Floor count) and the number of individuals collected from cave walls (Wall count), are shown.
Order | Family/Subfamily (common name) | Genus/Species | Total count | Floor count | Wall count |
---|---|---|---|---|---|
Blattodea | Blaberidae (Cave cockroaches) | Gyna sp. | 444 | 444 | 0 |
Orthoptera | Rhaphidophoridae (Cave crickets) | Spelaeiacris sp. | 55 | 25 | 30 |
Coleoptera | Tenebrionidae (Darkling beetles) | Eurychora sp. | 34 | 14 | 20 |
Araneae | Sicariidae (Violin spiders) | Loxosceles sp. | 33 | 4 | 21 |
Hemiptera | Reduviidae/Emesinae (thread legged bugs) | 14 | 2 | 12 | |
Coleoptera | Tenebrionidae (Darkling beetles) | Tenebrio sp. | 14 | 11 | 3 |
Pseudoscorpiones | Withiidae (Pseudoscorpion) | 11 | 11 | 0 | |
Hemiptera | Reduviidae (Assassin bugs) | 6 | 5 | 1 | |
Lepidoptera | Arctiinae | 5 | 0 | 5 | |
Hymenoptera | Sphecidae (wasps) | Likely Sphex sp. | 4 | 4 | 0 |
Araneae | Pholcidae (Cellar Spiders) | Smeringopus sp. | 4 | 1 | 3 |
Araneae | Selenopidae (Flat spiders) | Selenops sp. | 3 | 0 | 3 |
Sample sites differed significantly in terms of the overall number of macroinvertebrate specimens collected (χ2 (5), = 638.4, p < 0.001), the overall number of specimens collected from the cave floor and walls generally increased with distance into the cave, although the site at 40 m had a lower number of specimens than the site at 30 m and the site a 50 m (Figure
The four most abundant macroinvertebrate taxa; cave cockroaches, cave crickets, darkling beetles and violin spiders were not evenly distributed along the 50m transect. The cave cockroach abundance increased with distance into the cave (χ2 (5), = 72.7, p < 0.001) (Figure. 5). The number of cave crickets, cave beetles and violin spiders collected also varied across sample sites along the 50m transect into the cave, (χ2 (5), = 46.9, p < 0.001, χ2 (5), = 46, p < 0.001, and χ2 (5), = 15.5, p < 0.001 respectively). The relative abundance of macroinvertebrate taxa was greater near the cave entrance but decreased with distance into the cave. The proportion of the three taxa decreased strongly after the 20m distance into the cave, with the relative abundance of cave crickets and violin spiders reducing from over 20% each to less than 10%. Among the four most abundant taxa, darkling beetles were the most collected at the cave entrance (37%) but were not collected at sample sites past 30 m into the cave (Figure
Among the measured variables in the cave, pH ranged between 4.9 and 7.0, with a mean of 6.2. Electrical conductivity (EC) of the cave floor substrate ranged from 1179 to 12180 μS/cm with a mean of 6075 μS/cm. Moisture content of the cave floor substrate ranged from 15% to 35% with a mean of 12%. Moisture content of the floor substrate increased with distance into the cave (Figure
The total variation explained by the measured variables was 46% of which 36.8% was explained by canonical axis 1 (CCA) (Table
Proportion of variation in the distribution of cave floor macroinvertebrate explained by measured variables and the loadings of the variables to CCA axes 1–4.
Total | CCA1 | CCA2 | CCA3 | CCA4 | |
---|---|---|---|---|---|
Variance explained | 46% | 36.8% | 5.3% | 3.9% | 0.06% |
pH loading | -0.38 | -0.44 | -0.60 | 0.55 | |
EC (μS/cm) loading | 0.56 | -0.065 | 0.50 | 0.66 | |
Moisture (% in 10g) | 0.63 | -0.51 | 0.43 | 0.38 | |
Distance (m) | 0.83 | 0.36 | -0.17 | -0.39 |
Based on CCA analysis, the abundance of cave cockroaches (Blaberidae), darkling beetles (Tenebrionidae, Tenebrio sp.) and cave wasps (Sphecidae) was positively associated with axis 1, with cave cockroaches showing the strongest association with the axis. The adult cave cockroaches were strongly associated with floor substrate with high moisture levels whereas the abundance of the cockroach nymphs showed strong association with distance into the cave and EC levels of the floor substrate. (Figure
A CCA biplot of the relationships between cave macroinvertebrate taxa (triangles), sampling points (filled circles) and the measured environmental (variables lines with arrows). Environmental variables are presented as vectors, with arrow heads indicating their direction of increase. A positive correlation between the environmental variables and macroinvertebrate taxa and sampling points is indicated by their proximity to the arrowhead. Taxa and sampling points with a negative correlation with the variable axis plot on the opposite side of the arrowhead relative to the centroid, with the correlation strength increasing with distance of the symbols from the centroid.
The macroinvertebrate fauna that was collected from Gcwihaba cave during the current study include taxa which were previously collected from other caves in northwest Botswana. Macroinvertebrate groups such as cave cockroaches (Blaberidae) and assassin bugs (Reduviidae) were also collected by
Despite the collected samples indicating greater diversity of terrestrial macroinvertebrates for the cave compared to other caves in Botswana’s north west region, the macroinvertebrate diversity observed in our study is likely an underestimate of the total macroinvertebrate diversity that exist Gcwihaba cave. There is evidence that some macroinvertebrate species that occur in the cave were missed, for example a cave beetle (Ptinus peringueyi) previously reported by
In general, the collected macroinvertebrates taxa are those which can be expected from a cave in southern Africa. According to
Generally, cave systems have low macroinvertebrate diversity because of the limited variety of food sources at the base of their food webs (
The diversity of macroinvertebrates collected from the cave floor decreased with distance into the cave likely responding to environmental variables associated with proximity to the cave entrance. The measured abiotic variables, including pH, EC and moisture content of the cave floor, showed distinct trends that were associated with distance from the cave entrance (Figure
Cave macroinvertebrate communities are generally food limited; their diversity can therefore be expected to be greater near the cave entrance where food resources are likely more diverse (
This study is the first quantitative survey of Gcwihaba cave macroinvertebrates and has identified 12 species in 10 families and 8 orders of terrestrial macroinvertebrates. The macroinvertebrate diversity of the cave was found to decrease with distance into the cave with the deeper sites being dominated by cave cockroaches. Properties of the cave floor soil showed trends corresponding with distance into the cave; pH progressively decreased with distance into the cave whereas soil moisture content and electrical conductivity progressively increased. The environmental gradients may have played a role in the observed patterns of macroinvertebrate distributions in the cave. While the number of collected macroinvertebrates is comparable to that collected in other southern African caves, the number of macroinvertebrate taxa collected may be an underestimate of Gcwihaba macroinvertebrate diversity since they were collected from the cave’s north east chamber only. The preliminary survey is, however, an important contribution to our knowledge about cave biodiversity in arid regions, as it provides a baseline on which to build a future programme. Surveying further into the cave and conducting a more comprehensive macroinvertebrate survey is therefore recommended especially examining seasonal variability of the invertebrate fauna as the strong seasonality of the surrounding desert landscape is likely to influence seasonality in cave biodiversity.
We are thankful to our colleagues K Mogwera (Okavango Research Institute) and D Nkwe (BIUST) who helped in the fieldwork of this research. We are also grateful to the specialists who helped in the identification some of the organisms collected for this study, although from photographs; Dr Jan Andries Neethling (South Africa national museum) assisted in identification of Araneae, Dr. Mark Harvey (Western Australian Museum) assisted in identification of Pseudoscorpiones and Prof. Kippling Will (Essig Museum of Entomology, University of California) assisted in identification of Coleoptera. We are also grateful to Enrico Lungi and one anonymous reviewer for their very useful comments that greatly improved the quality of this paper This work is a result of a postgraduate certificate programme at University of Oxford and was co-funded by the Oxford PGCert-African bursary and the government of Botswana.