Background

The Thar desert represents one of the extremely fragile ecosystems in the Indian subcontinent. The biodiversity of the Thar is unique and adapted to its extreme climatic conditions. But the ecological scenario of the Thar desert is altering due to threats like climate change, invasion of exotic species, urbanization, unscientific sand dune stabilization methods, ecological consequences of the irrigation project, Indira Gandhi Nahar Pariyojana (IGNP)—like water logging, saline water intrusion, extinction of xeric biodiversity, etc. (Tembhurne et al., 2020). The fact that ecological studies in a changing ecosystem can forecast the reverberations in future, makes this study important for predicting the future of this desert ecosystem and planning conservation measures.

Spiders are appropriate for ecological studies as they are widely distributed across the world and inhabits vast range of niches (Marc, Canard and Ysnel, 1999). They are also the most profuse generalist predators which helps in pest suppression (Birkhoferet al., 2013; Michalko et al., 2018). Web weaving spiders can even check the pest population which they do not consume, using their sticky webs (Alderweireldt, 1994). Though the spiders reduce pollination success in plants, it can also reduce the seed damage caused by insects (Louda, 1982). They can also control insect vector borne diseases like Chikungunya, Filariasis, Dengue, Yellow Fever, Zika, Malaria, Japanese encephalitis, Tungiasis, Leishmaniasis and Trypanosomiasis (Ndava et al., 2018) by feeding those vectors. Another ecological benefit from spiders is that they indirectly assist in ecosystem services like litter decomposition and nutrient cycling (Lawrence & Wise, 2000). Published research works analysing the ecological aspects of spider assemblages in the Thar desert has been sparse till date. This study attempts to compare the dominance structure and constancy of occurrence of spiders in three different habitats of the Thar desert.

Methods

Study area

Major portions of the Thar desert lie extended in the states of Rajasthan, Punjab, Gujarat and Haryana of India and partially in Pakistan. It covers around twelve percentage of geographical area of India. Western part of the state of Rajasthan constitutes 61 percentage of the entire Thar desert. Hence, this study focus on desert area of Jodhpur district which lies in western part of Rajasthan. This region, like any other desert ecosystems, is known for extreme climatic conditions. Sometimes temperature during summer may raise up to 50 °C and may drop down to − 10 °C during winter (Sharma et al., 2021). Droughts are frequent in this region due to high rainfall variability (Rao, 2009). The average annual precipitation of the Thar desert has been about 290 mm during 1901 to 2019. During winter wind direction is towards northeast and speed is 3-4 km/hr. During summer and rainy season wind blows in southwest direction. Wind speed is relatively higher in summer (8–20 km/hr). Wind speed reaches 60–80 km/hr during intense dust storms (Sharma et al., 2021). Three different types of habitat (Sand dunes, Riparian habitat and Rocky desert) in the Thar desert were studied in the present work from March 2017 to February 2019 (Figs. 1, 2, 3, 4, 5).

  1. (a)

    Sand dunes

Fig. 1
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Map of study area

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Fig. 2
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Cheiracanthium melanostomum (Thorell, 1895)

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Fig. 3
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Indoxysticus minutus (Tikader, 1960)

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Thomisus lobosus Tikader, 1965

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Fig. 5
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Neoscona theisi (Walckenaer, 1841)

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Sand dunes with height varying from 9 to 30 m surrounding Osian village of Jodhpur (lat. 26°72′ 68″ N latitude and long.72° 89′ 85″ E) was one of the study area. Vegetation found here includes Acacia senegal, A. jacquemontii, Aerva javanica, Calligonum polygonoides, Capparis decidua, Cenchrus biflorus, Crotalaria burhia, Prosopis juliflora Tephrosia petrosa and T. purpurea.

  1. (b)

    Riparian habitat

Riparian habitat on the banks of Luni river, the only natural river that flows through the Thar desert of western Rajasthan was selected as second study site (lat. 26° 21′ 06″ N and long. 73° 06′ 39″ E). Natural vegetation present here includes Acacia senegal, Azadirachta indica, Calotropis procera, Prosopis cineraria, Prosopis juliflora and Salvadora oleoides.

  1. (c)

    Rocky desert

Igneous rock formations surrounding the Kailana lake of Jodhpur, Rajasthan was the third study area (lat. 26° 28′ 91″ N and long. 72° 97′ 48″ E). Vegetation prevalent in this area includes Euphorbia caducifolia, Anogeissus pendula Barleria sp., Tephrosia purpurea, Prosopis juliflora, Cleome viscosa, Indigofera cordifolia and Grewia tenax.

Methods of collection

Spiders were collected from the study areas using the following methods. Some collection methods using sweep net and pitfall traps were avoided due to limitations of the landscape like thorny vegetation and sand storms.

  1. (a)

    Hand collection

Collection of specimens from ground level to knee height and knee level to as high as collector can reach was done using this method. Specimens were gathered using aspirator or forceps or brush (Sørensen, et al., 2002).

  1. (b)

    Beating

Vegetation present in the study areas was beaten using a wooden rod while keeping a collection tray below (Tikader, 1987).

  1. (c)

    Litter sampling

Litter procured from the study area were collected in a plastic bag, sealed and carried to the lab. Spiders were sorted out from the litter by placing it on white canvas sheet or paper (Coddington et al., 1996).

Preservation and identification

Specimens collected were placed directly in 70% Ethyl Alcohol. Then it was properly labelled and sorted. Leica—M205C Stereozoom microscope was used to identify the specimens. The literature referred for identification process include works of Tikader and Malhotra (1980), Tikader (1982), Barrion and Litsinger (1995), Jocqué and Dippenaar –Schoeman (2006), Murphy (2007), Sebastian and Peter (2009), Prószyński (2017), Metzner (2020) and World Spider Catalog (2022) (Figs. 6, 7, 8, 9, 10).

Fig. 6
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Thyene imperialis (Rossi, 1846)

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Fig. 7
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Langona albolinea Caleb & Mathai, 2015

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Fig. 8
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Plexippus paykulli (Audouin, 1826)

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Fig. 9
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Oecobius putus O. Pickard-Cambridge, 1876

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Fig. 10
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Cyrba ocellata (Kroneberg, 1875)

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Data analysis

Class of dominance and constancy of occurrence of spiders were found using the methods described by Górny and Grüm (1993) and Silveira Neto et al. (1976).

  • (a) Class of dominance (D)

Class of dominance of spiders in different habitats were determined by calculating the percentage of specimens of a given species in total number of specimens collected from the habitat studied. The species with D > 10% were considered as eudominants (D5), with 5.1% < D < 10% as dominants (D4), with 2.1% < D < 5% as subdominants (D3), with 1.1% < D < 2% as recedents (D2) and D < 1% as subrecedents (D1).

  • (b)Constancy of occurrence

To identify the resident species in this study, constancy of occurrence was measured using the formula given by Silveira-Neto et al. (1976) and based on that they were grouped into three classes (Table 1).

Table 1 Criteria for classification of constancy
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$$C=\frac{P\times 100}{n}$$

P = Number of samples in which the given species were present and n = Total number of sampling performed.

Results

  • (a)Dominance structure of spiders in the Thar desert

Total 127 species of spiders under 61 genera and 17 families were recorded from the study area. Number of species collected from Sand dunes, Riparian area and Rocky desert were 76, 54 and 67, respectively. Nineteen species were common to all the three habitats. They are Neoscona mukerjei, Oxyopes gujaratensis, Neoscona odites, Afraflacilla sp.1, Menemerus bivittatus, Thyene imperialis, Neoscona theisi, Clubiona sp.1, Philodromus sp.1, Peucetia viridana, Langona albolinea, Indoxysticus minutus, Oxyopes chittrae, Thomisus onustus, Oxyopes javanus, Tmarus sp.2, Rudakius ludhianaensis, Cheiracanthium melanostomum, Neoscona nautica. The most abundant spider family in sand dune and rocky desert was Thomisidae with 139 and 302 individuals, respectively. In riparian area, Oecobiidae was the family with more number of individuals (151). Eudominant and Dominant species in Sand dunes constituted 5.26% of total species collected from there. 9.25% of total species collected were eudominants or dominants in the Riparian area. Only 2.98% of the total species collected constituted this category in rocky desert area. The dominance structure of spiders present in all the three habitats are given in (Table 2,3 and 4).

  • (b)Constancy of spiders in different habitats of the Thar desert

Table 2 Dominance structure of spiders present in Sand dunes
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Table 3 Dominance structure of spiders present in Riparian habitat
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Table 4 Dominance structure of spiders present in Rocky desert
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There were a total of 1725 individuals belonging to 127 species in the spider collection made from Sand dunes, Riparian and Rocky habitats of the Thar desert. The number of constant species was three in all the habitats studied. Mogrus rajasthanensis showed constancy in both sand dune and rocky habitat. Peucetia viridana and Tmarus sp.1 were the remaining constant species in sand dunes. Besides Mogrus rajasthanensis, Oxyopes chittrae and Tmarus kotigeharus were the constant species in rocky area. In riparian habitat Oecobius putus, Menemerus bivittatus and Menemerus brachygnathus constituted the constant species. Highest number of accessory species was reported from Sand dunes (11 species). The number of accessory species in riparian and rocky habitats was 7 and 4, respectively. Accidental species number were also higher in Sand dunes (62 species). It was followed by rocky (60 species) and riparian (44 species) habitats. At sand dune habitat, 14.47% of the species were accessory, 81.57% were accidental and 3.94% were constant. In Riparian area, 12.9% of the total species were classified as accessory, 81.48% as accidental and 5.55% as constant. Accessory species were 5.9% in Rocky desert. Besides that, accidental species were 89.55% and remaining 4.47% were constant.

Discussion

Analysis of data showed that 19 species were common to all the three habitats. However, each habitat had its own unique species more than common species (Sand dunes—29 species, Rocky desert—28 species and Riparian habitat—19 species). This may be due to the uniqueness of vegetation of each habitat. Schaffers et al. (2008) explored the arthropod assemblages including epigeic spiders at 47 sites in Netherland with different plant species composition and demonstrated a link between them. Plant species composition can affect the insect diversity and thereby indirectly affect the insect feeding spider community structure (Beals, 2006). Dennis et al. (2001) observed increased epigeal spider species composition in ungrazed grass land compared to grazed ones and related this to the presence of more plant litter below leaf stratum in ungrazed grass lands. Besides that, difference in environmental parameters like temperature and relative humidity might also have influenced the dominance structure. Range of temperature and relative humidity during the study was 18.8 to 40 °C and 12 to 54% for Sand dunes, 16.4 to 36.7 °C and 17 to 70% for Rocky desert and 18.6 to 36.5 °C and 27 to 77% for Riparian habitat. Jones (1941) studied the effect of temperature and humidity on the life history of spider Agelena naevia and concluded that mortality increases in low humidity—high temperature conditions but decreases slightly in high humidity condition with increase of temperature. It was also observed that relative humidity over 50% allowed growth of the spider. Almquist (1970) studied tolerance range for temperature in different dune dwelling spider species of Sweden in laboratory conditions and established that spiders collected during winter showed greater tolerance to low temperature. During winter, the spiders which are juveniles during the previous summer season increased their tolerance more than adults.

Results show that in all the habitats studied the number of accessory and accidental species was higher than constant species. This indicates that these habitats does not favour stable population of spiders in it. Adverse environmental factors like high temperature, low humidity, etc., may have forced the spiders to migrate frequently. In a study on the influence of diversity of flora on coleoptera population, Tahvanainen and Root (1972) mentioned that the presence of high number of accessory and accidental species indicates low complexity of environment which offers only few resources, thereby hampering the establishment of those species.

Due to the obstacles like unfavourable climate and vegetation type the collection methods, sweep netting and pitfall trapping were not done in this study. The prevalent thorny vegetation in the study site prevented the use of sweep net as it gets torned by the thorns. Also the frequent wind in the study site blows sand to the pitfall traps.

While estimating spider species richness in southern Appalachian cove hardwood forest, Coddington et al. (1996) found aerial and ground collecting yielded more productive result than beating and litter sampling. Although they also reported that number of species collected were unique to each method. Conducting nocturnal and diurnal collection by utilizing a combination of sampling methods in future may yield more acceptable result. There is also a need to conduct studies comparing the effect of different collection methods and time of collection on the spider catches. The effects of plant species composition and environmental parameters during different seasons on spider community structure of each habitat also have to be explored in detail.

Conclusion

Through the study of dominance structure of the spiders of the Thar desert, it is apparent that there was only one eudominant species in both sand dunes (Tmarus sp. 1) and rocky desert (Tmarus kotigeharus). Riparian habitat consisted of two eudominant species (Oecobius putus and Menemerus bivittatus). Analysis of constancy of spiders revealed three constant species in all the three habitats. The constant species in sand dunes were Mogrus rajasthanensis, Peucetia viridana and Tmarus sp.1. The most constant species in riparian habitat was Menemerus bivittatus followed by Oecobius putus and Menemerus brachygnathus. In descending order of constancy, the most constant spiders of the Rocky desert can be arranged as follows: Tmarus kotigeharus, Mogrus rajasthanensis and Oxyopes chittrae. The number of accessory and accidental spider species in all habitats was far higher than constants. This is due to the unstability of spider population. So, this study highlights the necessity of conservation of these habitats as they have unstable population of spiders.