Dominance structure and constancy of spiders in the Indian Thar desert

Background The knowledge about the species of a habitat (both resident and transient/dominant and rare) is a vital step to plan the conservation measures. Being generalist predators, spiders help controlling the population of their prey and maintain ecosystem stability. This makes spiders excellent bio-indicators for assessing the impact of anthropogenic disturbance factors on natural ecosystems (De, Siliwal, Uniyal and Hussain in Trop. Ecol. 63: 1–7, 2021). The aim of this study was to assess the dominance structure and constancy of spiders in three different habitats (Sand dunes, Riparian and Rocky) of the Thar desert. Study was conducted from March 2017 to February 2019 covering all seasons. Results The eudominant species in sand dunes and rocky desert belonged to the family Thomisidae. These species were Tmarus sp. 1 and Tmarus kotigeharus Tikader, 1963, respectively. There were two eudominants in Riparian habitat ( Oecobius putus O. Pickard-Cambridge , 1876 and Menemerus bivittatus (Dufour, 1831)). Through the analysis of constancy of all the species in the Thar desert, it was revealed that three species were constant in all the three habitats. The number of accessory and accidental species was far higher than constant species in all the habitats. Conclusions The number of accessory and accidental spider species in all habitats was far higher than constants due to the unstability of spider population. Therefore, this study highlights the necessity for conservation of these habitats of the Thar desert.


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 Page 2 of 12 Kashmeera and Sudhikumar The Journal of Basic and Applied Zoology (2024) 85:14 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.

Study area
Major portions of the 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).

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.

(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).

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

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 (D 5 ), with 5.1% < D < 10% as dominants (D 4 ), with 2.1% < D < 5% as subdominants (D 3), with 1.1% < D < 2% as recedents (D 2 ) 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).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.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.Plexippus paykulli (Audouin, 1826) (Fig. 8) SD

Sparassidae
Olios gravelyi Sethi & Tikader, 1988 SR Tetragnathidae Guizygiella indica (Tikader & Bal, 1980) R Guizygiella melanocrania (Thorell, 1887) R Indoxysticus minutus (Tikader, 1960) (Fig. 3) SR 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 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.

Fig
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, 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).

Table 1
Criteria for classification of constancy

Table 2
Dominance structure of spiders present in Sand dunes Dennis et al. (2001)ition 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 Table 2 (continued)

Table 3
Dominance structure of spiders present in Riparian habitat

Table 4
Dominance structure of spiders present in Rocky desert