Кавказский энтомологический бюллетень (Dec 2017)
Comparison of trophic spectra and hunting strategies of some large arachnids (Arachnida: Scorpiones, Solifugae, Aranei) in semi-desert biocenoses of Gobustan (Eastern Azerbaijan)
Abstract
Prey composition has been studied for the three syntopic species of arachnids. These were the mottled scorpion Mesobuthus eupeus (C.L. Koch, 1839), the common solifuge (camel-spider) Galeodes araneoides (Pallas, 1772), and the tarantula (wolf-spider) Lycosa praegrandis (C.L. Koch, 1836). The investigations were carried out in arid biocenoses of the interfluve of Pirsagat Jeyrankechmez (Gobustan, Eastern Azerbaijan). These species were chosen for study because of their high importance in communities, a wide range of methods of obtaining food, relatively high abundancy, co-habitation in this area, mainly the night activity and their quality as generalized predators. All this made them convenient model objects for studying the mechanisms of ecological segregation. Despite the fact that a significant number of publications have been devoted to these species in which various aspects of their biology and ecology have been examined, there is no holistic view of the structure of ecological niches of these arachnids in the literature. So far, some specific aspects of their biology which determine peculiar role of each species in nature have not been sufficiently studied. Ecological difference among these species discovered by previous studies does not allow us to fully determine the conditions for their cohabitation and to establish the presence or absence of any significant competition. Collection of data and visual observations were carried out from May to September in 2012, 2013 and 2016.The main part of the material was collected in the summer months in biotopes of three types: semi-desert loamy plain, stony slopes of the plateau, hilly valley with ridged clusters of residual rock. The composition and quantity of prey was estimated from the remains found in permanent shelters of the respective predators, as well as during direct observations of predators with a prey in chelicera. The remains of consumed organisms from the depth of predator’s burrows were extracted manually or by an aspirator (exhauster). Arachnids were weighed on electronic scales with precision to within 0.01 g before and after night activity. For this purpose, they were extracted from the shelters, placed in the plastic containers, weighed and then returned back. The next day they were caught again and re-weighed. In total, about 380 weightings of 126 individuals of arachnids were produced. Data on the mass of 90 specimens of arachnids (35 scorpions, 25 solifuges and 30 tarantulas) were the most representative. We examined 365 permanent shelters of arachnids (235 scorpions, 86 solifuges, 44 tarantulas). 1754 chitin residues and fragments of prey were found suitable for identification. They were determined mainly up to detachment and family, less often up to the genus. In addition, in the same areas invertebrate fauna composition was investigated by combining three traditional methods such as: 1) application of the biocenometer; 2) installation of soil traps; 3) visual accounting. The variety of diet (niche breadth) was calculated for each species using the reciprocal of the Simpson’s polydominant index. Comparison of food composition of arachnids was carried out with the Czekanowski-Sørensen similarity index as less dependent on the sample size. The evaluation of electivity was carried out by calculating by Ivlev’s electivity index. It has been established that arthropods of four classes (Crustacea, Arachnida, Chilopoda, Insecta) make up the basis of the ration of the arachnids. The taxonomic composition of prey in food spectrum was mainly determined by which invertebrates are having a similar rhythm of seasonal activity. Mesobuthus eupeus had a relatively wider range of prey, but its biomass was significantly lower than for G. araneoides and L. praegrandis. The feeding spectrum in the pairs M. eupeus – G. araneoides and M. eupeus – L. praegrandis, respectively, overlapped to a lesser extent. Most overlap was observed in the pair G. araneoides – L. praegrandis. The breadth of the trophic niche of arachnids, established according to the Simpson’s polydominantity index, had the highest values of this parameter for the tarantula on stony areas and for the solifuge on rocky sites. Dependence of niche breadth from the landscape features are well expressed only for the solifuge (for all three biotopes). The diet of arachnids in areas with different landscape conditions was determined by the forage conditions of the biotope, as well as the morphological and behavioral characteristics of potential prey. Differences in the prey composition of the studied species, partial overlapping of their trophic spectra are not only caused by the state of the habitat, but also by the food selectivity of the respective arachnids.
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