The abundance and species richness of the spiders (Arachinda: Aranae) associated with open savanna, woodland and false grassland in the Polokwane Nature Reserve, Limpopo Province

Abstract

Lack of professional expertise on the taxonomy and systematics of invertebrates in the country leads to significant problems in understanding the invertebrate biodiversity and causes under appreciation of species. Little knowledge exists about the diversity of arachnids in certain eco regions. South Africa is obliged to develop a strategic plan for the conservation and sustainable use of biodiversity since meaningful conservation cannot take place if the involved species are not known. The South African National Survey of Arachnida (SANSA) was initiated to make an inventory of the arachnid fauna of South Africa. Various projects are underway to prepare inventories of the spider fauna of the different floral biomes and provinces. Surveys of arachnid fauna are more important in conserved areas as conservation strategies are already in place. As part of SANSA, a study was initiated in the Polokwane Nature Reserve during March 2005 until the end of February 2006 to determine the species richness and diversity of spiders associated with Acacia tortillis Open Savanna, Acacia rehmanniana Woodland and the Pietersburg Plateau False Grassland at the Polokwane Nature Reserve in the Limpopo Province. This is the first survey in the area and provides information to SANSA in terms of conserved areas in the Savanna Biome. During this study four sampling methods (sweep netting, tree beating, active searching and pitfall trapping) were used to collect spiders from all the different vegetation layers. They were identified to morpho-species and a checklist of spiders collected was compiled, adding data to the Limpopo Province check list. The composition of the spider community, seasonal abundance patterns, fluctuations and the influence of vegetation structure on spider populations were determined. A total of 7 776 spiders were caught belonging to 33 families, 131 genera and representing 219 species. Of these species, 153 (69.8 %) were wanderers and 66 (30.2 %) web-builders. The Oxyopidae was the most abundant family (1575), followed by Eresidae 1 554), Thomisidae (1 411), Araneidae (582), Lycosidae (568) and Salticidae (527). The Araneidae was the most species rich family (33) followed by the Thomisidae (31), Salticidae (25), Gnaphosidae (20), Theridiidae (18), Lycosidae (16) and the Philodromidae (12). The seven most abundant species collected during this study were Stegodyphus dumicola (Eresidae) (1 549) followed by two oxyopids, an undescribed oxyopid Oxyopes sp.3 (535) and Oxyopes russoi (476), Runcinia flavida (Thomisidae) (437), another oxyopid Oxyopes pallidecoloratus (384) followed by a lycosid Evippomma squamulatum (282) and another thomisid Monaeses austrinus (201). Immature specimens dominated captures, while females and males peaked during December till March with the most diverse species found in April. The highest specimen numbers and species richness was caught in the Woodland site (3 520; 173) followed by the Open Savanna (2 823; 159) and the Grassland site (1 433; 115). The Shannon-Weiner diversity index value calculated for species caught in the three different sampling sites was the highest for the Open Savanna site (H’=3.777), followed by the Woodland site (H’=3.569) and the Grassland site (H’=3.273). During this study the highest number of specimens were caught by sweep netting (2 972), followed by active searching (2 572), tree beating (1 895), pitfall trapping (323) and only a few (14) were caught by one attempt of leaf litter sifting. High evenness values for the Open Savanna (HE=0.748), the Woodland (HE=0.692) and the Grassland sites (HE=0.689) indicated a relatively even distribution of species across these sites. The Bray-Curtis similarities calculated for the methods used to catch species occurring on the three sites was 48 % similar for species caught by sweep netting and tree beating. Species caught by active searching and pitfall traps were 44 % similar. Additionally the species caught by leaf litter sifting were only 8 % similar to that caught using the other four methods. Vegetation structure and the time of year influenced the spider community at the Polokwane Nature Reserve. Temporal changes influenced the species richness, diversity as well as the number of mature, identifiable specimens. No work have previously been undertaken in this area and the species collected represent new distribution records for all species collected.The present study made an important contribution towards increasing our knowledge of spider diversity in the Savanna Biome.