Towards a standardized and optimized protocol for rapid biodiversity assessments : spider species richness and assemblage composition in two savanna vegetation types

Abstract

A semi-quantitative inventory of spider diversity was done in the Blouberg Nature Reserve (BNR) and Western Soutpansberg Conservancy (WSC) situated in the Savanna Biome of the Limpopo Province, South Africa. Two hundred and ninety-six samples of one person-hour work each, comprised of five methods (vegetation beating, sweep netting, aerial hand collecting, ground hand collecting and leaf litter sifting) were divided between four relatively homogenous sites (plant communities) within a vegetation type of the BNR and WSC, respectively, an average of 37 per site. In addition, 25 pitfall traps were left open for a total of 20 days in each of the eight plant communities, 200 in total. We collected 1328 adult spiders representing 186 species of which 31% were singletons in the BNR vegetation type and 909 spiders in 222 species of which 41% were singletons in the WSC vegetation type. The number of species present was estimated using six estimators. The estimates varied between 233 and 307 for the BNR and 302 and 386 for the WSC. Inventory completeness was more than 70%. However, the fit to a lognormal distribution suggests that there are 370 species (750 000 individuals) and 445 (850 000 individuals) species in the universes (16 ha) sampled within the two vegetation types. Collector experience had no effect on the results of the inventory, whereas time of day had a very small yet significant effect. Seasonality only affected abundance and richness, but not assemblage composition. Sampling methods used had the biggest effect on our results. These results are used to design an optimized sampling protocol for standardized inventories in the Savanna Biome.