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Towards optimized and standardized biodiversity assessments in the Savanna Biome

Species richness and biodiversity assessments are instrumental in the process of conservation planning. Despite the abundance of invertebrates, this group is often ignored in conservation planning.

One of the major reasons for excluding invertebrates is the lack of standardized sampling protocols that allow the comparability of the data, and optimized protocols that improve sampling efficiency.

In a recent paper in the journal African Zoology, C·I·B student, Mulaloiry Muelelwa, and co-authors Dr. Foord (UniVen), Prof. Dippenaar-Schoeman (UP) and Dr. Stam (UP) set out to estimate spider diversity in a standardized manner that is comparable with studies in other regions of the world. For their study area, the researchers used the Blouberg Nature Reserve (BNR) and Western Soutpansberg Conservancy (WSC), both situated in the Savanna Biome of the Limpopo Province, South Africa.

Spiders were collected according to a protocol which was set forth by Coddington et al. in 1991. The protocol is defined by one-person hour worth of effort in sampling, making use of a selection of sampling methods. For the purpose of this study, the researchers used vegetation beating; litter sifting; sweep netting; ground hand collecting; aerial hand collecting and pitfall trapping.

The rhino-horned baboon spiderThe trapdoor baboon spider

Two of the species recorded in the survey, (a) the rhino-horned baboon spider (Ceratogyrus brachycephalus) and (b) the trapdoor baboon spider (Pisenor notius) (Trapdoor spider). (Photos: S. Foord)

A total of 1328 adult spiders were collected representing 186 species in the Blouberg Nature Reserve vegetation type and 909 spiders representing 222 species in the Western Soutpansberg Conservancy vegetation type. The Salticidae or jumping spiders was the most diverse family, with 42 species, followed by Thomisidae or crab spiders (28), Gnaphosidae or flat-bellied ground spiders) (27) and Theridiidae or cob web spiders (26). The most diverse genus was Oxyopes Latreille, 1804 (Oxyopidae or lynx spiders), 12 spp.), followed by Asemesthes Simon, 1887 (Gnaphosidae, 8 spp.) and Thomisus Walkenaer, 1805 (Thomisidae, 7 spp.). Since there had been no prior sampling in these areas, all species were new records for the two areas and have been recorded in species lists of Blouberg Nature Reserve and Western Soutpansberg Conservancy.

“Based on the identifications of specialists, this two week biodiversity survey can possibly include ten new species of spiders from two new genera” says Dr. Stefan Foord, project leader of the study. “We also recorded a rare purseweb spider, the Atypidae Calommata transvaalensis and newly described species, a specimen of which was last collected in the Soutpansberg in 1908.”

The data collected were then used to determine the sampling completeness in these two areas, assess the effect of season, time of day, and collecting methods on the abundance, species richness and community composition of adult spiders caught, and to explore the implications of these results for an optimized and standardized sampling protocol for spider inventories in the Savanna Biome.

Inventory completeness, an estimate of what percentage of the species present in a 16 ha universe were actually sampled and based on the fit to a lognormal distribution, was 50% for the two sites. This suggests that after spending ca. 170 hours of sampling in each of the two areas respectively that we only caught 50% of the species present and there were 370 species (750 000 individuals) and 445 (850 000 individuals) species in the 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. Branch beating had the most species unique to this method, followed by ground hand collecting. Sweep netting yielded the lowest number of unique species. These results were used to design an optimized sampling protocol for standardized inventories in the Savanna Biome. As a rule, spider diversity (alpha diversity) is grossly underestimated. The results of conservation planning based on the species richness of these mega-diverse groups should be interpreted with caution. This study emphasises the importance of alpha diversity in generating spider diversity, future work should probably focus on the role and scale at which beta diversity becomes a significant contributor to spider diversity and the identification of the processes that underlies this turnover.

Link to article abstract: M.I. Muelelwa, S.H. Foord, A.S. Dippenaar-Schoeman & E.M. Stam (2010). Towards a standardized and optimized protocol for rapid biodiversity assessments: spider species richness and assemblage composition in two Savanna vegetation types. African Zoology 45: 273 - 290.

For further details, e-mail Stefan Foord at Stefan.Foord@univen.ac.za