Surrogates of spider diversity, leveraging the conservation of a poorly known group in the Savanna Biome of South Africa (Arachnida : Araneae)

Abstract

The inclusion of spiders in conservation planning initiatives is confounded by several factors. Surrogates could facilitate their incorporation. In this paper we investigate the performance of a number of surrogate measures, such as higher taxa (genus, family), cross-taxon surrogates that are subsets of the spider assemblages (certain spider families) or non-overlapping groups (woody vegetation and birds), and the use of morphospecies. Birds and woody vegetation were included because they often form the focus of conservation planning initiatives. We assessed the surrogate measures based on their predictive power for species richness and extent to which conservation planning that maximizes representation of the surrogate is effective in representing spider diversity. A measure for the latter is the Species Accumulation Index (SAI). Generic richness as a higher taxon surrogate and the combined richness of the families Thomisidae and Salticidae were the best estimators of total species richness. Based on the surrogacy efficiency criterion, genera and the family Salticidae had species accumulation indices (SAIs) that were significantly larger than 95% confidence intervals of a random curve, while woody vegetation and birds turned out to be poor surrogates for spider diversity. The use of morphospecies as estimators is cautiously supported (adjusted R2 = 0.85, for species richness, SAI = 0.73). The surrogates identified here provide a viable alternative to whole assemblage analysis but should be used with caution. The use of genera is confounded by unstable taxonomy and the difficulty of identifying specimens up to genus level. Geographic location and varying sampling effort between surveys did not have an effect on the surrogate performance of the two spider families, viz. Salticidae and Thomisidae. The former family has seen a flood of recent systematic work, whereas the latter’s taxonomy is fairly well developed. These two families comprise ca. 20% of spider species observed in the Savanna Biome of South Africa and could provide a viable handle on spider diversity in this region.