Web spider abundance is affected by sheep farming in the Karoo

Abstract

Impacts of livestock farming include effects on arthropods and a range of related ecological processes. Grazing by sheep may indirectly affect web spider populations by changing vegetation structures available for web construction, or directly by trampling the spiders or their webs. We tested these two potential impacts by surveying spiders along transects and found that web spider abundance was 52% lower in a sheep grazing area compared with inside the adjacent Tierberg-LTER (Long Term Ecological Research) sheep exclosure. This reduction in web-spider abundance could be due to the 10% lower shrub cover in the sheep pasture than in the exclosure. Further support for the hypothesis that web spider abundance is affected by shrub cover came from spider abundance being higher on densely vegetated mounds, termed heuweltjies, than in the surrounding shrub matrix in both the exclosure and the sheep pasture, with heuweltjies showing cross-fence differences, consistent with grazing impacts. In addition, there may be a direct effect of trampling, as suggested by experimental removal of webs. By depressing spider populations, sheep grazing may affect a number of ecological factors, such as trophic relationships or nutrient cycling.

Keywords: grazing exclosure, Nemoscolus, shrub cover, trampling