Studying earthworms (Annelida: Oligochaeta) in South Africa

Abstract

Earthworms are an important component of southern African invertebrate diversity, due both to their influential roles in soil ecosystems, and the relatively large number of species. As of 2010, there were 282 indigenous earthworm species (most endemic) known to South Africa belonging to three families: Microchaetidae, Tritogeniidae and Acanthodrilidae. In addition, 44 introduced species from six families had been recorded. However, earthworms are rarely included in environmental monitoring or conservation programmes—partly because sampling and species identification are difficult and many sampling methods are destructive and/or toxic. In this paper we review the earthworm sampling techniques most commonly used by screening data from a digitised literature collection on South African earthworms and on-line global searches. By examining a case study sampling of three vegetation types, this paper highlights taxonomic challenges and the effort required to properly curate specimens. The study provides recommendations for future sampling and highlights some key priorities for future work on the group. From the literature review in early 2012, it is clear that collection techniques are often insufficiently recorded in published work. A total of 10 938 publications from the period 1950 to 2012 were found from the literature search and digitised collection and from these only 32 papers recorded the sampling methodology (mainly hand sorting) for South African research, pointing to the need to adopt standard sampling and reporting protocols. We also tested two of the most popular methodologies in the field. Sampling was conducted in January and February 2012 at four sites, with 24 plots at each site (12 digging and 12 using mustard extraction). A total of 2 094 earthworms collected could be assigned a species name, with introduced species predominating at both disturbed and natural sites. It took a team of three to five people digging and hand collecting all earthworm specimens encountered in a plot of 50 cm × 50 cm × 20 cm deep around 45 to 60 minutes. However, much more time was spent curating and identifying samples. While we recommend following the ISO (ISO11268-3, ISO23611-1) protocol for collecting introduced taxa, to get a complete inventory of South African earthworms a range of sampling techniques will be required; in particular, a large 1 m × 1 m × 20 cm plot is required for many large bodied native taxa, and the collection of giant earthworms will require different approaches. The identification of specimens requires skills that are scarce in the country and so there is an urgent need for training and funding for fundamental work on earthworm taxonomy. An atlasing project could serve as a focal point for future research. In providing some general recommendations based on the long and fruitful history of research on earthworms in South Africa, we are optimistic that a better understanding of the group will help us to both improve our usage of natural resources and provide insights into this vitally important edaphic group.