Do composting earthworms pose a threat to South Africa’s unique earthworm fauna?
Concern over the potential for non-native earthworms to become invasive, and have a negative impact on ecosystems has gained considerable attention within the global community in recent years. Earthworms play a major role as ecosystem engineers in both natural and agricultural systems. They are also increasingly being used for soil improvement, composting, waste disposal and as bait for angling. However, the short answer is that there is growing scientific evidence that the use of introduced alien earthworm species poses a risk to South African biodiversity.
I should point out that my intent in writing this article is not to provide complete coverage of this topic, nor am I trying to put the proponents of worm farming out of business – as a conservationist and ecologist, I simply want to introduce the topic to those who are making great strides to ‘go green’ by starting up a worm farm but who might not be familiar with the potential threats to the local fauna, and offer them some possible alternatives. My intent is to stimulate some discussion on what I’ve gathered thus far and I invite the worm farmers to contribute.
South Africa is known to have a unique earthworm fauna, found no-where else in the world. Of the 56 species of earthworm listed on Ezemvelo KZN Wildlife’s Strategic Environmental Assessment, these species are all potentially endemic to the province, and therefore are likely to be more reliant on conservation action for their long-term survival than other species in the province (Goodman, 2000). Many species of earthworms are habitat specific, only found in a particular vegetation type, i.e. grassland or forest.
Closer to home, a recent survey by Nxele (2012) in selected habitat types in Queen Elizabeth Park (a protected area for more than 50 years) resulted in the recording of 1127 specimens of nine earthworm species inhabiting the park, of which only one species (from the family microchaetid Tritogenia howickiana) is indigenous to South Africa. The other eight species are widely introduced and belong to four other families. Although coexistence of exotic species with indigenous species is noted, a decline in the endemics and dominance by exotics was observed. This is where the debate among researchers regarding the importance of the natural spread of established earth worm populations VS human-assisted introduction/spread gets the most attention.
Different invasive earthworms species have different habitat /feeding preferences (belong to different ‘functional groups’) and thus different impacts on the ecosystems into which they spread or are introduced. Different combinations of species can result in different impacts as well. Globally there are around 4400 species of earthworms, all can be classed into one of three morpho-ecological groups:
- Epigeic species live in organic material near and on the surface and ingest large amounts of un-decomposed and decomposing organic material. They tend to be small with relatively short lifespans and rapid generation times. They do not construct permanent burrows, and only produce ephemeral burrows into mineral soil to escape unfavourable conditions. These species play an important role in promoting decomposition and the incorporation of organic material into soils.
- Endogeic species feed on subsurface material containing large amounts of soil, with a preference for soil rich in organic matter. They build burrows that are mostly horizontal and repeatedly branched, and play an important role in decomposing roots, mixing and aerating soil.
- Anecic species build permanent, vertical burrows that penetrate the soil deeply. These species come to the surface to feed on partially decomposed litter, manure, and other organic matter. They have profound effects on organic matter decomposition, nutrient cycling, and soil formation.
Only a handful of earthworms species are commonly used in vermicomposting and almost all of these are Epigeic. Worms used for bait are mostly endogeic species. But it’s important not to make generalisations. The climate conditions in South Africa as a rule are harsh for earthworms, and of our SA species (about 300) none of them are top-dwellers. SA earthworms need a lot of territory in which to roam and are mostly underground soil dwellers. The threats posed by introduced species has not yet been adequately assessed – more work is needed in this area. However, Red Wiggler Worms (Eisenia foetida), which are the most common type of composting worm are very tolerant of variations in growing conditions and this increases the risk of them invading into habitats where their natural predators don’t exist.
The views on those in the world of vermicomposting with financial interests in, and reliance on, this field may be at least a little bit biased and possibly missing the point, as can be seen from the below argument (deliberately unreferenced):
- MAN has created the greatest imbalances on this planet. Name me ONE garden that is pristinely indigenous. Most gardens have exotic species of plants, grown with man-made synthetic fertilisers and are poisoned regularly at the first sign of pests. Our gardening and farming practices have killed the life of the soil.
- We consequently have polluted the planet with our indiscriminate use of chemicals and pesticides, we have poisoned the groundwater, polluted the air and have caused global warming to manifest. Not to mention our landfill sites – cancerous growths attached to ugly, hot, crime-ridden, rat-infested cities.
- The earthworm bins are there to HELP REDUCE the wet waste that goes to the landfill sites. Wet waste contaminates dry (and recyclable) waste, such as paper, etc.
- SURELY anyone in their right minds will see that the earthworm bins (and wet waste recycling) are here to help SAVE the ravages of human activities and lifestyle on the planet.
It is within this context, as well as the fact that the earthworms in South Africa have not yet been fully documented, that we should recognise the urgent need to build capacity and to conduct earthworm research. There are three PhD studies being undertaken under Dr John Wilson (Centre for Invasion Biology, Stellenbosch University), to assess the diversity and distribution of native and alien earthworms at the landscape scale in the Cape Floristic Region and KZN; and the introduction of and trade in earthworms in South Africa which will include an in-depth study of domestic vermicomposting and explore the socio-economic drivers of the industry with a view to produce guidelines for the public. We are uncertain as to what progress has been made to date with the issues considering there was a long time horizon for the results. It is thought that one of the PhD students withdrew from the programme.
Speaking to Dr Adrian Armstrong, Animal Ecologist: Conservation Planning Division of Ezemvelo KZN Wildlife, he had this to say:
There are indigenous invertebrates that can be used in composting. This is a field of research that needs to be urgently tackled, as spreading invasive alien species is, in my opinion, always going to have negative consequences for indigenous biodiversity. I do not know of any peer-reviewed scientific study that has properly demonstrated that the spread of an alien invasive species has not had any negative consequences on biodiversity in the long-term. However, simple composting is always an option, and I don’t really see the need for using and/or spreading alien species to assist with this, as there are always indigenous species (as well as alien species already present in the soil) that do the job of composting as a matter of course (as any gardener will testify). Temperatures differ from place to place, and this affects the rate of composting, but [composting] will happen nevertheless.
Another issue that arises is whether alien earthworms have been legally imported into KwaZulu-Natal. The import of alien species of earthworms requires an import permit under the Nature Conservation Ordinance of 1974, but I don’t know if any of these permits have been issued since 1974, and I have not recommended that any be issued since I have worked for Ezemvelo KZN Wildlife, as far as I recall. It would appear that an agricultural import permit would also be required. Have any permits been issued for the import of earthworms for worm farms? This is a question I would like to have answered.
Indeed, Plisko, who studied some of the introduced earthworms concluded:
Because no appropriate protection against deliberate or occasional earthworm introduction is yet applied in RSA, concern is felt about infestation of soils by further introduction of these, or other, competitive species. A unit, falling under government control, should be established to regulate the transportation of earthworm samples between countries, or nationally for vermicomposting, sugar cane cultivation, experimental study, or any other reason.
What is exciting to know is that there are indigenous alternatives that can be used in vermiculture and composting as can be seen from the poster above (click on it to enlarge). The larvae of the African rhinoceros beetle (Oryctes boas) can play an important role as soil ecosystem engineers and provides a promising option for converting organic wastes like cattle manure into strongly aggregated granular compost (Materechera & Mkhabela, 2002).
I for one would be interested in helping to develop the necessary protocols for composting using indigenous invertebrates so that the impact that introduced species is having on native biodiversity is reduced.
References
Armstrong, A. pers. comm. 20 November 2013. Animal Ecologist: Conservation Planning Division of Ezemvelo KZN Wildlife, Pietermaritzburg.
Do Composting Worms Pose a Threat as Invasive Species? – http://www.redwormcomposting.com/general-commentary/do-composting-worms-pose-a-threat-as-invasive-species/
Goodman, P.S. August 2000. Determining the conservation value of land in KwaZulu-Natal. KwaZulu-Natal Nature Conservation Service.
Nxele, T.C. December 2012. The megadrile fauna (Annelida: Oligochaeta) of Queen Elizabeth Park, South Africa: species composition and distribution within different vegetation types. African Invertebrates. Vol. 53 (2): 543–558.
Materechera, S.A. & Mkhabela, T.S. 2002. Casting of kraal manure by larvae of the African rhinocxerous beetle, Orycetes boas (Fabricius) and the physio-chemical properties of the casts. S.Afr.J.Plant Soil 2002, 19(2).
Plisko, J.D. 2010. Megadrile earthworm taxa introduced to South African soils (Oligochaeta: Acanthodrilidae, Eudrilidae, Glossoscolecidae, Lumbricidae, Megascolecidae, Ocnerodrilidae). African Invertebrates Vol. 51 (2) Pages 289–312 Pietermaritzburg December, 2010.
Red wigglers (also red worms, earthworms, Eisinia fetida) – http://www.squidoo.com/worm-bins-and-worm-farming
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