Earthworms as colonisers: Primary colonisation of contaminated land, and sediment and soil waste deposits

This paper reviews the role of earthworms in the early colonisation of contaminated soils as well as sediment and waste deposits, which are worm-free because of anthropogenic activities such as open-cast mining, soil sterilisation, consistent pollution or remediation of contaminated soil. Earthworms live in close interaction with their soil environment and are able to change it considerably by their burrowing and litter comminuting behaviour. While earthworms have been studied extensively, several questions still remain unanswered such as: What are the characteristics of successful early colonisers? Do they function well in dispersal, individual establishment or population growth? Do the negative environmental conditions in these kinds of anthropogenic soils hamper colonization or are these colonizers relatively resistant to it? To what extent does colonization change the characteristics of the colonized substrate? In short, do earthworms impact the soil? In this paper, the characteristics that make earthworms successful colonisers are briefly described as well as which species are the most successful and under what circumstances, and what do earthworms contribute to the total process of succession. We propose that it is not so much eco-type or r-K strategy that govern success and succession of earthworm colonisation but rather environmental flexibility not only towards pH, desiccation, and temperature but also towards contaminants such as heavy metals. Moreover, the formation of an organic litter layer, in close connection with re-vegetation of the area, is essential for establishing earthworm populations, which, at first, are mainly superficially and shallow active species. The burrowing and organic matter digesting activity of these earthworms changes the upper soil to a well mixed humus layer suitable for deep burrowing earthworm species.