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In this paper a simple model is used to study the dispersal of earthworm populations into new habitats. Simple models do not describe processes accurately, but can help gain insight into the functioning of ecosystems or processes in ecosystems. Using information on reproduction, survival and dispersal at the level of the individual, the velocity of earthworm population expansion was calculated. Dispersal of earthworms can be active or passive. The parameters of active and passive dispersal were calculated from field experiments in one of the Dutch polders. Parameters of reproduction and survival were estimated from published data. The effects of processes at the individual level on the velocity of population expansion were studied for two species (Aporrectodea caliginosa and Lumbricus rubellus). The model shows that passive transport has a major influence on the velocity of population expansion, which is strongly increased even if this transport involves only a very small part of the population. At a high level of passive transport, however, death induced by this mode of dispersal can have a negative influence on population expansion. In the discussion it is indicated that optimising growth conditions of the earthworms might be the easiest way to promote population expansion. However, promoting dispersal by passive mechanisms can also be very important.

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