ABSTRACT
Earthworm populations show a considerable amount of variability in time and space, with mean densities and biomass ranging from less than 10 individuals and 1 g m-2 to more than 1,000 individuals and 200 g n r12 under favorable conditions. However, within particular climatic zones, earth worm assemblages, with fairly characteristic species richness, composition, abundance, and biomass, can often be recognized in broadly different habitat types such as coniferous forest, deciduous woodland, grassland, and arable land. There is a considerable volume of literature describing the earthworm commu nities of such habitats and much of this has been summarized by Lee (1985). There is also a considerable volume of information describing the influence of various environmental and management factors on earthworm populations, but in comparison with insects, where the population ecology of many species has been subjected to quantitative analysis, earthworm population ecology is still largely at a descriptive stage. There are many reasons for this disparity, includ ing the relative ease with which key life history parameters of many insects can be studied compared with those of earthworms. The objective of this paper is to review, in a mainly descriptive way, the main factors influencing earthworm abundance. These, presented in a highly schematic way in Figure 1, fall into two broad categories; namely, external factors which determine the habitat (climate, soil, vegetation and litter supply, and management) and the biotic interactions within the communities to which earthworms belong (competition, predation, parasitism and disease, and food relations). 1
Climate affects earthworms directly by influencing their biology and life pro cesses, and indirectly through its effects on their habitat and food supply. Tem perature is a factor of primary importance in that it determines individual meta bolic rates and, on a global scale, it can have a major role in determining patterns of earthworm distribution and activity. The range of temperatures within which earthworms can function is narrow, with upper lethal temperatures being rather low (25-35°C) and optimum temperatures typically being in the range 10-20°C for cool temperate species and 20-30°C for tropical and subtropical species (Lee 1985). Few species can tolerate temperatures below 0°C, although many species have behavioral and/or physiological adaptations that enable them to survive unfavorable periods in areas with strongly seasonal climates.
