In general, there are considered to be around 6,000 species of earthworms (Fragoso et al. 1999; Lavelle and Spain 2001). Apart from extreme environments such as desert and glacial soils, earthworms can inhabit a wide variety of soil environments including agriculture, forest, and pasture ecosystems. The size of adult earthworms ranged from few mm to 2 m while their body mass changes between 10 mg and 1 kg. Giant earthworm species usually inhabit in tropical regions of southern hemisphere such as South America and Africa, Southeastern Asia, Australia, and New Zealand. Some other earthworms living in other regions of the earth are usually comparatively smaller and have lower body mass. The Lumbricidae are dominant species in temperate areas while remaining families inhabit predominantly tropical or subtropical areas (Lavelle et al. 1999). The number of different earthworm species living in a certain soil environment can be three or five and occasionally more than ten. In general, similar earthworm species exist in similar soil and climate conditions (Edwards and Lofty 1982a, b; Edwards and Bohlen 1996). The soil can contain 10-1,000 individual earthworms or 1-200 g earthworm biomass per square meter soil, depending on time and soil environmental characteristics. Earthworm biomass and diversity in soil is closely associated with plant vegetation and climate. Even forest type, deciduous versus coniferous can be significant factor affecting earthworm abundance and species diversity in soil. The temperate region soils were found to be predominated by 12 Lumbricidae species while 7 species were identified from Africa's soils (Edwards and Lofty 1982a, b; Edwards and Bohlen 1996). Tropical agroecosystems have been reported to be more diverse than these regions with 20 earthworm species (Barois et al. 1999). Lifespan of an earthworm usually ranges between 10 and 12 years but many species are eaten by the predator such as large insects, moles, and birds and therefore survive 1-2 years (Edwards and Bohlen 1996). The main soil characteristics that control the earthworm number and biomass are soil organic matter (SOM), texture, pH, water holding capacity, and soil temperature (Lee 1985; Bernier and Ponge 1994; Lavelle and Spain 2001; Karaca et al. 2010a).
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