Oligochaeta Earthworms

Earthworms are the most familiar and, with respect to soil processes, often the most important of the soil fauna. As observed by many farmers and gardeners and reported in the popular literature, the importance of earthworms arises from their influence on soil structure (e.g., aggregate or crumb formation, soil pore formation) and on the breakdown of organic matter applied to soil (e.g., fragmentation, burial, and mixing of plant residues). These observations have led to numerous studies of the potential benefits of earthworms in agriculture, waste management, and land remediation (Edwards, 2004).

While the scientific literature on earthworms officially began with Linnaeus's taxonomic description ofLumbricus terrestris more than 200 years ago, the modern era of earthworm research began with Darwin's (1881) last book, The Formation of Vegetable Mould Through the Actions of Worms, with Observations of Their Habits, which called attention to the beneficial effects of earthworms: "It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures." Since then, a vast literature has established the importance of earthworms as biological agents in soil formation, organic litter decomposition, and redistribution of organic matter in the soil (see Lee, 1985; Hendrix, 1995; Edwards and Bohlen, 1996; and Lavelle et al., 1999).

Despite the common reference in the popular literature to "the earthworm," there is great diversity and a wide range of adaptations to environmental conditions among the earthworm fauna. More than 3500 earthworm species have been described and it is estimated that considerably more await discovery and description (Fragoso et al., 1999).

Earthworms are classified within the phylum Annelida, class Oligochaeta, and order Opisthophora. Although there is not universal agreement on taxonomic classification, recent analyses suggest 16 families, 6 comprising aquatic or semiaquatic worms (cohort Aquamegadrili plus suborder Alluroidina), and the other 10 consisting of the terrestrial forms commonly known as earthworms (cohort Ter-rimegadrili) (Jamieson, 1988). Species within the families Lumbricidae and Megascolecidae are ecologically the most important in North America, Europe, Australia, and Asia; some of these species have been introduced worldwide by human activities and now dominate the earthworm fauna in many temperate areas. Likewise, several tropical species in the families Glossoscolecidae, Eudrilidae, and Megascolecidae have become pantropical in distribution. Such "peregrine" or "anthropochorous" species are highly successful in many agricultural or otherwise disturbed areas, and often show significant effects on soil processes (Lee, 1985, Lavelle et al., 1999). Different localities may be inhabited by all native species, all exotic species, a combination of native and exotic species, or by no earthworms at all. Relative abundance and species composition of local fauna depend greatly on soil, climate, vegetation, topography, land use history, and especially on past invasions by exotic species.

Whether introduced earthworms displace native species or occupy areas devoid of native species as a result of disturbance is a subject of debate (Kalisz and Wood, 1995; Hendrix and Bohlen, 2002). It is often suggested that the establishment of exotic earthworm populations proceeds through the stages of habitat disturbance, extirpation or reduction of native populations, introduction of exotic species, and colonization of vacant niche space by exotic species. Even in the absence of obvious habitat disturbance, some minimum habitat patch size may be required to maintain native earthworm assemblages; increased edges and potential vectors for invasion by exotic species into small ecosystem remnants may lead to displacement of native populations (Kalisz and Wood, 1995).

Earthworm Distribution and Abundance

As noted previously, earthworms occur worldwide in habitats where soil water and temperature are favorable for at least part of the year; they are most abundant in forests and grasslands of temperate and tropical regions, and least so in arid and frigid environments (e.g., desert, tundra, or polar conditions). Across this range of habitats, earthworms display a wide array of morphological, physiological, and behavioral adaptations to environmental conditions (Lee, 1985). Even in unsuitable regions, earthworms may inhabit local microsites where conditions are favorable (e.g., urban gardens, desert oases), especially if well-adapted species have been introduced (Gates, 1967). During unfavorable periods, many species are able to enter a temporary dormant state (aestivation or diapause) or produce resistant cocoons that hatch when conditions improve (Edwards and Bohlen, 1996). Within habitats, earthworms often show patchy spatial distributions corresponding with such factors as vegetation, soil texture, or soil organic matter; feeding preferences dictate vertical distributions of species within the soil profile.

Abundance and biomass of earthworms establish them as major factors in soil biology, leading Blakemore (2002) to remark: "And while birdwatchers get excited about a few kilograms of birdlife, or the grazier is concerned about a couple of 100's kg per hectare of livestock in a pasture, almost totally ignored is an underground biomass of earthworms often far in excess of those above that may total 2 or 3 tonnes per hectare." Earthworm densities in a variety of habitats worldwide range from less than 10 to more than 2000 individuals m-2, the highest values occurring in fertilized pastures and the lowest in acid or arid soils (coniferous or sclerophyll forests) (Table 4.10). Typical densities from temperate deciduous or tropical forests and certain arable systems range from less than 100 to more than 400 individuals m-2. Intensive land management (especially soil tillage and application of toxic chemicals) often reduces the density of earthworms or may completely eliminate them. Conversely, degraded soils converted to conservation management (e.g., no-tillage) often show increased earthworm densities and associated soil properties after a suitable period of time (Curry et al., 1995; Edwards and Bohlen, 1996). Biomasses of lumbricid species in temperate regions of the world, where they have been spread by human activities, often exceed that of other animal groups. In the Piedmont region of Georgia in the United States, for example, Hendrix et al. (1987) reported an earthworm dry-matter biomass of 10-g carbon m-2 in no-tillage agricultural plots, a value larger than all other fauna combined.

Biology and Ecology

Earthworms are soft-bodied, segmented animals, ranging in length from a few millimeters (e.g., the American log worm, Bimastos parvus),

TABLE 4.10. Typical Ranges of Earthworm Density and Biomass in Various Habitats

Habitat Earthworms m2 Fresh wt.gm2

TABLE 4.10. Typical Ranges of Earthworm Density and Biomass in Various Habitats

Habitat Earthworms m2 Fresh wt.gm2

Temperate hardwood forest

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