Agriculture Reference
In-Depth Information
Biomass is normally only a few mg dry weight and is unlikely to ever exceed 50 mg
(Petersen and Luxton, 1982).
4.3
Macrofauna
In terms of their abundances and their biological and pedogenetic roles, earthworms,
termites and ants are the most important macrofaunal components of soils. Indeed, the
importance of their activities has caused them to be designated 'ecosystem engineers'
(Stork and Eggleton, 1992; Jones
et al.,
1997). Macroarthropods and Mollusca are
constant inhabitants of litter and, to a lesser extent, of soils, but they have generally more
specific ecological roles.
4.3.1
EARTHWORMS
4.3.1.1
General Biology
Earthworms belong to the class Oligochaeta (Annelida: Clitellata) which started colonis-
ing terrestrial environments 600 millions years ago. Since that time, they have become
the predominant component of the soil fauna in all but the dry and cold environments.
From their aquatic origins, they have retained the primitive cutaneous respiration and a
narrow dependence on hygrophilous soil moisture regimes. They comprise 20
families, 693 genera and an estimated 6000 species of which 3700 have been so
far described (Reynolds and Cook, 1976; Fragoso
et al.,
1997). Most species are
relatively restricted in their distributions although a few peregrine species, generally
with parthenogenetic reproduction, have world-wide distributions and a clear preference
for soils affected by man
(e.g.,
Omodeo, 1952; Gates, 1970; Ljungström, 1972; Lee,
1985; Fragoso and Lavelle, 1992; Fragoso
et al.,
1997).
Earthworms are segmented coelomate Metazoa in which the coelom functions as
an hydrostatic skeleton. Their circulatory system is closed and they have a complete
digestive apparatus (Avel, 1959; Edwards and Bohlen, 1996). The body may be roughly
divided into two parts: an anterior pan with segments containing cephalic ganglions,
reproductive organs, gizzards, calciferous glands and hearts, and a posterior pan rear-
ward of the hindgut comprising a series of rather similar segments.
In length, earthworms generally range from a few centimetres to 2-3 m with most
species falling within the range of 5 to 15 cm. Size varies considerably within single
species populations and the largest adults may be more than 100 times those of newly
hatched individuals.
Most earthworm species are hermaphroditic and cross-fertilisation is predominant.
However, parthenogenesis is frequent in peregrine species which represent
ca.
3 % of
the total number of species (Lee, 1985). Bouché (pers. comm.), however, indicates that
the lack of spermathecae is not an absolute indication of pathenogenesis since a signifi-
cant number of species produce spermatophores. The reproductive apparatus is relatively
complex due to the presence of reproductive organs of both sexes and delayed external
fertilisation. In the family Lumbricidae, the reproductive apparatus has become even more
