SOIL ORGANISMS - Soil Ecology - page 287

Earthworms are semi-aquatic animals which extract water continuously from their
environment and must maintain their cuticle in a moist state to facilitate gas exchange.
They also add considerable amounts of water to the soil and litter they ingest to facilitate
intestinal transit and digestive processes (de Puytorac, 1954; Barois and Lavelle, 1986;
Daniel and Anderson, 1992; Trigo and Lavelle, 1993). Only part of this water is reab-
sorbed and the fresh casts and the cutaneous mucus that they excrete are moister than the
surrounding soil. Consequently, soil moisture status is a major limitation to earthworm
activities and distribution since they are unable to withstand prolonged periods of water
potentials less than -0.10 MPa (pF 3) (Lavelle, 1971). To survive extended dry periods,
certain species have developed such adaptations as quiescence or even diapause
(Saussey, 1966; Bouché, 1984); worms empty their gut and coil up into a tight ball inside
a chamber which has been previously coated with fine-textured casts. They may resist
drought better, but the reduction in mortality resulting from this behaviour does not
appear to be greater than 50 % in the case of the tropical endogeic species Millsonia
anomala (Martin, 1991). Other species, such as the pantropical endogeic Polypheretima
elongata, may have no resting stage and migrate down to depths of 1 to 2 metres to
escape dry conditions in the upper soil horizons, Where active populations are periodically
extinguished through water-logging ( e.g., in flooded tropical forests), or because of
severe drought or frost, the more resistant cocoons may survive to form new populations
when favourable conditions return (Bouché, 1977).
Fecundity is highly variable among species and populations; maximum values of
5.5 cocoons per adult per week have been reported for Eisenia fetida, a common species
in the farm-yard manure and compost heaps of temperate regions (Hartenstein et al.,
1979; Venter and Reinecke, 1988). Minimum values of one or less cocoons per year are
common in deep-dwelling tropical geophagous species (Lavelle, 1978). Tropical species
generally have low fecundities (1 to 15 cocoons per year) with the exception of a few
pantropical species with a parthenogenetic mode of reproduction, e.g., Pontoscolex
corethrurus and Polypheretima elongata which may produce up to 80-100 cocoons per
adult per year (Lavelle, 1981; Senapati, 1980; Lavelle et al., 1987). In temperate areas,
most values calculated for Lumbricidae are in the range of 20 to 60, with maximum
values for epigeic species and minimum values of ca. 10 for endogeic and large anecic
species. Substantial differences also exist between generation times which vary from
a minimum of 2-3 months in Eisenia fetida and the pantropical geophagous species
Pontoscolex corethrurus to 20-24 months in the deep-dwelling geophagous tropical
species. Such differences among species are a major consequence of the diverse
adaptive strategies found among earthworm species.
Digestion in the earthworm gut is mediated by a mixture of enzymes produced by
the gut wall and enzymes produced by the ingested soil microflora (Barois and Lavelle,
1986; Loquet and Vinceslas, 1987; Zhang et al., 1993, see Chapter IV.4). Assimilation
rates calculated as the difference in energy values between ingested food and excreta
may be as high as 30 and 75 % respectively for the epigeic species Lumbricus rubellus
fed alder ( Alnus sp.) leaves and one year old larch ( Larix ) litter (Dickschen and Topp,
1987). Geophagous species have much lower assimilation rates of e.g., 9 % or less in
the tropical mesohumic species Millsonia anomala (Omodeo) (Lavelle, 1978) and 1 %
in the temperate climate Lumbricidae Allolobophora rosea (Bolton and Phillipson, 1976).