Agriculture Reference
In-Depth Information
Water availability and temperature regimes usually become more favourable and
predictable with depth while food availability and quality, and exposure to predators
all decrease. The existence of inverse gradients in environmental predictability and
food availability down the soil profile is a constant feature of soil environments.
The way and the extent to which soil animals face these constraints largely determine
their abundances and ultimately the roles that they play in the soil system. Adaptive
strategies have been selected in response to the characteristic suites of environmental
constraints present in the soil environment.
4.4.1.2
Digestion
Soil invertebrates generally have relatively restricted enzymatic spectra. Animal cellulases,
for example, are rare and the cellulolytic activity detected in animal guts is generally due
to micro-organisms (see,
e.g.,
Devigne and Jeuniaux, 1961; Parle, 1963; Loquet and
Vinceslas 1987; Rouland
et al.,
1988a; Urbasek, 1990; Deleporte and Rouland, 1991;
Zhang
et al.,
1993). An association between invertebrates and micro-organisms is thus
necessary to allow digestion of resistant substrates. Depending on the intensity of this
association, digestion may occur: (i) by direct digestion; (ii) through the 'external
rumen'; (iii) through a mutualistic association with the soil microflora
in the animal gut; and (iv) through a symbiotic association with a specialised gut
microflora or microfauna.
(i) direct digestion occurs in predators but appears to be extremely limited among
saprovores. Higher termites have cellulases (Potts and Hewitt, 1974; Martin and Martin,
1979; Rouland
et al.,
1988b). Cellulases have also been reported in earthworms, but it is
still uncertain whether they are actually produced by the worm itself, or by associated
micro-organisms (Laverack, 1963; Loquet and Vinceslas, 1987) ;
(ii) in the external rumen type of digestion, invertebrates which do not produce
endogenous cellulase, periodically re-ingest their faeces and thereby take advantage of
the release of assimilable compounds due to external microbial activity (Swift
et al.,
1979). Most litter-feeding epigeic arthropods use this digestive system. They play impor-
tant roles in the comminution, fractionation and humidification of the ingested material,
thereby enhancing its microbial activity. Macrotermitine termites and ants of the tribe
Attini which cultivate fungi on specially prepared Wood or leaf and other materials also
belong to this group;
(iii) facultative mutualism with a non-specific microflora ingested with soil or litter
material occurs where invertebrates provide suitable conditions for microbial activity in
their gut. They then absorb the assimilable metabolites released by the enhanced micro-
bial activity. A digestive system of this type has been described,
e.g.,
in endogeic
geophagous earthworms (Barois and Lavelle, 1986). The ingested soil is mixed with an
equivalent amount of water and 5 to 16 % of intestinal mucus, a readily-assimilable mix-
ture of low molecular weight glycoproteins, amino acids and simple sugar molecules
(Martin
et al.,
1987). Intensive mixing in the gizzard disperses the soil and results in
the formation of an homogenous suspension of bacteria, readily assimilable substrates
(mucus) with soil mineral particles and soil organic matter whose physical protection
within aggregates has been removed. The soil microflora first increases its activity by
