Agriculture Reference
In-Depth Information
of galleries and chambers, have significant impacts on the evolution of soils at medium time
scales. Ants may be considered as part of this group, although the vast majority of them only
use soil as an habitat and have a limited direct impact on soil organic matter dynamics.
They will not be considered in this section because of the small number of studies devoted
to their effects on soil processes (review in Folgarait, 1998).
Whenever conditions are suitable for their activities, macrofauna, and especially
earthworms and termites, become the major regulators of microbial activities within
their spheres of influence (
i.e.,
the termitosphere of termites and the drilosphere of earth-
worms, Lavelle, 1984) in which they also determine the abundance and activities of
the smaller groups of soil fauna (Dash
et al.,
1980; Yeates, 1981). These 'Biological
Systems of Regulation' include the rhizosphere in which roots are the major determinant
(Lavelle
et al.,
1993) (see also IV.1.4).
The apparent looseness of the soil trophic structure may be due to the juxtaposition of two
fundamentally different types of relationships between invertebrates and micro-organisms
i.e.,
1. a 'classical' foodweb in which organisms of a given size feed on the smaller organ-
isms, at a lower level in the foodweb; and are eaten by larger organisms which comprise the
higher levels and 2. Biological systems of regulation which are systems of interactions based
on the mutualisms that occur between micro-organisms and invertebrates of different sizes.
The overall structure of the foodweb is further complicated by the effects of interac-
tions between these three guilds. For example, certain local conditions may favour high
levels of activity among termites or anecic earthworms and thereby significantly reduce
the resources available for the development of litter transformers and micropredators.
4.4.2
DETERMINATION OF SPECIES RICHNESS
Within individual communities, species richness within particular taxa is determined by
(i)
phylogenetic constraints,
(ii)
biogeographical factors and
(iii)
environmental conditions.
4.4.2.1
Phylogeny and species richness
Species richness differs greatly between taxa. Under favourable conditions, microarthropod
communities may comprise hundreds of species with diversity in Acari up to 400 to
500 species and that of Collembola 60 to 80 species. Under comparable conditions,
nematodes may comprise up to 90 species; protists, up to 60; termites, 60; Enchytraeidae,
22; earthworms, 15-17; Diplopoda, 15.
Two general trends seem to exist:
(i) species richness decreases in parallel with size (May, 1986; Stork and Brendell, 1993);
(ii) species richness appears greater in arthropod than in non-arthropod groups.
Some hypotheses may be proposed to explain these features. The effect of size may
be a consequence of the scale at which the organisms operate. A single soil macro-
aggregate may be the smallest environmental unit recognisable by an endogeic earthworm
although it comprises a large variety of microenvironments at the scale of a protist or
nematode. Thus, a greater diversity of potential niches for small organisms will
