Agriculture Reference
In-Depth Information
In tropical areas, an inverse relationship often exists between earthworm abundance and
that of termites at the scale of a region (Senapati
et al.,
1994; Decaëns
et al.,
1994).
Soil invertebrate communities are reliable indicators of land management. The pro-
portions of different groups of nematodes have been combined in an index which is
highly sensitive to changes of the ecosystem (Bongers, 1990). Larger invertebrates react
to changes brought about by soil management: cultivation, changes of vegetation,
decrease of shade and inputs of fresh organic matter. For example, the soils of humid
tropical agroforestry systems derived from the forest may conserve a large part of
the diversity and abundance of invertebrate communities of the natural ecosystem
(Figure III.63). At Yurimaguas (Peruvian Amazonia), palm-tree plantations with a legume
cover had an overall biomass of soil macroinvertebrates of 93.9 g f wt 74 % higher
than the original forest (53.9). Conversion of the original forest to systems which
comprise elements of both the original forest and an herbaceous strata often lead to larger
biomasses than occur in the original system; this is often due to the proliferation of
introduced exotic earthworm species (
e.g., Pontoscolex corethrurus)
which develop
large populations of 500 kg fresh weight or more. Pastures also have high macro-
invertebrate biomasses, because of their substantial earthworm populations.
In these systems, grazing transforms a significant part of the above-ground production
into dung which is a high quality resource for soil fauna communities and favours
the development of soil invertebrate communities, especially earthworms. When the
original vegetation was a forest, native species normally do not adapt to the new envi-
ronment and large populations develop through the immigration of opportunist colonists.
When savannas are transformed into pastures, local populations may develop and build up
populations up to 5 times larger than in the original savanna; this is the case
e.g.,
in the
Colombian llanos where earthworm biomass increased from 4 to 11 g f wt when the
original savanna was grazed; when African grasses and an herbaceous legume were substi-
tuted for the original vegetation, earthworm biomass increased up to 42 g Termite abun-
dance showed an exactly inverse trend with a significant decrease of abundance along the
aggrading sequence from the natural savanna to the improved pasture (Decaëns
et al.,
1994).
Annual crops always have severely depleted macroinvertebrate communities; this
has been noted for earthworms in a large variety of crops (rice, corn, beans, yam, yuca,
etc.) and the original fauna may disappear within a few weeks of clearing and planting.
This depletive effect of annual crops has been reported from a wide range of temperate and
tropical environments (see,
e.g.,
Lee, 1985 for earthworms).
4.4.3.3
Geographic scale: changes in overall community structure along a thermo-
latitudinal gradient
The relative abundance of the micro- and meso-fauna declines in tropical areas as a conse-
quence of: (1) an increased abundance of macro-invertebrates because of favourable
environmental conditions; and (2) a subsequent dominance of biological systems of
regulation based on mutualistic relationships between roots, soil ecosystem engineers and
the soil microflora. This contrasts with systems in which the microfauna (micropredators)
and meso-fauna (litter transformers) have
less-close associations with these micro-
organisms (Figure III.61).
