FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 472

4.2 The anecic drilosphere

4.2.1 DISTRIBUTION OF ANECIC EARTHWORMS

Anecic earthworms are unevenly distributed throughout the world for both climatic and
biogeographical reasons. They are absent from the acid soils of cold temperate regions
and coniferous forests in warmer temperate soils. They are dominant in most temperate
grasslands and deciduous forest soils where they may comprise up to 80 % of earthworm
biomass. In arable land, the proportions of anecics are much lower, although highly
variable, with average values of 10-30 % (Boström, 1988; Cuendet, 1983; Binet, 1993).
In tropical soils, they may be completely absent or have small populations. Savannas of
the Columbian llanos and, to a lesser extent, Amazonian rainforests at Yurimaguas, are
exceptions to this and anecic Glossoscolecidae may comprise from a few per cent. in the
forest to 15 % of the biomass in natural savannas, and 80 % in pastures derived from
these savannas (Lavelle, 1983a; Fragoso and Lavelle, 1987; Lavelle and Pashanasi,

1989; Jimenez Jaen et al ., 1998).
In cold regions, anecics are clearly limited by climatic and edaphic factors and they may
rapidly colonise soils when pH is increased to neutrality by an amendment (Toutain, 1987b).
Their absence from African and Central American forests seems to have a phylogenetic
reason i.e., the lack of an anecic adaptive strategy in the families Acanthodrilidae and
Eudrilidae that predominate in these regions. In contrast, in South American tropical
forests and savannas, the dominant family Glossoscolecidae, contains a large number of
true anecic species, as does the Lumbricidae in temperate areas. However, as indicated
previously, these distributions may also be explained in terms of soil nutrient concentrations
and it is not yet possible to discriminate between the two effects.

4.2.2 INGESTION OF ABOVE-GROUND LITTER BY FIELD COMMUNITIES

Anecic earthworms ingest a mixture of litter and soil although the relative proportions
of both components vary between species, showing a clear gradient from epigeics
to anecics to polyhumic endogeics (Figure IV.48; Table IV. 16). Anecics generally ingest
large debris whereas epigeics tend to feed on smaller debris, probably due to their
smaller size. The proportion of plant debris in the casts of anecics may be highly
variable since they ingest large proportions of litter when they are actively feeding at
the soil surface, and ingest a preponderance of soil when they are active deeper in
the soil, creating new galleries or avoiding unsuitable surface conditions. Tropical species
of a given ecological category generally ingest a much lower proportion of organic debris.
Earthworms also ingest less soil when litters are more palatable (Cortez and Hameed,
1988) and contain more nitrogen (Abbott and Parker, 1981; Lavelle et al., 1989).
Ferrière (1980) observed that co-existing populations of different species may separate
their niches by selectively ingesting litters of different plant species. Finally, tropical
anecics seem to ingest a smaller proportion of litter than their temperate equivalents
(Kanyonyo ka Kajondo, 1984). The large anecic Acanthodriline Millsonia lamtoiana ,
a common species in the savannas of Côte d'Ivoire, ingests 10 to 15 % organic debris
which consists of 69.2 % large grass particles, 19.3 % unidentified debris, 3.9 % seeds,