Agriculture Reference
In-Depth Information
4.1.1
DETERMINANTS OF EARTHWORM COMMUNITY STRUCTURE
The regional abundances of earthworms and the relative importances of the different
ecological categories are determined by large-scale climatic factors (mainly temperature
and rainfall), their phylogenetic and biogeographical histories together with such regional
parameters as vegetation type and soil characteristics. As a result, drilospheres differ
greatly at the scale of broad climatic areas and regions, and between patches with
different soil and vegetation.
Comparative analysis of a large number of earthworm communities of wet areas
has revealed that temperature is the main large-scale factor determining the nature of
earthworm communities (Lavelle, 1983c). Along a thermo-latitudinal gradient, population
density increases from a few tens to several hundreds while biomass achieves a max-
imum in temperate latitudes. The most striking feature is the gradual shift in dominant
community feeding habits: these change from largely litter-feeding (epigeics) at high
latitudes, to a co-dominance of liner and soil feeding (anecics) at temperate latitudes, to
a dominance by soil feeding (meso- and oligohumic endogeics) in the inter-tropical areas
(Figure III.45). Thus, with rising temperature, the activities of earthworm communities
become increasingly concentrated at greater depths as they become capable of exploiting
resources of lower quality; this starts with the high-quality resources found at the soil
surface (litter) and is lowest with the extremely poor-quality soil organic matter used
by the oligohumic species characteristic of deeper horizons (see Chapter III.4.3.1.2).
Phylogenetic and biogeographical factors may influence the functional structure
(
i.e
., the proportions of the different ecological categories) of earthworm communities.
Oligohumic endogeic species do not occur in such families as the Lumbricidae and
Glossoscolecidae although the anecic earthworms - species with deep, subvertical
burrows and a resistance to seasonal drought derived from a true diapause - are well
represented. In contrast, anecic species are not found within the more primitive families
such as the Megascolecidae although endogeic earthworms are more diverse, including
those of oligohumic and mesohumic feeding habits. Overall, it is difficult to determine
whether the phylogenetic differences do not simply reflect selection for the different
soil environmental conditions pertaining over the geographical ranges of these families.
At a regional scale, forests generally have smaller populations than grasslands.
In Europe, for instance, forests have live earthworm biomasses in the range of 400 to
680 kg fresh weight whereas in grasslands, values of 500 to or more
are common (Lee, 1985; Edwards and Bohlen, 1996). In the introduced pastures of humid
tropical areas, communities dominated by the peregrine endogeic species
Pontoscolex
corethrurus
and
Polypheretima elongata
may have live biomasses of
in contrast to the species-rich communities of much lower biomass
fresh weight) characteristic of natural forest soils (Fragoso and Lavelle, 1992).
Soil characteristics and rainfall are also important determinants of earthworm
communities within a given biome. In western Africa, earthworms do not occur where
average yearly rainfall is less than 800 mm (Lavelle, 1983a). In tropical rainforests
in America and Africa, earthworm biomass increases with rainfall up to values of
ca.
3,000 mm and then decreases with higher values, probably due to the low nutrient
status of soils typical of these high rainfall conditions (Figure IV.43). Nutrient status and
