Agriculture Reference
In-Depth Information
(i) epigeics live within the litter layers, a relatively harsh environment, where they
are subject to occasional drought, extreme temperatures and high predator densities.
They are small and homochromic worms, pigmented green or reddish depending
whether they inhabit grassland or forest. Epigeics balance a high mortality by using
a high-quality food (leaf litter) which permits rapid growth rates and a high fecundity.
They are typical
r
-selected (prolific) species in the sense of Pianka (1970);
(ii) anecics feed on surface litter that they mix with soil but pass most of their time in
subvertical subterranean galleries created within the soil. They are large worms with a dark
antero-dorsal pigmentation and a strong anterior digging musculature. Their demographic
profile is of the
K
type, with long lives, relatively slow growth rates but low mortalities;
(iii) endogeics are unpigmented geophagous worms that live and feed within the soil,
a better buffered and more predictable environment than the litter. However, food
resources,
e.g.,
decomposing roots and soil organic matter are generally of lower quality
than leaf-litter and are concentrated within the upper ten centimetres of soil and around
plant roots. Endogeics have developed different ways to use these resources (Lavelle,
1981); polyhumics exploit concentrations of organic matter by,
e.g.,
being small and
selectively ingesting organic particles, especially in the rhizosphere, or feeding at the
soil-litter interface; mesohumics are medium-sized worms that ingest the soil of
the upper 10-15 cm without selecting organic particles. Finally, oligohumics are large,
slow-moving worms that live deep in the solum in highly predictable and stable envi-
ronments with very poor food resources. The demographic profiles of endogeic species
change from
r
to
K
or
A
(
i.e.,
adaptation to adverse conditions,
sensu
Greenslade, 1983)
the deeper they live in the soil or become larger in size (Lavelle, 1981).
As a result of this wide range of adaptations, earthworms have highly diverse
functions in soils. Epigeics are efficient compost-makers but have no impact on the soil
structure. Anecics build dense semipermanent gallery networks, deposit casts at the soil
surface and translocate large amounts of leaf-litter into the soil. Finally, endogeics which
do not usually form permanent galleries, are major agents of soil aggregation (at least in
tropical soils where they are predominant) and soil organic matter stabilisation; they may
deposit large amounts of casts at the soil surface, as a response to soil compaction.
4.3.1.3
Density and biomass of communities
Earthworms comprise the major animal biomass in the wide range of ecosystems that
experience less than a few consecutive months of drought or frost in an average year.
Density is often in the range of 100 to 500 individuals and may be as high as the
2000 ind. reported from such environments as temperate pastures in New Zealand or
irrigated orchards in temperate Australia (Lee, 1985). Live biomass commonly ranges
from 30 to 100 g Bouché (pers. communication) using a combination of formalin
extraction and wet sieving of soil evaluated at 110 g the average earthworm biomass
in 55 sites representative of French soils. Maximum biomasses of 200 to 400 g have been
recorded in fertilized pastures in temperate regions (see
e.g.,
Cotton and Curry, 1980)
and tropical pastures (Barois
et al.,
1988).
