Agriculture Reference
In-Depth Information
FOOD
There is little doubt that earthworm populations are often food limited; this is evident from the fact
that populations often increase following organic amendment. The response to organic amendments
can be particularly marked in disturbed habitats of low organic matter content (Edwards 1983;
Lofs-Holmin 1983; Lowe and Butt 2002), but significant population increases can also occur in
favorable habitats, such as permanent pasture, following the application of high-quality organic
materials like animal manures (Curry 1976; Cotton and Curry 1980a,b; Edwards and Lofty 1982b).
In the absence of other constraints, it is likely that the earthworm-carrying capacity of most habitats
could be increased considerably by increasing the food supply. Hartenstein and Bisesi (1989)
estimated from laboratory studies conducted by Hartenstein and Amico (1983) that a biomass of
could be sustained under conditions of unlimited food supply in soil
irrigated with livestock wastes. However, it is unlikely that such a high earthworm biomass could
be sustained under field conditions (Schmidt et al. 2003).
The main source of the organic matter on which earthworms feed is litter from aboveground
plant parts in most ecosystems, although dead roots and rhizodeposition can also be important food
sources. Some species, including
L. terrestris
up to 0.5 kg m
Ï2
, are found in close association with
roots, and some species are known to ingest living roots (Baylis et al. 1986). Earthworm populations
in woodlands can be limited by the amount and continuity of the litter supply; this was apparent,
for example, in afforested coal mine dumps in Germany, where the epigeic species (
Allolobophora chlorotica
Dendrobaena
spp.,
) flourished when a well-developed litter layer was present but declined
in importance when the litter layer was depleted by the action of anecic species such as
Lumbricus rubellus
L. terrestris
(Dunger 1989). Zicsi (1983) concluded that the continuity of food supply was of cardinal importance
in determining the suitability of deciduous woodlands in central Europe for the survival of large-
bodied, anecic earthworms. However, it appears to be the quality rather than the actual quantity of
litter that most often limits earthworm populations (Satchell 1967; Swift et al. 1979; Bostrm and
Lofs-Holmin 1986).
Much of the litter input into soil is poor in nutrients, with nitrogen in particular often in short
supply. Satchell (1963) calculated that the nitrogen requirement of the
L. terrestris
population in
an English deciduous woodland (about 100 kg ha
) was at least equivalent to, and was
possibly in excess of, the nitrogen supply from litter. Nitrogen is often considered the critical factor
year
−
1
Ï1
limiting earthworm populations in many ecosystems, both temperate (Satchell 1967) and tropical
(Lee 1983).
Nitrogen content can be a useful indicator of food quality when comparing widely different
types of litter but may be less useful as a predictor of earthworm performance on more palatable
residues from agricultural crops and deciduous trees (
Table 6.1
). Bostrm and Lofs-Holmin (1986)
and Bostrm (1987, 1988), for example, found no consistent relationships between nitrogen content
and the growth rates of
cultured in soil amended with plant materials, with nitrogen
contents ranging from 0.37 to 4%, although adult growth rates and cocoon production rates were
significantly lower on unfertilized barley straw (0.35% nitrogen) than on meadow fescue (2.57%
nitrogen) and lucerne (2.3% nitrogen) residues.
Particle size has an important influence on the quality of plant materials as food for endogeic
species, such as
A. caliginosa
A. caliginosa
(Bostrm and Lofs-Holmin 1986), but not for large anecic species
such as
L. terrestris
. Boyle (1990) reared juvenile
L. terrestris
and
A. caliginosa
in mixed cultures
(1
per 1-L container) in a peat/mineral soil medium with chopped
(8-mm pieces) or milled (<1-mm pieces) ryegrass (
L. terrestris
plus 1
A. caliginosa
Lolium perenne
) provided as a food source
either added to the soil surface or mixed into the soil every 2 weeks. Predictably, in view of its
endogeic feeding habits,
A. caliginosa
grew faster on milled than on chopped grass, whereas for
L. terrestris,
which gathered the food and dragged it into the mouth of its burrow, particle size was
unimportant. Also predictably,
L. terrestris
fared better when the food was placed on the surface,
