Agriculture Reference
In-Depth Information
2.5
Processes in the litter system
2.5.1
COMMINUTION
Some epigeic invertebrates ingest large amounts of litter. In an Oak (
Quercus robur
)
wood in western France, Isopoda with a mean annual biomass of 20 kg live weight
annually ingest 210 kg or 7 % of the dry mass of leaf litter and egest 160 kg
of faecal pellets (Mocquard
et al.,
1987). Larvae of the sciarid fly
Bradysia confinis
were
estimated to have ingested 2.5 % of the leaf litter accumulated in an Oak-Beech
(
Quercus robur, Fagus sylvatica
) wood in Brittany over a period of two months,
i.e.,
121 to 219 kg dry mass (Deleporte, 1987). The activities of these larvae are
concentrated in small areas where all the 'old' litter of the F and H strata may be trans-
formed into faecal pellets in a few weeks. Slugs also ingest significant amounts of litter,
commonly in the range 1.3-8.4 % of total accumulated litter mass (Jennings and Barkham,
1979; Phillipson, 1983) although overall litter ingestion may be as high as 23 %
(Kurcheva, 1967).
In a Mediterranean forest, the millipede
Glomeris marginata
(Diplopoda) may
ingest 2.6 g of dry litter per gram of live body mass. As a result, a population of
50-100 individuals weighing 7.2 to 10.8 g may annually ingest 250-280 kg dry litter
i.e.
8-11 % of annual litter production (Marcuzzi, 1970). In a temperate climate
forest near Orleans (France) consumption of Oak (
Quercus robur
) litter by the millipede
Cylindroiulus nitidus
was estimated at 10-14 g
i.e.,
3-4 % of annual litterfall
(David, 1987). In relatively dry environments, these invertebrates may ingest a much
larger proportion of litter: in miombo woodland in Zimbabwe, for example, Dangerfield
and Telford (1989) estimated annual consumption by millipedes at 88.1 g dry matter
equivalent to 30.6 % of annual litter fall.
From these few examples, it is clear that a variable but significant proportion of
annual litter production is ingested by epigeic animals and re-ingested by the same or
increasingly-smaller organisms. Diversity of species implies some diversity in feeding
preferences; the palatability of litter to saprophagous invertebrates differs greatly among
species, with different responses to the type of litter and its state of decomposition.
In temperate forests, for example, Diptera larvae are among the earliest colonisers of
freshly fallen litter (Mollon, 1982) whereas anecic earthworms usually prefer litter that
has already undergone some degree of fungal decomposition (Cooke and Luxton, 1980;
Cortez and Hameed, 1988).
As a result of fragmentation, the overall surface area of the decomposing litter is
increased by a factor of 50 to 200 after having been egested and re-ingested by a sequence
of increasingly smaller animals (Bachelier, 1978; Bertrand
et al.,
1987). These animals
may range from large saprophages such as Diplopoda to smaller micro-arthropods and
even nematodes.
2.5.2
TRANSFERS
Transfers within the litter system involve: (i) leaching of water soluble elements and
other materials towards deeper horizons or laterally in runoff; (ii) passive burial of
