Agriculture Reference
In-Depth Information
of soil; saprophages play a major role in comminution of surface litter and a diverse
community of predators regulates their populations.
4.3.4.1
General Biology
Isopoda
Isopods are relatively consistent components of litter systems (Sutton, 1972; Warburg,
1987). Their size ranges from a few millimetres to 1-2 cm and their fresh weight is of
the order of a few milligrams. Respiratory organs range from the gills of aquatic forms
to closed, lung-like structures (Warburg, 1987). Most species are highly susceptible
to water-loss and are thus restricted to moist, sheltered habitats although a few
drought-resistant species colonise desert habitats. They also have little resistance to
cold temperatures. As a result, they are absent from tundra soils and, in temperate
environments, they have long periods of quiescence during winter.
Isopods may live for several years. They are saprophages that preferentially ingest
decomposing material with a low phenolic content (Neuhauser and Hartenstein, 1978).
Coprophagy is frequent in Isopoda and this external digestion seems to be obligatory
for normal development (Hassal and Rushton, 1982; Gunnarsson and Tunlid, 1986).
They need calcium for their exoskeleton and copper: a relationship between the concen-
tration of this element in litter and isopod abundance has been established (Wieser, 1966).
Many, but not all, species may be limited by the availability of these elements.
Daily ingestion rates (in mg dry matter ingested per mg fresh weights x100) are
generally less than 5
%
but may vary from 0.5 to 20 % of body weight, depending on
the species and its diet (Bizé and Célérier, 1986). Assimilation rates may be relatively
high with values in the range of 4 to 45 % with an average of 25-30 %, depending on
the species, the stage of decomposition and the quality of the litter (Rushton and
Hassal, 1983; Bizé and Célérier, 1986).
Myriapoda
The larger myriapods are divided into two separate classes with quite disparate
ecological requirements and roles in the soil. The Chilopoda are mostly carnivorous
whereas the Diplopoda are saprophages, with a few exceptions. Three main categories may
be distinguished: (i) large-sized macro-saprophages (Iulida, Glomerida and Polydesmida),
(ii) small micro-phytophages (Craspedosomatida) or (iii) intermediate forms with a liquid
alimentary regime (David, 1987). Some large tropical Diplopoda may ingest soil and
egest compact faecal pellets (Aouti, 1978).
Diplopods and chilopods differ substantially in size with average individual weights
varying from 0.5 mg f wt in a Swedish coniferous forest to 560 mg in the litter layer of
a mixed deciduous forest in North Carolina. Some giant tropical Iulidae weigh up to
10 g fresh weight, although most values are in the range 10 to 25 mg (Petersen and
Luxton, 1982).
Diplopods and chilopods are found on the soil surface, in the litter layers, in the
sub-cortical habitats of tree stumps and decaying logs where their food occurs, and where
the humid air protects them from desiccation. Most have a limited ability to penetrate
the soil which they move through by displacing it in all directions ('bulldozing').
