Agriculture Reference
In-Depth Information
III.4
INVERTEBRATE COMMUNITIES
The biology and ecology of the major groups of soil invertebrates are also considered in
this chapter. Emphasis is given to the way in which these organisms adapt to the
constraints imposed by living in soil, the factors affecting their abundances and
distributions and the patterns of the community structures.
Soil invertebrates have been classified on the basis of size and water relations (see
Table III.l). They may also be classified in terms of the nutritional resources they use,
their microhabitats and possibly, related morphological and demographic characteristics.
Special attention is paid in this chapter to ecological classifications based on such criteria.
They are useful in that they permit preliminary evaluation of the assessment of the impact
of invertebrate communities without the need to consider each individual population.
4.1
MICROFAUNA
4.1.1
PROTOCTISTA (FORMERLY PROTOZOA)
4.1.1.1
General biology
Taxonomic classification
Soil protists belong to three phyla within the Kingdom Protoctista: Rhizopoda (or the
former Sarcodina) which include the naked Amoebae and the Testacea (amoebae protected
by a resistant test or outer covering), the Zoomastigina (formerly Mastigophora, or flagel-
lates) and the Ciliophora (or ciliates) (Dommergues and Mangenot, 1970) (Figure III.26).
The parasitic Apicomplexa (formerly the Sporozoa) also occur in soils as many infective
stages must pass through the soil (Stout
et al.,
1982).
For convenience, it is proposed to refer to the above soil dwelling Protoctista in
general as protists.
Microhabitat
Soil-dwelling protists occupy the pore space outside the microaggregates and their
growth rates have been reported to decrease when pore space has been reduced by
compaction (Coûteaux, 1985, Vargas and Hattori, 1986, Coûteaux
et al.,
1988). However,
this is not a general rule as those protists that possess fluid body forms such as the
pseudopods of the amoeba
Acanthamoeba castellanii,
may penetrate into micro-
aggregates to ingest bacteria colonising this microhabitat (Vargas and Hattori, 1986).
Size, metabolic rate and growth patterns
Soil protists are generally much smaller than those of aquatic habitats and have a size
range from a few micrometres to less than 100 micrometres. The larger species are
253
