Agriculture Reference
In-Depth Information
(Sohlenius and Sandor, 1987)
Temporal variation
Samples taken at monthly intervals generally show some degree of seasonal variation
with different patterns emerging with changes in climate, vegetation (Arpin, 1983) and
with the trophic group under consideration (Seastedt
et al.,
1987). Large inter-annual
variation has also been observed (see,
e.g.,
Arpin
et al.,
1985).
In a
Brachiaria humidicola
tropical grassland, Smith and Wallace (1976) found
centres of permanently high and low populations of
Helicotylenchus dihystera
which had
similar patterns of seasonal variation, but which fluctuated asynchronously at shorter
scales of time. Short term fluctuations may be expected to be frequent and intense since
the generation times of nematodes vary from 5 to 50 days. Further, in desert soils, Freckman
and Womersley (1983) have observed that anhydrobiotic microbivorous nematodes
may resume activity 15 minutes after rainfall has started and lay eggs immediately.
The species composition of nematode communities may also change with time.
In sugarcane plantations in the Côte d'Ivoire, Cadet and Debouzie (1990) showed that
population changes appear to depend more on changes of the environment (soil
microclimate, plant activity) than on interspecific interactions.
Nematode communities are highly sensitive to environmental change. The addition
of nitrogenous fertilisers results in decreases of the density of predaceous and
omnivorous nematodes whereas bacterivorous and phytoparasitic populations increase
(Wasilewska, 1989).
4.2
Mesofauna
MICROARTHROPODS
Microarthropods are constant inhabitants of leaf liner and soil interstices that, as a group,
function mainly as epigeic decomposers. Apterygote Collembola and Acari are generally
dominant by far, both numerically and in terms of biomass. However, other Apterygota
(Protura, Diplura and Thysanura), Micromyriapoda (Symphyla, Pauropoda and
Polyxenidae) or small Diptera larvae and Coleoptera may be locally important.
All respond, although differentially, to the dry funnel ('Berlèse') extraction technique.
4.2.1
COLLEMBOLA
4.2.1.1
General Biology
The Collembola or springtails, are a very diverse taxon with 21 families and twenty
thousand currently-described species. However, the total number of existing species is
expected to be close to 300,000 (Schaller, 1970; Massoud, 1976). They are apterous
hexapods close to the true insects, small and elongate with a characteristic saltatory
organ that, when fully developed as in surface-dwelling species, allows rapid jumping
