Agriculture Reference
In-Depth Information
the ecosystem. They have therefore developed specific adaptive strategies whereby they
rely more on mutualistic relationships and suffer less from antagonistic interactions
with other species (Lavelle, 1985).
4.4.1
ADAPTIVE STRATEGIES OF SOIL INVERTEBRATES
4.4.1.1
Principal constraints to soil invertebrate activities
Soil invertebrates have adapted in different ways to the major constraints of their envi-
ronment. Some major accommodations to their environment include: (i) the derivation of
their nutrient energy requirements from a suite of largely poor-quality organic resources;
(ii) adaptation of their locomotion to suit the dark, confined and sometimes compact soil
environment; and (iii), modification of their metabolism to cope with the occasionally
unfavourable temperature and moisture conditions.
Food resources
The nutritional value of most available soil resources is relatively poor. Most comprise
decomposing materials with characteristically low nutrient contents and substantial
concentrations of highly-polymerised and resistant molecules such as lignin, humic
acids and tannin-protein complexes. Higher quality resources such as the microbial
biomass or freshly-dead invertebrates are much less abundant, dispersed in the soil
profile and rendered partly inaccessible by physical protection in microhabitats
(see Chapter I.3.2).
Movement
The mineral soil environment is compact and movement for most species is only
possible through a labyrinth of pores and channels. These comprise a relatively small pro-
portion of the whole soil volume which decreases with increasing depth (see Chapter I.1.3.1).
Moisture regimes
The semi-aquatic soil environment is theoretically suitable for those faunal groups
with high moisture requirements. However, water availability is highly variable in time
and space due to climatic variation and capillary forces which, depending on the pore
size distribution, may keep much of the water beyond the extraction capacity of many
animals (see Chapter I.2.2.3.2).
Temperature
Through its overall extremes and short term changes, temperature is the ultimate
regulator of metabolic activity in soil organisms which, with few exceptions, are
poïkilothermic. Temperature and moisture regimes determine the intensities and temporal
distributions of the seasonal periods of activity. In that respect, they largely determine
the success of the various adaptive strategies. Strategies based on the use of low-quality
resources will only be satisfactory where long and intense periods of activity are
possible; if they are reduced, life-cycles must be completed in a shorter time, through
the use of higher-quality resources.
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