SOIL ORGANISMS - Soil Ecology - page 351

complex and physically unprotected organic matter of the ingested soil. This mechanism,
typical of the priming effect defined by Jenkinson (1966, after Broadbent and Norman,
1946), is equivalent to that observed in the rhizosphere. In the rhizosphere, exudates are
the functional equivalent of mucus and root elongation replaces the movement of soil
through the gut.
It has been demonstrated that the mutualistic digestive system of earthworms becomes
increasingly efficient at higher temperatures. Here it is hypothesised that the same
phenomenon exists in the rhizosphere because of the broadly similar processes occurring
in both systems.
The mutualism between termites and their intestinal microbiota (Section III.4.3.2.1)
may also become more efficient at higher temperatures. These insects rely partially
(although not entirely, as shown in this chapter) on micro-organisms for the digestion of
the lignocellulose-rich plant materials that they feed on. Their species diversity and envi-
ronmental significance are greatest in the tropics and this may result from the increased
efficiency of their mutualistic digestive systems in these warmer climates (Lepage, 1983).

4.4.3 DETERMINATION OF SOIL INVERTEBRATE COMMUNITIES

4.4.3.1 Local responses to the vertical gradient of food resources and environment

predictability: trade-offs in adaptive strategies

Within a single zoological group, possessing similar phylogeny ( i.e., with the same basic
morphology, anatomical and physiological organisation), individual species may apply
different solutions to the problem of exploiting the resources of a given soil. This is
illustrated by the adaptations of three sympatric earthworm species to a sandy alfisol in
the humid savannas of Lamto (Côte d'Ivoire) (Table III.18). This environment is
characterised by the contrasting gradients of food of diminishing quality and increasing
environmental predicability that exist from the surface to deep soil horizons. The species
present have adapted to distinct sections of this gradient by having characteristically
non-overlapping size ranges and utilising different nutritional resources. The three species
discussed below show the most contrasting adaptive strategies.
Leaf litter, the highest quality nutritional resource, occurs largely on the soil surface
and organic matter concentrations diminish markedly with depth. Further, mortality due
to environmental unpredictability and exposure to predators tends to decrease with
depth. Thus, Dichogaster agilis, a litter feeder, has a relatively rapid growth rate since it
uses a high-quality resource although mortality is also high so close to the surface.
Therefore, individuals must complete their life cycles in a short time to maximize their
reproductive success; this is achieved by limiting their body size, which allows a higher
basic metabolic rate.
In contrast, Millsonia ghanensis lives in the deeper horizons where the soil organic
matter it feeds on is a very poor quality resource, although risks of mortality are reduced.
It completes its life cycle over much longer periods and has a relatively-low metabolic rate.
Its large size confers a selective advantage as these worms must possess considerable
strength to penetrate the relatively dense soil. This and similar species ingest large quan-
tities of soil since the organic matter at its preferred depth is both sparsely-distributed