Agriculture Reference
In-Depth Information
of the mineral soil using macropores or the galleries of burrowing animals;
(iii) tunnelling through the soil. Only the larger soil animals have sufficient strength to
do this and this behaviour only occurs in anecic and endogeic earthworms, termites, ants
and a few insects with fore-legs modified for digging,
e.g.,
nymphal Cicadidae,
Gryllotalpidae (Orthoptera), Cydnidae (Hemiptera) or Scarabaeidae (Coleoptera).
The ability to dig gives such invertebrates a great selective advantage. Like anecic
earthworms or termites, these invertebrates are able to feed in leaf-litter (where the
highest-quality resources are found) and avoid the frequently unfavourable environmen-
tal conditions and high predator densities that occur in this environment. Some of these
invertebrates are also active root-feeders.
4.4.1.4
Adaptations to temporarily unfavourable climatic conditions
Smaller soil organisms are able to survive drought and sub-zero temperatures since
they can shelter in favourable microsites when overall conditions become unfavourable.
This is especially true of bacteria and protists which may remain in the water films or
micropores 1 or less in diameter, that is in soils with matric potentials as low as
-5 MPa (pF 5). Protists and nematodes have also developed such remarkable forms
of drought resistance as anhydrobiosis and encystment.
Large invertebrates such as earthworms do not survive long in very dry soils since
their water-conserving mechanisms are poorly developed in most cases. However,
certain organisms modify their environments to obtain better protection. This is
especially true of such social insects as ants and termites, which may build nests with
highly-buffered temperature and moisture conditions.
4.4.1.5
Response to soil constraints: adapt to, or modify the environment?
The efficiency with which soil organisms exploit particularly low quality soil resources,
their ability to move and their resistance to unfavourable environment and moisture
conditions are largely determined by their sizes and their individual or social behavioural
responses to the constraints considered above. Small animals cope better with tempera-
ture and moisture extremes although they are less able to use low-quality resources and
modify their environment. Conversely, large soil animals make better use of low-quality
food but are poorly adapted to extreme environmental conditions. However, some groups,
notably the termites, have a great ability to transform their environment.
Ants are an exception to this in that they normally utilise high-quality resources
through strategies of predation, nectar and seed feeding.
Individual size partly determines the ability to use microflora in mutualistic digestive
systems. A minimum size is necessary to allow transport and temporary or semi-permanent
storage of an associated microflora in the gut:
(i) the microfauna can only use micro-organisms as food;
(ii) the mesofauna also contains many microbivores. However they also transport
micro-organisms and partly utilise an 'external-rumen' type of digestion;
(iii)
the macrofauna has the ability to develop the external-rumen system on a large
