Agriculture Reference
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scale. Alternatively, a large-scale utilization of soil resources by a facultative or obligate
symbiotic microflora may occur in their guts.
As a result, the capacity of soil invertebrates to use low-quality resources normally
increases with size, other environmental conditions being equal.
Size also determines the mobility of invertebrates. The smaller they are, the less
effectively can they move and the more closely they depend on the distribution and
continuity of water films and/or the pore space. Conversely, large animals may range
much further provided that they can dig through the soil when their diameter is greater
than the average pore size.
Finally, size partly determines the ability of invertebrates to survive temporarily
unfavourable conditions and the larger invertebrates are generally less successful at this.
These differing responses may partly explain why the distributions of micro- and, to a
lesser extent, mesofaunal taxa are continuous whereas macrofauna, especially termites
and earthworms have highly discontinuous distributions at both regional and geograph-
ical scales.
The behaviour of individuals and populations may modify this general pattern.
Aggregation and social organisation are most important in this respect:
(i) aggregation is frequent in most groups of mesofauna; although sometimes interpret-
ed as the result of purely biological features (concentration of egg-laying sites, response
to the heterogeneity of resource distribution); its adaptive value has been pointed out by
several authors (Joose, 1970; Usher, 1975; Verhoef and Nagelkenke, 1977). In contrast,
the non-social large invertebrates which do not rely on external-rumen types of digestion
(
i.e.,
predators and earthworms with an 'internal-rumen' type of digestion), generally do
not have such highly-aggregated patterns of distribution.
(ii) social organisation is an efficient way to enlarge the scale at which individuals
operate, and thus the range and abundance of resources they can utilise. Co-ordinated
efforts among attine ant and macrotermitine termite workers allow the construction and
maintenance of fungal gardens in which cellulose and lignin are transformed into
secondary resources readily assimilable by individuals. Such fungal gardens represent
the most highly evolved and efficient external rumens found among soil invertebrates.
Social organisation also allows species to modify their environments to an extent that
far exceeds the capacity of a single individual. Termites are individually extremely
sensitive to low humidities but nevertheless survive well in semi-arid and arid environ-
ments. The nests, galleries and surface sheetings within which they live have a constantly
humid atmosphere due to acquisition of water from the water-table which may sometimes
be many metres deep.
4.4.1.6
Functional guilds: microfoodwebs, litter transformers and ecosystem engineers
Soil invertebrates have a continuum of strategies from the smallest microfauna that
colonise the water-filled pore space in the same way as micro-organisms, to macrofauna
that modify the soil environment to accommodate their Own needs. The size of inverte-
brates and the aquatic or aerial nature of their respiration, reflect their way of adapting
to spatial constraints. Three groups have been distinguished (Bachelier, 1978; Swift
et al
., 1979):
