FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 511

lines, or by concentrations of particles too large to be transported by the termites ( e.g.,
Williams, 1968; Wielemaker, 1984; Johnson, 1990).
Alternatively, where surface erosion removes the fine materials brought to the surface, a
coarse-textured surface soil may result (Wielemaker, 1984; Humphreys, 1994).
On sloping surfaces, downhill transport and sorting of materials is hastened by the actions of
termites and other burrowing animals through the exposure of soil materials to rainsplash and
overland water flows. Where prevalent, termites therefore contribute materially to the for-
mation of catenas (Nye, 1954,1955abc; Wielemaker, 1984).
Net annual increments in the thickness of fine-textured, termite-derived surface
soils have been estimated to range from 0.01 to 0.10 mm (Lee and Wood, 197 la). This is
an important pedological and ecological process in the large areas of the tropics and sub-trop-
ics where the termite fauna is dominated by species that build soil materials into epigeal
mounds and other above-ground structures, Beneath the surface, changes in texture - and
therefore water holding capacity - and the distributions of organic and mineral matter also
reflect the composition and functioning of local termite communities.
5.3.1.2 Effects on soil structure
Termites have profound effects on soil structure through their mound-building activities,
the formation of aggregates and other structures and through the creation of extensive
systems of subterranean nests, galleries and storage chambers. Epigeal mounds are
highly-visible structures of substantial importance in terms of pedogenesis and plant
growth, both directly and in terms of their erosion products. However, the many species
that nest underground also have substantial effects on soil structure. Humphreys (1994)
considered that the four termite species present in the sandy ultisols and entisols at
his study site in southeastern Australia were more important as sub-surface bioturbators
than mound builders.
While a range of termite-derived structures are known from epigeal mounds and
gallery walls (see, for example, Sleeman and Brewer, 1972; Garnier-Sillam el at, 1985),
less is known of their contribution to soil aggregation away from the immediate
environs of the mounds, or of the role of subterranean-nesting termites in this regard.
Termite galleries act as networks of horizontal and vertical macropores influencing bulk
density, aeration and water infiltration and forming conduits for the movement of mate-
rials upwards, downwards and laterally within the soil (see, e.g., Wielemaker, 1984).
Movement of soil materials through the galleries is mediated both by the activities of
the termites and by physical processes.
Termites influence soil structural stability in the vicinity of their mounds although
the effect is dependent on the ecological strategies and nesting locations of the termites
involved. Garnier-Sillam et al. (1988b) recorded the contrasting effects of two species
in a humid African forest environment. The soil-feeding species Thoracotermes
macrothorax increased the structural stability of the soil in its area of influence while
the fungus-cultivating species Macrotermes muelleri reduced it. The effects of termite
activity on drainage properties are considered below.