FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 524

5.4.2 INDIRECT EFFECTS

5.4.2.1 Termites and the local distribution of vegetation

Termites interact with vegetation to influence its composition, distribution and
productivity, although this interaction is also modified by other factors including
herbivores, saprophytic organisms and environmental factors such as rainfall and fire.
However, an important local factor is the presence of epigeal termitaria, particularly
those of grass-harvesting and litter-feeding termites. The mound materials of these
species may be substantially enriched in plant nutrient elements in comparison to their
surrounding soils. Where these mounds are populous, it is partly through the dynamics
of their construction and breakdown that termites influence the nature and distribution of
the vegetation present. A major effect of the presence and erosional reduction of these
termitaria is an increased nutrient patchiness in the landscape (Salick et al., 1983);
in some savanna environments this provides sites for species requiring a higher soil
nutrient status than that available in the general environment (Spain and McIvor, 1988).
In degraded areas, the same process may provide locally fertile sites for the initial
regeneration of vegetation and the re-establishment of nutrient cycles.
Such erosional breakdown of mounds is not always beneficial. In parts of northern
Australia, erosion of mounds of the litter-feeding termite Amitermes laurensis may lead
to the deposition of an annulus of fine-textured soil materials around the mound bases.
These deposits dry to form an impermeable crust, inhibiting the growth of the surround-
ing vegetation.
Subterranean termites may also influence vegetation growth. Whitford (1991) reported
increased soil N concentrations 14 years after experimental eradication of the termite
Gnathamitermes tubiformans from plots in a Nearctic desert environment. Further, the
reduced water infiltration and storage reported apparently led to the eradication of
the perennial grass Erioneuron pulchellum and higher litter production in the deep-
rooted shrub Larrea tridentata. The increased litter mass present on these plots has
also provided a habitat and food source for selected micro-arthropods, notably cryp-
tostigmatid mites and psocopterans.
In sandy soils (alfisols) overlying laterite in Niger, areas of both poor and enhanced
growth of the woody species Faidherbia albida have been found (Miedema et al., 1994).
The greatest growth occurred on soils close to abandoned Macrotermes mounds, but not
on the mound materials themselves. The structure and texture of the materials support-
ing the greatest growth had higher clay and organic matter contents and therefore greater
water-holding capacity and an enhanced soil nutrient status.
As shown in semi-arid environments of Burkina Faso, termitaria may influence both
long and short term landscape dynamics in sites with banded vegetation distributions
(Ouedraogo, 1997). In this environment, termite mounds cover, respectively, 2.7 % and
2.3 % of the soil surface in unvegetated and vegetated strips. Termite activities and
structures:
affect soil porosity by creating galleries that open at the soil surface (375 holes
in the vegetated zones where termites are highly active: almost none in the unvegetated
zones) and considerably loosen the soil;