Agriculture Reference
In-Depth Information
Effects of subterranean nests and epigeal mounds
Epigeal termite mounds are usually externally hard, massive structures and shed almost
all of the water that impinges on them. Through the energy of rainfall impact and runoff,
a steady erosion of mound surfaces occurs, even while they still contain active popula-
tions (Bonell
et al.,
1986). There is a considerable sorting of the eroded material: finer
materials are transported furthest leaving coarser-textured materials close to the mounds
(Janeau and Valentin, 1987) (Figure IV.64). Sometimes, sufficient fine-textured disper-
sive material is deposited close to the mounds to form surface seals which may
continue to modify local infiltration and drainage patterns until disrupted through
the activity of the soil fauna (Janeau and Valentin, 1987), or other agencies.
The bioturbing effects of termites have been used as part of systems for rehabilitating
certain degraded soils in Burkina Faso. Roose
et al.
(1996) found that surface mulches
attracted termites, locally disrupting surface crusts. In experimental studies of these
crusted soils, Mando (1997) found that these burrowing activities increased infiltration
rates and saturated hydraulic conductivity, reduced bulk density and cone penetration
resistance. These effects were not evident in mulched soils from which the termites had
been removed by pesticide application.
Mound erosion may exclude some epigeal mound-building termites from high
rainfall areas, particularly those species whose mounds lack specific adaptations for
water shedding. The repair of these losses must entail substantial energy costs for
the termite colony, both during mound building and repair. The soil materials added
to epigeal termitaria during both processes are initally soft and particularly susceptible
to erosion, at least until hardened by curing or internal repacking processes.
After the colony has died, or the mound has been broken up, the resistant mound
materials erode slowly and may continue to modify local drainage patterns for some
time. Where mounds are common, an area equivalent to the entire soil surface will have
been covered by their bases over quite short geological times. As stated above, the
median area occupied by the bases of the mounds recorded from the northern Australian
sites discussed above is 0.8 % of the land surface area. If this percentage were to remain
constant over time, an area equivalent to the whole surface will have been covered
by the bases of termite mounds in approximately 125 generations of mounds. If the
standing mass of the mounds is assumed to be a constant 20 Mg and turnover time
of the mound materials is assumed to be of the order of 30-50 years, this is equivalent
to the distribution of 2500 Mg (or a layer of soil
ca.
20 cm thick with bulk density
1.3 Mg ) over the surface during a period of 3730-6250 years, a short time in terms
of soil formation.
Simple calculations of this type patently underestimate the effects of termite mounds
on the soil. Many incipient colonies build small mounds but soon die out through
the effects of predation, competition or other factors. Further, the masses of soil
materials brought to the surface to form runways and to cover food supplies have
only occasionally been quantified. The few data available suggest that this may be in
the order of 1 Mg (see, for example, Lepage, 1981b, Bagine, 1984).
The turnover times of mound soil materials are poorly known but are substantially
longer than the life spans of the colonies that construct the mounds. An individual mound
of the Australian grass-harvesting termite
Nasutitermes triodiae
in northern Australia
