SOIL FORMATION - Soil Ecology - page 176

The leaf-cutter ants of the tribe Attini (Chapter III) occur throughout much of the
Neotropical and the southern part of the Nearctic regions. Certain members of this group
excavate large quantities of soil materials during nest construction, much of which may
be deposited on the surface. The pedogenetic effects of these ants are indisputable.
Alvarado et al. (1981) reported an amount equivalent to 460 Mg of AB and B horizon
subsoil spread over the soil surface and Boulet et al. (1995) estimated at 0.21 to 0.23 mm
the thickness of the layer annually deposited at the soil surface.
Other ants construct epigeal nests on the soil surface and these may become abundant
in certain environments. Population densities of the mounds of Lasius flavus recorded by
Wells et al. (1976) from British grasslands were locally equivalent to 5000 mounds per
hectare and their bases occupied 15.6 % of the site surface area. The selective nature of
their contribution to vertical translocation is evidenced by the finer texture of the soils
comprising the mounds.
Apart from these single nests, compound structures built by ants may achieve
several metres in diameter and rise well above the local topography; these have entirely
different soil characteristics from those surrounding (Cox et al., 1992). Construction of
these large mounds occurs through generations of ants building nests in the same
location, starting from a local topographic high point and gradually extending through
a combination of vertical and lateral transport of materials selected from the underlying
and adjacent soils.
In addition to translocating soil materials to the surface, ground-dwelling ants return
organic foodstuffs to their nests. In particular, the downward translocation of organic
materials into the nests of certain leaf-cutting ants may be substantial; reported
consumption estimates range up to the equivalent of several tonnes per hectare per
year of dry plant materials (Fowler et al., 1986). Further, ants can have considerable
effects on profile morphology. Eschenbrenner (1994ab) reported the presence of
chambers that had been back-filled with loosely-packed mineral soil in attine
ant nests in Martinquan Andisols while Alvarado et al. (1981) found similar structures
infilled with organic matter and mineral soil in Costa Rican inceptisols. The latter
authors also reported large modifications to the number and type of the horizons present
because of the large amounts of soil translocated. Underground galleries and chambers
may be infilled by illuviation and other transport processes after their abandonment.
Because of their abundances, particularly in the inter-tropics, ants indirectly influence
soil processes through predation and other trophic interactions with the rest of the biota.
Non-trophic interactions (ecological engineering; Jones et al., 1994), include habitat
creation for other animals. Humphreys (1994) reported constructs formed by small
mesoarthropods active within infilled ant galleries excavated in soils formed from
quartzose sandstones in southeastern Australia.
Faunal induction of translocation and creep
Where they occur together, termites and earthworms may have complementary effects.
In forested areas of Nigeria, Nye (1954, 1955abc) described a topographic sequence of
soils with two primary horizons: a surface horizon of soil material moving slowly
downhill under the influence of soil creep (Cr) and an underlying sedentary horizon (S).
The Cr horizon is subdivided into three separate layers namely, a CrW, formed from