Agriculture Reference
In-Depth Information
4.3.3.8
Effects of ants on soils
While arboreal ants have few direct effects on soils, many ants nest on and within
soils where they may build epigeal mounds and construct systems of galleries, nesting
chambers and other voids. Through their contributions to bioturbation (Levieux, 1976;
Cowan
et al.
1985; Lockaby and Adams, 1985), they add their effects to those of
other soil-based ecosystem engineers and to the landscape level processes of soil and
biomantle formation discussed in Chapter II.
Ground-dwelling ants, particularly the mound-building ants can be considered
as ecosystem engineers (Folgarait, 1998) in that they modulate the availability
of resources and alter the soil and surface environments in ways that affect other
organisms (Jones
et al.,
1994). Where abundant, ants modify the physical structure of
the soil through the creation of systems of galleries and chambers and this influences soil
porosity, aeration, infiltration and drainage and creates habitat for smaller soil organisms.
Changes to texture and other soil properties occur through the vertical and horizontal
transfer of materials of different particle sizes and the creation of new soil horizons
(Alvarado
et al.,
1981; Gotwald, 1986). They also produce characteristic structures in
soils, different from those found in the surrounding soil matrix (Humphreys, 1994).
The activities of ants can also influence the chemistry of the soil, notably by
increasing the amounts of organic matter, phosphorus, potassium and nitrogen in
the mounds (Petal, 1978; Carlson and Whitford, 1991; McGinley
et al.,
1994) although
such increases are not universal (Lobry de Bruyn and Conacher, 1990). Wagner (1997)
showed that soil materials associated with the workings of
Formica perpilosa
in a North
American desert environment had a higher organic matter status and elevated phosphorus
and nitrogen concentrations compared with the surrounding soils. Where these materials
were associated with the bases of
Acacia constricta
shrubs, these plants set approxi-
mately twice as much seed as those without ant workings. In southeastern Australia,
a nearly fifty
per cent.
increase in the growth of
Eucalyptus obliqua
seedlings was
obtained in mound materials of
Aphaenogaster longiceps
compared with that in more
distant surface soils (Andersen, 1988). The physical changes and elevated chemical
status of many soil materials associated with ant mounds induce greater mineralisation
activities by decomposers (McGinley
et al.,
1994) and heightened root and mycorrhizal
growth, as shown in mounds of the harvester ant
Pogonomyrmex occidentalis.
Plant communities differing from those of the general environment may occur on or
surrounding ant mounds as a response to the altered physical and chemical conditions
pertaining (King, 1977; Culver and Beattie, 1983; Carlson and Whitford, 1991).
4.3.4
MACROARTHROPOD COMMUNITIES
Soil macro-arthropod communities include a wide range of taxa. The most important
groups are Myriapoda, Coleoptera, Isopoda, Diptera larvae and Araneae although
some Homoptera (larval Cicadidae), Heteroptera (Gryllidae), Thysanura, Blattoidea,
endogeic Hemiptera or lepidopteran larvae may occasionally be important. Most of these
macro-invertebrates have a much lesser capacity to move through the mineral soil than
earthworms, termites or ants, Thus, most live in the litter or in the upper few centimetres
