SOIL ORGANISMS - Soil Ecology - page 317

their surface. However, because of their feeding habits, they may perhaps be of less
general importance in directly regulating soil function than the termites or earthworms.
Tropical American Atta leaf-cutting ants (Tribe Attini) represent a spectacular exception
in that they make huge subterranean nests and their harvesting may lead to the incorpo-
ration of large amounts of organic matter and nutrients into the soil. Many other ants nest
in the soil although they may also form part of the aerial food-web. In some locations,
ants may be important agents of bioturbation (Levieux, 1976; Cowan et al., 1985; Lockaby
and Adams, 1985). A number of species also concentrate plant nutrients in their nests
and associated soils ( e.g., Wagner, 1997).
Ants have a wide latitudinal distribution extending from the arctic to the tropics; they
occur in all but the most extreme of the world's ecosystems although native ants are absent
from some isolated oceanic islands. Where significant populations occur, they often
dominate the activities of the biota below, at and above the soil surface. Most species are
effective predators although many also feed at least partially on materials of plant
origin and a number of species are parasitic. In contrast to the termites, the ants have
not specialised in directly utilising cellulose as a food resource.
Ants have achieved their ecological dominance through their wide taxonomic diversity
and numerical abundance, their successful development of a stable sociality and the
broad diversity and pliability of their ecological strategies (Wheeler, 1910). As effective
predators, ants influence herbivore populations and therefore plant productivity.
Hölldobler and Wilson (1990) attributed the success of the ants to a range of causes.
They were the first predaceous, eusocial insects to live and forage primarily on the ground
and within the litter layers. Their successful evolution of sociality during the Cretaceous
and their later radiation within the Tertiary period thus pre-empted the development of
other groups. Perhaps a major part of their successful adaptation to soil dwelling was
the development of the metapleural gland, an organ that produces antibiotic secretions.
These secretions are spread throughout the ants' nests and appear to protect them
from pathogenic micro-organisms that might otherwise affect them in the humid
soil environment.
A broad range of relationships exists between ants and plants. These vary in degree from
mutualisms in which, for example, certain plants (myrmecophytes) provide shelter or
food to a resident population of ants in exchange for possible protection from herbivores,
to herbivory in which plants are damaged either directly by consuming their tissues or
indirectly by eating their seeds. Ants of the tribe Attini culture fungi on collected food
materials in a way an analogous to the macrotermitine Isoptera (Weber, 1982).
As with the termites, ants also modify soil chemical and physical properties by
transporting food and soil materials during such activities as feeding and mound and
gallery construction. These activities affect soil developmental processes and fertility
and may modify the nature and distribution of the vegetation, at least on a local scale.
Through their manifold activities, ants frequently impinge on man and his interests.
Many ants have highly-efficient defense mechanisms and use their effective stings and
mandibles to repel man and other vertebrates from their areas of influence. In agricultural
situations, ants may protect such crop herbivores as honeydew-secreting Homoptera or
act directly as herbivores themselves. Nonetheless, their beneficial role as effective
general predators should also be noted (Way and Khoo, 1992). In parts of the Neotropical