Agriculture Reference
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carbohydrates are passed on to the midgut where further enzymatic degradation occurs
(Febvay and Kermarrec, 1986). Proteases acquired from the symbiont are passed through
the worker gut largely unaltered (Martin, 1984) and are applied to the harvested leaf
material facilitating its utilisation by the symbiotic fungus.
Apart from the above, the only non-pathogenic interaction between ants and micro-
organisms so far described is that between the harvester ant Pogonomyrmex occidental-
is (and certain other Pogonomyrmex species) and the elevated abundances of the hyphae
of vesicular-arbuscular mycorrhizal fungi that occur in the mound materials of this
species (Friese and Allen, 1993).
P. occidentalis clears all vegetation from circular areas around its nest and such areas
may cover up to 10 % of the ground surface. Through the return of food materials to their
nests, these areas become enriched in nutrient elements and are heavily exploited by very
dense mats of VA mycorrhizal-infected roots of the shrub Artemisia tridentata. Ants clip
the VAM-infected root tips, presumably to prevent them from obstructing galleries
although it is unknown whether the ants ingest any of this material. Following colony
death, rapid invasion of the cleared areas by A. tridentata and a mycotrophic grass is
facilitated by the presence of the highly infective VA inoculum of the mounds.
4.3.3.5
Geographic distribution of ants
Ants are spread widely throughout the world and the approximate numbers of species
known from the major biogeographical regions of the world are indicated in Table III.17
(from Bolton, 1995). As stated by these authors, many species still remain to be described.
Because of their marked thermophily, ants have radiated most widely in the tropics
and in a parallel with the termites, the number of species declines steadily with increas-
ing latitude. Ants are absent from Iceland, Greenland and Antarctica (Hölldobler and
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