Agriculture Reference
In-Depth Information
to be the softer materials. Wheeler (1910) records that a number of hypogaeic ant
species feed on underground plant parts including the cotyledons of germinating seeds.
These materials are probably eaten more for their more readily-digestible components
than for their cellulose.
Many of the relationships between ants and plants have co-evolved over time to
produce an apparent mutual benefit to both groups. However, in reviewing a number
of studies of this phenomenon, Becarra and Venable (1989) reported that only 43 % of
the 28 associations they examined were beneficial to the plant and that many of these
associations may be facultative.
Seed harvesting
Seed harvesting is a way of life for a number of species of the subfamilies
Ponerinae,
Formicinae
and most notably the
Myrmicinae,
which feed wholly or largely on seeds
which they collect and transfer to their nests. These are known as harvester ants and
dominate in deserts and drier grasslands in temperate and tropical environments;
seed harvesting may buffer them against temporal changes in the availability of food
(Handel and Beattie, 1990). While most seeds gathered by harvester ants are destroyed,
some are lost during transport and others may germinate in the nests or on refuse dumps.
Seed-harvesting ants are therefore simultaneously agents of destruction and dispersal
for the plant species whose seeds they feed on and they may substantially influence
the distribution of vegetation in space, and over time.
Seed harvesting ants are also active in such humid environments as rainforests.
Levey and Byrne (1993) reported that
Pheidole
spp. harvested small seeds from frugi-
vore faeces on the forest floor of a Costa Rican lowland rainforest. Depending on the
species, 25-32 % had seed cached in their nests and 24-32 % had discarded seed on their
refuse piles. In glasshouse studies, seedlings of one genus grew substantially better on
the refuse piles than on local topsoils, illustrating the advantage to the surviving plants.
Many other ant species occasionally use seeds as food materials but contrast with
the more specialist seed feeders. Even such prominent carnivores as the army ants may
collect and consume appreciable quantities of seeds (Gotwald, 1974).
Elaiosomes (arils)
Seed dispersal by ants is a widespread feature in all continents where they occur (Handel
and Beattie, 1990) and plants whose seeds are spread largely by ants are termed myrme-
cochores. Berg (1975) lists more than 1500 species of these plants from Australia and
similar numbers have been recorded from South Africa (Bond and Slingsby, 1983).
The evolution of structures on seeds that contain materials attractive to ants has
occurred many times and the structures involved originate from a diversity of tissues.
Elaiosomes are large bodies attached to seeds that contain such materials as lipids,
proteins, starch and sugars that are attractive to ants (Beattie, 1985). On being shed from
the plant, the seeds and their attached elaiosomes are transported by ants towards their
nests, the elaiosomes eaten and the seeds discarded undamaged, often within or in the
vicinity of the nests.
In addition to elaiosome-related transport by ants, compound seed dispersal mechanisms
are known that also involve other animals. Clifford and Monteith (1989) report the three
