Seed dispersal

Some plants have developed relationships with animals to help them spread their seeds. Ants are the most commonly involved insect in seed dispersal. Seeds that are dispersed and buried by ants have a number of advantages that make seedling growth more likely.

Seed dispersal is the removal of seeds from a plant to another location. Seed dispersal influences the structure of plant communities and seedling survival.

Many insects, and their young, feed on seeds. These include beetles (e.g. weevils), bugs (e.g. seed bugs), wasps, ants, thrips and some moth species. In the process of feeding, seeds can be dispersed by simply knocking the seed from the plant to the ground or by being carried great distances from the plant.

A few insects use seeds for other purposes. For example, the seeds of the Cadagi Eucalypt contain a resin that is highly sought after by meliponine bees - for use in nest building. Bees collecting the resin, inadvertently carry away the sticky seeds. The bees discard the seeds as soon as they can because they can cause their nests to become blocked. Bees forage a long way from their nests, so seeds can be dropped some distance away from the parent plant. This is beneficial for the plant if the seed lands in a suitable habitat.

Ants are generally regarded as the most effective at seed dispersal. Seed dispersal by ants is very important in Australia, with many plants relying on, and consequently encouraging, ants to disperse their seeds. Australia has a high diversity of seed-collecting ants that occur in most habitats across the continent. Outside Australia, seed-collecting ants are generally restricted to arid regions.

Five potential benefits of seed dispersal by ants to both plants and animals are that it:

1. Reduces competition between young plants and their parents. By distancing the seed from the parent plant and sibling seedlings it lowers the likelihood of competition for resources.
2. Reduces the amount of seeds lost to predation. By moving the seeds into ant nests, it is more difficult for other seed-eating animals to get to them.
3. Provides favourable conditions for seedling growth. Soil in ant nests is less compacted and richer in nutrients than surrounding soils. This is a great advantage to seedlings in arid environments like the Australian interior, which generally have hard, infertile soils.
4. Provides protection from harsh environmental conditions. By moving the seeds below ground they are protected from fire and high summer temperatures.
5. Provides protection for eggs of other insects. Some insects have exploited the seed dispersal behaviour of ants. Stick insects, for example lay eggs that mimic seeds. These seed-like eggs are taken back to ant nests where they are guarded or discarded by the ants. When the young stick insect hatches, some species look and behave much like an ant. This method acts to disperse the stick insects as much as it does the seeds they mimic.

Many plants actively encourage ants to disperse their seeds with chemical attractants and nutritional benefits. The ant gains a reward for dispersing the seed and the plant species has a greater chance of survival. This partnership is referred to as myrmecochory. In Australia this process is practiced by more than 1500 species of plants.

Ants are especially attracted to seeds that have food bodies known as elaiosomes. These food bodies are generally rich in fatty acids, amino acids and sugars. They can be easily removed and digested by the ants and are often laced with a chemical attractant that stimulates collecting behaviour.

The food body usually differs in shape and colour to the main body of the seed in such a way that it is easier for the ant to carry the entire seed. The ants grip the food body with their mouthparts and carry the seed back to the nest. The food body is then removed and fed to the larvae. The remainder of the seed due to its size, unwieldy shape or hard coating is usually left intact in the nest.

Five ways that seeds may be dispersed by ants:

1. At the time of collection. Seeds collected directly from the plant may be dropped onto the ground.
2. During transport back to the nest seeds maybe lost.
3. Before the food body is removed at the nest. Ants, for a variety of reasons, may vacate their nests, abandoning stockpiles of unconsumed seeds.
4. At the time of removing the food body. Often when the food body has been removed, the remainder (i.e. the seed) is too unwieldy for the ant to move.
5. After the food body has been removed. Many ants relocate seeds to abandoned galleries or refuse piles. Some ants use the seeds in the construction of their nests.

References

• Anderson, A.N. 1991. Seed harvesting by ants in Australia. In: Huxley, Camilla R; Cutler, David F (eds). Ant-plant interactions. Oxford University Press, Oxford. pp. 493-517
• Berg, R.Y. 1975. Myrmecochorous plants in Australia and their dispersal by ants. Australian Journal of Botany 23:475-508.
• Boyd, R. 2001. Ecological benefits of Myrmecochory for the endangered Chaparral shrub Fremontodendron decumbens (Sterculiaceae) American Journal of Botany 88(2): 234-241.
• Gullan, P.J. and Cranston, P.S. 2004. The insects: an outline of entomology. Blackwell Publ. pp. 270.
• Hölldobler, B. and Wilson, E.O. 1990. The ants. Springer-Verlag, Berlin. pp. 549-551.
• Yen, A. and Butcher, R. 1997. An overview of the conservation of non-marine invertebrates in Australia, Environment Australia.
• Zhang J, Drummond F. A., Liebman M, and Hartke A. 1997. Insect predation of seeds and plant population dynamics. MAFES Technical Bulletin 163: 1-32.