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Pollination and Seed Dispersal

Acacias are essentially insect pollinated; beetles, wasps and bees being mostly involved. Occasionally birds and mammals may be implicated (New 1984, Breeden & Breeden 1972, Knox et al. 1985) and although ants may be sometimes observed on the flowers they are probably not effective pollinators. However, there appear to be no specific pollinators and those that are involved are mostly generalists. For example, in a study (Bernhardt 1996) on bee pollination of acacias many native bees were observed to take Acacia pollen and also visit other flowers and carry their pollen, and presumably the pollen of other nearby flowering acacias. That there are relatively few Acacia hybrids suggests the existance of strong barriers to cross-pollination in Acacia. The European bee (Apis sp.), which collects pollen is now a common pollinator. Although the flowers lack nectar, many of the insects and some birds and aminals consume the pollen which is a rich source of protein. Insects, such as mites and thrips, feed on the flowers themselves, and some of the beetles and wasps appear to prey on these smaller insects, and may only incidentally serve as pollination vectors. Some honeyeaters have been cited by Ford & Forde (1976) as probably effecting pollination when visiting the nectaries (glands) of phyllodes near the inflorescences of Acacia pycnantha, either to take the nectar secretions or the insects associated with the secretions. In this case it was argued that the honeyeaters were more likely to facilitate outcrossing by visiting other nearby, as well as, more distant plants than would be the case with insects.

Acacia seeds are dispersed mostly by being ejected from the legume when it opens, usually under the influence of the hot sun. In some cases the seeds may remain hanging by their red or orange-coloured funicles from the open legume, the coloured funicle and aril acting as a bird attractant. The birds disperse the seed while its passage through the bird's gut may assist in germination. Emus and Mallee Fowl are also known Acacia seed dispersers.

Ants have been observed harvesting fallen seed. It is thought that the ants consume only the fleshy aril. This ant attracting structure, known as an elaiosome, is found in a large number of Australian xerophytic plants whose seeds are dispersed by ants. Such plants are known as myrmecochorous plants, and the benefits derived by such plants appears to be mainly in burying the seed and protecting it from predation rather than the actual removal of the seed further from the parent (Berg 1975). The soil covering will also protect the seed from the extreme temperatures of wildfires and possibly ensuring germination only after penetrating rain, although this is doubtful for deeply buried seed. Some seed, along with other detritus, may be deposited around the nest entrance, and this may provide a rich source of nutrients for germinating seedlings. See also Auld (1996).

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Written and compiled by Phillip Kodela, Terry Tame, Barry Conn, Ken Hill, Linn Linn Lee
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