Pollination
The pollination ecology of Banksia has been well studied, because the large showy inflorescences make it easy to conduct pollination experiments, and the pollination roles of nectariferous birds and mammals makes the genus a popular subject for zoologists.
Visits to Banksia inflorescences by Western Honeybees and nectariferous birds are often observed and are obviously important to pollination. Also important are visits by nectariferous mammals, although such visits are rarely observed because these mammals are usually nocturnal and reclusive. Studies have found that Banksia inflorescences are foraged by a variety of small mammals, including marsupials (such as possums and Yellow-footed Antechinus, Antechinus flavipes), and rodents (such as the Pale Field Rat, Rattus tunneyi). These carry pollen loads comparable to those of nectariferous birds, making them effective pollinators. Other studies have shown that the relative importance of vertebrates and invertebrates for pollination may vary from species to species, with some Banksia species exhibiting reduced fruit set when vertebrate pollinators are excluded, while others are unaffected by the exclusion of vertebrates and set some fruit even when all pollinators are excluded.
Almost all Banksia species studied so far have shown outcrossing rates among the highest ever recorded for plants; that is, very few Banksia plants are found to occur as a result of self-fertilisation. There are a number of potential reasons for this:
One possibility is that Banksia flowers are simply not exposed to their own pollen. This is highly unlikely for two reasons. Firstly, the morphology of the Banksia flower makes it virtually inevitable that the stigma will be exposed to its own pollen, since it functions also as a "pollen-presenter". It has been suggested that this problem would be avoided if the flowers were strongly protandrous, but the evidence so far supports only partial protandry. Moreover, the question of protandry of individual flowers is probably irrelevant, because the sequential anthesis of flowers means that each inflorescence will typically contain flowers in both male and female stages at the same time. Observations of foraging patterns in pollinators have shown that transfer of pollen between different flowers in the same inflorescence is inevitable.
Another possibility is that the high outcrossing rate is due to self-incompatibility, due either to a failure to fertilise or abortion of self-fertilised fruit. Studies have shown self-compatibility of pollen to vary between Banksia species, with some but not all species inhibiting the growth of pollen tubes for pollen from its own flowers. A more likely form of self-incompatibility is the spontaneous abortion of fruits that have been self-fertilised. These could be caused either by the expression of lethal genes, or the expression of genes that, while not lethal, cause the maternal plant to abort. Genetic causes are thought to be a common form of self-incompatibility, because of the high genetic load of the genus. However abortion rates are difficult to assess because the ovaries are deeply embedded in the "rhachis" (woody spine) of the inflorescence.
Finally, there is the mechanism of "facultative" abortion of fruits, where a maternal plant without the resources to mature all fruit aborts the least vigorous ones. This is thought to be common in those taxa that are generally self-compatible, since even these have high outcrossing rates. For example, Banksia spinulosa var. neoanglica, one of the most self-compatible Banksia species, has been shown to set far more cross-pollinated than self-pollinated fruit.
A few species, such as B. brownii, are exceptional in having low outcrossing rates. In all cases these are rare species that occur in very small populations, which increases the probability of self-fertilisation, and may discourage visits by pollinators.
Read more about this topic: Ecology Of Banksia