Pollination and Pollinating Vectors.
Pollination is the delivery of pollen (containing males gametes and manufactured by the anthers of a flower) to the female stigma of a flower of the same species. This results in fertilisation and the production of seed. Pollen may be transferred to a flower on the same plant (self-pollination) or a different plant (cross-pollination). Pollen may be dispersed by wind and water, however, more than 80% of flowering plants, together with some seed plants such as cycads and pines, depend upon biological vectors, or pollinators, for pollination. Biological vectors are much more reliable and efficient than mere chance. Insects are the most effective pollinators, and are responsible for pollinating an estimated 65% of flowering plants, including most commercial crops. Plants attract pollinators using scent and visual cues, and by the promise of food, in the form of nectar or pollen. Some flowering plants have developed intimate relationships with specific pollinators, using a suite of sophisticated adaptations to ensure their genetic material is successfully transferred.
Insect pollinators include bees, wasps, ants, flies, thrips, butterflies and moths, and beetles. Flying insects are ideal pollinators as they can visit many flowers in an extended area over a relatively short time span. The insects seek out nectar and pollen as a food source, and in the process become covered in the latter, some of which is then transferred to other flowers. Obviously, plants that attract pollen feeders must produce copious amounts of pollen, in order to satisfy the appetites of the pollinators as well as guarantee successful pollination.
Bees are undoubtedly the most important pollinators. Bees collect pollen and nectar, and visit large numbers of flowers in the process. They are typically furry and carry an electrostatic charge which assists pollen grains to adhere to their bodies. In addition, many bees also have an arrangement of special hairs on their hindlegs and abdomens which is designed to carry large amounts of pollen (the scopa). Honey bees and bumblebees lack scopa, but instead possess pollen baskets or corbiculae on their hindlimbs. Nectar provides the bees with a concentrated energy source, which is converted to honey. Pollen is high in protein and is gathered to feed their young (the brood). Beekeepers who provide pollination services will therefore manage their bees so that the timing of pollination coincides with the bees being in a ‘building’ state, with large quantities of brood to feed, meaning that the bees will actively forage for pollen. One third of the human food supply consists of crops that are pollinated by bees.
Amongst the other insect vectors, hoverflies are also important pollinators. Butterflies and moths consume only nectar, which is lapped up by their elongated ‘tongue’; moths are important pollinators of tobacco and some wildflowers. Cycads from inland Australia utilise thrips for pollination, while some beetles are important for pollinating specific plants. Ants may also contribute to pollination, but often tend to consume nectar without transferring significant amounts of pollen.
A variety of vertebrates may also act as pollinators, including bats, which are valuable for some tropical flowers. Birds such as hummingbirds, honeyeaters and sunbirds use their long beaks to access nectar from deep-throated flowers, and are accomplished pollinators. Monkeys, lemurs, possums and rodents, along with reptiles like snakes and lizards, have all been known to assist in pollination.
Pollinators provide a critical service, both in maintaining natural biodiversity and the productivity of crops. Unfortunately, many currently appear to be in a state of decline, posing a significant threat to global food webs and the health of the human race. This results from a combination of factors including exposure to pesticides, pests and pathogens, lack of resources and reduction in habitat.