Academic Reading — Test 51

Reading passage

Across the heathlands, woodlands and coastal scrub of Australia, the banksia stands out as one of the continent's most distinctive plants. Named after the eighteenth-century botanist who first collected specimens of the genus, banksias produce dense, cylindrical or dome-shaped flower spikes that can hold hundreds or even thousands of individual flowers. These spikes glow in shades of yellow, orange, red and bronze, and each one functions as a single advertisement to the animals that feed upon it. Although insects visit banksias, the plants depend most heavily on a particular group of birds for the transfer of pollen between flowers. Chief among these are the honeyeaters, a large and varied family of birds found throughout Australia and neighbouring regions. The relationship between the two is built on an exchange. Banksias offer a generous reward of nectar, a sugary liquid secreted at the base of the flowers, and in return the birds carry pollen from one plant to another. A honeyeater feeding on a banksia spike pushes its head and beak deep into the mass of flowers, and as it does so its forehead and throat become dusted with pollen. When the bird moves to the next plant, some of that pollen is brushed onto the receptive female parts of the new flowers, achieving cross-pollination. Because the birds typically travel considerable distances in a single day, they spread pollen far more widely than a crawling insect ever could, which helps banksia populations maintain genetic diversity and avoid the weaknesses associated with inbreeding. Several features of banksias appear to be adaptations that encourage bird visitation rather than insect visitation. The flower spikes are sturdy enough to act as a perch, allowing a bird to grip the structure while it feeds. The colours that dominate the genus, particularly reds and oranges, are highly visible to birds, which see these tones well, whereas many pollinating insects are far less responsive to red. Banksia flowers also tend to produce little scent, since birds rely chiefly on sight rather than smell, and they secrete nectar in comparatively large volumes to satisfy the high energy demands of a warm-blooded visitor. The long, curved style of each flower projects well beyond the surrounding parts, positioning the pollen exactly where a feeding bird's head will brush against it. The honeyeaters, for their part, are well equipped for a diet rich in nectar. Many species possess a slender, down-curved beak that can probe between tightly packed flowers, and most have a remarkable tongue tipped with a brush-like structure of fine bristles. This brush draws nectar up through capillary action, allowing the bird to harvest the sugary liquid quickly and efficiently. Nectar alone, however, cannot supply all the nutrients a bird requires, and honeyeaters supplement their diet with insects, which provide the protein necessary for growth and for raising young. In this sense the birds are not wholly dependent on banksias, yet during the cooler months, when many banksias flower and few other food sources are available, the plants can become a critical reserve of energy for the birds that remain active. This mutual dependence carries risks as well as benefits. A pollination system that relies on a narrow set of partners is vulnerable to disruption. If honeyeater numbers fall, perhaps because of habitat clearance or competition from introduced species, the banksias they service may set fewer seeds, and over time whole populations of plants may decline. Equally, because certain banksias flower in winter when little else is in bloom, the loss of those plants could leave honeyeaters short of food at the very season when they need it most. Researchers studying these interactions have noted that the fate of the two groups is closely linked, so that a threat to one is, in effect, a threat to the other. Conservation efforts that protect banksia woodlands therefore tend to safeguard the birds at the same time. The banksia and the honeyeater are often presented as a classic example of co-evolution, in which two kinds of organism gradually shape one another over long spans of time. It would be an oversimplification, however, to imagine that each banksia is served by a single dedicated bird. Most banksias are visited by several honeyeater species, and most honeyeaters feed from a range of flowering plants beyond the banksias. The partnership is therefore better described as a flexible web of relationships than as a rigid one-to-one pairing. What remains clear is that the bright spikes of the banksia and the brush-tipped tongues of the honeyeaters represent one of the most striking and ecologically important alliances in the Australian bush, a quiet transaction in nectar and pollen that has helped to shape the vegetation of an entire continent.