Academic Reading — Test 53

Reading passage

In the dense, humid rainforests of north-eastern Queensland lives one of the world's most extraordinary birds, the southern cassowary. Standing up to two metres tall and weighing as much as sixty kilograms, it is a flightless creature with glossy black plumage, a vivid blue and red neck, and a tall, helmet-like crest known as a casque. Although its striking appearance often draws attention, the cassowary's true significance lies in a role that is far less visible: it is among the most important seed dispersers in the entire ecosystem. Without this single species, the structure and diversity of these ancient forests would be profoundly different. The relationship between the cassowary and the rainforest is built around fruit. The bird feeds almost exclusively on fallen fruits, consuming the produce of more than two hundred plant species over the course of a year. Many of these fruits are large, brightly coloured and rich in flesh, having apparently evolved to appeal to a big-bodied animal capable of swallowing them whole. Some are simply too large for any other forest creature to eat in their entirety. Because the cassowary moves across a wide home range, often several square kilometres in extent, it transports the seeds it has eaten well away from the parent plant before depositing them in its droppings. This movement matters greatly, for seeds that fall directly beneath their parent tree frequently fail. They must compete with the adult for light and nutrients, and they are more easily found by insects and fungi that attack seeds gathered in one place. The benefits of passing through a cassowary extend beyond mere transport. For a number of plants, the digestive process itself improves the chances that a seed will sprout. The bird's gut is relatively gentle, and seeds generally emerge intact, but the surrounding flesh is stripped away during digestion. Removing this pulp is valuable because it can otherwise inhibit germination or attract decay. Seeds are also deposited together with a quantity of nutrient-rich dung, which acts as a ready-made fertiliser for the young seedling. A handful of rainforest species are now thought to be so closely tied to the cassowary that their seeds germinate poorly, if at all, unless they have first travelled through the bird. The cassowary plum, a tree bearing a large purple fruit that is toxic to humans, is frequently cited as an example of a plant strongly associated with this avian gardener. This dependence highlights a vulnerability. When two species rely heavily upon one another, the decline of one can endanger the other. The southern cassowary is today regarded as endangered in Australia, with only a few thousand birds remaining in the wild. The principal threat is the clearing and fragmentation of the rainforest for agriculture and housing, which reduces the area available to a bird that requires a great deal of space. Roads cutting through cassowary territory present a further danger, as vehicle strikes are a leading cause of death among adults. Domestic dogs, disease and competition with feral pigs for fallen fruit add to the pressures. Conservationists warn that if cassowary numbers continue to fall, the plants that depend on the bird may gradually disappear too, even though those plants themselves face no direct threat. The consequence would be a slow unravelling of the forest, unnoticed at first because mature trees can live for many decades after they have ceased to reproduce successfully. Researchers studying these forests describe the cassowary as a keystone species, a term used for an organism whose influence on its environment is disproportionately large relative to its abundance. Removing such a species can trigger changes that cascade through an entire community. Efforts to protect the bird therefore aim to safeguard far more than a single iconic animal. Measures include establishing wildlife corridors that link isolated patches of forest, lowering speed limits and installing warning signs on roads through known habitat, and encouraging landowners to preserve and replant native fruiting trees. Some programmes also work to keep dogs under control near reserves and to discourage the feeding of cassowaries by people, a practice that draws the birds towards roads and settlements where they are most at risk. Although the outcome remains uncertain, these initiatives reflect a growing recognition that the survival of the rainforest and the survival of its largest bird are bound tightly together. To lose the cassowary would be to lose the quiet labour of a gardener that has shaped these forests for millions of years.