The New Nature Page 11

  Some biologists want camphor laurels (imported from China) retained as weeds because rainforest pigeons now dote on the fruit. In New South Wales and southern Queensland topknots and white-headed pigeons migrate in winter from highlands to lowlands. In the valleys, paddocks and towns now stand where rainforest once grew. The pigeons have had to switch diet, as Harry Frith observed:

  In 1955 there was a failure of the fruit crop in the rainforest [around Lismore] and great numbers of Topknot Pigeons came out into the open dairy country, searching widely for food. They began feeding on the ripening berries of camphor laurel, which was widely planted as a shade tree after the destruction of the native rainforest. This has now become an annual event and each winter many Topknot Pigeons can be found feeding in the open on this tree, although they return to the rainforest to roost. Continued growth of camphor laurel may now begin to compensate, to some extent, for the destruction of the natural feeding habitat.

  This is what has happened. Pigeons are multiplying on camphor laurel, and also on privet (Ligustrum lucidum), wild tobacco (Solanum mauritianum) and lantana. White-headed pigeons are now thriving. Pioneers working the Big Scrub around Lismore seldom saw them but they are common today. Harry Frith wrote in 1982:

  I have the impression that the bird was always one of the least common of the rainforest pigeons but, whatever its former status, it has certainly increased greatly in some places. In the Richmond River district it began to appear commonly in the open country in the mid 1940s and by the 1950s my father (who had been born in the district in 1885) considered that it had never been so common or widespread before in his experience. It has increased further in numbers since then.

  It is increasing still – and expanding its range. A white-headed pigeon turned up in Canberra in 1998, in my friend Doug’s garden. Privet grows next door.

  Most national parks are high in the mountains, and pigeons desert them in winter when fruit grows scarce. The number of pigeons returning each spring depends on the weed bounty found below. Topknots play a vital role in the rainforest economy by distributing the seeds of trees, and without the weeds they feed on in winter – camphor laurel and privet especially – few would return to do this work. Which means that the upland rainforest now depends on these two Chinese trees. This is a terrible state of affairs, because these weeds ruin small remnants of lowland rainforest by invading in vast numbers. We should restore the lowland rainforests and evict these ghastly trees. Unfortunately they are thoroughly entrenched and eradication is an impossible dream.

  When birds spread seed, they get the habitat they need. In this way they operate as ecosystem engineers. We may blame ourselves for our weed woes, for bringing the plants to Australia in the first place, but birds are often the ones sowing the seeds. They attach symbiotically to new plants. Many of our weeds are pests only in proportion to the number of birds hankering after their berries. The birds themselves recognise no weed crisis. Weeds often bear more fruit than native plants because they aren’t attacked by their natural insect predators. This in turn means that insect-eating birds miss out. Fruit-eaters are the winners, insectivores the losers. More than eighty of our native birds feed on exotic fruits. Those most culpable for our weed woes include silvereyes (for bridal creeper, asparagus ferns and lantana), currawongs, figbirds, and exotic blackbirds and starlings. Other culprits include honeyeaters, seagulls and emus; also flying-foxes, possums, lizards and ants. Australia gets ten new weeds each year, and birds contribute to this statistic. One newcomer, neem tree, a source of insecticide, is seeding prolifically in the tropics because bowerbirds are scattering its seeds.

  The claim is often made that birds take foreign fruits only when their natural foods are scarce, and that native foods are more nutritious. Neither claim is true. Birds can’t tell native from foreign fare and don’t care. Asparagus ferns and bridal creeper came from South Africa, where Cape white-eyes (Zosterops pallidus) – related to silvereyes – thrive. Both birds look and sound so alike it would be remarkable if they didn’t like the same foods. The idea that animals dislike weeds flows from the belief that animals are perfectly fitted to their environments. That may hold true for insects adapted to feed on particular plants, but swallows like bridges, albatrosses like offal, and currawongs may well prefer privet to anything Australian.

  Like foreign fruits, foreign seeds, tubers and leaves are favoured foods for Australian birds. Australia’s parrots crush up and digest millions upon millions of weed seeds. This work is seldom appreciated, however, because parrots sully their reputation by attacking crops as well. In Victoria’s grain belt long-billed corellas rely on foreign plants for 90 per cent of their diet. After the summer oat harvest they feed on the stubble, switching over to onion-grasses (Romulea) in autumn when they consume millions of the weedy bulbs brought up where soil is tilled. This is all harmless, but when farmers sow their grain the corellas dig it up. The staple food of these birds was originally yam daisy (Microseris lanceolata), now a scarce resource. Some of our prettiest parrots are declared ‘vermin’ for the havoc they wreak in fields.

  Foreign weeds also keep rare parrots going. The turquoise parrot was streaking towards extinction until it learned to eat exotic grass seed, and the endangered Norfolk Island green parrot relies on feral olives, guavas (Psidium cattleianum) and lantana. Confined to 5 square kilometres of forest, it would soon die out if its foreign foods vanished. There are other species for whom exotic weeds are luxuries rather than staples.

  In 1993 I went looking for one of Australia’s rarest wallabies, the Proserpine rock wallaby, a primitive marsupial that lingers on in a few scraps of hilltop rainforest north of Mackay. Naturalist Barry Hillary led me to a mountain stream and we hiked upwards. We encountered black snakes and eels but no wallabies. I was disappointed, but so sublime was the rainforest I could only admire the wallaby’s taste in choosing such charming living quarters. In the fading light we stumbled back to the car, where Barry suggested a quick look at another site. There rock wallabies hopped in front of us, behind us and all around. It was a new housing estate. The wallabies were cropping exotic grasses on footpaths and verges. We nearly ran them over. They were also raiding nearby gardens and grazing lawns.

  Most of the butterflies in our gardens are fed by exotics, the caterpillars fattening on oleanders, camphor laurels, citrus leaves and weeds. Garden plants, pruned regularly and watered during droughts, produce soft new shoots ideal for baby caterpillars. Most of Australia’s glasswings (Acraea andromacha) and plumbago blues (Syntarucus plinius) are foreign-fed. The only native food of the latter, native plumbago (Plumbago zeylanica), is not a common plant. But the caterpillars also eat petals of exotic garden plumbagos (P. auriculata), which are bred to flower profusely year round. My guess is that because of this, 90 per cent of Australia’s plumbago blues now flit about in gardens. You can see them in parks out at Cunnamulla, far from the woodlands where their native food grows. They also flutter about in nurseries, their larvae hitching rides on potted stock.

  In the 1940s Charles French found it ‘remarkable how many of the native insects are changing over from their natural food to that offered by cultivated alien plants which they find to be as palatable, or more so’. Emperor gum moth caterpillars (Antheraea eucalypti) will accept leaves of roses, apples, plums, apricots and elms in place of their true-blue gumleaf diet. The well-known bladder cicada (Cystosoma saundersii), often found on willows and privets in gardens, has yet to be found on a native food plant. There are even mistletoes that feed on foreign plants. The creeping mistletoe (Muellerina eucalyptoides) attacks camphor laurels and grows on the American plane trees in central Melbourne.

  Foreign leaves feed many a marsupial, grub and duck. Koalas often munch on American cypress pine needles and camphor laurel leaves. (They also like to perch in camphor laurels in summer for the cool shade they throw.) Exotic foods, and I don’t just mean weeds, are thoroughly enmeshed in foodwebs. Most of Australia’s birds of prey take exotic meats. A stud
y around Mildura found that young rabbits were the staple food (60–92 per cent by weight) of eagles, goshawks, harriers, kites, and falcons – eight species in all. That was before calicivirus struck. Wedge-tailed eagles will eat feral cats. In Western Australia little eagles moved into the south-west when rabbits arrived, then retreated after myxomatosis struck. House mice feed hawks, snakes and owls in central Australia, making up to 97 per cent of barn owl diets. During mouse plagues owls and kestrels multiply madly. Foreign rats are also important foods. There must be tens of thousands of brown snakes, including my garden visitor, relying on exotic rats and mice. In most disturbed habitats foreign rodents and rabbits outnumber native mammals.

  Examples of foreign food sources go on and on. Exotic earthworms and mosquito fish feed many a native predator. In city gardens native ladybirds subsist largely upon exotic aphids. Birds flip over toxic cane toads and tear out their stomachs or strip flesh from their thighs. Toads sound like meals of desperation when they’re really just a cheap and easy resource, exploited by all and sundry. I have seen forty-nine dead toads around a small dam with only their tongues removed. And let’s not forget exotic sea creatures. The globefish in Port Phillip Bay feed largely on exotic shellfish (Corbula gibba) and crabs (Pyromaia tuberculata), and Tasmanian abalone like to browse Japanese kelp (Undaria pinnatifida).

  As we saw with lantana, foreign species are also good for shelter. Hermit crabs covet New Zealand screwshells (Maoricolpus roseus), well-outfitted crabs parading wherever screwshells abound. Prickly gorse, lantana and blackberry afford excellent shelter for small animals. A study around Canberra found that several small birds do best among tangles of woody weeds. Southern brown bandicoots near Adelaide survive only where prickly blackberry brambles protect them from foxes. To block a foreign predator a foreign shrub may work best.

  Some of the least promising-looking exotic habitats can harbour native wildlife. Pine plantations, for example. I used to loathe the gloomy rows of pines that now lend a sombre air to vast tracts of Australia. Countless colourful heathlands of priceless worth were razed to grow pine for chairs and bunks. These plantations are deserts, ecologists say, but that’s not quite true. Rainbow lorikeets, crows and cockatoos like eating pine seeds, and in Western Australia thousands of endangered short-billed black-cockatoos are now plantation-fed. Pine cones have replaced, to some extent, the banksia and grevillea seedpods they relied on. The federal government Action Plan for Australian Birds (2000) warns that these pines ‘will inevitably be harvested leaving a possible food shortage’. So here we have an endangered species threatened by the harvest of plantation pines.

  Endangered velvety peppercress likes pines too, as we saw in chapter 2. In Tasmania four rare and threatened plant species have been found growing among pine needles. Powerful owls, nankeen night herons and spectacled flying-foxes roost among pines because they afford more privacy than scantily clad eucalypts. Olive whistlers around Canberra use pine plantations with a native understorey, similar in structure to their rainforest habitat. Invertebrates like pines too. The leaves are attacked by a native caterpillar, the pine looper (Chlenias). Bob Mesilov found ‘particularly impressive’ numbers of certain centipedes and millipedes in Tasmanian plantations. He found a ‘huge expansion of native litter invertebrates from minuscule bush fragments (a few tree-ferns in a gutter) into the new, closed, exotic forest’. I could offer more examples, but I still view pine plantations with some disdain. My examples don’t show that every grove brims with life; nor am I a fan of the timber industry.

  A similar story can be told about sugar. Captain John Mackay was exploring central Queensland in 1862 when he came to an ‘expanse of open plain’. No open plains remain today around the town he gave his name to, unless you count the shiny fields of swaying sugarcane. He evidently saw a blady grass (Imperata cylindrica) flat, a habitat that disappeared when farmers came. Luckily many animals could shift from blady grass to sugar, swapping one grass for another. They could endure burning by cane farmers because they came from a habitat created by Aboriginal fire. When today’s farmers torch their cane, the creatures fleeing the melee include taipans, carpet snakes, bandicoots, grass owls, pheasant coucals and hundreds of native rats. These rats are cane’s worst pest, costing millions of dollars each year in bacteria-infected gnawed stems. The main culprits are canefield rats (a native species despite their name) and grassland melomys. They sustain hungry snakes and owls, plus the kites that come when cane is burned. Cane is now an important habitat for the grass owl, which roosts under the stems and preys on rats at night. Cane is also attacked by wallabies, possums, brush turkeys, swamphens and millions of native beetles (major pests). In the Mackay area alone browsing agile wallabies and swamphens ruin many thousands of tonnes of cane, with cockatoos and magpie geese adding to the losses. Cane attracts plenty of flying insects that in turn feed swallows and bats. It is even a butterfly foodplant, feeding the evening brown and iris skipper. Now that farmers on the Ord River in Western Australia are growing cane, many animals there are thriving. Freshwater crocodiles and turtles have moved into the irrigation channels, and grass owls, once rare in the area, are multiplying. Star finches, endangered in eastern Australia, flit about the fields harvesting grains of sabi grass (Urochloa mosambicensis), a common weed. They hide inside cane when sparrowhawks approach.

  Farmers live with the hard fact that native animals are often keen on crops. Statistics on crop losses are disturbing, and so are those on animals killed in defence of food. Some seventy native birds count as crop pests, including honeyeaters, silvereyes, bowerbirds and cuckoo-shrikes. Possums, flying-foxes, wallabies and native rats are troublesome as well. Each year farmers lose tens of millions of dollars, and hundreds of thousands of animals lose their lives. Most consumers have no idea of the massive slaughter that goes on to protect our food. Victorian grape growers sometimes lose 60 per cent of their harvest. Almost anything eats grapes: regent bowerbirds, spotted bowerbirds, cuckoos, currawongs, cockatoos, rosellas, lorikeets, red-winged parrots, noisy miners, Lewin honeyeaters, wattlebirds, figbirds, orioles, crows, ravens, bats and more. Around Orange, in New South Wales, the major grape pests include silvereyes, friarbirds, wattlebirds, yellow-faced honeyeaters, noisy miners, currawongs and crimson rosellas (and exotic starlings as well). The birds are discerning consumers of different breeds, damaging up to 85 per cent of some chardonnay crops.

  Native insects – many of them hiding behind exotic-sounding names: pumpkin beetle, apple-root borer, cherry borer, light brown apple moth – are even worse than birds. Many of Australia’s leading pests are native insects, including Queensland fruit flies, fruit-piercing moths and moth larvae. Cutworms (Helicoverpa punctigera and H. armigera) are the worst enemies of summer crops (especially tomatoes, corn, lucerne and linseed), worse than anything from abroad. They are also cotton’s worst bane, and it’s because of these moths that so much spraying goes on, harming human health. The famed bogong moths (Agrotis infusa) that descend upon Sydney and Canberra are also reviled pests, sometimes decimating wheat and maize. Aborigines once ate them, but now we feed them instead. Locusts, blowflies and most household termites are indigenous. The biblical plague locust (Locusta migratoria) is found naturally all the way from Australia to Asia and Africa.

  A taste for foreign food has helped Australian insects conquer the planet. Cottony cushion scale (Icerya purchasi) is a global pest of orange groves, and Australian spider beetles (Ptinus tectus) attack stored foods everywhere. Our fern weevils (Syagrius fulvitarsus) blight ferns in Hawaii’s rainforests, and our carpet beetles (Anthrenocerus australis) attack – yes – carpets in Europe. In return, foreign animals now exploit Australian plants grown abroad. Brazil’s eucalypts are blighted by 220 native insects, including leaf-cutting ants that dismantle whole plantations. I’ve seen gum trees in Kenya with bark stripped away by hungry monkeys. Hummingbirds in California also target gum trees. Endangered stitchbirds on one New Zealand Island rely on the nectar of an Australian weed,
Cape wattle (Paraserianthes lophantha). Florida’s vast groves of paperbark tea-tree (Melaleuca quinquenervia) harbour hordes of opossums and raccoons, judging by all the roadkills. Australian paperbarks provide habitat for nine American mammals, forty-six birds and thirty-four reptiles and amphibians.

  Thousands of Australian golden wreath wattles (Acacia saligna) now sprout in paddocks around Cape Town in South Africa and, like pine plantations back home, these areas are not as barren as they seem. In one patch I could hear Cape canaries trilling while a jackal buzzard floated over, guinea fowl rushed past, and two tortoises lumbered by. The ground was ploughed by mole rats. I later saw masked weavers stringing nests from wattle stems. I must say that few images compare with a weaver on a wattle, the green and gold of one harmonising with the same hues on the other, creating a picture that is all the more striking because it was never meant to be. I tallied up twenty bird species in a two-hour search of wattles nearby, although what they were eating I cannot say. I do know that wattle seeds feed baboons, bustards and striped mice. Africa’s acacia pied barbets, by excavating nest holes inside the stems, have extended their range into formerly treeless regions.

  There are more spectacular examples abroad (with no Australian connection) of native animals exploiting foreign foods. Indonesia’s komodo dragon, the world’s most monstrous lizard, lives mainly on exotic prey: feral deer, pigs, goats, sometimes people. Fossil remains imply that the dragons once ate pygmy elephants, now extinct. In Gir Forest in western India Asia’s last lions live mainly upon cattle poached from peasants. I searched for these beasts with forest rangers and we flushed a pride from their kill, which proved to be four small cows. I found this out when a ranger pointed out a cow’s burst stomach under my feet. The head lay nearby. No wonder the lions looked angry.