单选题
Invasive weeds are a serious problem in Australia Weeds【C1】______the biodiversity of Australia's unique waterways, National Parks, and National Heritage listed areas. By out-competing native plant species, weeds threaten their survival and reduce the plant diversity needed to【C2】______indigenous insects, birds and animals. Changes to the natural flora cover can【C3】______other negative environmental impacts, including increased soil erosion and greater bushfire intensities that further damage the land and the native organisms that【C4】______ it. Agriculturally, weed species can reduce or contaminate crop yields【C5】______poison or injure livestock, decreasing productivity. Farmers are often forced to invest significant amounts【C6】______ controlling invasive species on their properties, adding to the costs of production. Most weeds in Australia were, and still are,【C7】______introduced for agricultural or ornamental purposes. 【C8】______these plants have formed the basis of important industries and beautiful gardens, many have also gone on to become serious weeds — 70% of Australia's weeds are【C9】______garden plants that have "jumped the garden fence" . Some important commercial species, such as the radiata pine and the European olive, are also serious environmental weeds. More than 27,000 species of【C10】______ plants have been introduced to Australia since European colonization, with new species being introduced every year. About 10% of introduced species become naturalized in their new habitat, meaning they can【C11】______ their populations in the wild without human cultivation. One in ten naturalised plants become weeds, and have a negative【C12】______on native biodiversity, agriculture, or both. The price paid for species which become weeds is tremendous, both in economic and environmental【C13】______. Economically, weeds have been【C14】______to cost Australian agriculture a total of $ 4 billion every year: One in every seven dollars【C15】______farm income is spent on the control of weeds. The health costs of weeds are estimated at over a billion dollars annually, when medical treatment, drugs and time【C16】______work are added up — and money can't measure the human suffering caused by【C17】______health problems. Environmentally weeds have caused the【C18】______of four known species of Australian native plants, and 57 more are, or will soon become, threatened because of invasive introduced plants. Species like rubber vine in North Queensland have the potential to destroy entire native ecosystems【C19】______supplanting indigenous vine thickets, threatening native animals as well as plants. Vast areas of Australia and nearly every kind of native ecosystem【C20】______affected by introduced plants, from oceans and waterways to rainforests, grasslands and deserts.