TPO63

Reading

Native to Europe and Asia, cheatgrass is an invasive species of grass that is causing problems in North American fields. The plant quickly dominates fields that it has invaded and drives out other plants. This can cause, among other problems, severe damage to animal habitats and to scenic areas. Several solutions to the cheatgrass problem have been proposed by ecologists.

One option is to encourage animals such as cattle to feed on cheatgrass. Cattle and other livestock are known as grazers because they graze, or eat, small portions of grass or other plants throughout the day. If grazers were released in fields where cheatgrass is prevalent, the cheatgrass would be reduced. That would create room for native species to re-establish themselves and flourish. This plan is appealing because cheatgrass is most prevalent in areas of North America where cattle and other livestock are raised.

Another option is to burn the cheatgrass off the fields with controlled fires. This plan has the advantage of eliminating vast amounts of cheatgrass in a short time. Cheatgrass, it turns out to be a highly flammable plant: it burns much more easily than the native plant species that have been crowded out. Strategically set fires could burn away the cheatgrass where it has come to dominate, creating space so the newly cleared fields could be reseeded with native grasses and other plants.

Still another option is to introduce a fungal parasite that specifically attacks cheatgrass. In Europe and Asia, where cheatgrass is a native species, there is a species of fungus that has the ability to prevent cheatgrass from reproducing. Introducing this fungus in North America fields where cheatgrass has proliferated could slow the spread of cheatgrass, making it possible for native species to better compete against cheatgrass.

Listening

听力原文

The methods mentioned in the reading are not likely to work very well in controlling cheatgrass. Here’s why.

First, cheatgrass is not the plant the grazers prefer. This means that if grazers are released in a field that has a lot of cheatgrasses, but that also has other kinds of plants, the grazers will eat the other kinds of plants first. The grazers might eat some of the cheatgrasses, but only after the native grasses and plants, the grasses and plants we are trying to protect, have been destroyed. So, releasing grazing animals in the field with cheatgrass will probably have the opposite effect that one intended. There would be fewer native grasses, but plenty of cheatgrasses around.

Second, fire will destroy cheatgrass plants on the surface, but that doesn’t mean cheatgrass won’t quickly come back. Cheatgrass produces many, many seeds. The seeds can germinate even a few years after falling to the ground. Many seeds get push down into the soil below the surface. If the seeds are below the surface, fire cannot harm them. So, after the fire has burned away cheatgrass plants and seeds on the surface of the field, the seeds that are buried in the soil below the surface sprout and give rise to new plants, and the field is soon again filled with cheatgrass. 

Third, the fungal parasite. Here you have to understand that cheatgrass and fungal parasites have lived together in their native habitats for thousands of years. During this time, cheatgrass plants have been able to develop some resistance against the fungus. So, while the fungus has the ability to harm cheatgrass, in reality, it only harms cheatgrass plants that are already weak or sickly. The healthy and strong cheatgrass plants can usually resist the fungal affection. So, introducing the fungal parasites into North America will probably not be efficient.

1 thought on “TPO63

  1. 二老师 Reply

    帮忙捉个虫~
    听力最后一段倒数第二句话应该是 infection 不是 affection

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