Earthworms have a beneficial effect on the physical and chemical properties of soil. Earthworms change the structure of the soil and improve its ability to cycle nutrients. The burrowing of earthworms aerates the soil and improves water drainage. As earthworms dig
through compacted soil, they ingest the soil and grind it up. They move organic matter from the surface and mix it deeper into the soil. In the forest, this organic matter is called duff, mostly litter from fallen leaves and decaying branches.

Earthworms benefit plants in every conceivable way. Their burrowing creates channels through which plant roots may more easily penetrate the soil. Earthworms aid in the formation of humus, the organic matter in soil. A high level of humus is associated with soil
fertility, so worms are beneficial to forests, agriculture, and gardens. Worm excretions are a valuable source of nitrogen, which assists in plant growth. The bodies of earthworms are rich in proteins, minerals, and vitamins, and when worms die these nutrients are released into the soil.

Earthworms are an important link in the food web because they are a food source for numerous species of animals. They are preyed upon by many species of birds, snakes, and mammals, as well as invertebrates such as beetles and snails. Earthworms are also used as bait in sports fishing and as animal feed in poultry farming. With the help of gardeners and sports fishermen, earthworm populations continue to spread, sometimes at the speed of ten meters a year. Wherever they go, earthworms are a positive force in the ecosystem.

Summarize the points made in the lecture, being sure to explain how they refute specific points made in the reading passage.

Some people think that we learn our most important lessons in school. Others think that the knowledge we acquire outside of school is the most important. Which view do you agree with? Use specific reasons and examples to support your opinion.

Key points:

• The lecture states that earthworms are causing significant damage to some forests by destroying the soil cover. This refutes the point in the reading that earthworms have a beneficial effect on the soil.

• The lecture states that earthworms are a threat to wildflowers. This refutes the point in the reading that earthworms benefit plants in every conceivable way.

• The lecture states that worms have a negative effect on animals and, in turn, the whole forest ecosystem. This refutes the point in the reading that worms are a food source for animals and a positive force in the ecosystem.

Now listen to part of a lecture on the topic you just read about.

We used to think earthworms were helpful, but now we know otherwise. One problem is that earthworms are causing significant damage to some forests by destroying the soil cover. I know a forest ecologist who’s studying the soil near a popular fishing lake. She noticed the forest floor was changing rapidly near the shoreline, right where fishermen dump their unused bait worms. Right there, a thick layer of spongy duff was disappearing. When she took a shovel and looked at the soil, she found large numbers of earthworms. It was the worms that were destroying the soil cover.

Another problem is the threat earthworms pose to wildflowers. The worms are taking organic matter from the surface—duff—and moving it down deeper into the soil. The worms are cleaning the forest floor so well that the duff disappears in just a few weeks. And wherever the duff disappears, so do the wildflowers. Duff contains nutrients that are slowly released into the soil—the nutrients that plants need to sprout. But earthworms are literally eating the forest floor right out from under the plants. This has a devastating effect on the wildflowers that once adorned the lakeshore and attracted photographers.

A third problem is that the disappearance of the duff hurts animals too. The duff is the habitat for a lot of different animals—salamanders, insects, and spiders, to name a few—so when the duff disappears, so do these animals, and in turn, the other animals that feed on them. So, the negative effect of the earthworms is being felt across the whole forest ecosystem.

Summarize the points made in the lecture, being sure to explain how they refute specific points made in the reading passage.