TPO 75

The reading lists three approaches to prevent the invasive Northern Pacific sea stars from spreading. However, according to the listening, while these approaches can help to control the population, they cannot really stop the spreading of the invasive sea stars.

First of all, the reading points out that quicklime can be used to kill the invasive sea stars without being harmful to the commercially important mollusks. Nevertheless, the listening argues that while quicklime is beneficial to controlling the invasive sea stars, it can also hurt the native sea stars, which is critical to the native ecosystem. Specifically, after the invasive sea stars are killed by the quicklime on the seafloor, native sea stars will move to their habitats and consequently be killed by the quicklime as well. As a result, quicklime may not be suitable to prevent the spreading of the invasive sea stars.

Secondly, in view of the author, introducing parasites that attack the reproductive systems of male sea stars will be useful in controlling the sea stars’ population. But the listening also disputes this by saying that those parasites cannot completely destroy the reproductive systems of male sea stars. In that case, male sea stars can still fertilize the eggs of female sea stars, while a female sea star can lay 20 million eggs within a single year. Thus, controlling the population by introducing parasites will not show the effect as the reading material describes.

Thirdly, the passage suggests that the spreading of the invasive sea stars can be controlled by treating ballast water. Nevertheless, according to the lecture, while sterilizing ballast water can kill the sea star larvae in the ballast water, there are also other ways for sea stars to spread. For example, they can attach themselves to moving objects, such as the bottom of small fishing boats. Thus, treating ballast water will not prevent the invasive sea stars from spreading.

However, Instead, Nevertheless, In that case, But, Thus, Moreover, Also, … as well.