Considering the negative impact of large population of cane toad in Australia, the reading passage offers three methods to reduce the spread of cane toad in Australia. However, the professor thinks that these methods will not be successful, and they even bring about some damage for ecosystem. 

Firstly, even though the reading passage suggests that building a national fence is a good idea, because it can prevent toads from moving other parts of Australia, the professor claims that the fence is not effective. This is because toads' eggs lay in the river, which can carry toads' eggs into any part of Australia. Therefore, the fence can not block the spread of the toad.

Secondly, the reading passage states that volunteers can help to capture and destroy the toads, because it is easy to catch toads' eggs and young toads. Nevertheless, the professor asserts that untrained volunteers may destroy other kind of native frogs, which are very important to environment and extremely rare. cane toads and other frog is very similar. normal people can not easily distinguish between them, especially when they are in the young stage. Therefore, using volunteers is harmful.

Thirdly, the professor believes that employ virus will cause some damage for American toads. Since some researchers study in Australia may bring virus to America, American toads will be infected. Cane toad is native animal in America, and it is a really significant component of American ecosystem. Therefore, if American toad are influenced by virus, the whole American ecosystem will suffer.