Answer to Question 1
The risks of nuclear power are primarily some type of loss-of-coolant accident and the release of radioactive materials into the environment, and the need to isolate radioactive wastes from humans and the environment.
As to whether or not we are overly concerned or not concerned enough, this is an opinion question that will depend on the individual's risk assessment. Some individuals will feel that we are too concerned, whereas others will feel that we are not concerned enough. A justification for one's opinion is important. Are the consequences of exposure to radiation too high? Are the human and environmental costs higher from coal?
Although the same exact accident that happened in Chernobyl or in Japan could not occur in the United States, of course it is possible (and it has happened) that reactor operators respond poorly to events taking place within the reactor. It is possible for the material in the rod storage pools to be released to the environment in a way that would be similar to what occurred at Chernobyl because the pools are not within a containment building.
Answer to Question 2
Removing a top predator (which can typically be a keystone species) can have a major impact on an ecosystem. Removing such a predator would reduce environmental resistance on their prey, which would in turn lead to potential J-curve growth of the prey. This has a trickledown effect on many organisms in the ecosystem. On the other hand, if humans add species to an ecosystem we run the risk of the non-native species becoming invasive and competing with or destroying native species in the ecosystem. This could also have a trickledown effect on many organisms in the ecosystem.
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