Reply to Agelbert:
Regarding DDT:
1. The environmental harm of something must be weighed against benefits or desirable effects. In the case of DDT, at the time it was introduced there were almost no effective ways of preventing malaria. DDT was highly effective in some contexts; e.g., wikipedia: "For example, in Sri Lanka, the [DDT] program reduced [malaria] cases from about one million per year before spraying to just 18 in 1963." In other words, DDT essentially eradicated malaria in Sri Lanka. Is that worth something? Yes, of course it is. It is not worth environmental devastation. But did DDT devastate the environment? No, it did not. Further, other and better ways of preventing malaria came online since that time, and DDT is no longer needed. For all its drawbacks, which are considerable, it is a good thing that DDT is now generally banned.
2. Can you provide evidence that DDT caused the extinction of thousands of species? I cannot find reference to this in standard reference material. The eggshell-thinning effect is mentioned, but it seems that different bird species have different sensitivity to this effect. No extinctions were mentioned.