I'm guessing by the time of these stories the ecosystems have already been hopelessly corrupted. This could be especially catastrophic when some alien biochemi

I'm guessing by the time of these stories the ecosystems have already been hopelessly corrupted. This could be especially catastrophic when some alien biochemistry that uses (say) dioxins as building blocks gets introduced. It makes all the "let's blow up planets" more understandable, I suppose.