The most important environmental problems involve endangered species other than
Why this is right
While the extreme term "the most" would normally scare us on Necessary Assumption, here the idea of "most important environmental problem" is actually one of the New Concepts in the Conclusion. Was the author thinking that the most important environmental problems involved large mammals, or species other than large mammals? Definitely the latter! After all, the evidence is saying, "Sure, publicity campaigns would have no trouble with large mammals -- we could arose people's sympathy for them and thus have a lot of impact on problems relating to them. But ... it's hard to elicit sympathy for other types of organisms, and thus it's unlikely that publicity campaigns will have much impact on the most important problems". We predicted this missing link: "soil microorganisms are an example of a species connected to the most important environmental problems". This answer is just giving us this link in a weird way. Instead of saying "soil microorganisms are among the species involved in the most important problems", this is saying, "large mammals are not among the species involved in the most important problems". If we negated this answer, it would definitely weaken, because we'd be saying "Yo, author -- the most important problems involved large mammals. So what's the problem with publicity campaigns? Why wouldn't they have much impact? Didn't you say it's easy to evoke sympathy for large mammals?"
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.