Necessary, because if this were not true — if the most important environmental problems do NOT involve endangered species other than large mammals — then the premise concerning the difficulty of eliciting sympathy for animals besides large mammals would provide no support to the conclusion.

Not necessary, because the concept of “pain” and organisms having “feelings” is irrelevant to the reasoning. Although the author refers to the ease of “attributing feelings” to large animals, that just means people perceive large animals as having feelings. That doesn’t commit the author to any opinion about pain or whether organisms actually have feelings.

Not necessary, because the conclusion concerns publicity campaigns “for endangered species” — this is a narrower group of campaigns than those that are “for the environment.” Also, the author doesn’t have to assume anything about when publicity campaigns are the “most effective.” Although it is true that the author assumes some kind of relationship between eliciting sympathy and impact of a publicity campaign for endangered species, this doesn’t imply that these campaigns are the “most” effective when eliciting sympathy.

Not necessary, because “ignore” is too extreme. Although the author does think that a campaign for endangered species is more likely to be effective when it can elicit sympathy for an organism, that doesn’t commit the author to thinking that people will “ignore” environmental problems if they don’t have sympathy for the creatures affected by it. Perhaps those people will just donate less money, but not entirely ignore it.

Not necessary, because the reference to large ecosystems and agriculture was simply part of an example of how animals besides large mammals can be important to the environment. There may be other ways in which an organism can be environmentally important besides affecting large ecosystems or agriculture; the author doesn’t deny this possibility. Those other organisms would simply be additional examples of how an animal, including ones besides large mammals, can be important. Also, the argument concerns the “most important” environmental problems; so we don’t know what the author thinks about important problems that are not the most important.