I'd personally agree with the view that small, simple life-forms which prey on humanity -- or on other large, complex organisms -- can be destroyed with impunit

I'd personally agree with the view that small, simple life-forms which prey on humanity -- or on other large, complex organisms -- can be destroyed with impunity. The Green Party's original manifesto rejected this view; their position has some logical consistency, but I don't think that the continued existence of smallpox and rinderpest, or Guinea worms, or mosquitoes (http://www.nature.com/news/2010/100721/full/466432a.html http://www.nature.com/news/2010/100721/full/466432a.html), or Dutch elm disease (which doesn't really harm humans, but elm trees certainly suffer), is worth the suffering and premature death they cause. So, I guess I could reduce my position to saying that if a species can only exist by causing harm or premature death to something more biologically complex than itself, it shouldn't exist. Do you think that this is reasonable? I'd add two corollaries: that predation is natural enough that we shouldn't kill all predators; and that a species should only be wiped out if it has to cause this sort of harm in order to live -- if it's parasitic or infectious, or possibly (as with mosquitoes) if it's an extremely serious disease-carrier. And I'm not at all interested in driving mostly-harmless species to extinction for purely economic benefits...