Wild animals suffer more than they flourish. This should be obvious to vegetarians. As it is obvious that our own prey suffer more than benefit from their consumption, it should be obvious that wild prey suffer more than their predators benefit. We are not perfectly energy efficient, nor is any predator. Our energy goes to maintaining our bodies and dissolves into heat. We, like all predators, consume our body weight many-fold on a yearly basis. Although predators are more complex than their prey, this is not by the same order of magnitude. So on average, predators are living unethically. If we could remove all predators, it would be of net benefit. So, if it was possible, we might destroy those at the top of the food chain first. The sharks, the lions and son on. We would then kill those on the next rung down. Once each rung is removed, the next would be a predator that, all things being equal, has an unethical diet and ought to be killed. We would keep going until we had doubts about the consciousness of these prey. But even then, would we wish for bugs, for instance, to persist in eating grass. Probably, but I am not so sure. For although I feel personally certain that grass is unconscious, I don't know quite how I could convince you of this. I would be loathe to allocate the possibility of conscious grass a probability of zero. Ants also seem unlikely to be conscious. But perhaps they are behaving unethically by eating a large volume of grass by mass when they are not so much more complicated than it. After all, they're made of the same sorts of Eukaryotic cells, they have the same nuclei containing similar DNA and so on. Then, we would destroy the entire ecosystem, except the grass. Then, what's next. After all, the grass itself depends on carbon dioxide from the atmosphere for maintenance and growth. But after all, this plant requires an awful lot of carbon dioxide to carry on living. Perhaps it should not be allowed to do so. Consider, in particular, that the carbon dioxide is made of the very same protons, electrons and neutrons as that selfish plant! All those plants should be killed.
Note I don't think this is a true representation of utilitarianism. But I would like you to point out where I'm wrong scientifically or philosophically. What is the proper expected-value based approach to this type of problem?
Lastly, note that the killings are assumed to be not very painful or expensive. This is not currently possible, but the argument requires you to imagine a future possible world in which this has become possible.