Would killing the lion in order to save the gazelles be the most ethical thing to do? The idea that opposing predation means killing the lions is often thrown at us as a reductio ad absurdum when we raise issues of wild animal suffering. And we ourselves often recoil, and explain that we would favour other, “gentler”, steps, such as feeding the lions with special vegan preparations, or progressively altering their genes (through gene drives for instance) so that they might cease needing and wanting to kill, or phasing out their existence through sterilization. But we don't want to kill the lions. Fine animal activists we would be, if we were advocating killing animals!
This is despite the simple consideration that one lion does kill a large number of other animals during his or her (I'll say “her”) existence. By not killing one lion, we are killing many gazelles. From a consequentialist point of view, it would seem better to kill one lion rather than kill (indirectly) all those other animals; and better to do it right away, rather than to rely on more lengthy solutions.
The waters can be muddied by the other consequences that might (or might not) follow, such as an overpopulation of gazelles. Such issues should be addressed in their own right. It remains that we do have strong inhibitions towards the idea itself of killing the lions, independently from any indirect consequences. I think that those inhibitions are unfounded, and are an effect of the the different way we describe the situation in the case of predation on one hand, and human interactions on the other.
It is generally recognized that all humans have a right to life. But this right is mostly a liberty right, not a claims right. The distinction between claims rights and liberty rights is an important one. A liberty right is, for instance, the right to marry. It means that you are free to marry, if you want and can, but not that society has an obligation to find you a spouse, it you cannot find one yourself. You cannot claim that your right be fulfilled.
Libertarians tend to recognize mostly liberty rights. In their view, your right to life just means that no one can kill you. It doesn't mean that society must feed you if you are starving, or give you antibiotics if you have a life-threatening but curable infection but not the means to pay for them. Libertarians like Ron Paul believe that it would be right to simply let such a person die (see this video). They believe that letting a person die is not the same as killing.
For most people who are not libertarians, however, the right to life is somewhat a claims right, in the sense that you should be given food or antibiotics if you need them. They would say that to withhold them is an act of killing. However, and this is my point: even for non-libertarians, the right to life is only a claims right up to a point. If you need a heart transplant to keep you alive, no one is obligated to donate their heart and hence their life; or even to donate a single kidney, which isn't lethal. If you refuse to donate your kidney, no one will say that you have killed. It is viewed as an act of letting die, not of killing.
So here we get back to the lions and the gazelles. All have a right to life. If we view their right to life as we normally do for humans, it is a liberty right, and only in a limited sense a claims right. The lion should receive antibiotics if that is what it takes for her to stay alive. But can a lion claim that the gazelle must donate her organs – in effect, her whole body – in order to fulfil her right to life? I don't see how this could be. Applying the standards we apply to humans, we should not kill the lions; but we should not let them eat the gazelles. If they cannot survive without eating the gazelles, they will die. But that doesn't mean that we will have killed them, just that we have let them die.
When we are accused of wanting to kill the lions, perhaps we should answer that if there is no other option – such as vegan lion food – then we should not kill the lions, but let them die. Letting them eat the gazelles is not an option; the gazelles are not theirs to eat.
The reason we don't usually view things this way is, I think, an effect of status quo bias. It appears normal that the lion should eat the gazelle. In contrast, it is not in the status quo, and not viewed as normal, for one human to harvest the organs of another. But if the lions had previously been herbivores, but then suddenly, perhaps as a result of some virus, had turned carnivorous and now needed the flesh of gazelles to survive? Would the gazelles suddenly be available for them to eat? Why should they be?
It may be objected to this view that it would be more humane to kill the lion than to let her slowly die of starvation. This may well be true, in which case euthanasia might be the right thing to do. Compare to the case of, say, a domestic cat who is dying of heart failure, but who could be saved if we sacrificed another cat and took her heart. If at some point we choose to put the cat out of her misery, we will call it mercy killing. We will not say that we killed the cat by withholding the heart of another cat.
This discussion may seem purely abstract; we are not, at this time, into implementing either vegan lion food or the prevention of lion predation. It is probably better, strategically, to concentrate on the predation committed by humans, that is on their consumption of meat. However, the way we view predation, and the hypothetical solutions, is not without consequences. It has strong symbolic value, I believe, to assert that it would be justified to prevent predation, even at the expense of the life of the predator. It can also allow us to feel better about the limited interventions we can practice in nature, in which for instance we protect a mouse from an owl. We may feel uneasy, wondering if in the Kantian sense we could will that the maxim of our acts be universalized, which would imply that the owl would starve. Accepting that yes, we can will the universalization of that maxim can make it easier for us to act.