Hey, fact is wars, conflict, natural catastrophes and disease have always kept the human population in check it has been a normal method of controlling our numbers. It is a scientific fact that due to our improved medical practices, being heather and improved longevity along with fewer large scale conflicts has caused a problem the world has reached a point where we will soon be unable to support our population. And I fear the next global conflict is going to be over food not oil.
Due to us being a civilized race we will not nor cannot come up with a rational method to deal with this problem. We can not just eliminate people to solve the problem. So there will be wide spread famine.Then a great global conflict and those that survive will live. And with all the screaming about genetically altering food to feed the masses it is inevitable that this will happen.
And your alien scenario just could happen if they feel its the only way to reduce the population to a manageable number of humans. Of course the all out war that would result if they did declare an open season on us. And that would soon reduce our population to manageable numbers. But then I would hope that perhaps they would just move our surplus population to another planet.
Of course we are unable to do the same with our deer as they are thick as fleas on a dogs back and there's no place left to move them to. Oh and the huge numbers of deer are a result of farmers growing more foods that feed us people. So we will shoot'em gut'em and eat'em!!! And if you want to pet them that's alright with me, to each his own!!!