1) Yes, I think it is natural to hunt.
2) I doubt the ecosystem would collapse because I believe Mother Nature is much more resilient than we give her credit for. For example, the speed of wildlife recovery in the zone impacted by Mt. Saint Helens has absolutely shocked scientists. That being said, I certainly expect that we might see some changes to the ecosystem.
3) Species overpopulation may impact other species. For example, if animal X and animal Y compete for similar food, an increase in the population of X may have a detrimental impact on animal Y. Of course, an increase in animal X might benefit predators of animal X. So I think the impacts can be both positive and negative and also quite complex.
4) I'm glad you asked this as it's an important question. Something is NOT morally right just because it is natural. Lying, cheating, stealing and murdering are surely natural activities... yet most of us agree these are not morally acceptable activities in most circumstances.
Though there is wide agreement with respect to certain moral values, I think hunting is an area with quite a bit of gray.
People have all kinds of reasons why they think hunting is ethical. Frankly, I’ve yet to read one that I find very convincing. Sure we need to eat, but we certainly don’t need to hunt to eat. Sure it’s natural to hunt, but that doesn’t mean it’s morally right to hunt. Sure hunting helps control animal population, but it’s not necessarily the most humane way of doing so. Etc.
I think self-defense is probably a pretty widely accepted ethical reason for killing an animal, but that’s not hunting. By definition, hunting is actively seeking to kill game, not protecting your self from an unexpected attack.
In my opinion, my personal choice to hunt is nothing more than one of many somewhat arbitrary decisions in my life. I choose to hunt because the pleasure I gain exceeds the moral concerns I have with taking an animal’s life. I don’t believe that I have the moral or natural right to kill an animal, I just do it because I want to do it. Now I do believe that I should try to harvest animals in certain ways to limit the suffering of the animal, but again the rules I adopt for myself , where I draw the line so to speak, are/is somewhat arbitrary.
So I have plenty of respect for somebody that chooses not to eat meat because they don’t like the idea of inflicting pain and suffering on an animal. I don’t think my decision to hunt is any more moral or right then their decision not to eat meat.
Some people of faith argue that hunting is condoned by their religious tradition. Of course, matters of faith are open to interpretation. Certainly, there are those that feel being a good steward to God’s earth means not killing animals. Again, I respect those that disagree with me on this issue. It seems unreasonable for to assume that everybody ought to agree with me on this issue.
So in summary, I don’t think hunting is clearly morally wrong or morally right. Like many aspects of life, it’s a grey area open to many perspectives.