Well this just depends on how you see man in the world. Is he a part of nature or is he above that of nature. Of course if you see him as a natural creature, then everything he does is natural. But if everything he does is outside of nature then you start looking at the positive and negative effects of what he does.
Also goes to the debate of free will/fate as slimebeast brought up. Are we doing this based on it is predetermined or is this something we choose to do.
My view on it, is everything in nature is a resource. We are a part of nature not outside of it. Thus everything we do is done for best survival. We hunt and kill animals for the purpose of survival. We build houses for the purpose of shelter. We make medicine for purpose of survival. We even educate and study numerous amounts of thing in nature for the purpose of survival. At the heart of most things in life aside from the pursuit of pleasure, survival is our main purpose.
So we have to start thinking of what we do is best for survival. Thus for best survival we need to be able to grow food and kill prey. However, is it best for our survival that we kill all the prey? Of course not. We'll have then exhausted that resource. Thus as we except today, primary food sources are "grown" like a vegetable and used to support human necessity of survival. We don't simply kill all the chickens or cows or pigs... we use the resource properly and assure that it is renewable.
However, if I simply looked at it that way, then I'd be very naive. There are numerous other things that we need to think about. Which is why I completely disagree with sqrl in his argument against scientists/environmental scientists. They do years upon years of study on ecosystems and how they interact with each other. Thus they know that when you over hunt (not enough to kill them all) of a certain prey you then weaken one of their predators. If one of their predators doesn't have a food source they start to starve. Also on the flip if you kill too much of a predator then the prey becomes over populated and similar things happen. Even more on a smaller level, such as microscopic, you can destroy the balance of nature. Which is why when humans do things they need to make sure they are acting within that balance... otherwise problems will exist at the top of the food chain.
Which comes to a few other questions... well what about survival of the fittest. Although true, we all know from Darwinian evolution that adapting to changes in the environment take thousands of years to be able to do so. Quick environmental changes cause extinctions. Thus you can go back to the original topic on that.
So when thinking about this kind of thing, make sure you are looking at facts from science and not some hackneyed pathetic political twist on it for their own self-interests. These environmental scientists aren't saying all this because they want to hug trees on a daily basis... for the most part they say these things such that the natural balance of nature isn't offset. Do we need extinction? Well in nature it isn't a great thing, but life will continue. But if humans continue on the trail they are on... they might find themselves being the one to go extinct. Have to act in what is best to survive and make sure we don't upset the balance of nature. You might think affecting things at the bottom of the food chain doesn't hurt us... but it does.
Another thing I'd like to throw out there is, we need to also assess the problem that is human overpopulation. Ask any scientist and they'll say a lot of the issues facing humans right now is over population. The environment simply can't handle how many people there are. Thus eventually these actions will lead to a decline in our populations. If nature can't support us then eventually it will take us all out. Whether it be not enough food, not enough space, or water depletion... eventually we'll go. But as always there will always be something to live on after us... those best fit for the environment. That's what humans have to take into consideration... despite all our intelligence there is nothing stating we are best fit to survive or even adapt to a changing environment. And unless we actually try to work with the "laws" of nature, then we could very well lead to our own demise.
