This is more like a science question than an art question. Overproduction? Since we humans are on top of the food chain, we could possibly eliminate any other species. It also is a problem because there are limited resources.. For example, oil. Oil is expensive and we humans are disregarding the fact that we already have discovered most of earth except the bottom of the sea. This is where NASA takes place. We are trying to discover habitatable planets for us humans. I could go on and on but I hope I gave you some ideas.