If earth's population continues to increase without limit, how might this affect natural ecosystems?

Well we already have problems with out ecosystem. If if kept increasing without limit, we would have even more like this..

1.) The overuse of water drains natural aquifers and alters the local water table.
2.) Pollution can negatively affect wildlife populations. Whenever humans enter a habitat, they tend to reshape it to fit their own needs, destroying the resources the animals use, which drives them out.
3.) Fishing is another big problem. Humans often put extreme pressure on apex predator species, such as tuna. When the numbers of these predators dwindle, the lesser the predators consume grow in number, which then puts pressure on the species further down the food chain. Additionally, industrial fishing operations can damage habitats and kill unrelated species that are caught along with the targeted fish.

There are so many more reasons, but this is just a few.

Hope I helped!