By eating less beef? The people hardly using resources aren't eating much of that anyway.
By biodiversity loss? We'd do a better job with that if we stop looking at in terms of effect on people and start caring about wildlife for its own sake, and that's the perspective I take when I say the planet is crowded: not just crowded for people, but people overcrowding the rest of nature.
But if you insist on focusing on people, we're all going to regret the loss of fish from the oceans.
Side note: I'm not sure who you meant but in some of those third-world countries that "hardly use resources," there are people doing major damage to biodiversity by burning down rainforests and hunting endangered animals. Their countries just aren't burning much fossil fuel.