This is a fictional conversation with a person who does not think human overpopulation is an actual thing. In reality I think the non-believers really don't want to hear anything we have to say. They are either in denial out of fear that this is an actual concern, or they want to have lots of children and do not want to consider that doing so might be one of the worst things for the planet.
Them "Prove to me that humans are overpopulated.".
Me "Look around you, what do you see?"
Them "I see the city, the buildings."
Me "Right, so what was there before?"
Them "It was farm."
Me "Right, and what was there before it was a farm?"
Them "It was a forest I guess..."
Me "Right, and that proves we are overpopulated. At one time all this land was a forest, nature lived here. We had species here that are now extinct, such as the passenger pigeon. There was a swamp with animals, there was a forest, and from year to year things pretty much stayed the same. Then we overpopulated where we (humans) were living and we needed food, so we cleared the area and made it into a farm to grow food for ourselves. Where did nature go? Then our population grew more and we needed to fill in the swamp and take over the farm to turn it into a city."
Them "But we still have lots of open spaces where we could fit more people."
Me "Don't you get it? Every time we fit more people, we displace nature, we displace trees, we displace birds, we displace deer, cougars, and everything else that lived here before."
Them "We can fit all the people on the earth into the state of Alaska and each person would have 2000 square feet of space!"
Me "So go live in Alaska and hope your 2000 square feet of space is not on a mountain, and good luck growing enough food for yourself in that space, as well as housing yourself, and disposing of your waste."
Them "Well we would have to grow food elsewhere and dispose of waste elsewhere."
Me "So what was the point of saying we could all live in Alaska if you have to grow food elsewhere and dispose of waste elsewhere? If we have to do that then we are not just living in Alaska and you still forget that a lot of the space you mention is not inhabitable at all." Pause "You are forgetting that the human population does more than just take up space... we deforest areas for things we think we need, such as coffee, gold, and diamonds. We deforest space so we have lumber for houses and to make way for more and more farm land, and our pollution has rendered some areas uninhabitable for humans or other animals. We have reached a point where we rely on non-renewable resources and are consuming renewable ones at a rate faster than they can be renewed."
Them (getting defensive) "Well, if we are overpopulated you should just fix it by killing yourself."
Me "Saying we are overpopulated does not mean we need to resort to killing ourselves or killing others, we just need to be more responsible and not have kids when we are young, and not have more than one child per couple, and even that is growth as our children do not replace us when we are still alive."
Conversation ends.. I have been blocked.. hung up on.. or walked away from.
Typically people who think that human overpopulation is not an issue do not want to take into account that other species have a right to live here (on the planet) too. They tend to think that "food" is the only issue in regards to concerns about human overpopulation. They often have very little worldly experience, and refuse to consider what might happen in the future due to population concerns.