Sure, those areas that have too many people to sustain them have many problems, including "waste management, spread of diseases, civil unrest, and not to mention animal abuse."
But that's MORE the case in places that have a few rich people. A Millionaire (not even a billionaire) in the UK consumes well over 1,000 times what the AVERAGE person (not even the poor) in Sub-sahara Africa. So that millionaire can adopt a bus-load of kids, but to make it even with someone from sub-sahara african they would have to adopt nearly 900 kids from your average family. A real solution would be to get rid of the rich, since globally, 10% of the world population consumes at least 80% of the resources, INCLUDING animal abuse/destruction, creating of disease. Just because they can afford a shower doesn't mean that they aren't dirty with global destruction (including the suffering of other human and non-human animals).
In some cases, if initiated from the communities themselves, the real issue *is* overpopulation... but in most, stability, distribution, and the basics of unfair economics is what causes starvation/etc. It is also not the number of people, but most often culture and capitalism that fuel the destruction of animals for food/entertainment and encroachment and destruction of wild animals and habitat. In those cases, increasing female education is key (reduces birth rates while also giving a path towards democracy and political stability), but so is no longer being colonialists and market exploitationists in the Global South.
I just spent some time reading the whole white-supremacists websites whose main fear is that brown people are reproducing too quickly. Frankly, I know that such an argument is different than the one made by well-meaning liberals who want to curb population growth in poor communities in the Global South and for poor people in their own countries... at least, what I argue is the same, even if their arguments are remarkably similar. "Dirty <insert unsavory people here> that spread disease and have no real culture to protect <insert value>..."
Sure, we need to curb population growth... but my alarm is that we are making more and more rich people with less and less democracy and education. It is quite easy for a poor person to curb birthing more children with an education and basic fair economic distribution, but that is nearly impossible when rich people exist to the point of severe lack of justice in the economy.
So sure, I'll give you that point. Adding 1,000,000 more poor people would put a lot of strain on planer earth and animals... at about the same rate as one billionaire. Cutting back on those poor people will help put less strain on planet earth, other humans, and other animals. Is this the part where you'd say economic justice is only a small part of the equation, or not the focus? Is this the part where it's "selfish" for poor people to have kids and where we don't talk about economic inequality and it's effects on the planet? (And for that matter, probably make the birth rates rise by decreasing stability and increasing the unfair distribution of education.)
Instead of the question: "Is it selfish to have your own children" I'd rather ask "Is it ethical to have an economy where such economic injustice can exist?"... 'cause frankly, even if you are rich you can't change things by just giving away your money... what we need is deeper than anther market solution (like giving away money) and it's not just folks on the internet in rich countries that have all the answers. IMO it's not the rich that are the problem, but the system that allows such incredibly injustice to be the ideal.