I saw a very interesting experiment in the effects of overpopulation many years ago, conducted at some university, I can’t remember where. They created this perfect rat habitat. It was the size of a whole wall, maybe 8 x 10, two feet deep with a wall of Plexiglas in front of it, so that the rats could be observed. It was filled with little cubbies and tunnels and runways and wheels. Anyway, they found the perfect number of rats to live in this habitat to match the space and the resources (the food and water made available). The rats lived in perfect peace while these numbers were maintained.
Then, the researchers proceeded to add more rats, one at a time, while upping the food and water. They noticed that more fights started happening. They kept adding rats over a long period of time until the society went completely berserk. They reported that homosexuality went through the roof, infanticide became a regular occurrence, cannibalism started happening, cliques of rats started ganging up and killing weaker members and whereas before there would be normal struggles between males for sexual dominance, now they would fight to the death, etc. etc. etc. The test was regulated for different levels of resources, but finally the researchers concluded that the critical factor was that each animal had to have a particular amount of space. Once they reduced the population to the “golden mean,” peace returned.