We'd carve a place out of the wilderness—"
"But there is no wilderness, Mane said. "Even without war, even if you found a space not already cultivated, you would be forced to occupy a region, delineated in space, time, and energy flow, already exploited by another portion of the ecology."
It took some time for Emma to figure that out. "Yes," she said. "There is bound to be some environmental impact. But—"
"Other species would find reduced living space. Diversity would fall. And so it would go on. Soon the world would be covered from pole to pole by humans, fighting over the diminishing resources.

Europe is overpopulated, the world will soon be in the same condition, and if the self-reproduction of man is not rationalized... we shall have war.

Earth's population was still rising. Some day, with all that the Cities could do, the available calories per person would simply fall below basic subsistence level.

If people should ever start to do only what is necessary millions would die of hunger.

Look at us—thousands more of us every day. . . In a century or so, we shall be in the Age of Famines. We shall manage to postpone the worst one way and another, but postponement isn't solution, and when the breakdown comes there'll be something so ghastly that the hydrogen-bomb will seem humane by comparison. I'm not romancing. I'm talking about the inevitable time when, unless we do something to stop it, men will be hunting men through ruins, for food. We're letting it drift towards that, with an evil irresponsibility, because with our ordinary short lives we shan't be here to see it. Does our generation care about the misery it is bequeathing? Not it. 'That's their worry,' we say. 'Damn our children's children; we're all right.' ... [...] Like any other animal that overbreeds we shall starve; we shall starve in our millions, in the blackest of all dark ages.

What would happen to the world if we were human?

Population growth and the multifarious forms of pollution: the human animal was overgrazing his range. The human animal had destroyed everything in sight, killed off the other big animals, used up millions of years of deposits of fresh water, oil, coal, and minerals. Fragile ecosystems had been pushed to the limit, some of them beyond recovery. The death of the earth was undramatic, but it was happening, and fast. And the governments continued to squabble and defend their various economic, religious, and social doctrines. The wealth of nations large and small was bled into the continuing efforts to increase the size, ingenuity, and ferocity of the armed forces. The humans were killer ants, devoting all their time to breeding more powerful mandibles.
Something had to be done immediately to preserve and maintain the great interlocking global system of ecosystems that sustained all life upon the earth. Only by managing the entire earth as a single unit could the basis of life continue on beyond the next century or two.
But twentieth-century civilization was locked into its self-destructive groove. Nothing could be officially done until the various threats became much more threatening. But by the time that stage had been reached, it would probably be too late to do anything.

Is the Farmageddon scenario – the death of our countryside, a scourge of disease and billions starving – inevitable?

When they face desperation... human beings become animals.

[The animals] are now especially bred for eating purposes, and if the demand decreased, the supply would decrease also. Further, how is it that we are not overrun by wild animals of all sorts? … People need not worry about the future welfare of the bovine race, if they would only be a little more humane in their treatment of its present representatives!

Human survival, in a situation of resources degradation, hunger, poverty and uncontrolled climate migration, will make conflict an inevitable result.

There are clear and predictable consequences for the world if human beings continue to rape the earth and plunder its resources; to exploit, oppress, and dominate the weak and the poor for the sake of greed and the hunger for power; to depend on ever-rising levels of violence and ever more lethal instruments of death and destruction in order to secure positions of power and privilege.

Look when the world hath fewest barbarous peoples, but such as commonly will not marry or generate, except they know means to live (as it is almost everywhere at this day, except Tartary), there is no danger of inundations of people; but when there be great shoals of people, which go on to populate, without foreseeing means of life and sustentation, it is of necessity that once in an age or two, they discharge a portion of their people upon other nations; which the ancient northern people were wont to do by lot; casting lots what part should stay at home, and what should seek their fortunes. When a warlike state grows soft and effeminate, they may be sure of a war. For commonly such states are grown rich in the time of their degenerating; and so the prey inviteth, and their decay in valor, encourageth a war.

If the world were a paradise of luxury and ease, a land flowing with milk and honey, where every Jack obtained his Jill at once and without any difficulty, men would either die of boredom or hang themselves; or there would be wars, massacres, and murders; so that in the end mankind would inflict more suffering on itself than it has now to accept at the hands of Nature.