Overpopulation is not the real problem
The general idea was that the demographical growth of populations will increase the use of natural resources, putting the world on route to collapse. There is however another side to it.
Decades ago there was great concern about over-population. The idea was that the demographical growth of populations would increase the use of natural resources, putting the world on route to collapse. This led to many programs around the world for the reduction of birthrates, some of them leading to catastrophic results, like in China, for example.
This idea was especially strong during the 1970s, and as in other situations Srila Prabhupada positioned himself strongly against it. On SB 9.20.21, for example, he wrote:
"The small ant is supplied a grain of sugar, and the elephant is supplied tons of food, but everyone is able to eat. Therefore there is no question of overpopulation. Because the father, Kṛṣṇa, is fully opulent, there is no scarcity of food, and because there is no scarcity, the propaganda of overpopulation is only a myth."
His idea is that the planet can produce any quantity of food grains and sustain any amount of human beings, as long as people are Krsna Conscious. He mentions that there are 8.400.000 species of living beings in this Universe and everyone is eating by the mercy of Krsna, from the elephants to the ants. Similarly, every human being also has his quota and the planet always produces enough for feeding everyone, as long as human beings perform their duties and live a simple and natural lifestyle.
The idea is that when people become religious, the planet produces more food and other resources, and thus a larger population can be sustained. In Srimad Bhagavatam, for example, we have the story of King Vena. Because the king was so sinful and stopped all religious activities, people became sinful and as a result, the planet stopped producing grains and people started to starve. As soon as King Pṛthu came into power and restored the execution of religious activities, the Earth became fertile again and there was enough food for everyone.
In other words, the problem is not over-population, but the fact that people become sinful and lead an artificial lifestyle. This leads people to want to consume more than their quota, resulting in degradation of the environment. Even today, more than enough food is produced on the planet, but there is still hunger because not only a great part of it is used to feed livestock in rich countries to maintain people's habit of eating meat, but many people eat far more than necessary. Over-eating not only results in health problems but also in hunger since if one eats more than he needs, another has to go hungry. Because of the law of karma, the roles get inverted in the next life, perpetuating the cycle.
From the scriptures, we get the information that when people are sinful the planet restrict the supply of food, and when people become pious the supply increase. This means that the climate becomes more suitable, there are fewer problems with pests, the rain becomes more regular and the land more fertile, allowing anyone with a modest piece of land to provide for his family through his honest labor.
While the amount of flat land suitable for mechanized agriculture on the planet is limited, there is much land available that could be used to grow food using natural methods and animal labor. There are about 150 million square kilometers of dry land on the planet, which could sustain a population much greater than we have nowadays.
Another way to consider this question is to examine the meaning of "Krsna Conscious". Nowadays our movement has, according to some sources, more than one million members. There are a lot of people out there who are practicing Krsna Consciousness, at least to some extent. However, although we are practicing Krsna Consciousness, most of us are still very much associated with the consumerist society. We may call it different names or use different excuses but the fact is that we are still part of it. We criticize the consumerist society, but we still don't have a better model to offer. The way things are, even if the whole population of the world would become "Krsna Conscious" on the same level we are, not much would change. We would still have a consumerist society that would continue polluting and exhausting natural resources.
If we go a little deeper into the teachings of Srila Prabhupada, we find that he often speaks about a Krsna Consciousness person not harming any other living being, understanding that everyone is part and parcel of Krsna. This seems to be a deeper level of Krsna Consciousness than most of us are practicing at the present moment and it works better along with the concept of sustainability.
Under this definition, a Krsna Conscious person will not do things that cause harm to others. This means a Krsna Conscious person will not pollute and misuse natural resources. A Krsna Conscious person will aim for a natural lifestyle in harmony with nature. Srila Prabhupada was frequently speaking about "simple living, high thinking". It seems that both ideas are strongly connected.
If everyone in the world would become Krsna Conscious at this level, then it's quite possible that the world would be capable of sustaining a population many times bigger than the present one, without environmental destruction.
Again, the problem is not over-population, but the fact that people are living an unnatural lifestyle. Such an unnatural lifestyle originates from the fact that people are not Krsna Conscious (or are following Krsna Consciousness on a superficial level). However, when people come to a deeper level of practice of Krsna Consciousness, they aim for a natural lifestyle and stop degrading the environment. At this point, humanity stops being a burden to the planet and the concept of overpopulation becomes irrelevant.
As Srila Prabhupada mentions on SB 3.5.5: “According to the Vedas, population experts are wrong in their crucial assumption that earth cannot supply the needs of a large population. If people are God-conscious, there is virtually no limit to the population the earth can comfortably support.”
As long as people would live on the land, cultivating their own food and making houses and other products with local materials, without much industrial activity, people could live happily without pollution. In fact, human beings lived like this for millions of years before the start of the industrial revolution. The Vedas mention that in previous yugas the planet used to sustain a far larger population than nowadays without exhaustion of natural resources. The problem is therefore not so much about the number of people, but about their lifestyle.
Natural resources are not exhausted by people living on the land, but by people living a consumerist lifestyle in the cities, sustained by industrial production and transportation of goods through great distances. People of demonic mentality push for an artificial lifestyle based on the exploitation of natural resources, and then push for the reduction of population, so there is more left for them to continue their exploitation. When one pushes the idea of controlling the birthrate in order to preserve natural resources, what he is actually saying is that others should have smaller families, so there are more natural resources available for him to continue his exploitative lifestyle. It comes from the desire of getting more for oneself and not the desire for the common good. The reality is that the problem is not overpopulation, but the lack of God Consciousness.