Every planet and moon has its own Population Limit. If this limit is passed, pressure on the environment is too high.
Population limit explained Edit
In the most simple way, population limit is affected by the Solar Constant (the amount of energy a planet receives from its host star) and its derivate value, Visual Constant (the amount of visible light a planet receives).
In order to survive, plants need visible light, both in red and blue spectra, while green light is almost not used. If a plant receives only red or only blue light, it will not survive. I made experiments with plants, giving them limited access to light, to mimic conditions expected in the outer Solar System, like an Outer Planet. Results are that plants can survive as far as the orbit of Neptune, but their productivity will be severely affected. Agriculture, as we know it, can exist as far as the Asteroid Belt. Beyond, we will need huge fields to produce little food. Earth plants can survive as far as Neptune. For more details, Plants on new worlds and Agriculture are useful articles.
On the other hand, human activity produces heat and heat produces vertical currents, that can affect layers of greenhouse gasses, destroying the climate on a planet.
Also, humans need not only food, but places to live, places for recreation, industrial centers and places for trade. We need energy and transport. Every human activity affects the environment.
In case of an Inner Planet, there are other risks. First of all, there is a risk that Anti Greenhouse Technology used to cool the planet can fail. Anti Greenhouse Gasses, Micro Helium Balloons or any other shielding devices can be affected by human activities, like heat currents. Anyone can guess that if the anti greenhouse shielding devices fail to protect, the planet will enter a runaway greenhouse effect.
However, the population limit differs on many other things, from planet to planet, the solar constant is only one and the most important factor to take into account. Also, to be noticed, is that resource consumption and pollution levels are important to take into account. Earth could support 10 billion inhabitants if they were to use resources rational, but could not support a single billion of people that eat only the best meat and live a luxury life.
Local density Edit
Population limit applies in two ways: on a global and on a local scale. If we concentrate all Earth's population and industry on a small area (for example in South America), this will have worse effect then if people were spread all over the globe. This is because too much pressure on a small part of the biosphere can trigger effects that the rest of the biosphere cannot cope with. In our example, if we move all humans and all human activities to South America, soil will be overrun, desertification will start, water will become too polluted and air will be too heated. The biosphere will lose a continent and the effects will be felt immediately. Toxic water will affect the oceans and will destroy plankton, allowing unwanted species to multiply. Nature will have no way to absorb the shock.
If population is spread all over the world, nature has the power to heal itself. It is the difference between having one big, lethal wound, compared to having many small wounds. In case of a big wound, all local resources are not enough and the infection will spread through the body. In case of many small wounds, they are not so deep, so that the body can work repairing them faster, using local resources.
If we look on Earth, at China and India, we see this very clear. Desertification and pollution to incredible levels are common. Soil is exploited beyond natural regeneration limits, so that desertification is inevitable. And when Chinese rivers reach the sea, they are so polluted, that the effect is felt as far as Japan. This has destroyed the plankton and as a result fish has vanished and was replaced by nature with the toxic giant jellyfish species.
On Earth, we see giant cities with 30 million inhabitants. This is close or perhaps above the limit Earth biosphere can support without major side effects. Based on this, we can say that the population limit for a city should be 0.4% of global population limit.
Also, on Earth, we see that in rural areas of India and East China, fields are feeding even 1000 people per square km. However, a value that should be safe for soil erosion and would not have major side effects for the biosphere, should be around 300 mouths per square km. Given Earth's population (at a safe value, or 5 billion inhabitants), the needed farmland would be 5% of planet surface, including water.
Global population Edit
The global population limit is the largest number of humans a planet or moon can support without major impact on the environment. For Earth, it should be 5 billion people. We already are more then that, but also we see the consequences.
When a part of a planet is overpopulated, measures should be token. From increasing taxes in crowded areas and a tax-free regime in low-populated areas to resettlement programs, there are many things that must be done.
If global population gets closer to the limit, immigration should be limited. And if the population limit is passed, there are other politics that can help. A tax increase is very efficient. Also, rationalization of resources will limit consumption and will make the planet support more people. In time, there must be token actions to decrease population.