City size versus carrying capacity
In order for a tribal society of hunter/gatherers and basic agriculture to survive, there must be methods of controlling the temperature with shelter, insulation, and fuels. The easiest shelter to heat is one that is below ground, because if at even a short depth, the Earth's temperature is naturally 50deg F (10 deg C). As long as you can raise that for your comfort with an exhaust of the fumes, then you can use minimal fuel towards that purpose. Since very few people understand or even desire to live this way, few people are likely to do so.
The most likely means of heating is with wood. This means a high availability of forests, cutting tools, and periods of dry weather to season it. A lot of people cannot realistically heat their homes based upon sourcing wood or cutting enough or too much moisture to adequately dry it.
Animal manure has been used in history as a heating fuel. Because animals require a large amount of silage to consume and complex carbs, most cities of any size cannot heat this way. One must expect a certain level of disease based upon livestock proximity and fecal matter.
Peat has been used as a combustible fuel, but only those around those bogs and knowing how to source it can burn it. Coal can be mined and was mined in very small operations in places where it was located in relatively close to the village. Most likely these seams of coal have been used up. We can't expect mining operations to continue in a collapse.
Most liquid fuels can't be created in sufficient capacity to regularly use them as a heating source, not in the chaos post-collapse. Think about every liquid fuel, and then consider all of the drilling needed plus distillation. Even if one possesses the capability and technical ability, then one needs source materials.
Any city over 5,000 without forests is likely to be doomed. They won't have enough fuels to burn to heat their homes, so unless they go underground, then there's a high likelihood of exposure.
Most tribal societies began directly next to water sources. All must have them, and since it's so important, the proximity of clean water that regularly falls as rain to renew itself is key to survival.
Much of the time, people adapt to living in arid environments as long as they can create wells or have a water reservoir to pull from. If not, then the carrying capacity severely limits tribal size.
Water prior to the early 20th century was terribly polluted. It's the primary cause of ill health. We take for granted water purification. As the size of a city increases, so do its sanitation woes. They must go somewhere, and the most likely case is leaching back to the lowest elevations. Since water collects in the lowest elevations, the pollutants find their way back.
Water has to be distributed in a city of any size. Not everyone can carry it regularly. This means either wells are dug, or multiple water sources exist in a city.
Rain can distribute water, as long as materials to create those systems are possible, the average rainfall supports its use, the rainy season is regular, and dehydration of the rainfall is not high. Since the water may be channeled, there's potential for molds, bacteria, bird fecal matter, grit, etc to collect into the rainwater vessels and cause disease.
Water is needed for animals since they produce skins, protein, manure, and other potential products (hooves and bone). Other animals are used to produce work (oxen) or transport since they can carry more weight and do more work. Large animals requires way more water than humans.
Water is needed for agriculture. Any city can't survive without crops. Most cities can't exist without some foraging for medicinals, dyes, and some edibles. A staggering amount is needed for this and livestock to occur.
Any city of over 500 without high amounts of rain and a water supply won't make it. Even a tribe of one needs water. Large cities with their sanitation issues, lack of a working delivery system and purification, regular seasonal rainfall, fields for crops, room for raising sufficient feed and for animal living space, won't be able to survive post-collapse.
People in civilized and organized communities mostly ate plants to survive. They couldn't produce enough animals to eat meat, drink milk, eat eggs, etc on a regular basis. Small tribes could both grow crops, harvest livestock, forage for plants, hunt game, and fish. Because of an increase in the efficiency of agriculture, large cities were possible. Because most farms are corporate today, and lacking in manpower and organization and supply chains to provide seed, fertilizer, feed, etc plus the means of dealing with sanitation, then most likely many people in a city of more than 5,000 will not survive.
The only way that animals can exist as livestock in a tribe is with organization. Their food must be grown and specific to the animals needs. On occasion they need additives to vary the diet based upon their maturity and to produce offspring. Otherwise, the immature creature or its mother will die. Many can be preyed upon by other animals. Others can get disease based upon the intensity of the livestock operations. Increase that operation even higher, and the result is severe stress to the animal and a severe effect on human health.
Even on the frontier, the amount of fishing, hunting, and trapping was very limited. As humans moved in, animals left the area because the effects of the proximity of humans interfered with their habitat. In some cases, the local foraged plants were suddenly eaten by humans and the animals. Animals were hunted or trapped at younger and younger maturity and didn't grow fully into adulthood. That means they didn't produce young creatures. Changes in water availability meant death.
Fishing on a large scale means intervention by experts to restock the area. Otherwise it's a drain on the ability of a species to replenish itself.
The drought and other seasonal changes will mean cycles when the animal population will decline and not be available for consumption. Many animals will be sick and may cause disease when eaten.
In a collapse, since most of the animals are raised on corporate farms, we can expect severe death and pollution from their decay. There are not that many game animals in any areas of large sizes. While there are nuisance animals like feral pigs (see previous postings), in a collapse there will be a lot of hunters.
Again even small cities of 5,000 or so will quickly exhaust their livestock and game animals. Should the animal population get too small, it's very possible that it could take a decade to manage their increase. Mostly their increase will be directly proportional to human population.
While there are a very high number of specialists in urban areas like doctors and nurses, they won't be able to get medicines. These are produced from pharmaceutical operations around the globe and must be shipped. Most of what medical specialists do is either dispense the correct medicine and dosage to patients, or do surgery. Some of what they do is nurturing care under controlled circumstances. None of that is possible in a collapse.
Some medicinal herbs exist, but most doctors are leery of them. They don't know how to use them, their concentration of phytochemicals varies based upon location, or they can't locate them. There are not enough grown herbs to treat everyone. Much of the bottled herbs in stores come from foreign countries. Farmers are paid to grow those crops and then they are shipped in.
Since the concentration of a human population means high need for medical care, and the main means of delivering the care is from complex drugs, most people will become much more ill than the norm with a much higher capacity for decline and death.
Patients can't often take care of their day-to-day care, and certainly not their survival care, and so many if not most will die. That's based upon history. A long time ago,patients were dispensed advice and medicines, but nurtured by their families. Since most families don't know any medicine, we can expect high deaths.
In order to control our environment, we pipe in natural gas, water, sewage into our homes and bring electricity in as well. These allow us to heat, cool, and provide clean water and lighting. Other than this, it creates communications and luxury use of electronics and electricity.
When the utilities go out, it's because there's not enough personnel to operate them or supplies to generate them.
In a collapse, there are existing security officers to maintain order. As the above issues occur, then their own families' security will be compromised. Some will depart to protect their families. It will become unclear how they will be paid or compensated. More will leave.
Security usually deals with issues after it occurs. As more and more crime happens, and less and less officers are available, it's possible for military intervention. However if it's happening in many places, those military folks must be deployed by transportation and by movements which produce clear secure transportation hubs.
The same issues of compensation versus soldier security will determine how long they support civilians in troubled areas. In a country-wide collapse, most likely cities of under 100,000 will not get military units of sufficient troop strength to create order. We don't have that many domestic soldiers to deploy other than hastily created “levies” which were utilized for cities during the medieval period. That means untrained citizens drafted into military operation to create order. There's a high likelihood of an attempt to try this in a collapse.
We do have large numbers of soldiers world-wide, but the simultaneous withdrawal of them from diverse places would create disorder in those places from enemies. Still in a collapse, it's likely to see that happen, and leave chaos in those countries currently supported by air and water and land operations.
Still, they must be transported back home, and this will take an enormous amount of fuel, time, effort, supplies, etc.
I don't think most people have considered the ramification of all of the above events. They assume they'll wing it by themselves or by bugging out or have very unrealistic expectations about their ability to forage or raise food. A lot think they'll loot enough to survive.
Ponder these questions, “How long would the people in the aftermath of Hurricane Sandy survive without outside support from transported supplies, specialists, and support personnel? How long could they live by looting from contaminated food and supplies? How long before disease occurred from exposure and natural Winter illnesses?”
Tribes of under 500- 5000 in a village will have great difficulty increasing their census from the forces listed above. What will happen is a decline in the availability of food, water, and medicines. Exposure and malnutrition will create ideal conditions for disease and contagion.
Fear of death from lack of supplies will lead to competition for those resources. Violence will occur as those with strength and force multipliers wrest away supplies. History has taught us that the larger the city, the higher the crime in most circumstances.
When a disaster is localized, then we can deliver what's needed in time based upon political leadership and administration of supplies and personnel in supply chains. When wide-spread, resources will get to the worst areas with the highest capacity for open chaos.