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This isn't strictly related to a writing project, but I have been lurking for a while and have been amazed by the different questions that the people here are able to answer.

If there was a major disaster, (people going off to war, alien abduction or whatever), assuming a "regular" city, what percentage of the population could be lost and have the economy remain roughly normal. Assume that the missing people are a fair cross section of the population, so it isn't that all the doctors, or all the children are gone, for example.

Also, assume that whatever happened, happens everywhere, so the needed people can't just be shipped in from somewhere else. (Obviously there would be major disruptions to the personal lives of many individuals, but that is out of scope.) Thanks.

To clarify, as requested. I am talking about humans on modern earth. Pick your favourite city of at least 500,000 or so. Something happens. That something isn't specified. If I instantly remove 100 people, life can go on for the city mostly unchanged. There will be mourning, etc. but if you consider the city as a whole, not much will happen. If 80% are instantly removed, the other 20% will be alive, but there is no chance it will be anything like life as normal. Where is the line that will just allow life and the economy to continue as normal? My gut instinct is that it's something like 10 to 20%, but I don't know.

Edit 2: Roughly normal means that most people still go to work, most business are still open. I'm not concerned about where supplies are coming from. I'm mostly concerned with relatively short term. (a month?) I'm not sure exactly were to draw that line, but considering it similar to a recession is probably not a bad idea.

Edit 3: It was my intention to separate and ignore psychological effects at this stage, and make this question more about logistics. Maybe that can't be done. People can put up with a lot if they are motivated, but if they panic, even small things can destroy law and order, as mentioned in an example given below.

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closed as too broad by sphennings, Slarty, L.Dutch, StephenG, Mołot Oct 24 '17 at 6:31

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Could this question be related to this, possibly? It's not the same, but some of the ideas could be helpful. worldbuilding.stackexchange.com/questions/3/… $\endgroup$ – Inflationary_Bubble Oct 23 '17 at 22:16
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    $\begingroup$ See also worldbuilding.stackexchange.com/questions/91443/…. I apologize, but I must vote to close this question unless improved. It is currently too broad. How quickly are people disappearing? Are they disappearing randomly or more so from a particular region or demographic? What is the naturre of the society? Are we talking humans or aliens? If aliens, what is their nature? Etc. As asked, the question is too broad. $\endgroup$ – JBH Oct 23 '17 at 22:24
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    $\begingroup$ Yes I agree with JBH. Although it is an interesting idea it needs to be focused. What constitutes "roughly normal"? Is a recession or a depression normal? $\endgroup$ – Slarty Oct 23 '17 at 22:31
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    $\begingroup$ You should be concerned with sources of supplies. Modern cities cannot survive without daily inflow of them. Normal life will be disrupted the moment the distribution chain starts to malfunction. $\endgroup$ – Olga Oct 23 '17 at 22:44
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    $\begingroup$ @user3034958, how important is it to keep the life 'normal' and economy fully functioning? $\endgroup$ – Olga Oct 24 '17 at 1:57
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As I said in this answer to another question, city operation basically depends on (a) stability, (b) food imports, (c) utility infrastructure, and (d) law and order. But, before we go into stuff like that, let's get a trivial condition out of the way.

Trivial Answer: You can have a functioning society of two people. But, such small societies are very, very different in their nature than a society of even 1,000 (much less the 500,000 you state in your question). Small societies operate on individual independence1. Large societies operate on group interdependence2. I'm going to assume you mean the greatest loss of population that allows a large group-interdependent society to remain a group-interdependent society and not shift to individual independence.

1 individual independence is, for example, the ability and need to hunt/garden for your own food, which you can do because there aren't so many people that the food source will become depleted.

2 group interdependence is, for example, the need to depend on others to provide food for the whole because if everybody went out hunting the deer would be depleted so quickly that starvation would set in. A more efficient food supply is required: bulk farming and ranching. Population density is a harsh taskmaster that we will encounter again momentarily.

Food: As we're limited to just a single city, we can assume (b) isn't a problem as unaffected areas can send the afflicted city enough food to survive and prosper. The only caveat to this is that you need enough people in your city to receive and distribute the food. A quick Internet search did not afford specific insight into what percentage of a city's population was devoted to food distribution (everything from truck drivers and regional distribution centers to that excellent taco trailer in your company's parking lot). However, based on my personal experience, I doubt any city devotes more than perhaps 5% of its population to food distribution. And that might be generous. Further, the smaller the population the easier the distribution (struggling people will come to you, after all). Therefore, food isn't a limiting issue.

Children & the Aged: Before we go further, let me point out that children and the aged are (ruthlessly) the easiest demographics to reduce without affecting the city. For example, the U.S. Census estimates that in 2016 about 23% of the population are age 18 and below and 15% are age 65 and above. Not to demean anyone, but the odds are so much in my favor that an 18-year-old and a 65-year-old don't hold society-critical jobs that the entire 38% can be removed without having any affect on the city's stability. Quite the opposite. By freeing trained adults from education, child care, geriatric care, and other services supporting children and the aged, by reducing the food and utility needs required by children and the aged, etc., etc., etc., you're actually increasing the stability of the city. (And I just grew horns and a bifurcated tail for pointing it out.)

City Infrastructure:  This is the second biggest problem, but it's also very flexible. The problem is the random chance involved that one of the few highly-trained individuals who run water, sewer, gas, petroleum, electricity, police, fire, medical, etc., services are removed. They are not easily replaced. Remove enough of them and the city loses those critical infrastructures and quickly collapses. This group of people may only represent 10% of your adult workforce, but that 10% can't be reduced by much (say, from 10% to 8%) and your city is teetering on the edge of the abyss. Utilities are among the most sensitive.

Law and Order: This is the big one. It would seem you could reduce a city's population almost forever, but the 1977 New York blackout proved that basically good people become basically bad people in droves when law & order loses control. As I said in my previous answer: The average number of cops to citizens is about one for every 250 people. It doesn't take much panic at all before the police are completely overwhelmed. As the population depletes, the number of abandoned houses and buildings increases, accompanied by crime and the chaos it brings. Reduce the population to far and a criminal can run almost anywhere and hide. Since the people supporting surveilance cameras, etc., are also depleting (as are the people fixing and installing them, because they're getting shanked in the growing lawless areas where there isn't sufficient police protection) the situation spirals quickly.

You'll notice I didn't include the politicians in any of that discussion. Yes, they serve a purpose, but when the chips are down, politicians only control the flow of regulation, which can be and would be compressed mightily to accommodate population decrease. In other words, while not entirely inert like children and the aged... they're pretty darn close....

Population Density: Here's where population density is so important. One of the many reasons why the ratio of police-to-residents is so low is that high population density is easy to police. Yes, a high population density is one meal away from panic and chaos, but that noose around all our necks is also what allows the police to keep so much control. We're dependent on each other, and so very little actual policing is required to keep order. If the density drops (lots of abandoned buildings between the police station and John and Jane Doe's house), then the ability to effectively police drops considerably. Your population would of necessity need to move closer-and-closer to the center of law and order to retain a functioning group-interdependent society... with the outer ring of buildings becoming more-and-more lawless. (You'd better hope Grandma is willing to leave her home.)

Finally, there's the issue of how quickly people are disappearing: This last issue is important. Even in a city of 500,000, if you lost 1,000 people in a day, the panic would be instantaneous. Lose 50,000 people over five years and the bank's ruthless lending practices are blamed — but the city adapts, reorganizes, and continues. Let's assume, then that the depletion is slow enough the city can rationalize it and reorganize to compensate — but cannot import new talent to replace critical functions.

Conclusion: With 38% of the city's population being "inert" and assuming 100% random depletion (no geographic or demographic focus of any kind, a very unselective virus...), that means your city is roughly 38% inert population, 10% critical population, and 52% compressible population. My assumption is that you begin to lose stability when that 10% critical population drops to 8%. That's a 20% loss of population overall.

But it's the bottom number that's interesting. That number represents rise in the crime curve associated with abandoned buildings and general panic. I'm pulling this number out of my left ear with by right big toe... it's that well supported by non-existent facts. But my gut tells me at 40% you have so many abandoned properties that too many "good people" become opportunistic thieves.

Therefore: 20%-40% reduction, after that the city collapses like Godzilla was on the horizon.

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  • $\begingroup$ Does your prognosis satisfy the condition of 'normal life'? $\endgroup$ – Olga Oct 24 '17 at 1:55
  • $\begingroup$ @Olga, As was asked before, what's "normal life?" The loss of any individual will have a negative impact on some number of others. By that standard, no single individual can be lost without life losing normalicy. If we measure it as "normal" so long as 50% of the remaining poeple are unaffected by the loss. We all know, what, 30 people in our cities well enough to not be "normal" by their loss? By that definition, no more than 16%-17% can be lost before "normal" is gone. If "normal" is defined as "the city continues to supply services without loss of law and order," then my answer stands. $\endgroup$ – JBH Oct 24 '17 at 2:14
  • $\begingroup$ The OP gives this definition of normal: 'Roughly normal means that most people still go to work, most business are still open.' ・・・I cannot find the source, but I read somewhere that 10% absenteeism could lead to a dramatic decrease in functionality of public services and businesses. Of course, this assumes that 10% of workers do not show up. But if people are removed randomly, 16-17% will result in comparable absenteeism. $\endgroup$ – Olga Oct 24 '17 at 2:29
  • $\begingroup$ Therein the OP needs to clarify. People can be pulled into jobs to keep the city running. I assumed we're looking for catastrophic failure (the city cannot continue running). That's triggered by the impact of losing the 10% or so jobs that are not easily replaced or quickly retrained. 10% absenteeism my lead to companies closing, but it would not lead to the city ceasing to function. If the OP clarifies the issue, I'll modify my answer. $\endgroup$ – JBH Oct 24 '17 at 2:33
  • $\begingroup$ I agree. The normalcy point must be clarified. ・・・I wanted also to mention that I think that children are important. While they do not work right now, they are going to. Their presence ensures the stability of the city in a long run. $\endgroup$ – Olga Oct 24 '17 at 2:43
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Without social and psychological effects, I would say 75% of the population can disappear before the city should substantially cut down its services and business closures become irreversible.

If we look at most of the businesses and government agencies, all operations can be easily scaled down to be supported by about 25% of the staff, below that is typically a "breaking point" where previous activity just can't be maintained. For example, at school, 25% of teachers can teach 25% of children, but 25% of janitors and groundskeepers would have to work very hard to keep the whole school operating.

But real people would not behave orderly in the face of event like this. disappearance of 1% of the population would lead to widespread panic and many people would try to leave the "cursed" place. If they can't leave, many people would rather stay home with families than go to work.

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  • $\begingroup$ I disagree. Nothing that works in an assembly-line fashion can scale appreciably. Furthermore, few small businesses will be in a position to survive that kind of cutting without taking out someone who is the only one in the business who knows some particular aspect of it. In many cases the business will continue to function for long enough to meet his requirement, though--there will be serious holes in it's command structure but it will keep going as it was, it just can't correct course. $\endgroup$ – Loren Pechtel Oct 24 '17 at 3:00
  • $\begingroup$ Real world example: Company of a few hundred, one important guy was forced out for political reasons. A year later I'm asked: "Who is keeping our price list up to date?" Some jaw drops when I answer "Nobody." $\endgroup$ – Loren Pechtel Oct 24 '17 at 3:04
  • $\begingroup$ @Loren Pechtel - I would agree that some industry enterprises would not scale down well, and would agree that many small businesses would shot down - but I would say that reduction in number of small businesses is actually a city-wide scaling down. $\endgroup$ – Alexander Oct 24 '17 at 3:35

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