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I'm imagining a world populated with a species of intelligent, social, and "modern" (in terms of technology) humanoids. For the purposes of this question, they and their world can be near identical to us, except they are obligate carnivores.

I think this may be a mildly controversial point, but it is my understanding that the amount of food used to grow livestock could feed a much larger population of humans than the livestock themselves will feed (see here and here for two simple examples). Similarly, top carnivores are frequently the least common animals in the food chain, as a simple consequence of the fact that there needs to be more of everything else for them to find enough to eat (if you pardon the gross oversimplification).

This leads me to believe that, all else being equal, a species of technological carnivores would have more trouble maintaining as large of a population as we do on the same world, due to the logistics of acquiring sufficient food. However, I'm not actually sure if this is true, nor am I sure what the main limiting factors would be (increased land use for food, increased environmental destruction, increased transportation needs etc...) and if they limiting factors might cause more short-term or long-term problems (aka New York city is simply no longer possible for logistic reasons, or modern cities are possible but increased environmental destruction causes more sever global warming). Of course, another possibility is that this won't be an issue at all. To summarize though:

  1. Would being an obligate carnivore make it harder for a technological society to enjoy the same sustained population growth that we have had?
  2. If not, what are likely to be the primary limiting factors?
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Food logistics is still a primarily limiting factor on human population growth now; I see no reason for it not to be with a carnivorous species as well. I think the real question you're asking is whether a purely carnivorous species will be limited more or less than we are, and the answer can only be more, because you're in essence limiting the options of what a species can eat and that kind of specialisation is always punished by evolution on earth as it changes.

Human success is at least in part based on the fact that we can eat almost anything. Vegetables of course, fruit, other animals, and some foods that would kill other animals. Have you ever seen those posters up in your local vet about all the foods you can't feed your dog, and why? Dogs, which as a species are pretty flexible in what they can eat, can only eat a small subset of what we can. In practice this means that humans can grow as a species because wherever they go, they can find something they can eat.

In places where they can only find things they can't eat (like grassland prairies), there are plenty of animals that we've domesticated that can eat the grass, and then we eat the animals. So, we still make use of the local resources.

Finally, humans can now grow in even bigger numbers because of distribution technologies. We can get massive amounts of food into our cities every day, offsetting the fact that those cities don't produce much of their own food. In point of fact, our society is capable of growing to its current size because we need so much smaller a percentage of that population to actually produce the food that we all eat.

Moving on to your carnivores, if they have the same level of technology, then you probably only need the same small percentage of it to produce food for the others, so in that respect the limiting factor is the technology, and is the same as what we currently face. The bigger problem is the fact that you're limiting yourself to a specific food type, meaning that you can only move to places and areas where that food type can be supported.

Many graziers actually do plant crops; they just plant crops of tall grasses for their cattle, sheep or otherwise. They do this so that they can support a larger population of meat stock on their lands.

I'd expect that your carnivores would do the same thing, but the food your meat animals eat might not grow everywhere, meaning that you have a limitation in the amount of food you can produce. For us it's not a problem because if the land doesn't support pasture cropping, we just grow something else on the land; something else we can eat. Your species is going to find that difficult.

So yes, your food supply will ultimately be the limiting factor, because it's our limiting factor as well. Also, because you're limiting the food types that can be consumed, your growth is likely to be smaller as well simply because as the world changes its harder to support specialised food supplies. As such, your technological species had better be good at adapting.

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    $\begingroup$ afaik the things (according to industry advice) you're mostly not supposed to feed to dogs are largely either bulbs or things which dogs did not evolve side-by-side with, or those which humans and most other animals also have some issue with.. that and vets and pet food stores issue advice to avoid 'tummy upsets' rather than 'medical' advice. $\endgroup$ – Giu Piete Mar 8 at 13:34
  • $\begingroup$ Whilst a hunter-gatherer would eat whatever grew naturally, as soon as technological agriculture becomes a thing & takes over as the predominant source of food for a society, the diversity of foodstuffs in a diet diminishes in favor of whatever is easiest to grow/catch, for this reason and it's corollaries... $\endgroup$ – Giu Piete Mar 8 at 13:40
  • $\begingroup$ Then that the crop selection is very different, a grazier does not plant what crop the herd prefers(or what the miller will buy), but what is most reliable. $\endgroup$ – Giu Piete Mar 8 at 13:48
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Doubtful

Assuming this species did not just appear overnight and that they were able to reach modern levels of technology. I don't think that logistics of food will limit a population size.

With modern logistics I can get my "product" shipped to my door overnight from a warehouse in a different city. Might not be the exact same with food but we're getting pretty darn close. Logistics are getting better and better with the inclusion of modern technology (computers, AI, robots, multitude of transportation options).

Creation of food might be a limiting factor (still doubtful considering economic forces would push the market to keep up with demand), but being able to move it from point of creation to point of consumption with modern technology would not be an issue.

We have refrigeration and preservatives (might not be the healthiest thing but it does extend the shell life off food). Keeping food from spoiling would also not be an issue in this modern era.

Keeping things as parallel to todays world as possible one of the biggest differences I would see would be that the average family would spend a larger % on food.

If logistics include the creation of food then imagine all the research done for plant GMOs (genetically modified organisms) now being redirected towards animal GMOs. Might have more moral / ethical questions but I am very confident that the volume of food created would be able to keep up with population growth.

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    $\begingroup$ there's perhaps not really any need to redirect the focus of research, as the (food) animals can just as well eat the modified crop as humans can. fuel efficiency in vehicles isn't just a matter of the vehicle's qualities, but also of all the improvements in technology from the (under)ground up, from drilling to piping to refining to etc. Think it's worth considering also that in a (pre-urban) environment everybody can have food animals that not only simply live off of garden product, but eat food and perhaps other organic waste. $\endgroup$ – Giu Piete Mar 11 at 15:32
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    $\begingroup$ In the contemporary world an enormous amount of effort and manpower is put into cutting lawns, clearing growth etc, in the UK(first) gardening accounts for ~0.5% of employment, and this doesn't include (the enormous) investment of time and effort by farmers, biopharma, etc in clearing fields of unwanted growth which is almost totally unnecessary for a civilization that is entirely focused on meat output (and the corollary of utilizing animals where feasible for any role they might be applied to because of the increased preference for having animals that reliance on meat entails.) Speculative $\endgroup$ – Giu Piete Mar 11 at 16:03
  • $\begingroup$ *Speculative, but people keep animals as pets and enjoy zoos for emotional reasons, it seems highly likely in my view that a species of technological carnivores would be engineered in such a way as to get a much wider and more appreciable range of emotional inputs from contact with animals and thus prefer them in their environment. It's been argued before that essentially the only animals humans suffer to live when they have the choice are those they use. It seems probable that, given a reliance on meat products, evolved carnivores would still prefer variety, variety among animals. $\endgroup$ – Giu Piete Mar 11 at 16:09
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I suppose in some ways, yes, but there are really three parts to consider here: technology, planet size, and livestock size.

Technology

Exactly how technologically advanced is this species? Is it the same as humans? It is entirely possible the species may have ways of producing synthetic meat, or using cloning technologies to produce meat. This is perhaps even a point to build culture around, with synthetic meat being considered a poor persons food, and natural meat for richer people. You know, the kind of thing white suburban moms would brag about at the PTA meeting: "AT LEAST I DON'T FEED MY CHILD FAKE MEAT KAREN!". You get the idea.

Planet Size

How large is the planet the species is inhabiting, especially compared to current population size and required living space? Perhaps the species has indoor farms in skyscrapers, producing food right in the cities where it is needed. Perhaps there is something similar to the Japanese roof gardens, where people in cities have areas on top of buildings where they can grow a communal garden.

Livestock Size

How big are the most common livestock, what do they eat, and how fast do they reach maturity? All of these are things to consider in your determination of whether or not the society can produce enough food

Conclusion

There are many factors in deciding how food would limit population growth, but ultimately, any society will eventually be limited by it's food production capabilities.

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A strictly carnivorous humanoid species' population size would be limited by the amount of available arable land in the same way that an omnivourus species would be. In order to farm animals like cows or goats you will need meadows/pastures for them to graze in. If this fictional humanoid species lived on a relatively small rocky island like Japan the amount of farm land might be extremely limited. You might be able to mitigate this somewhat by factory farming like is done with pigs, but at the end of the day every area has its carrying capacity. If this fictional humanoid species had advanced enough technology then cultured meat might be a viable alternative.

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  • $\begingroup$ You should explain what does terms mean. You don't need to add a deep explanation but a brief introduction, like, what is cultured meat or factory farming. People shouldn't be "forced" to see external links in order to understand an answer. $\endgroup$ – Ender Look Mar 11 at 16:47
  • $\begingroup$ @EnderLook: People are also expected to have at least a basic understanding of what terms like "arable land", "factory farming" and "carrying capacity" mean. $\endgroup$ – AlexP Mar 11 at 17:19
  • $\begingroup$ animal herding is absolutely not restricted to arable land or for that matter, easily navigable land. not all herds (for one thing) need to be moved for culling even annually. Organized food distribution methods have been around since antiquity. $\endgroup$ – Giu Piete Mar 12 at 19:09
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Indirectly limited

Thomas Malthus had the highest misfortune of describing existing trend exactly in moment when it finally started loosing its relevance. Humans are at latest from the green revolution ('50s-'60s) no longer realistically constrained by food production in any way. If somewhere is a famine, it is not caused by technological or environmental inability of producing food, but political issues (read: Mao's Great Leap Forward as last big example).

Right now we are as fussy as we want. Beef? Salmon? Avocado? Ecological food? No big deal.

So if such specie reached the same technological threshold, sooner or later they would be fine. Nevertheless, you have a very good argument why their population should be much lower than humans - all their history, until somewhere around XIXth century equivalent they were drastically restrained by food production. Low food -> low population -> less scientists -> slower growth. If they made it anyway, then they are fine. However if at their year 1900 equivalent their global population was 500 mln, then right now their population may be reaching 3 billion. Not because they are hampered by their diet right now, but because their population haven't grew so much from times when it mattered.

Moreover as result of historical legacy their population could be a bit more dispersed. There are plenty of inhospitable lands (or sea shores in sub-polar regions which are rich in fish) that may be considered by a carnivore as quite tempting.

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Land

To produce meat capable of providing 1 kilocalorie of energy one needs to feed the chickens/pigs/cattle anywhere between 3 and 20 kilocalories of vegetable food, for a conversion ratio from 30% (for broiler chicken) to 5% (for luxury beef). Thus, in a first order appoximation one can say that to reach the same population as us omnivores, the obligate carnivores would require anywhere between 3 and 20 times more land dedicated to agriculture.

Going into more details is complicated, because while chicken and pigs eat more or less stuff which we could also eat, ruminants (and horses) do not have to compete with us -- they can eat grasses, which humans cannot digest. In practice, cattle grown for meat are fed a lot of grain, which of course comes into competition with human food. On the other hand, we use a quite lot of grain to make dubious products such as ethanol (to drink or to burn)...

The problem is of course that there is no such undeveloped land available on Earth; with very advanced technology we might be able to increase the area under cultivation by, maybe, 50%, but increasing it by a factor of 3 or 4 or 5 is out of the question.

The sad conclusion is that at best the man-sized obligate carnivores cannot reach a population level higher than one quarter to one third of the present human population. However, this is not the saddest conclusion.

The really sad problem is that I see no plausible way for them to sustain a pre-modern civilization. With pre-modern agriculture primary productivity was low, and feed conversion ratios were dismal. Consider that during the antiquity and the middle ages, the average human was lucky to get 5% of their energy from meat; the other 95% came from vegetable sources, mostly cereals and pulses. But it may well be that this can be handwaved somehow.

Logistics

Meat is much more energy dense than vegetables, especially in terms of volume. So there is no reason to believe that the logistics of bringing all the food into the cities in the form of meat will be more complicated than bringing it in the form of vegetables; if anything, the logistics will be simpler.

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  • $\begingroup$ They would be heavily dependant on the salt trade to preserve the meat.... $\endgroup$ – Efialtes Mar 14 at 0:38
  • $\begingroup$ Or refrigeration in place of salt. As was done with huge volumes of lamb from New Zealand to Europe. $\endgroup$ – Andrea Williams Mar 15 at 22:57
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Will food logistics limit the population of a civilization of exclusive carnivores with modern technology?

If you mean transportation and storage, maybe. If you mean, the geopolitical system of food production, very likely.

Would being an exclusive carnivore make it harder for a technological society to enjoy the same sustained population growth that we have had?

Yes, indeed. The agricultural revolution was key for population growth, and hence, ancient technology.

If not, what are likely to be the primary limiting factors?

In decreasing order: climate change/energy sources, peasant labor, drinking water and arable land.

Explanation

This is a complex question, so I'll try to explain some points and then come back to the answers.

According to Systems Ecology, the limiting factor for the population growth of any species is always the amount of energy it can derive from its environment in a useful way.

For most species, this means that the food consumed delivers more energy to the body than the energy invested in basal metabolism and other activities, including of course, those activities that are meant to obtain food (i.e. grazing, hunting and gathering). For example, if gazelles develop a way to run faster, the cheetah population would narrow to the fastest cheetahs. Then the latter species might either evolve into some sort of faster supercheetahs (who need and get more energy from their environment) or dissapear altogether due to famine, sickness, poor genetic diversity, etc. but that is a different story.

For those species that hunt in group, the rule applies for the whole group: those packs of wolves that are able to catch faster gazelles would thrive, but the species might be endangered for the same reasons pointed out in the case of cheetahs.

For those species that are able to produce their food, i.e. transform their environment in order to catch or grow their food, the energy rule holds as well but it gets more interesting. Bees, ants, termites, spiders, beavers, among others, modify their environment to obtain and store their food, so the amount of energy invested in running the production of food must be lower than the amount of energy delivered by the food produced. The energy needed to maintain the glands and synthetize the complex molecules that form the thread of a spiderweb must come for the preys that the web catches.

When it comes to humans, things get even more complex because our species has been able to use exosomatic energy (energy outside the human body) to produce food. In other words, the energy invested in producing, storing and transporting food does not necessarily comes from the energy delivered to the human body through food digestion. Agricultural tools, beasts of burden, natural and chemical fertilizers, machinery, gasoline, etc. are used in the production of food, but the energy required to produce them comes mostly from fossil fuels, biofuels (bioethanol mainly) and primary electricity (hydro, solar, wind, nuclear, biomass, etc.). These exosomatic sources allow us to grow food in deserts, mountains, and other places where it was impossible to produce it a hundred years ago. If you plot the world population and the world production of fossil fuels over time, you will see that both lines start growing roughly at the same time.

Land, water, soil and the "usual" limiting factors are not an obstacle to our food system AS OF TODAY. Each of these factors can be supplied, enhanced or substituted using the exosomatic energy sources mentioned above. In fact, the current food system produces more food than required by the world population (according to FAO), so edible crops are used for other purposes. There so much corn, for example, that it is used to make ethanol for cars, to feed livestock and to sweeten all kinds of things (that's why you can find high-fructose corn syrup in your favorite soft drink). Since the system is based on fossil fuels, it is clearly unsustainable in the long run but that is a different story.

Feeding humanoid carnivores is a bit harder than feeding humans. However, while it seems doubtful that a carnivore population of 7.2 humanoids would survive under the current energy available to us, it is equally doubtful that they would grow at the same rate that omnivore Homo sapiens did because the amount of energy and biomass required to grow carnivore food is larger.

In a trophic pyramid, only ten percent of the energy contained in one level can be transferred to the next one. Since life is basically made of the same molecules (carbohydrates, proteins, etc), this number is also an approximation of biomass transfer. In other words, you need 100 lb of grass to feed 10 lb of herbivores (even if it's 1 lb of insects), and then feed 1 lb of carnivores. For carnivores, agriculture would only be the step in the food system. In fact, carnivore humanoids might never had developed such a labor intensive practice in the first place! Why take all that time and energy in a system full with weather uncertainties when you can go around hunting game happily? In fact, hunter-gatherer communities were healthier than agricultural societies, not to mention absence of slavery, plagues, wars and other niceties that characterized agricultural empires.

On the other hand, hunting carnivore humanoids might go extinct after they hunt down every last game available. In fact, humans are held responsible for the extinction of megafauna in all continents except Africa, where elephants, rhinos and hippos evolved together with sapiens, so they "know" that our weak appearance is deceiving and that sapiens are not to be trusted.

Assuming our humanoids did develop agriculture as a desperate means to "grow" their own chickens or something, you would have a though problem feeding carnivore slaves and peasants. Anyways, let's assume peasant labor was fed on a mix of dry insects, blood and bones for some time until some ancient civilization found coal (energy resource) and built some steam engine (technology) that they could apply to agriculture (note we are talking here about two major processes in human history: agricultural and industrial revolution). Even in that case, the rate of growth of the population would be lower because these humanoids would still be entirely on the third level of the trophic pyramid, rather than partly on the second level as our agricultural ancestors were.

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