That means of course after all the land has returned to some sort of "wilderness". And the absence of competition from other large predators, nowadays decimated in Europe, may mean slightly higher densities. Auferre, trucidare, rapere, falsis nominibus imperium; atque, ubi solitudinem faciunt, pacem appellant.
In this paper, we follow the definition used by animal ecologists and refer to the space used by an individual or social group as its home range. The home range, H0 (area in km2), required by an individual to meet its metabolic requirements is determined primarily by the rate of resource supply, R (in W/km2). Because humans feed on animal and plant foods, rates of resource supply, like most biological rates, increase exponentially with ecosystem temperature. Ecosystem temperature affects rates of biological production and interaction at multiple levels, including biomass production, ontogenetic and population growth rates, timing of life history events, and interactions with parasites and diseases. Much of this variation is captured by the Boltzmann factor, e-E/kt, where E (in eV) is the activation energy for the rate limiting biochemical reactions of metabolism, k (8.62 x 10-5 eV K-1) is Boltzmann's constant, and T (in K) is temperature (16-18).
I wouldn't be surprised if there were enough rabbits within a mile's radius of where I live to feed the entire village over the summer.
Since it's about a mile between villages, that would be a start towards self-sufficiency.
But that's to feed around fifty people - not fifty thousand.
The UK has a population density of over 600 per square mile. Oops... When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes
While the rabbits here are plentiful over the summer, over the winter there's really nothing at all available for picking off a tree - unless you grow it, preserve it, or both.
In medieval times, 15-30 acres (6 to 12ha) were considered enough to support a family. Modern farming would be - at a guess - 5 to 10 times more productive.
So you could homestead successfully and possibly also be energy self-sufficient on around 3-5 acres, or perhaps an acre if you're a sustainable farming god.
But it would be organised seasonal growing and storing - foraging wouldn't be a big part of it.
Your self-sufficiency still depends on reasonable weather. If it's too wet or too dry or too stormy or too sunny, your crops fail and you starve.
The most useful thing civilisation does is manage storage and distribution of food to create a surplus and a safety net. (For most of the world.) The second most useful thing is management of infrastructure, including irrigation, drinking water, drainage and power distribution. The third is centralised access to expert services, including R&D/innovation, and medical care.
If everyone is a smallholder, all of those become more complicated and potentially less effective.
