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Let's say that tens of thousands of years pass and earth effectively becomes a giant city.

What species of organisms survive our conquest? How do these species evolve to adapt? How long would human progress have to stagnate for any organism to become a competitor or threat?

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  • $\begingroup$ Bacteria will survive and superbugs may eventually become a threat. $\endgroup$ – Samuel Feb 13 '15 at 20:24
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    $\begingroup$ Any species that evolves to be extra cute would probably survive. $\endgroup$ – Twelfth Feb 13 '15 at 21:00
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    $\begingroup$ Tens of thousands of years? Don't I wish. The cold, hard truth is that the only survivors are likely to be extremophile bacteria. Humans, of course, will be among the extinct species. And on current trends, this will most likely happen in closer to 500 years. $\endgroup$ – jamesqf Feb 13 '15 at 23:19
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    $\begingroup$ There is more forest in developed countries now than in 1900. $\endgroup$ – Oldcat Feb 13 '15 at 23:55
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    $\begingroup$ @Oldcat: Irrelevant, as the OP specifically is discussing an Earth that is one giant city. Also untrue, or rather an instance of lying by telling misleading partial truths. There MAY be more scrub woodland than in 1900, but it will take many centuries for that to become true forest. $\endgroup$ – jamesqf Feb 14 '15 at 6:01
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You can extend what is already happening in our cities today. Cities generally do still have fairly extensive green areas, with paved areas and houses between them. Inner cities obviously are harder to survive in but even there you have lots of discarded human food to live of.

You should expect to see a lot of scavengers. The animals that already adapt well such as foxes, rats, seagulls and other birds. Large grazers or carnivores clearly would not do well. Small animals like squirrels and other rodents would generally do fine and you might well find isolated populations developing in different areas. For example a specific breed of squirrel might evolve in one park, distinct from another park because there is no easy way to cross between them.

Cockroaches and other similar scavenging insects may also succeed, although we may well have developed efficient ways to wipe them out and as a result they are consigned to zoos.

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  • $\begingroup$ Single add to Tims list...thieving animals such as monkeys would be happy in this. Apparently Sloths do well as we tend to think they are oh so cute and welcome them with open arms. $\endgroup$ – Twelfth Feb 13 '15 at 21:32
  • $\begingroup$ If the planet's one giant city, though, where do the humans get the food they discard? $\endgroup$ – jamesqf Feb 15 '15 at 18:05
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You are thinking about something like Trantor. It is extremely unlikely something like this to happen. Reason is a simple energy balance.

You have to feed your population, and provide it with energy. For that you need fields: area where you produce food, open to sun. That is area where all kinds of animals can live. Most of such land is marginal, and will not be used for agriculture. And even more area will need to be devoted to producing carbon-neutral energy. Without carbon-neutral energy, your climate will collapse and productivity of your field will collapse, and your population will starve.

To increase population density, you will have to import food from dozens of other planets, and those planets have to be also habitable to be able to export food. What would Earth export in return? And where the energy to lift Earth's export will come from? And if humanity has access to such huge reserves of energy and many habitable planets, why not to move there?

Trantor was a center for huge fictional galactic empire based on Roman Empire, with many worlds devoted to farming, and with no robots to do the farming and other tedious work.

I think that your ability to supply carbon-neutral energy for your over-populated will run out sooner than area for farming and wild places like mountains.

Humans do not have to stagnate - they need to "get to the stars". Plenty of space in Space.

Some more thinking... I think OP ever-estimates plausibility of future overpopulation. Even today, person in technologically advanced society needs half year of education (with no income) for every year of work for income - elementary school to graduate degree. In the future, time invested in education to gain skills necessary for paid work will increase, and need for low-skilled work is steadily decreasing. Such pressure has same effect in all developed societies: decrease of female fecundity. USA is exception, not a rule: most other advanced societies have decreasing population, and not increasing.

r/K selection theory distinguishes between two population strategies:

  • r strategy: produce as many offsprings as possible, and spend little resources supporting them
  • K strategy: have few descendants, but spend lots of resources to make sure they succeed

I claim that K strategy is better suited for technological future of humanity. In the future, maybe even right to have a child will have to be earned (because r strategist cannot expect responsible taxpayers to support his or her r strategy, while limiting own strategy to K because of limited resources). Humanity will have to face limited resources, and adjust morality accordingly. Will it be forced sterilization after second child for a woman on welfare? Effective contraceptives in public drinking water? If every child has right to be loved and has access to opportunities, it forces K strategy on humanity.

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    $\begingroup$ There was a parody of Trantor in "Bill the Galactic Hero" by Harry Harrison where they had ships importing Oxygen to keep everyone alive. $\endgroup$ – Oldcat Feb 13 '15 at 23:56
  • $\begingroup$ Sounds like a slightly more horrid version of China... $\endgroup$ – Serban Tanasa Feb 15 '15 at 23:04
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    $\begingroup$ I fail to see how that 1 year of ed for 1 year of work calculation works out, unless you live in Greece... $\endgroup$ – Serban Tanasa Feb 15 '15 at 23:05
  • $\begingroup$ It will work other way around: mid-career your education will become obsolete and you need to go to school for few years. $\endgroup$ – Peter M. - stands for Monica Feb 16 '15 at 13:47
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    $\begingroup$ This answer assumes that people still rely on normal agriculture for food, if people ate algae grown extremely efficiently through some sort of industrial process it might no longer be the case that you need a large amount of farmland to support urban areas. $\endgroup$ – Vakus Drake May 7 '16 at 4:46
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Plenty of species already evolve with us in urban environments (at least 'evolve' in the sense that they thrive and benefit from the urban environment).

  • pigeons
  • hawks
  • rats
  • mice
  • squirrels
  • raccoons
  • feral cats, dogs
  • domestic pets
  • even bears, moose (when in proximity to urban areas)

Species that are adept at changing environments and opportunistic in terms of leveraging human environments will thrive. Others will not.

While not urban-centric, the PBS documentary on Dogs talks about one particular species that has thrived with humans: http://www.pbs.org/wnet/nature/dogs-that-changed-the-world-introduction/1273/

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Let's say that tens of thousands of years pass and earth effectively becomes a giant city.

Ok. Given a compelling enough economic rationale, it's doable. Even without doing a single calculation, you can already know you'd have to replace all cultivated fields with massive hydroponic food factories, so you'd still have vast amounts of the city (say a few levels) dedicated to agriculture. You'd have to modularize the city to make it resistant to earthquakes, and you'd have to capture and channel much of the waterflow that currently flows in rivers. None of this would be cheap. The question is whether there's a compelling reason to have that many people close together.

A dense city like Mumbai or New York will usually top out at something like 30,000 people per sq. km. With proper technology, you can easily double that, perhaps even expand tenfold. But for comparison's sake, let's stick to Mumbai levels of today. With a land surface area of 149,000,000 sq. km, and given a 60% utilization rate (avoiding tall mountains and frozen tundras) you get a population of 2,682,000,000,000. That is about 2.7 trillion which is about 400 times more people than you have on the planet today. The energy production levels needed to support such a population, even with advanced hydroponics, are currently beyond our technology. Perhaps with massive space-based agro-habitats and solar plants linked to the Earth by space elevators, or with a multi-level arcology of which a non-trivial portion is reserved for agricultural purposes.

What species of organisms survive our conquest? How do these species evolve to adapt?

With 100% certainty, vast numbers of single-celled organism species, such as bacteria yeasts, molds.
With almost 100% certainty, cockroaches, ants and mites.
With >95% certainty, mice, rats, cats, dogs.
With >90% certainty: pigeons, sparrows and other climate-equivalents.

How long would human progress have to stagnate for any organism to become a competitor or threat?

Unlikely to ever happen. By colonizing the entire planet, we will likely drive into extinction or zoos all other large mammals that could possibly compete against us. With 2.7 trillion people, you'd be in no risk of being numerically overwhelmed anytime soon. The only threats to such a megalopolis would be internal strife upsetting the complex networks required to keep each portion of it alive and thriving.

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