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There was an orbital station; a huge torus, as known from sci-fi. It was a luxurious place for people from dusty planets to relax at, and spend hard-earned money in environments resembling Earth from the legends. Forests, meadows, hills, rivers and lakes. Everything powered by solar energy, protected from radiation by artificial magnetic field, and maintained by a horde of autonomous robots that would repair anything broken, repair each other if any of them breaks, and go into nearby space to hunt comets and asteroids to provide raw material for all the repairs (and fuel for more such voyages) in case they'd be running low.

Something happened, and the human population abandoned the station, leaving almost all the autonomous systems running. Someone, maybe in a fit of wry humor, switched "ecosystem regulation" off (while leaving maintenance; water, humidity, temperature etc), so it would no longer be artificially maintained at status quo, but could evolve, and also disabled the spin motors of the station.

The station took a couple thousand years to de-spin due to tidal forces, the artificial gravity gradually dropping to zero. Meanwhile, the ecosystem thrived in the changing conditions, allowed to "run wild"; robots faithfully maintaining the infrastructure, bringing resupplies of air in place of leaks, fixing breaches from meteorites, etc. And the ecosystem (oh, pick anything you like that is reasonably varied, say, moderate climate forests, or African savanna, just please, not "bottom of Pacific"), kept evolving, adapting to the new conditions.

The station is "rediscovered", say... 100,000 years later. How will the different classes of species (predators like wolf or leopard, larger herbivores like deer or antelope, small herbivores (rodents etc), birds and insects look like? Or will one or more class of these go extinct?)

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    $\begingroup$ How big is your station? $\endgroup$ – PatJ Jul 4 '16 at 9:06
  • $\begingroup$ @PatJ: "Enough". ;) Let's say 100km^2 of original green terrain, plus second that much of various "engineering" areas that got taken over to greater or lesser degree by expanding wildlife. $\endgroup$ – SF. Jul 4 '16 at 9:38
  • $\begingroup$ (also, all "business/staff" areas were hijacked - hotels, shops, living quarters etc. At first by animals looking for shelter, later animals brought seed, carcasses, nesting material in amounts sufficient to support plant life; it rolled from there. Water taps operate on proximity sensors (and sinks don't sink water without gravity) so plants get water; lights are on in most areas.) $\endgroup$ – SF. Jul 4 '16 at 9:45
  • $\begingroup$ If there's no gravity, what is keeping the water in place? The soil? What prevents the station from being filled with a giant lightning storm / duststorm that blocks light and abrades or shocks/burns everything? $\endgroup$ – Urausgeruhtkin Jul 4 '16 at 21:17
  • $\begingroup$ @Urausgeruhtkin: Water is held by viscosity and surface tension, moves with air current, binds dust. The "climate" is still being maintained artificially, never mind there's no planetary scale atmosphere to create such violent storms - and with no gravity, the stresses normally observed in atmosphere would be much more mild Normally, air cooled in upper layers condenses and descends towards the surface; air heated over deserts heats up, expands and climbs; evaporation rises as clouds, then falls as rain. Without gravity such trends don't occur; violent weather is unlikely. $\endgroup$ – SF. Jul 4 '16 at 21:53
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My own exploration into this (which is how I came across this question) shows a bias into climbing and swimming adaptations. Animals that have grasping paws are able to grip onto areas of the station, allowing them more control over their location. Swimming adaptations allow the animal to move and control their location in a low-gravity environment without relying on pushing off of the walls.

Flying adaptations are also useful, but their surface area will likely decrease due to the lack of gravity (as the density of the air remains pretty much the same). Swimming adaptations (such as fins) may increase in surface area to compensate for lower density, but as there is also much less drag that is uncertain,

Longer limbs and necks will likely be selected for higher ease when capturing food and water (as both will float in this environment)... If the fans are off, there might also be adaptations around the head area to prevent Carbon Dioxide or other gasses from building up & suffocating the creatures.

Less energy/muscle mass is needed to function in this environment, which may prompt larger animals. Said animals will also likely be fairly flexible to allow for turning in mid-air.

Other then this I'm unsure as to which adaptations will be needed. I focused on the physical/visible changes rather than the changes in behaviour (to which there'll be many) but this should hopefully give a general idea.

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See: link

I think the outcome looks bleak. Assuming all base life support holds ...

The biggest hurdles to overcome are

  • Loss of bone structure, resulting in osteoporosis.
  • Loss of muscular structure, including potential heart problems.
  • Fluid distribution is difficult, which leads to issues long term.
  • ...

So, who lives

I would think invertebrates have an advantage, so maybe squid- and jellyfish like creatures? Snails, that sort of thing.

Insects with a hard exo-skeleton would also be ok, I guess. So beetles and the like.

Maybe soaring birds who don't need a lot of muscle action to keep going? That's assuming there's enough atmosphere left to have lift.

UPDATE: Apparently birds are in trouble, because they depend on gravity to swallow their food.

So, who dies

Pretty much everyone. Minimal or close to zero gravity has effects that make sustainable live practically impossible.

PS: I'm not a (bio)scientist, so take this as the (non-educated) opinion that it is.

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    $\begingroup$ Extreme cold, low pressure, oxygen shortage, high UV exposure. Gravity hardly affected. Entirely different set of adverse conditions. $\endgroup$ – SF. Jul 4 '16 at 11:48
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    $\begingroup$ Humans can live on ISS for over a year with no serious adverse effects (the major adversities resulting from osteoporosis and atrophy appear only upon return to gravity; it's actually eyeball distortion that puts the practical limit on life in freefall, the astronauts slowly going blind.) In the conditions you described, without protective clothing, rebreather, and a lot of specialized gear, human won't survive a night. Microgravity alone definitely isn't harsher. $\endgroup$ – SF. Jul 4 '16 at 12:38
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    $\begingroup$ We've gotta agree to disagree. Entirely untrained mice are doing fine. $\endgroup$ – SF. Jul 4 '16 at 13:14
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    $\begingroup$ @SF "untrained mice" - I lol'ed $\endgroup$ – AndreiROM Jul 4 '16 at 13:40
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    $\begingroup$ @Spikee: You are still ignoring the evolution part - conditions changing gradually over thousands of generations. $\endgroup$ – SF. Jul 6 '16 at 6:14

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