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I'm making a planet which hosts three distinct layers of floating islands, each higher than the last one. The islands should be reasonably thick, ranging from a couple hundred metres to three or four kilometres in average, depending on the size of the island and its relief. The layers need some separation between them of course. Taking all of this into account, it's notable the planet is going to need a lot of vertical space, furthermore, inhabitable vertical space, as the islands should house all kinds of lifeforms.

I've been playing around with Artifexian's "Earth-like atmospheres" spreadsheet of this video of his and tuned the planet so, with a similar composition to that of Earth's atmosphere, lower gravity and 4,2atm of pressure at planet's surface, we get a pressure of ~0,9atm at an altitude of 15km, which I think should do the trick for the most part.

Could complex life exist and develop in such high altitudes? Would the difference in pressure prevent life from the surface from living on the high islands and viceversa?

I'm sure there are a lot of important factors that make a difference as altitude increases I'm ignoring, like temperature decrease, solar radiation, wind currents and more. Please do point these out so I can take them into consideration.

Note that the lifeforms considered in this scenario are adapted to the conditions of their place of origin on the planet and do not strictly follow human endurance limitations when it comes to adapting to harsh conditions (i.e.: minimum and maximum oxygen partial pressure). With that said, human limitations may be used as a guide.

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    $\begingroup$ Is this earth with floating islands? Because why are they different? I like the idea of a gas giant with a stratified atmosphere that was different at different altitudes. Life would be different depending on constituents of the atmosphere on a given level. $\endgroup$
    – Willk
    Mar 27 at 18:57
  • $\begingroup$ @Willk despite being Earth-like, the planet in question isn't a tuned up Earth, it is a different planet, with higher mass, a bigger radius, lower gravity (around 0,86g) and 4,2atm at sea level. The gas giant idea is cool, but the planet we are dealing with here isn't one. Plus I'd like (if possible) for there to be movement of living things among layers, so the conditions shouldn't be absolutely dividing from a layer to another. $\endgroup$
    – El Nitromante
    Mar 27 at 19:38
  • $\begingroup$ What about your planet's star? All of these answers are modeled upon a sun like star. Different size stars have different UV output and could theoretically fudge the habitability one way or another. Smaller stars put out less UV so plants/life may br able to survive at higher altitudes. Note:UV is just one factor in play here that determines plant growth. $\endgroup$
    – Hippeus_Lancer
    Apr 26 at 23:39
  • $\begingroup$ @Hippeus_Lancer That would be an F9 star with values relative to the Sun's of 1.120 (mass), 1.086 (radius) and 1.487 (luminosity) $\endgroup$
    – El Nitromante
    Apr 27 at 11:21
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Altitude can be a proxy for latitude.

You can design your floating island ecosystems by thinking about ecosystems from varying latitudes on Earth. In places where there are big altitude differences this is how ecosystems work. A rise in altitude is a proxy for an increase in latitude - going up in elevation is like moving away from the equator and toward the poles. An example is the Sierra in Ecuador which is on the equator.

https://en.wikipedia.org/wiki/Geography_of_Ecuador#The_Sierra

The climate in the Sierra is divided into levels based on altitude. The tropical level—400 to 1,800 meters —has temperatures ranging from 20 to 25 °C and heavy precipitation. The subtropical level—1,800 to 2,500 meters —has temperatures from 15 to 20 °C and moderate precipitation. The temperate level—≤2,500 to 3,200 meters - -has a year-round temperature in the range of 10 to 15 °C and an annual rainfall of 1,000 millimeters . The temperate level experiences rainstorms, hailstorms, and fog. Winter, or the rainy season, lasts from January through June, and the dry season or summer from July through December. Most rain falls in April. There also is a short rainy period in early October caused by moisture penetrating the Sierra from the Oriente. Quito and most other populated areas in the Sierra are located at this temperate level. The cold level extends from the temperate zone to 4,650 meters . Here, average temperatures are 3 to 9 °C , and the precipitation often appears in the form of rain, hail, and thick fog. Above 4,650 meters (15,256 ft) is the frozen level, where peaks are constantly capped with snow and ice, and temperatures range from below 0 to 3 °C.

In Ecuador you will find species at altitude that you would find at lower elevations in regions farther from the equator. This would be a fine shorthand for you to use in designing your islands of various heights. They happen to be floating islands of different elevation but they could as easily be sites on a mountain range at different elevations. The lowest are steamy, hot and tropical. The highest are dry and cold, with bright sun.

Your high atmospheric pressure would tend to exaggerate the elevation differences we see on earth. I like the idea that on the lowest level with an oxygen atmosphere and a pressure of 4 atmospheres, the only reason everything does not burst into flame is that there is always 100% humidity. I think I will stay a few levels up from that one.

As regards the circumstances under which intelligence is adaptive for a creature, I think that is a question separate from altitude and ecosystem.

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