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What would be the natural reason for the surface of a planet to be toxic or otherwise uninhabitable to animals but not plants? This planet is not Earth and life would have evolved this way from the beginning.

Ideally the animals would have to live higher up (trees, rocks, maybe even floating islands) and would all have evolved to fly or find another way to navigate treetops/the air.

Plants would still thrive on the ground and animals would still be able to go down to the ground but not for long.

(when I say animals I mean animals and sentient species)

What I've thought of so far:

  • a layer of CO2 coats the surface (or higher % of CO2) which means plants can grow but animals can only go down for a quick time (or have developed ways of holding their breath for a bit longer).
  • the ground burns for some reason, but plants have evolved to resist it
  • the ground is toxic to touch, whatever the toxicity is it's something plants use

I'm looking for complete and plausible ideas or reasons animals can't live on the ground but plants can.

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  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Commented Jun 8, 2022 at 9:35
  • $\begingroup$ Are you asking what can cause the lack of animal life at ground level, or what could cause your explanation of it? Because both questions are present, and we prefer to answer only 1 question per post $\endgroup$
    – L.Dutch
    Commented Jun 8, 2022 at 10:08
  • $\begingroup$ I've edited the original post to be more clear. I'm looking for reasons animal life would have to evolve to escape vertically away from the ground. $\endgroup$
    – Juliette
    Commented Jun 8, 2022 at 11:01
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    $\begingroup$ If you're determined to stick with "animals" then whatever you come up with will have to work for coral, tardigrades, and cockroaches...which seems like a tough ask. $\endgroup$ Commented Jun 8, 2022 at 19:54
  • $\begingroup$ Is there anything that would restrict an animal to the air? I think they gotta come down to the ground and rest at some point. Is a bird restricted to the air? Is a whale restricted to water? $\endgroup$
    – Wyck
    Commented Jun 9, 2022 at 19:29

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Pollen from the plants (maybe just one extremely common plant family) is extremely toxic to most animals and there's an awful lot of it in the air. Perhaps it's a defense mechanism these plants evolved to stop animals from eating them. Most animals can cope with inhaling a small amount of pollen but more than that will kill them. At higher altitudes there is less pollen.

Not sure why there are no animal species that evolved a resistance to pollen. Maybe it disrupts some very basic chemistry common to most animal life on the planet. Or perhaps it's not the chemical toxicity but the physical shape of the pollen - perhaps the particles are hard and have jagged edges that damage the animals' internal organs.

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  • $\begingroup$ This is a great idea and is very flexible in terms of plant diversity and landscape. Inspired by your suggestion: the pollen chemically affects basic animal respiratory functions (such as the transportation of O2), and is relatively heavy meaning that it doesn't float upwards more than 5-10 meters. $\endgroup$
    – Juliette
    Commented Jun 9, 2022 at 12:56
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    $\begingroup$ "Not sure why there are no animal species that evolved a resistance to pollen." Maybe they will, they just haven't yet because this is a new change in the plants. $\endgroup$
    – Michael W.
    Commented Jun 9, 2022 at 15:40
  • $\begingroup$ Can pollen damage human lungs in real life over time? $\endgroup$ Commented Jun 9, 2022 at 19:09
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Big Smelly Mangrove

enter image description here

Animals cannot come to the ground because there is no ground. Only an endless swamp with nothing to stand on.

The air is thick with CO2 from decomposition. It will not kill you but you might pass out and drown. Or get eaten by the Arapaimas that lurk below the brown surface.

The decomposition is part of the world's ecosystem. Animals live in the sky, die, fall into the swamp, decompose and get sucked up by the trees and so returned to the sky.

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    $\begingroup$ +1 "...the circle of liiiiiiife" $\endgroup$ Commented Jun 8, 2022 at 17:57
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    $\begingroup$ I like the decomposition cycle a lot, although I would still like to have a varied landscape and plant life across the continent. I will most likely have a biome type that is inspired by this, thanks a lot! $\endgroup$
    – Juliette
    Commented Jun 9, 2022 at 12:52
  • $\begingroup$ @Juliette Then you should keep the swamp part but have different types of "mangrove" trees that look different on top but have the same roots and life cycle on the bottom. $\endgroup$
    – Daron
    Commented Jun 9, 2022 at 16:35
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There is one type of animal that lives on the ground.

ants

source

Your plants are myrmecophytes or the alien equivalent. The plant - insect alliance is so successful that all existing plants are allied to one degree or another with the Hive, and the denizens of the Hive control all ground spaces in this world. I picture the Hive as like the depicted army ants. If you touch down you are bit within seconds, and you have a few more seconds to get well into the air before the wave arrives - both from the ground and from adjacent vegetation. The Hive spans the entire planet, like a hive of Argentine ants spanning all of california. These creatures have conquered the planet.

There are places where plants use chemical wiles to trick their Hive associates into helping those plants over other competing plants and this happens in our world: Trees Trap Ants into Sweet Servitude. There are nest parasites and things which lead a sneaky parasitic existence in the Hive. But the Hive eliminated all other animal competition millenia ago. Those species that survive took to the air and airborne refugia where they (hopefully) stay out of reach. Many of these are predators on Hive members and will watch and swoop down to snatch one and take it away - usually at the cost of some bites from Hive members that jump on when it is low and ride off with it.

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    $\begingroup$ Great answer, it's making me itch $\endgroup$ Commented Jun 10, 2022 at 13:42
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    $\begingroup$ Even better if there are a handful of different ant species in a state of constant war; that way there is perpetual environmental and evolutionary pressure for the ants to be in peak combat performance. Also, FYI, this happens on Earth: forbes.com/sites/quora/2016/08/12/… $\endgroup$
    – Zags
    Commented Jun 10, 2022 at 16:05
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The simple answer, though it may not be the one you are looking for, that is the plants are carnivorous. They evolved slowly to become more and more dangerous, the animals evolved to escape from them into the air. After some millions of years it has settled to just the right amount of separation for your story.

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  • $\begingroup$ Carnivorous plants are less efficient at being plants. However, what if part of being carnivorous means that the plants themselves will fight each other on occasion? This would explain why almost all plants have the ability to move. Normally plants are in a tense truce with each other because war means loosing a lot of energy. Of coarse, when an animal comes by they will eat them for caloric benefit. Normally the plants fight over resources such as water and sunlight. $\endgroup$
    – philn
    Commented Jun 10, 2022 at 20:04
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All the vegetal life on the planet produces a toxic sap, which kills any animal interacting with it.

Therefore the only way out for animals is to stay above the ground, away from the sap and its lethal effects.

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The plants predate on the animals. The ground is covered in the things, and the only safety available for animals is to remain airborne.

I don't know if this means your plants aren't sedentary photosynthesizers or not. Perhaps all of their nutrition comes from predation. Perhaps it just supplements photosynthesis and allows for a more energetic metabolism. But whatever you do, don't land or crash.

The animals then give live birth or lay eggs in such a way that those can be cradled on their backs. Their young learn to fly quickly, lest a gust of wind make them food for Audrey II (feed me!). The niches not colonized by plants (rims of active volcanos) would become crowded. Some plants would become safe long enough for animals to pollinate, others would fake this condition... ouch for the poor hummingbirds who fall for it.

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enter image description here

Source

Simple, no ground at all! Although this answer does not answer the question specifically about ground, it is one idea with no ground. A typical "no animal zone" might include some dangerous plants as well as large amounts of C02 (already mentioned). Water hemlock is on such plant, which grows in marshy, swampy areas; and along banks of streams and rivers and mainly large bodies of water. The toxin directly attacks the central nervous system, causing nausea, violent convulsions, grand mal seizures and death.

Source

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Beneath the surface there is a volcanic system which either constantly or periodically releases large amounts of carbon dioxide in sufficient quantities to kill all animal life in the vicinity.

In 1986, such an emission occurred at Lake Nyos in northwestern Cameroon in Africa. The result was the death of 1,746 people and 3,500 livestock.

Elsewhere in Africa, between Rwanda and Congo is Lake Kivu, which again, due to volcanic activity associated with the East Africa Rift can violently explode releasing vast quantities of both carbon dioxide and methane. As with Lake Nyos, such eruptions have the potential to kill all animals in the region.

Also, in 1984, Lake Monoun in Cameroon killed 37 people after it erupted carbon dioxide.

You planet doesn't need to have violent eruptions of volcanically sourced carbon dioxide, it can simply be a constant emission of sufficient quantities of gas to cause suffocation.

Additionally, to make it more difficult for animals to move around on land a significant proportion of the vegetation could be similar to horizontal scrub, often described as impenetrable. If traversed, one can find oneself clambering over branches to eventually find oneself several meter above the ground. Occasionally, people have been known to fall through and die.

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  • $\begingroup$ Thanks for your response! Wouldn't this also release enough ash to block sunlight and therefore end up killing plants too? $\endgroup$
    – Juliette
    Commented Jun 9, 2022 at 12:50
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    $\begingroup$ Not necessarily. The volcanic magma chamber can be very deep with vents leading off it & instead of lava erupting carbon dioxide escapes. One of the arguments climate change deniers use against anthropogenic warming due to human caused emissions of carbon dioxide is volcanoes constantly emit carbon dioxide, even when they're quite. See the section under Volcanoes. $\endgroup$
    – user81881
    Commented Jun 9, 2022 at 13:47
  • $\begingroup$ Late memory revival. The Lake Nyos event wasn't associated with a volcanic eruption or explosion. As I remember, everyone went to sleep, but they didn't wake up. The event occurred during the night while they were asleep. There was discussion at the time that an earthquake or a slump of ground under water, in the lake, caused a disturbance of the lake sediments that might have released the carbon dioxide. $\endgroup$
    – user81881
    Commented Jun 9, 2022 at 15:04
  • $\begingroup$ Also more than 400 milion years ago high concentrations of CO2 allowed giant plants to grow and thrive. If the vegatation in this world is able to absorb and transform the CO2 into oxigen fast enough, you could have this 2 atmosphere layers $\endgroup$
    – Sxubach
    Commented Jun 10, 2022 at 10:57
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There Is No Ground

Turns out all the landscape has been changed to something more dangerous, like extending cave systems with no end, which also explains why tall mountains are so common. Plants have adapted with their more durable and less mechanic parts, their roots extends to deepest parts of the caves and all cavities filled with waters that contaminated with toxic CO2 dissolved in it.

Higher parts of the plants such as leafs, can be eaten by animals. And daring animals can go deeper more for the prize, fruit which has more nutrients but far deeper in the caverns.

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There's research showing that plants release chemicals into soil as part of a mutually beneficial interaction with microorganisms (e.g. here).

In this world, perhaps, evolution has favoured those chemicals to become increasingly toxic to complex ground-dwelling animals - and for said animals to evolve towards long term flight.

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  • $\begingroup$ And I might add : Some plants use communication chemicals to trigger stronger defensive measures against predators from leaves to leaves. Defensive measures which can be quite deterrent. And in this case including toxic soils. $\endgroup$ Commented Jun 10, 2022 at 14:36
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The cell types that make up animals are fundamentally different from the types of cells that plants are made of. One major difference is that plants have a compound in their cells that causes them to combine - or become welded - to the cells of other plant cells. Somehow, this doesn't result in one giant plant species on the ground that is fused together, but it is a feature of life in this world that plants fuse to other plants - permanently or semi-permanently - when contact between the two is made.

Animals, a long time ago, by definition, developed the ability to touch a plant cell and not become fused, at least not instantly.

Animals can walk on the planets surface - of which almost every part is covered by plants - but if they remain in contact with a plant for an extended amount of time, they too begin to fuse (although, ultimately, unsuccessfully, because their cell types are so different, it just kills the animal).

The current state of the world is at a time when animals have yet to invent a technology for staying on the ground, but it will happen.

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In Dr. Stone's anime there's an example of a valley full of sulfuric acid (gas state) denser than air, so if anyone goes below a certain height they will die. In this reddit post someone explains that some kinds of algae generate it when decomposing.

If your plants generated sulfuric acid when decomposing or simply the atmosphere had a % of it in the lower layer it would very well explain your phenomena.

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