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The animal species on my planet has a highly sophisticated communication method and part of the reason for this is that they had no enemies on the planet except themselves.

So far, the reason for this might be:

  • The planet is really very rough and doesn't allow for much to live, that might be because the planet is a rogue planet.

Now there should be one type of plant they eat and one type of bug that eats dead plants or the dead bodies of the animal species. The animal species, on the other hand, cares for the plants and some of their degradation products are essential for the plant to survive.

So animal and plant need each other and animal as also plant knows they need each other to survive.

Is such a scenario possible and what could make this happen?

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    $\begingroup$ Of course the scenario is possible. St. Exupéry's Little Prince is from planet B 612, which has only an animal (the Little Prince himself), a silly rose and fast-growing baobabs. The Little Price had come to Earth in search of a sheep to eat the baobabs before they take root and destroy the little planet. It is a very successful little book; it has been translated in hundreds of languages, has spawned countless adaptations and interpretations, and it is considered a masterpiece of world literature. $\endgroup$
    – AlexP
    Aug 12 at 10:39
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    $\begingroup$ Are their bacteria and microorganisms? $\endgroup$
    – John
    Aug 12 at 13:44
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    $\begingroup$ @AlexP It should be noted that in works of fiction, popularity does not imply realism. $\endgroup$ Aug 12 at 18:15
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    $\begingroup$ @EthanManess: The character sequence r e a l does not appear in the question. On the contrary, the question explicitly asks if such a scenario is possible. I have the example of a famous and widely acclaimed book which uses such a scenario, and for very good reasons. (It makes full sense in the context of the book, which, in a way, is all about the virtues of simplicity and the sins of complexity.) $\endgroup$
    – AlexP
    Aug 12 at 19:13
  • $\begingroup$ Frame Challenge: You've just recreated the banana problem. All the bananas we eat are grown on trees that are clones of each other. The wrong virus, and no more eating bananas. Of that variety. $\endgroup$
    – NomadMaker
    Aug 12 at 21:35

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Stopping evolution, or there will be more than three!

There are several issues with such an ecology. So for a moment we'll assume this is engineered somehow for that specific effect. This eliminates the need to explain how the ecology evolved to that particular state.

Given that we assume life to rely on some information-carrier substrate to direct the development of new organisms (DNA), and given how in all foreseeable circumstances mechanisms like that must (on a geological timescale) occasionally fail, it's difficult to imagine how evolution has been arrested. Any malfunction that causes a variant of an organism to appear (a mutant) carries with it the chance that this mutation will be advantageous. More such mutants should show up in subsequent generations.

Now, if any of these organisms are distributed worldwide (or even continent-wide), then there is enough range that the organisms are isolated from each other. They should (over a geological timescale) start to diverge into separate species.

Thus, if we're picking some arbitrary time period in the existence of this world, there's no reason to expect it to have only 3 extant species. You end up needing to visit it quite soon after whatever it was that caused there to be only 3 in the first place (or there won't be just the 3 anymore).

If you want this to not be true, you have to come up with a mechanism that arrests evolution, and I struggle to imagine anything that could do that.

Viable ecology?

Next, let's talk about how 3 such organisms could even comprise a viable ecology. It's true that you can find experiments (and I suppose, works of art) where someone has sealed up a big bottle or perhaps a sphere of glass with some plant in it, and some other animal (snails, perhaps, or shrimp, etc) and that these little ecosystems can chug along for decades (or longer, no one knows if there are limits with any certainty).

Your world has several advantages over these. In the "bottle garden" (that's what Wikipedia is calling them it seems), if the plant dies for whatever reason, it is either the only plant, or one of a very small number of them. This is disastrous. The dead plant won't magically resurrect, and even if it isn't the only one it can wreck the homeostasis of the bottle, causing the other plants to die. Generally, the animal life is a bit better off, you'll have more individuals and they are fast-breeding and perhaps even hermaphroditic/parthenogenic... a single individual dying is much less likely to cause a collapse. But with a small number of each, a death of any plant or animal is just a risk that doomsday is in the cards for this tiny little microcosm.

Your world has no such problem. Even if something sparked a local die-off, the planet is large enough that there is a reservoir of organisms to re-colonize that desert and restore some balance.

But we can't discount how precarious that balance is. The bottle garden doesn't suffer just from a shallow bench, so to speak... there's only three positions sitting on that bench. Each extra species (that fills a new niche) extant on this world brings a little more stability to it. Each extra species adds a slightly different nutrient cycle to the world, each extra species can be provide a backup nutrient cycle if some other is interrupted.

These overlapping cycles don't only substitute for others when disaster strikes, they more finely tune homeostasis. With many animal species, you don't have to worry so much about plants going nuts and over-oxygenating the atmosphere (they tend to look at the plants as food and some minor boom-bust cycle is about to occur). With many plant species, you don't have to worry about some overly-gluttonous animal deforesting the entire world. In the bottle garden, the physical size of the environment itself limits those behaviors... but you've given them an entire planet.

Finding the right balance between two species in a bottle garden is difficult and requires experimentation, skill, and more than a little intuition. Finding the balance on your planet-sized bottle garden will be even more challenging, though there's nothing to suggest that it is outright impossible.

Can any of this even work without decomposition?

The bottle gardens might only include a single plant species, and a single animal species... but that's far from the same thing as including only two species. Whether deliberate or unintentional, whether they were aware of it or not, those who have made these little tiny closed ecosystems were also putting bacteria (and possibly other microbes) into the bottle.

Thus, when one of the little fairy shrimp die, or when a leaf falls to the bottom, those get to work and turn the detritus back into something that the remaining plants can absorb as fertilizer.

Your world needs something like that too, and it's non-negotiable. Now, maybe they weren't included in your list because they're boring or it was an oversight, but you'll need to keep this in mind as you explore the idea.

What could even lead to just three species?

From what little we can speculate on the subject, it seems that at one time in our own planet's history there was probably only a single species. It was unicellular and microscopic. And unlike modern microbes, it may not have reproduced quickly... it might not have been "very competent" in that job (after all, it would only have to succeed once).

But after that, evolution kicked in, and it almost certainly diverged into multiple subspecies quite quickly. Whether this was hundreds, thousands, or tens of thousands of generations, this is still only a difference of a couple magnitudes of order, and whether or not a generation was measured in 20 minutes or 20 days... well, it just didn't take very long for there to be more than one type of little microscopic squiggly thing.

And then for those to proceed up through to the sorts of multicellular life we know today. To have the divergence of some photosynthesizing plants and scuttling arthropods, a skulking vertebrate... well, for that to have happened, evolution had to produce more than just three models.

Even if you could imagine a disaster that wiped out all but 3 of those, how likely is it that the 3 remaining would constitute a viable ecology by themselves? Why just 1 plant instead of 3 plants and no animals? Why only 2 animals and 1 plant instead of 3 animals and no plants? Why 1 herbivore and 1 insectivore instead of 2 carnivores?

The only way to reconcile this requirement is to suppose that the arrangement is artificial. Someone engineered this planet to be this way. Maybe something that looked like a terraforming effort, maybe K3 quasi-gods just wishing it into existence in the same way we'd run a simulation. But no natural set of circumstances could produce the result you're wanting.

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Very unlikely. Even monoculture like coffee or bananas rely on multiple bacteria and other organisms. Not counting that by reproducing they will end up diverging anyway.

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If the life is bioengineered, sure.

As others have noted, this wouldn't happen via evolution. The animals would branch out and fill slightly different harsh niches, and numerous organisms would have similar successful routes to life.

But, suppose this was a terraforming project by an alien race, later abandoned. They designed particular organisms to improve the planet, went off for some reason. A million years later, one plant one animal and one bug is what you have.

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    $\begingroup$ A millions years is enough time to get new species, they need to have abandoned it within thousands of years.. $\endgroup$
    – John
    Aug 12 at 13:42
  • $\begingroup$ @john thousands of years is enough time to get new species, they need to have abandoned it within decades. $\endgroup$
    – fectin
    Aug 12 at 18:16
  • $\begingroup$ Bioengineered organisms can be designed to be extremely resistant to mutation. $\endgroup$
    – Nepene Nep
    Aug 13 at 21:21
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If we're talking hard sci-fi, and basing it off our current knowledge of life, I would say no - at least not by natural processes.

Life as we know it evolved from single-celled organisms, but evolution isn't like leveling up in a game. Some of the first single-celled organisms evolved over time to multicellular life, while others kept evolving as single-celled organisms. Evolution means branching off, and in building stable off-shoots life creates ecosystems. We, as humans, are very dependent on bacteria still today, many millions of years of evolution later. As far as we know, humans cannot survive without our multitude of gut bacteria, and while there perhaps may be some odd animal species that don't have gut bacteria, no species survives long-term without a stable ecosystem.

Healthy ecosystems tend to have variation and buffers. It also provides challenges which create evolutional pressure, which in turn lead to diversity. Over time, most species branch out or die off.

TL;DR: No, because life, uh, finds a way.

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    $\begingroup$ Descolada.......... $\endgroup$
    – DKNguyen
    Aug 12 at 15:06
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    $\begingroup$ Actually monocellular life on Earth still dwarves multicellular animal life, at a 80:2 Gt C scale. Humans are at 0.06 Gt C, plants at 450 Gt. (Source: christiankull.net/2019/11/01/biomass) Conclusion: Higher animal life is a byproduct of bacteria evolution, with the "goal" being plants... $\endgroup$
    – toolforger
    Aug 12 at 17:24
  • $\begingroup$ @toolforger - I knew that humans account for a very low percent of the total biomass on Earth, but I've never seen these specifics. It's an interesting way to look at success which brings my mind to Yuval Noah Hararis' Homo Sapien, and how he posed the question if we 'tamed' wheat, or wheat tamed us. $\endgroup$ Aug 12 at 22:34
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Resisting the urge to make a really silly joke about a really small planet :p

Having a single animal or bug species on a planet isn't really possible due to divergent evolution. Having a single invasive plant could happen though. At least two of earth's epochs were ended by incredible algae blooms turning the oceans green, suffocating near all non-plant life with oxygen, then dying off and suffocating near all remaining life with carbon dioxide.

Plants have several methods of reproduction: seeds, spores and duplication. As long as you use a plant that uses one of the duplication methods of reproduction which doesn't allow for genetic variance, you could land up with your one plant world - so long as its inhospitable enough to kill off all other plant life.

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    $\begingroup$ Even duplication give You smal genetical drift. After 10-20 generations You can have more plant species from base one. Even faster when in interstellar trip flooded in radiation. $\endgroup$
    – Kamitergh
    Aug 12 at 13:26
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Terraforming, ad hoc.

The residents of your planet had to leave their original planet in a hurry. A big hurry. They did not have time to thoughtfully pack. There was a student of this particular bug who brought some with. That was fortunate for all involved even though these aliens now more or less hate the bug. They hate it more for what it represents than what it is.

On their new miserable rogue planet home, they had this bug and their food plants and each other. They made do. It is less than ideal in many ways but somehow it has been working. It is not at all certain it will work for the long term. These folks are concerned about the short term for now.

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As others have mentioned, evolutionarily it's pretty unlikely. I won't reinvent the wheel explaining why, so I'll just give an alternate explanation on how this could come about without evolving on planet.

Panspermia: It could be that long ago some intelligent life was shuttling by and crash landed on the planet. These intelligent creatures were killed on impact, but for food and sustenance they had created a self sustaining ecosystem out of 1 animal, 1 bug and 1 plant.

It could even be that there were many plants, animals and bugs, but most could not adapt to the new environment and died out leaving only one of each.

Since this ecosystem was very fragile with low diversity, over time evolution would select for the animals who had behaviors that aided these plants and bugs in adapting.

Obviously this would be easier with communication, and so intelligence was selected for as well.

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