I have, what I hope to be, a fairly simple question. I am designing a planet where there are no carnivores.


  1. The creatures would all be very complacent and would feed and live in very vulnerable positions, because they have never learned otherwise.
  2. The creatures would mostly be brightly colored, because the pressure to attract a mate would be much stronger than the pressures to stay hidden.
  3. For the same reason as the above point, mating rituals would become very elaborate.
  4. Finally, the point I am most unsure of: The creatures would quickly acclimate to a newcomer to their area.

What I am looking for in an answer

  1. Are my assumptions correct?
  2. What would the impact be on the local plant life (assume earth flora). Could it sustain itself?
  3. Could the population be controlled without predators to keep them in check?
  • 3
    $\begingroup$ Your second and third assumptions occur on Earth. $\endgroup$
    – Frostfyre
    Feb 27, 2016 at 3:13
  • $\begingroup$ I would encourage you to consider what a carnivore is. Strictly speaking, a carnivore is simply a species that derives most of its nutritional needs from consuming specimen of other species of animal. For example, many species of birds -- even "songbirds" -- are carnivorous. Consider the, at the very least, radically different evolutionary pressure that results from this seemingly simple change. Also, what happens to animals after they die? $\endgroup$
    – user
    Feb 28, 2016 at 16:52
  • $\begingroup$ "what happens to animals after they die?" en.wikipedia.org/wiki/Decomposer $\endgroup$
    – RonJohn
    May 16, 2017 at 21:42
  • 1
    $\begingroup$ The Giants series by James P Hogan (specifically The Gentle Giants of Ganymede but it's worth reading the trilogy not just the 2nd book) had just this. It was a planet where early evolution had happened to make flesh toxic to eat - no carnivore was able to adapt and the toxicity was an essential side effect of their systems so even without carnivores to reinforce it the toxicity remained. $\endgroup$
    – Tim B
    Feb 26, 2018 at 15:40
  • $\begingroup$ Just becasue it has to be said, It is completely impossible for a planet like this to occur naturally nor could it persist for very long. Predators would evolve fairly quickly. You may want to put some acknowledgment of the handwave needed to have this planet in your question. $\endgroup$
    – John
    May 13, 2018 at 21:24

8 Answers 8


The assumptions are reasonable, but I might change a few

  1. The creatures would all be very complacent and would feed and live in very vulnerable positions, because they have never learned otherwise.

They would not feed and live in very vulnerable positions. Rather the definition of vulnerable shifts with the environment. Think about how you left your house this morning. Let's say you left out the front door. Did you open it a crack and take a sniff to make sure there were no predators nearby? Did you listen to the birds chirping to make sure it was safe before turning your back on the world to lock the deadbolt? Were you vulnerable? No. You were balanced within your environment. These animals will be balanced within theirs. It may look to us as though they are vulnerable, but in fact they are plenty safe.

However, complacency will never be seen. There is still going to be competition for resources, and it will be aggressive! Nobody's going to be sleeping on the job on this planet!

  1. The creatures would mostly be brightly colored, because the pressure to attract a mate would be much stronger than the pressures to stay hidden.

I think its unclear whether bright colors would be used to attract a mate. Generally speaking, getting a mate to see you is easy. The bright colors are to impress. But why is it impressive? That's harder to say. Its entirely possible the lack of predators may make colors less interesting. With no reason to not be bright and flashy, its not exactly a good tool for judging the fitness of a mate. They may have to find more subtle ways to pick mates.

That's not to say the planet couldn't have bright and flashy animals trying to attract mates, just that it's not a foregone conclusion that they must be that way.

  1. And finally, the point I am most unsure of. The creatures would quickly acclimate to a newcomer to their area.

What does it mean to acclimate? If I had no predators, and the only thing limiting my ability to live and reproduce is finite resources, I'd acclimate to their presence real quickly: I'd be guarding those resources with all my might. Just because we don't have predators doesn't mean the plants aren't going to develop herbicides to kill off competition, and the herbivores wont aggressively defend their turf. Death is part of the circle of life. Getting rid of predators wont change that.

Could the population be controlled without predators to keep them in check?

Predators are just one tool evolution has unearthed that limits a population. Fundamentally, its the quest for finite resources which really generates the limits. The competition for resources will be fierce on this planet, and many will die simply because they cannot find the resources they need to continue living.

Two fun case studies. Yeast is a magical compound. Not only does it make beer work, but it has a terribly interesting lifecycle. When yeast enters a fertile area, it multiples like mad in a haploid state which reproduces asexually. This is an extremely fast way to consume the newly found resources. Eventually, the haploids start to starve themselves for resources. With a quorum sensing like behavior, they undergo a transition, combining two haploids into one diploid cell. Once in this form, they undergo sexual reproduction. If the resources continue to dwindle, these diploid cells can form spores, which are hardened against the environment, ready to be transplanted into a more suitable location.

The other case study, Desulforudis audaxviator, is a fascinating bacteria. It is found deep in cracks in goldmines in South Africa. It is known as a "single species ecosystem." Not only does it not have any predators, it indeed is the ecosystem. There is no DNA present in the mines besides that of D. audaxviator. The fact that it can exist in balance tells me that you can find a balance without predators.

Random factoid: D. audaxviator is the only known nuclear powered organism on the planet. One of its several metabolic channels is specialized at using hydrogen peroxide generated by the decay of Uranium atoms nearby.

I think the biggest challenge your ecosystem faces is why there are no predators. With all of the competition for resources, it would be very reasonable for animals to try to take each other's resources. It doesn't take long for that to turn into predatory behavior. You're going to need to come up with an interesting mechanic to ensure predators don't evolve on your planet.

  • $\begingroup$ Bright colors demonstrate fitness in more ways than making it hard to hide. They can convey a resistance to parasites and an ability to gather the resources necessary to produce large, pigmented plumage. Also, intraspecific competition for food and mates could also make stealth an advantage even when predation is not a concern, so demonstrating that one does not need to rely on stealth is a good display of mating fitness. $\endgroup$ Feb 26, 2018 at 10:48

An ecosystem containing herbivores only, appears stable but it is not. The evolutionary history of Earth proves that in the absence of carnivores/omnivores, some of the herbivores would evolve to utilize this free source of food. Examples include:

1- Dinosaurs. While the pioneering genera like Nyasasaurus were omnivores, they gradually split up into pure herbivores and pure carnivores.

2- In mammals, whales are closely related to hippopotamuses, which means they were herbivores. They later settles on shorelines and gradually shifted from herbivory to carnivory.

So no. In the long term (a few tens of millions of years), herbivores will shift on to carnivorous lifestyle if such creatures are not already present in the ecosystem.

  • $\begingroup$ Are my assumptions correct? (Quoted from op's post). That is what he/she wanted to know and my answer addresses that direct inquiry directly. $\endgroup$ Feb 28, 2016 at 19:58
  • $\begingroup$ Let OP decide that? $\endgroup$ Feb 28, 2016 at 20:09
  • $\begingroup$ Thanks for the upvote, but you are stating that my answer is either a- not directly to-the-point of the question or b- invalid in assuming axioms about the universe. OP posted an ecosystem and asked if such ecosystem is realistically possible. I have explained in my answer that it is not possible (according to earthly history of evolution). I consider it directly answering the most crucial part of the question presented by OP. Hence my suggestion that it is OP's authority to decide whether my answer directly answers his/her question or not. $\endgroup$ Feb 28, 2016 at 20:23
  • $\begingroup$ 2) Hippos are not a carnivores. What do you mean? $\endgroup$ Feb 26, 2018 at 11:25
  • $\begingroup$ @NuloenTheSeeker that hippos are not carnivores. $\endgroup$ Feb 26, 2018 at 17:34

The fauna in a world with no carnivores will not look like that of earth with the carnivores removed. Disease and both intra- and inter- specific aggression will likely be the major causes of mortality (popular science link re. disease: http://www.bbc.co.uk/earth/story/20150327-ten-scary-diseases-of-animals).

Predation pressure is a major factor driving the grouping behaviour of animals, so without predators, many species may never evolve group living (whether that be organised sociality such as seen in many primates, for example, or the passive aggregations of savannah-dwelling herbivores.). Add to this the issues of disease - social transmission, decaying corpses of dead members of multiple species - and the likely outcome will be small groups or solitary individuals, possibly highly mobile (to avoid the rotting bodies).

If the rotting corpse issue is explained away (perhaps the abundance of this resource has selected for bacteria that decompose corpses on an accelerated time scale), then the biology and behaviour of the herbivores will be shaped largely by whether their resources are defensible. Population sizes are likely to be large - given no predation - so territoriality will not be feasible as time/energy costs will be too high. Instead, individual resource patches would be defended, and this will favour large groups, which in turn require access to multiple resource patches to meet their collective nutritional needs.

So intense inter-group competition, selecting for large-bodied, aggressive, 'weaponed' animals.

With that in mind, and addressing the question:

  1. Are my assumptions correct?

The creatures would all be very complacent and would feed and live in very vulnerable positions, because they have never learned otherwise.

Clearly not. In some if not many cases, the herbivores may look/behave much more like 'carnivores' (of Earth fauna). The nature of vulnerability will change (vulnerable to inter-group violence, rather than predation), but it will still impact behaviour.

The creatures would mostly be brightly colored, because the pressure to attract a mate would be much stronger than the pressures to stay hidden. For the same reason as the above point, mating rituals would become very elaborate.

It would be easier for such mating rituals and colouration to evolve, given the absence of predators, but whether or not it does depends on intensity of mate competition, and the balance between inter- & intra- sexual selection.

Finally, the point I am most unsure of: The creatures would quickly acclimate to a newcomer to their area.

If the logic above is correct, then the opposite.

  1. What would the impact be on the local plant life (assume earth flora). Could it sustain itself?
  2. Could the population be controlled without predators to keep them in check?

Simple answer, yes. Population numbers will adjust to the available resources through the mechanism of competition. Intense foraging pressure might lead to localised ecological changes - grass, for example, is much more resistant to consumption than arboreal leaves - and there may also be selective changes on the plants to resist high levels of consumption by high numbers of herbivores. So thornier, tougher, higher levels of tannins and other 'anti-feedants', which in turn will decrease available resources, and increase the intensity of the competition between the herbivores.

  1. I agree with your first three assumptions, but I'm not so sure about your last one. It's really hard to tell unless we know whether this species is naturally aggressive or passive.

  2. Three factors can limit the speed of photosynthesis: light intensity, carbon dioxide concentration and temperature. Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.

Basically, this quote is from here and it tells us the easiest way to speed up photosynthesis is to increase light intensity. So I suppose if the light intensity was high enough, the flora could photosynthesize faster. Also, if the plants had photosynthesis-speeding enzymes in their body, they could photosynthesize faster. Or perhaps the plants chemosynthesize, in which case none of the above is relevant. Either way, I think if you added one of the modifications I suggested, you should be fine.

  1. Sure! You simply just have to create a natural system to keep the population in check. Perhaps, every four months or so, some sort of natural acid rain occurs. This could kill any organisms feeding off of the plants. If these organisms would've adapted to the acid rain, then simple overpopulation will kill them off. Without room to live and enough food to eat, some will starve to death, so in a way, nature controls the number of these organisms on the planet at one time.

That's exactly the situation in James P Hogan's Giants series. Initial observations of drawings and other representation are taken (by us) to be cartoon characters and are presumed to be unrealistic.

As I recall, complex life (equivalent to our vertebrates) developed a secondary circulation system for carrying waste, and this developed into a toxic defense rendering all such life inedible.

(Yet there were canned fish noted in the original novel, so there's another story there.)

Now in this story, animals were also very vulnerable to their own toxicity, and even a minor cut would be fatal.

I note that just because animals can't eat each other, they can still kill each other in competition for resources or reproductive success! Look at lethal conflict within our own species: it's not due to predation. But people still must protect themselves.

You ask about plant life being able to sustain itself. Autotrophs are the base of the food chain now. Each generation of predation yields only 10% of the energy, so the amount of biomass enabled by predation is 10% of the 10% that are herbivores. IOW, not much of the total biomass.

This doesn't consider the decay processes, which might exist if predation does not. Likewise for disease and parasites. These might all have different considerations and ways around the blocks.

Look at the effects of the carboniferous period, where for t0 million years after the appearance of "wood" nothing could eat it.

What if something similar happened to a major branch of animal life. Something essential that's part of every cell is indigestible and even toxic to all other life. That would mean that not only would animal cells not be etable but it could not decompose either. Actually wood decayed except for the particular lignin and suberin proteins which piled up.

If animals had something in their metabolism that could not be broken down, digestion would just work around it. If it were actually toxic, how could normal metabolism work? So maybe it's just toxic if existing enzymes try to break it down, so early filter feeders simply rejected the cells that contained it and never learned to digest "meat" so that never emerged. Decay processes manage to separate it out so leave that protein behind.

A complex organism could exist like the way a cow eats grass. But it's contrary to such a design to have it run down prey, and it doesn't get the benefit of fast energy anyway: it might be less efficient than eating plants.


To answer question #3:

And also answering, “How could this scenario exist?” could be: an intelligent species on the planet kills any being that eats other than light or plant life, in order to feed, or anything like a cookoo bee. So the evolutionary tree is tended like a shrub or bonsai.


What would the impact be on the local plant life (assume earth flora). Could it sustain itself?

If animal food is not plant also. The animal feed itself by Photo-synthesis (like plant). Therefore, not much impact on local plant live and animal can sustain itself. ( [Starcraft Protoss][1] can photo-synthesis so they don't need food)

Could the population be controlled without predators to keep them in check?

WAR. They killed each other so we don't need predator. You can create the scenarios where WAR is a MUCH.

For example: Honor: war is good for both side because who die in battle will live in honor with their god (?!) Mating: their is unbalance sex (male or female) so the others have to fight for it.

(read more about the Pig in Speaker of the Dead for some idea)

Spoiler: After die (life 2), pig become tree (life 3). Who die honor (being plant) will become Father Tree, which can mate.

Question about the pig:

Does “planting” have any biological impact on the Pequeninos (piggies) lifecycle?


Controlling population of animal life on a plant with no predators could be achieved if all the females had long gestation periods and/or only 1 offspring per litter, or if females only produced a few eggs per lifetime, or if females where only receptive 1 day per year - many variations are possible to create a viable world without predators.

Dead animals add nutrients back to the soil so more plants can grow and produce fruits, beans, nuts, vegetables etc... result.

  • $\begingroup$ The problem with this answer is that it's short-term and unstable. There's a huge adaptive advantage for having extra offspring, so a tendency to do that would very quickly evolve, and you're back to an uncontrolled population. $\endgroup$
    – Mark Olson
    May 13, 2018 at 21:25
  • $\begingroup$ With respect, and thanks for your answer to my thought - I suggest that it would take much longer than you think for such adaptations to occur, especially if there are no pressing environment reasons for the adaptations. Extra offspring is a response to the environmental stress of carnivores. Without predators there is much less need and pressure for extra offspring. Actually - the opposite might be said - that there is more pressure for less offspring. You might find populations stabilizing. $\endgroup$ May 13, 2018 at 22:15
  • $\begingroup$ Evolution happens all the time, and it is driven solely by comparative advantage in producing descendents. If a mutation increases the number of children even slightly, it will spread through the population by the simple fact that in each generation, creatures with that gene will have slightly more children that creatures without it. As far as evolution is concerned, being better adapted (being "fitter")isn't about being a better fit for the climate or the food supply, but in having more descendants. Evolution is a blind and ruthless master and cares about nothing else. $\endgroup$
    – Mark Olson
    May 13, 2018 at 22:20
  • $\begingroup$ Maybe, but it seems if that were true, as generations passed each individual animal would produce more and more offspring. And that is not what is happening in our world - The gestation periods of animals are the same now as they were since the start - animals like elephants still have only 1 baby at a time etc... The reproductive systems of most animals on earth have not evolved to the point which you describe. $\endgroup$ May 14, 2018 at 11:40

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