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An idea for a story I’m working on involves a far away world that was once inhospitable. Scientists - likely alien ones - terraformed it so it could support life (coincidentally, Earth-like life). They then introduced new species to the world, species that had been genetically engineered to have certain minor modifications. In short, they built a habitable world from a desolate planet and populated it with new plants and animals.

The researchers decided to do one thing, though, which was to mess with the single, planet-wide food web. Instead of the web being built up from plants to herbivores to carnivores to more carnivores in the normal ecological pyramid, they made all the animals omnivores, designed to eat both the plants and most other species - disregarding cases where one animal could not physically eat another animal, such as a frog eating a giraffe. In short, the food web is now a food loop. Any animal can eat either plants or animals of another species.

Can this ecosystem sustain itself? I know that in a normal food web, there are fewer predators than prey, but that equilibrium seems to be broken here, because all animals are predators and all animals are prey. I would guess that eventually eating habits would change to reach an equilibrium state where some species are higher up on the chain than others, but I don’t know for sure.

Essentially, can every animal eat most other animals? Or are the scientists in trouble?

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Taking our world's species as an example, a fly would have an awfully hard time eating a whale. At some point along the way, you run into the issue of any one species not being able to eat another specific species simply due to difference in size or habitat. If you are willing to relax the requirement that they all have to be able to eat any other one, though... – Michael Kjörling Mar 28 at 15:56
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RE: The small eating the large (fly <- whale, frog <- giraffe). Not a problem, just wait until one dies and begins to rot a bit, then scavenge it tiny bit by tiny bit. – cobaltduck Mar 28 at 16:56
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@cobaltduck no need to wait for the large one to die first. See army ants. – Dan Neely Mar 28 at 17:15
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"in a normal food web, there are more predators than prey" Isn't it the opposite? When predators start to outnumber prey, some of them starve. – tubes Mar 28 at 17:17
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@tubes: Not if those predators are omnivores, who'll eat meat if they can catch it, but subsist on plants if not. Indeed, it wouldn't surprise me to find that omnivorous mammals (as for instance humans) outnumber pure carnivores or pure herbivores. – jamesqf Mar 28 at 18:09
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From the plant's point of view the herbivore is a predator. The only thing that matters as far as that goes is that more plants grow than the herbivores can eat, otherwise they will run out of food and die.

You have the same problem in your case, with one additional problem: the cycle of energy.

Basically, plants capture and use solar energy to grow. When a herbivore eats that plant it converts that energy (which it is able to process) into tissue. When the predator eats the herbivore it converts the tissue into energy yet again. That energy will eventually be used for other purposes. Also, when the predator dies its body will decay and, to some extent, feed the plants. No energy is created or destroyed, however much of it will be rendered "useless" in the form of heat, etc.

Due to the bleed-off of energy you will require the constant input (and conversion) of solar energy in order to maintain the ecosystem. If your creatures are not sufficiently herbivorous this will not work out!

The problem, as I see it, is that it would be very difficult for a balance to be created. In my opinion some species will prove very well suited to hunting a few other weaker species. Since the caloric intake of eating meat is much higher (and possibly tastier) than eating plants, there should, in my opinion, exist a bias toward behaving in a predatory, carnivorous fashion.

What this would mean is that a few blood thirsty and well adapted species will first hunt some weaker species to extinction before they resort to eating plants (and as a result their own population will die out due to a famine - plants will not be as nutritious as a hunk of meat, and the competition for those plants will be very high, with many individual animals simply withering and dying out).

Eventually the system will probably balance out, but your planet will see many many species become extinct in the first couple of years.

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Related to this answer: What efficiencies make a realistic food chain? – Michael Kjörling Mar 28 at 15:53
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@TheAnathema - my first thought was that most herbivores expend less energy than predators, and as such can afford to survive on plants rather than meat. However, thinking of deer, or large migrating herds I thought again. There are predators that expend a lot of energy, and some that lie in ambush and barely move at all. Each creature is different, yet they all have one thing in common - they are well suited to hunting their prey. Giraffes, for example, can reach the tops of trees - deer cannot. In the end, however, the balance of carnivorous and herbivorous creatures will make the difference – AndreiROM Mar 28 at 15:54
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@MichaelKjörling - my God. I can't possibly hope to match that answer. – AndreiROM Mar 28 at 15:57
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I agree that energy is not created or destroyed, especially from a physics perspective, but it is important to note that while all the materials are preserved within the biosphere, that some energy is "used up" from a more biological perspective. In other words, it is turned into other forms that no biological creature can efficiently harness, like heat. That is the reason a constant solar input is required, to balance out real energy losses in the biosphere. – wedstrom Mar 28 at 16:34
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My only point is the paragraph preceding that quote sounds like a net-energy neutral process, and though you suggest it is not, it is not explicitly stated. You have to infer it from the solar input statement. It's not much of a leap, just a potential misunderstanding for some readers. – wedstrom Mar 28 at 17:22

To a certain extent, this is already the way things work in the real world. The reasons why not all animals eat plants are not the same as the reasons why not all animals eat meat.

Not all animals eat plants because digesting plants is difficult. Plant cells have thick cell walls which call for specialized biological machinery to break them apart into something useful. Additionally, even when that machinery is present, the energy return per unit plant mass is less than the energy return per unit meat.

Not all animals eat meat because catching and killing other animals is difficult. Plants largely just sit there, maybe they'll try to poison you but that's about as bad as it gets. Animals will fight back, potentially injuring you and thereby preventing you from reproducing. If you want to eat meat regularly, you have to be very sure that you will be able to kill animals without there being any significant chance of them killing you back, otherwise you will not survive long enough to mate. But if an animal is already dying or dead, pretty much any digestive system can handle breaking it down into useful parts as long as you're able to break off pieces of meat small enough to fit down your throat.

The changes you suggest should have absolutely no effect on prey species whatsoever, they can already eat meat in principle, they are just bad at doing so in practice. Predators will become somewhat more resistant to famine, since they will be able to subsist on plants even when prey is scarce, but will likely still preferentially target other animals for consumption.

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Short answer: Yes.

More complex answer: The current "Food Web" is already a "Food Loop". Herbivores eat plants, Carnivores eat Herbivores, Scavengers eat anything that dies. Plants absorb nutrients from the decomposing.

That said, an artificial ecology could be set up to have a much tighter loop. You could even make plants take a more active part by making carnivorous plants quite common.

Your loops would likely shift from "all animals can eat all other animals" to "all animals eat other animals and are eaten by yet other animals".

So you could have Animal A that feeds on Animal B that feeds on Animal C that feeds on Animal A. For example, a large whale-type creature (A) may feed on swarms of piranha-type creatures (B) and be generally immune to retaliation because of size. Creature A, however, is hunted by packs of Orca-type creatures (C) which are large enough to pose a threat to A, but are not large enough to be immune to the swarming attacks of B.

Such a cycle is quite flexible, so a small number of B might easily be eaten by C and an A might have survived an attack by C, but has large enough wounds that it is no longer immune to a swarm of B

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In the short run, many of the omnivorous species will tend to drift towards one end of the spectrum or the other due to competitive pressures. If every creature is like a bear and can both hunt and eat berries, then after a while some will find it more advantageous to become specialized in eating nuts and berries, while others focus on hunting.

Omnivores have the evolutionary toolkit to selectively adapt towards a more specialized lifestyle quickly and efficiently (and can recover from poor choices; you may thrive on nuts and berries, but can still scarf down some raw salmon or a slow witted deer if needed). Carnivores like cats can evolve the other way as well (there is some evidence that the Domestic Short Hair can handle a much higher percentage of vegetable matter and proteins than a wild cat, due to generations of them eating "cat chow" rather than hunting mice and birds), but from a biological perspective this is much harder to do. Herbivores will have to do genetic handstands to evolve into carnivores, a far more difficult proposition.

The primary change in this ecosystem of all omnivores is that since all animals have the potential to kill and eat each other, evolutionary pressures will be driven towards adaptations and behaviours which can minimize the possibility of becoming prey while maximizing the ability to gain resources from the local environment. On Earth, this seems to have been done through the development and evolution of social groupings and pack behaviours: Packs of wolves, pods of whales or troops of chimpanzees are good examples of this sort of behaviour (and members of the dog family and chimps are omnivores as well).

Complex social behaviours could lead to the development of communications, intelligence, tool using, language and eventually a fully fledged sentient species like Homo sapiens.

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A big herbivore will spend much of their time eating and digesting. Their stomach(s) and intestines are optimized to break down large amounts of tough plant matter.

A big carnivore will spend much of their time resting between hunts. Their high energy diet is required to power the bursts of energy.

Omnivores like humans, bears, or pigs often specialize on high-energy plants like nuts, berries, or fruits. An ecosystem where all animals go for the nice stuff could have problems.

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Let's start from the most obvious and basic pattern: a single type of animal spread over the planet.

They are cannibalistic, and can photosynthesize, so there is no problem with it being a zero sum game; there're essentially infinite resources, but the only resources are sunlight, and eating each other.

Simplistically, you can pretend that there are just two areas where evolutionary improvements can happen, the two we care about: fighting, and photosynthesis.

If an evolutionary change can be developed which has a significant upside for one of these, but no significant downside for the other, you can assume all members of the population have that advantage (those that don't will basically die out). If a change confers a significant disadvantage and no advantage, assume no members of the population have it (those that develop it will die out).

So the only evolutionary changes we are interested are those which improve one area, while reducing ability in the other.

Over time, cooperative behavior will emerge. Herd members do not eat members of the herd, so only those on the outskirts of the herd are members of the constant battle for meat-resources, so everyone's average risk goes down; cooperation is adaptive, herds thrive.

Herds can also cooperate as packs - a group attacking a singleton will typically win, and can then share the spoils.

It is advantageous to the herd for those on the outside of the herd to be better fighters (defensively at least). It is advantageous for those on the inside to be better at photosynthesis (since they will never get to fight anyway).

If the majority of the population is specialized towards photosynthesis, it is advantageous for some smaller herds to specialize in combat, to pick off and eat those wimpy herbivores with ease.

So specialization cannot help but happen even in a homogeneous population with only two traits.

If that's the case, then a pre-distinguished group with an infinitude of traits will just specialize faster. Even if you make cows and sheep truly omnivorous, they are never going to become hunters, and their efficiency at converting grass to energy will drop, possibly below the level of survivability. Equally, being a cat takes far higher energy resources than can be provided by simple harvesting of nuts and berries, so while an omnivorous cat might eat them if it sees them, it's more likely to try to eat the birds eating the berries instead; and the adaptations to allow it to eat berries might make it less able to catch, eat, and efficiently digest those birds.

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