So Blake, what are your thoughts on changes we might see in herbivores and plant life if animals stopped eating other animals (including their eggs and any other qualifier to be able to offer up opinions on which direction that might push things).
I agree entirely with John Mace: The line between plant and animals isn’t rigid, and if there was no predation then there could never have *been *any animals.
Animals are *defined *as multicellular organisms that engulf their prey. Yet according to the scenario dictated by the OP, as soon as the first multicellular organism attempted to engulf another organism, it was rendered sterile. As such animals could never have evolved.
What we are left with is a world occupied exclusively by plants and fungi. There is simply no starting point from which animals can be produced and no advantage to *being *an animal.
Thank you for correcting my misinformation. Wonder where I picked that up from, then.
Okay, touche. I was envisioning something more like this, though:
(1) Godlike alien being arrives well after animals are established (let’s say it’s five million years ago, so we can leave hominids out of it).
(2) Any creature that comes as close or closer to sentience as a mayfly does, that intentionally kills any other creature that also comes that close or closer, is sterilised.
(3) Sit back and watch what happens for a few tens of millions of years.
(And yes, this qualifies as selective breeding, fine. :P)
Let’s just play by the the spirit of the OP and leave the nitpicking aside. Without predation that leaves two major macro level pressures: Food sources and sexual selection. Since now all animals are feeding exclusively (more or less) on plant material, it is likely that we will see a more advanced evolution of their food sources as well. I suspect you might see a world of specialists that each consume only one, or a few plant types. A whole world made up essentially of pandas.
Not necessarily. For example, a monkey might say F off, these fruit trees are mine. His larger size would be an advantage in fighting off the other monkeys.
Predating? Is that what pre-teens do? The word you are looking for is preying. 
But I’m sure you know that “fitness”, in evolutionary terms, mean reproductive fitness. The ones who pass on their genes most effectively. Using food resources more efficiently may or may not be correlated with that.
Ha. I thought the same thing. A poison in which evolution was unable to counter.
I liked the scene where a gentle giant had a cat hop on his lap, lay down and purr. It made him uncomfortable 
I consider that as encompassed in efficienct use food resources.
Efficiency is the problem word here. I’d be better off making the point that animals still have to compete for available food. And I find predating to be a perfectly cromulent word.
I like predatorizinating better.
OK, given the new stipulations to the increasingly convoluted hypothetical.
It is impossible to stop predation over geological timescales without also stopping evolution. That is an absolute statement that I can say incontrovertibly is correct, which very few absolute statements are. Life cheats, so unless your aliens keep coming back and adding new clauses to the rules, something is going to become a de facto predator.
See, other animal are are amazingly nutrient dense. They are absolutely jam-packed with all the nutrients that an organisms needs to live, all in one easy-to-use package. It doesn’t matter how you try to twist the rules, any organism that can kill another organism is going to have a huge advantage. And I mean HUUUUGE. The advantage to a predatory lifestyle is simply to huge to possibly get around by imposing arbitrary rules.
According to the current rule, we can’t have any creature that “intentionally kills any other creature”. That seems fine, except that it doesn’t stop predation in any way at all. Animals don’t *intend *to kill anything. Intent to kill is really only a human trait. Maybe some other primates have an intent to kill, but I can see no evidence at all that other animals have any intent to kill. Their sole intent is to obtain a meal. If the other animal dies as a result, that’s not something that they care about. It’s not their intent.
Just look at some of the examples: dogfish take bites out of their prey, and the prey usually goes on living. Crows will peck the eyes out of sick animals and the animal will live for weeks. Foxes will eat the vulva out out sheep and cattle that are having difficult births and the animal will live for days. None of these animals is intentionally killing another creature, so they will go on to produce just fine. They are going to get bigger and bigger and do more an more damage, but since the selection is solely for intent, all you are going to do is breed predators with a mental blind spot, nothing more. Your aliens might arguably exterminate all the lions and hawks, but within a few tens of millions of years their niches will be filled with the descendants of foxes and crows, and they will be killing just as freely, but they won’t be intending to kill. Rather than selecting against an ecological niche, all you’ve done is select for a mentality.
So, your aliens come back and they impose rule 1.1. Now sterility is the fate of any animal that actually eats any part of any live other creature. That means no parasitism, no taking bites out of living prey. Nothing.
Great, but once again, life cheats. Coyotes have been known to herd large prey towards busy roads so they get killed by traffic. Dolphins herd fish onto beaches and mudflats, where they are very likely to die. So instead of having predators that deliberately and intentionally kill their own prey, you have produced a system where not-at-all-predators-really construct traps that cause other animals to die. And they will then stand guard over the dying not-at-all-prey-really, and as soon as it expires they will eat it. Your new system will have just as many predators as we have today, they just won’t kill their own prey, they will construct an environment in which death is inevitable, but they won; do any killing. Spiders will be one of the obvious short-term winners in this new system, since they just need to sit in their perfectly functional webs and wait for the prey to die of dehydration.
So, your aliens come back and they impose rule 1.2. Now sterility is the fate of any animal that actually eats any part of any other creature with which it interacted, directly or indirectly while the creature was alive. That means no parasitism, no taking bites out of living prey, no trapping other creatures so they die, no following around creatures waiting for them to die. Animals can not eat any other creature if they were aware of its existence prior to death or if the creature interacted with any trap, lay, snare of obstruction that the predator implemented.
Getting complicated isn’t it? 
But all that predators have to do to negate that ruling is engage a middle-man. A great many animals, ranging from spiders to hawks to wolves, will offer gifts of dead prey to prospective mates. Even more animals will bring prey back to their mates and offspring without eating it themselves. So very soon the Earth’s ecosystems will be dominated by this sort of Strangers-On-A-Train predation. A wolf will kill a deer, but instead of eating it, it will swap it with another wolf. That neatly sidesteps the ruling.
So, your aliens come back and they impose rule 1.3. Now sterility is the fate of any animal that actually eats any part of any other creature with which it interacted, directly or indirectly while the creature was alive or if it make use of the corpse of such creature as payment or reward.
I hope the animals have a good lawyer who can explain this convoluted ruling to them.
But once again, nature has already got around that through kin selection. Animals live adjacent to their own relatives, and any event that benefits other members of their own family benefits their own genes. A Komodo dragon bites a deer, the deer dies of septicaemia a week later and is eaten by a related dragon who never had any knowledge of the original attack. The genes for biting the deer are selected for. A wolf kills every animal that it possibly can, it then sent-marks the corpse to ensure that it doesn’t accidentally eat a corpse that it killed itself and walks away. Since all the wolves in its territory are either its parents or its siblings, the corpse will provide food for its genes, if not for itself. Just as importantly, its relatives will have left a supply of carrion around that it can eat. There i never any intent to consume involved here. The wolves don’t even need to be aware why they are killing these animals. They just have to be programmed to do so. Wolves will remain just as effective as predators as they ever were, they will just never eat what they kill themselves.
So, your aliens come back and they impose rule 1.4. Now sterility is the fate of any animal that actually eats any part of any other creature with which it interacted, directly or indirectly while the creature was alive or if it make use of the corpse of such creature as payment or reward or if the corpse is consumed by any related organism.
So now, wolves will not eat any prey animal killed by other wolves. Wolves still kill every animal they come across, they just won’t eat them any corpse killed by another wolf. And wolves will not tolerate any other predators in their territory, except for bears. And bears will not eat any prey animal killed by other bears. Bears still kill every animal they come across, they just won’t eat them any corpse killed by another bear. And bears will not tolerate any other predators in their territory, except for wolves. So the bears leave lots of corpses lying around, and they get eaten by the only other predators in the area: wolves. And wolves leave lots of corpses lying around, and they get eaten by the only other predators in the area: bears. The most successful bears are obviously those that live in territories with the healthiest and largest wolf packs. And the most successful wolf packs are likewise those that live in territories with the largest and healthiest bears. Neither animal preys on other animals, or uses the animals it kills as a bribe, or allows the animals it kills to benefit its own genes. But bears and wolves as just as effective predators as they ever were.
So, your aliens come back and they impose rule 1.5. Now sterility is the fate of any animal that causes the death of any any other creature, directly or indirectly.
At this stage, all animal life ceases to exist.
All animals kill other creatures. A mother goat that drives its offspring away at the beginning of the next breeding season is causing the death of its own offspring. Not 100% of the time, but wolves don’t kill every dear they hunt either. And of course if the offspring were never weaned, then the offspring would kill the mother. At some stage all animals, by virtue of competition, must kill other animals, at least indirectly. It’s a zero sum game. The planet has finite resources, animals compete to maximise their share of those resources, and as a result some other animals must fail to obtain sufficient resources to survive.
Your aliens simply can not stop predation from occurring.
You can’t stop animals from killing other animals. You can stop them killing them directly, but they will simply kill them indirectly or vicariously through various co-operative or incidental mechanisms that can never be controlled.
You can’t stop animals from benefiting from the death of organisms that they kill. You can stop them eating other organisms, but they will then derive benefits vicariously through trade, kin selection, environmental selection or, at the last extreme, their use as fertiliser. Animals are such a dense nutrient source that there will always be a way to benefit from their deaths.
And as long as you are unable to stop organisms from killing other organisms, and unable to stop them benefiting from their deaths, there will be de factor predators that look and act identically to the predators we have right now. A man that eats a salmon won;t look very different to a man that catches a salmon and uses it to fertilise a maize plant. Technically the maize grower may not be a predator of salmon, but the pratical distinction for the biology of both the man and the salmon is undetectably small.
Sort of a meaningless question IMO.
I think you’re over-thinking this. What about the scenerio I mentioned from The Gentle Giants of Ganymede? Predation/scavenging simply doesn’t work, because all animals are toxic to everything but themselves.
How does that work biochemically?
That’s a work of fiction. IRL, some organism would become immune to the toxicity.
It takes two stages of digestion to make the toxin safe. The first stage is by a fungus. The second stage is by bacteria that live in nodules in the roots of all plants.
Why can’t a would-be predator develop a symbiotic relationship with those two organisms?
Like eating grass, yeah that might work
It is impossible in any system that even vaguely resembles the Earth. Consider the following:
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How can such a situation come to be in the first place? Toxins are the result of an arms race. They don’t spring out of the Earth fully formed. They have to develop from precursors, and the precursors can’t be toxic to “everything but themselves”. Simply ask yourself, if it were possible for any organism to be “toxic to everything but themselves”, why don’t we have even a single organism on Erath that comes even remotely close to that? It’s simply because it’s evolutionarily impossible. Evolution doesn’t produce deus ex machina toxins. It produces toxins that are marginal improvements on existing chemicals, chemicals that other organisms already have a degree of ability to digest.
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How does the decomposition process even work physically? Someone proposes that “It takes two stages of digestion to make the toxin safe. The first stage is by a fungus. The second stage is by bacteria that live in nodules in the roots of all plants.” So how does the toxin itself get broken down? At the grossest scale, it is bound up inside the skin of a dead deer. And that skin can not be eaten by scavengers and it can’t be digested by microbes. So it sits there as a watertight bag literally forever. The toxin can’t be removed until the cell contents of the skin are exposed to the roots nodules of a plant, and the cell contents can’t be exposed to a root nodule of a plant until the toxin is removed. It’s a catch-22. If the skin is literally indigestible to anything aside from a bacterium in the root system of a plant, then the skin can never be digested. The alternative is that the fungus somehow manages to digest the proteins in the skin and extract nutrients and energy from that, allowing the toxin to leach into the soil. But that means that the fungus is immune to the toxin. That means that the fungus *itself *will very rapidly evolve into a predator. It doesn’t need to be able to digest the toxin to make a good living chasing after and catching animals. It just needs to be immune to it. Most predators can’t digest all of the prey they chase. They just spit out the indigestible bits?
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Why no symbiosis or horizontal gene transfer? We already have chemicals on Earth that animals can’t digest. Cellulose is the most common one. You know what animals do? They kill the organism that produce cellulose, then they eat the organisms that can digest cellulose and steal the digestive enzyme sin that manner. In some more advanced cases they actually permit the organism that can digest cellulose to grow in their own guts. Animals steal toxins and enzymes from their prey constantly. Inevitably some herbivore is going to “realise” that it can eat just a little meat if it eats fungus and root nodules first. And that is all that it takes for your system to collapse utterly.
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How is the decomposition process is so complex? Why has no free-living bacterium evolved to digest the toxin. If the decomposition process actually requires fungus and root nodules, then in arid environments there will be vast amounts of dessicated carcasses lying on the ground. Once again, we can compare this to indigestible cellulose on earth. Since it’s unable to be directly digested by animals, in arid regions you get thousands of tonnes of cellulose accumulating during the dry seasons when microbes can’t effectively operate, and that is in the presence of animals such as termites and rabbits that have hijacked the microbial chain. Under the system proposed, once an elephant carcasse dessicates, since it can not be broken down by scavengers at all, it will just lie there for years. Rain won;t be able to penetrate the hide to provide for fungal growth so the carcasse will only decay at a few hundred grams a year. The Serengeti would be literally covered by animal carcasses. Yet we are expected to believe that in 3 billion years not a single microbe has managed to evolve a way to digest these toxins? No horizontal gen transfer, no enzyme evolution. Nothing. This is an utterly preposterous idea that simply is not possible in an Earth type system. We know the toxin is biodegradable. We know the genes for the biodegradation exist in that environment. There is no way to prevent a microbe from developing the ability to digest it.
I could keep going all day pointing out the glaring problems with such a proposal. Far from overthinking it, you are underthinking it. I don’t think you have considered even the most basic problems associated with how such a system could come to be, or how it could continue to exist.
It’s a plot hook from a sci-fi story. Like space warps or time travel or little green men or any other plot hook. It’s meant to e accepted in order for the story to proceed, it isn’t evidence that the hook itself is even vaguely plausible or well thought out. In fact it’s utterly ludicrous.