How would evolution proceed without predators?

I was watching The Lion King with my daughter, and sort of smirked at the self-serving explanation Mufasa (the lion king at the beginning of the movie, for the two people who haven’t seen it) gives his son and heir apparent about why it’s okay that they eat antelopes and so on. He says that it’s a “circle of life” and that when they (the lions) die, their flesh rots back into the soil and feeds the grass for more herbivores to eat. (He kind of leaves out how it’s probably more about their feces and the feces of the scavengers that pick at their corpses, but the basic idea makes a certain amount of sense.)

My scoffing though is related to the fact that predators are not needed for this “circle of life” to function–they are more or less just middlemen unnecessarily horning in on the action. If you just had herbivores and scavengers, everything would work just fine with no need for predation. Sure, we always hear about how predators keep the herbivores’ gene pool “strong” by culling the slow and weak, but isn’t that sort of begging the question? Doesn’t this just keep their strengths focussed on evading predators?

I mean, if there were no predators there would be no real reason for gazelles or any herbivore to be able to run fast. Nor would their young need to be able to stand and run within minutes after birth. And surely they are sacrificing other potential evolutionary strengths when the selection pressure is so oriented around evading or fending off predators. If predators disappeared from the planet, they would still be competing with other herbivores, and would still need to survive the elements and so on, so there would still be natural selection of a different kind.

So I wondered: what would the animal kingdom look like in such a world? I realise that it’s hard to imagine evolution proceeding for millions of years without predation re-arising even if it were eliminated once; so let’s just stipulate that this is the science project of some super-advanced race of vegans. Every time any critter of any sort starts to have a diet that is even partly based on killing any other non-plant life form (scavenging meat from an animal who died of natural causes, OTOH, is fine), that animal is instantly made sterile. Seems like that would “discourage” evolution from heading down that path (I use those quotes because I know that evolution is in fact mindless, but it often is useful to describe it as if it had intentionality or purpose).

Unless you count microbes as “predators”, then resistance to disease and parasites would be a major evolutionary pressure. Beyond that, probably sexual selection driven by competition for mates. The ability to withstand prolonged droughts where little food or water was available would be another. Maybe increased body size? Herbivores seem to trend to economy of scale.

What happens to the plant eaters when they inevitably pick up a few insects along with their greens?

That’s not evolution by natural selection. That’s a form of selective breeding.

Actually I’m reminded of The Gentle Giants of Ganymede by James P. Hogan. He postulated a world where due to a quirk in how multicellular life evolved (basically, all animals had blood that was poisonous to any other animal), there was no predation, and the animal life that evolved there was hopelessly defenseless by Terran standards.

I think we need to define predation better, since the plant/animal dichotomy is an arbitrary classification, and also an artifact of our particular evolutionary path.

Really having no predation would mean organisms could only ingest either non living material directly (gas, water, minerals) or naturally deceased organisms.

I suppose we could say that mobile organisms could only eat non mobile ones. We might end up with a lot of sloth like creatures.

But I think that even without predation, there will still be a lot of what appear to be selections for fitness that are really the equivalent of peacocking. Even without interspecies violence, you are still going to have competition for mates.

You seem to have entirely missed the point, wasting a giant quote box on a two sentence non contribution. No one is saying the alien interference is natural selection. The question is, given that one artificial constraint, what might natural selection do?

Predation is a major selective pressure, but it’s not the only one by a long shot. This experiment has more or less been done many times, by the way. There are numerous examples of islands that had never been colonized by large predators until humans showed up. You probably couldn’t say that everything there was vegan, but there were certainly creatures at the top of the chain that lived a predator-free lifestyle for a good long time.

Not sure what you mean by that. I think the more you tried to eliminate predators, the more you would select for proto-predators because you’d make it that much easier to become a predator. Right now, if you’re an organism that isn’t a predator, and something changes in your environment that pressures you into becoming one, you have to compete with all the existing predators. But if there are none, and all these fat, slow meals-on-the-hoof are out there, you don’t need too much change in your phenotype in order to dine on them.

I don’t know why you are distinguishing between large and small predators, but perhaps you could name some of these examples of mostly vegan islands.

Imagine attacking such a world. They’d never expect it!

Careful, Bryan. Jackdavinci might scold you for “wasting” space.

Just thinking out loud here and I’m no expert, so this is just pure speculation that could be completely wrong:

No predation means herbivores will increase to the point of exceeding the food supply.

With limited resource available, seems like there are a few directions to go:

  1. Smaller more numerous offspring to avoid requiring as much food
  2. Faster more aggressive herbivores to out compete for limited food supply
  3. New species taking advantage of underutilized food sources (not sure what these might be)

Thoughts on response by plant life:

  1. Faster life cycle due to the slow ones to reproduce getting eaten before reproduction
  2. More defenses to provide needed time for reproduction
  3. Grow down or up to try to escape reach of herbivores

The proliferation of Doritos and PlayStations is having that effect in our ecosystem.

Don’t herbivores have digestive systems different from omnivores and carnivores, though? And in any event, if the first time you deviate from veganism you become infertile, I’d think that would not make it too easy to select for proto-predators or any other kind.

I am tempted to think of giant herds of really slow, large herbivores. But would there even be herds, without the need for safety in numbers? Would they still grow horns, to fight over territory and females?

The fittest would be the ones who use food resources more efficiently. They would be less subject to death by starvation. There would be many different animals who achieve the superior efficiency in many different evironments. They wouldn’t be directly predating on other animals, but they cause their death in the end. Unless there are endless resources, evolution will continue much like it does now.

Predators are necessary to keep the Alien population in check. If not, all life throughout the galaxy would eventually succumb to face-huggers. This pan-galactic monoculture would eventually crash, leaving the universe entirely devoid of life, and more importantly, devoid of Sigourney Weaver in her undies.

Hoards of locusts circling the earth, hoards of ants that eat the locust’s dead bodies.

Oooh, you might have something there!

We do have something of a hint. The Kiwi, of New Zealand, evolved without natural predators, until rats were introduced recently. As a result, the most successful birds laid the biggest eggs.

Why? Evolution doesn’t “remember” its mistakes. Also, you are allowing scavengers. It’s a thin line between scavenger and predator, especially if there are no other predators around.

Females. Worth playing for? You bet!

Kiwis evolved from ostrich-sized ancestors. They also evolved with large numbers of predators, primarily lizards, frogs and other birds. All of those predators lack the ability to gnaw trough eggs: they have to either swallow them whole or pick them up and drop them. That’s not a problem for full-sized ratites, since the eggs are so large they can’t be lifted, and the young are born so well developed that the egg shells can be astonishingly thick. The problem came when kiwis evolved to fill the niche of a smaller bird. There was no barrier to altering the adult form, but the eggs could not become any smaller without succumbing to predation. So as a result the kiwi produces ludicrously oversized eggs, something lie 1/3 of the hen’s normal body weight.

The large eggs are a successful defence against predators, nothing more. It remains a successful defence against predators, which is why kiwis still exist when almost every other flightless, terrestrial bird in their size range is extinct. Large, thick eggs are a very good defence against stoats and rats, the threat from introduced predators is to the newly-hatched chicks, not to the eggs. Only about 1% of eggs laid succumb to predation, usually from pigs.