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As the tittle says what would the ecosystem look like if predator and prey were to switch places? The cause of this change is either handwavium or a very unlucky sorcerer. The transformation is mainly in diet: prey become carnivores and predators herbivores (omnivores are unaffected so you can leave them out for the time being). This change in diet will then change the niches that each animal occupies over time. With even more time their bodies will change (mostly their teeth to accommodate for their diet) but their overall body stays the same.

For example a giraffe affected by this spell will become a carnivorous giraffe. The animals teeth become sharper and it uses its long neck to prey on other animals. A lion for example would become a peaceful herbivore with flat teeth.

So how exactly would this change the ecosystem?

Edit: Yes this change is supposed to change the animals instincts and their physiology to some extent. Hooves might become claws or herds may become packs, but the overall animal stays the same. And to answer a specific comment, no I did not leave out these details out of fear for my idea being stolen, but rather to avoid making the question overly complicated.

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    $\begingroup$ A carnivorous giraffe would have a very hard time catching prey and will go extinct. $\endgroup$
    – Alexander
    Commented Jan 8, 2020 at 19:20
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    $\begingroup$ It would be a brief bloodbath, as plants consume all the animals on the first day...and that's all. $\endgroup$
    – user535733
    Commented Jan 8, 2020 at 19:22
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    $\begingroup$ What research did you do before you posted this question? $\endgroup$
    – kleer001
    Commented Jan 8, 2020 at 19:32
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    $\begingroup$ There is no such thing as the ecosystem. There are only many many specific ecosystems. So that the first thing to do is pick and focus on a specific ecosystem, for example, the Serengeti grassland ecosystem. Then find out how many lions etc. live there (a few thousand) and how many wildebeests, giraffes, gazelles etc. (hundreds of thousands). Then figure how hundreds of thousands of former herbivores will survive by hunting a few thousand former carnivores. $\endgroup$
    – AlexP
    Commented Jan 8, 2020 at 21:31
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    $\begingroup$ “If cars and drivers switched places, how would this affect transportation and traffic?” $\endgroup$
    – RBarryYoung
    Commented Jan 9, 2020 at 8:49

6 Answers 6

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In a normal ecosystem, the number of prey animals is normally much greater than the number of predators.

(Basically, predators like lions need to make kills one a week or so. If lions live an average of, say, five years, then they need around 250 kills in their lifetimes. Now it's true that a pride will share most kills, so decrease that by a factor of 10. In the course of a lion's life, the lion will require about 25 giraffes (or whatever). Thus the population of giraffes must be at least 25 times longer than the population of lions. You can play with these numbers and get differing results -- for example, the relative lifespans of the prey and the predators matter -- but in the end, prey must greatly outnumber predators or the predators will starve.)

So if you suddenly had a savanna with 2500 carnivorous giraffes and, say 500, grass-eating lions, there'd be a bloodbath, and in the course of no more than a month or two there'd be (a) no more grass-eating lions and (b) a rapidly shrinking population of starving meat-eating giraffes.

This result is pretty robust, though there's doubtless always some way to gimmick the result. (E.g., choose something to become the new predator which can't actually successfully hunt prey. Then the new predator starves (again) and as soon as the new predators are extinct, the new prey undergoes a population explosion and strips the savanna of food before staving in larger numbers.)

Either way, it's not a pretty sight.

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    $\begingroup$ Pedantically: you don't need the number of herbivores to be X times as many as the carnivores. You need their birth rates to obey that relation. That is, for every predator born, X prey animals must be born. The number of prey animals actually alive at a given time can be less than X. In any case, however, you're going to have a disaster. $\endgroup$
    – Matthew
    Commented Jan 8, 2020 at 20:39
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    $\begingroup$ The lions would still be around. The giraffes do not have the hunting abilities for offensive weapons. They are built to run away. The giraffes would mostly eat each other. $\endgroup$
    – SRM
    Commented Jan 9, 2020 at 0:56
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    $\begingroup$ @SRM on the other hand a lion does not have instincts or sensory organs to warn him of approaching predators. E.g. they sleep without much alertness when someone approaches, so the hungry giraffes would probably slaughter a pride of lions at night. $\endgroup$
    – Falco
    Commented Jan 9, 2020 at 13:31
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    $\begingroup$ @Matthew The ratio isn't about individuals, it's essentially about total biomass. For each kg of antellope mass, you need about 10kg of grass. Each kg of lion mass needs about 10kg of antellope mass. That's the primary reasons why predators who prey on other predators are almost non-existent on land (the math is very different in water, for many obvious and less obvious reasons). $\endgroup$
    – Luaan
    Commented Jan 9, 2020 at 13:43
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    $\begingroup$ @Luaan: In the end it’s about conversion rates, not biomass. Carnivores require a certain amount of energy to survive. The energy comes from the sun and has to be processed by plants and herbivores. If your plants and/or herbivores grow very fast (and efficiently) they can sustain a relatively large group of carnivores. That’s why insects can be a good source of meat, because they grow fast and don't require much energy themselves. $\endgroup$
    – Michael
    Commented Jan 9, 2020 at 15:35
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The vast majority of all things non-omnivore would go extinct in short order

Animals are, for the most part, specialized life forms which are designed for limited sets of behavior. There's more to being a predator than just digesting meat and there's more to being prey than digesting grass.

Birth rates, for instance. Rabbits breed like rabbits because of how many of them die, yet lions do not breed at anywhere close to that rate. Another important point is tracking - predators must be able to track down their prey. Cats have developed senses to track and hunt prey, and part of that is actually a set of patient stalking behaviors. Mice will never achieve that level without drastic adjustments, so if the transformation was just mainly in diet, entire swaths of the food chain would go extinct.

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    $\begingroup$ Given how many holes there would be in the food chain, even the omnivores are going to have difficulty. I mean... bees are herbivores. If they're suddenly needing meat, how do they even eat that?! So the bees ... and most other pollinators .. die. That kills a lot of plants within a year. $\endgroup$
    – SRM
    Commented Jan 9, 2020 at 19:52
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    $\begingroup$ Flies eat meat. Mosquitoes sort-of eat meat. That said, good point on the sudden lack of pollinators doing Bad Things to the environment. Of course, that's just one of MANY problems. Realistically, if someone actually managed to do this somehow, they'd better fix it darned fast, or there might not be much life, of any sort, left... $\endgroup$
    – Matthew
    Commented Jan 9, 2020 at 21:21
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Mass extinction of most (ex)-herbivores

I'm assuming, based on your question, that all carnivores that become herbivores have all the digestive tracts and the like required to properly consume enough plant food to stay alive, and all the ex-herbivores get the proper digestive tracts to digest meat in order to stay alive.

That means the only barrier to staying alive is getting the food. Let's also assume that the animals innately understand that they suddenly need to eat something else, to prevent them from instantly going extinct because they keep trying to eat food they can't digest.

All carnivores are now built like carnivores, but can only digest grass, all herbivores are now built like herbivores, but can only digest meat.

The problem here is that if their bodies don't change besides dietary requirements, almost all creatures that used to be herbivores will quickly go extinct.

Capturing, killing and devouring meat isn't that difficult for a hippo, they're already a very dangerous herbivore (and now they're a dangerous carnivore), but most ex-herbivores aren't that lucky. While there are plenty that can most likely kill something, the vast majority of herbivores simply aren't designed to kill other animals.

It doesn't matter that a lion can no longer digest meat, it's still going to win a fight with an antelope. In fact, most ex-carnivores will still be vastly more dangerous than the ex-herbivores, so the ex-herbivores will have no real option but to either become scavengers or prey on other ex-herbivores that are lower on the food scale than them.

Before long, most of the species that were once herbivores will be dead. Assuming a lion understands it now needs to eat grass or leaves or something (I have no idea how you are handwaving their evolutionary digestive tract and what type of herbivore they are now), it should have no problem finding that food now that the (previously more) plentiful ex-herbivores are no longer consuming it.

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    $\begingroup$ The Killer Rabbit of Caerbannog is a joke after all, a bunny suddenly being carnivorous doesn't make it all that dangerous. $\endgroup$
    – Spoki0 - Reinstate Monica
    Commented Jan 9, 2020 at 10:09
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    $\begingroup$ A hippo might be dangerous because it can kill most things rather easily, but if it has to kill for food rather than defense it needs to hunt or capture their prey, first, which will be a huge problem. $\endgroup$
    – Thomas
    Commented Jan 9, 2020 at 12:41
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    $\begingroup$ @Thomas Hippos are one of the few herbivores I honestly give a good chance of surviving. They'll easily kill most of the stuff in the lakes they live in, and they've got a pretty good chance of taking over from crocodiles as ambush predators near watering holes, they can run pretty fast if they have to, are already adept at lurking underwater, etc. They'll most likely still be in trouble though eventually because of all the other ex-herbivores going extinct and ex-predators being too few to provide enough food. $\endgroup$
    – Theik
    Commented Jan 9, 2020 at 12:59
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    $\begingroup$ "to prevent ex-carnivores from instantly going instinct because they keep trying to eat meat they can't digest" (you probably meant "extinct") Herbivores can digest meat just fine. It's the other way around that's hard (for obvious reasons, and with obvious exceptions e.g. fruit, nuts etc.). Most herbivores will happily eat meat or eggs when presented with the opportunity (e.g. deer routinelly eat the younglings/eggs when a bird nest falls to the ground). They just suck at hunting :) $\endgroup$
    – Luaan
    Commented Jan 9, 2020 at 13:46
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    $\begingroup$ Sure, but a lion cannot actually attack a bunch of antellopes and win. You'd be surprised how gruesome the results can be for a predator when he attacks a group of "prey" and misjudges the attack. And imagine that you're the lion, and the antellopes just keep coming - and as soon as you try to attack them, they run away. You can't just ignore them, but every feint makes you more and more exhausted. Humans hunted this way for ages, long before we had any weapons comparable to lion's - or indeed, those of antellopes. $\endgroup$
    – Luaan
    Commented Jan 9, 2020 at 14:06
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If the only changes in morphology are the teeth and digestive system, it's very likely that the new predators would be completely inadapted and would go extinct.

But let's put that aside for a moment, and consider the point made by Mark Olson. Actually, in the most commonly used mathematical model of the evolution of a prey-predator system (the Lotka-Volterra model), the theoretical behaviour is that no matter the initial condition, the system will eventually become periodic. This is observed in real life, where some species have population booms every few years.

Of course, the Lotka-Volterra model may become unrealistic in a number of situations (notably if the population of one species falls below a certain threshold), but it's very possible that you would indeed observe alternating cycles of low and high populations for both species in that situation (sudden reversal of the population of preys and predators).

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  • $\begingroup$ If the world survives the initial upheaval, then yes, the long term expectation would be to end up back at some sort of equilibrium with predator-to-prey ratios similar to today. Kudos (and upvote) for being the first to remind us of that... the OP's question isn't entirely clear if it's short- or long-term consequences that are of interest. (And in the very long term, things ought to wind up looking not so much different than now...) $\endgroup$
    – Matthew
    Commented Jan 9, 2020 at 21:31
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TL;DR: Only omnivores, plants, and some former carnivores will be left after a few years. There will be some ex-herbivores, but they will be relatively rare.

Long answer:

If the plants become carnivores, here is the progression of events:

(1) Previously non-carnivorous plants would consume most of the previously carnivorous plants like Venus Flytraps and pitcher plants. Those that survive will die of malnutrition, since carnivorous plants are carnivorous due to a lack of available nutrients. Liverwort might survive, but that's about it.

(2) When the plants run out of other plants to eat, they will then consume most of the land-dwelling reptiles (snakes, anoles, geckos, etc), since they cannot easily avoid plants.

(3) All bacteria will die of malnutrition. Bacteria are, by default, carnivores; there is nothing else for them to eat.

(4) The vast majority of fauna will be caught unaware by the suddenly-hostile flora. As a result, most of the suddenly carnivorous plains and jungle herd animals will die. The former carnivores like lions will also suffer casualties, but they will probably suffer less due to their usage of caves and outcroppings as home bases. Mountain animals like goats and sheep will survive relatively unscathed for the same reason.

(5) The vast majority of the surviving ex-carnivores will now die of starvation. While herbivores can eat animals easily, the same does not go for carnivores. A lion can eat fruit and nuts in a pinch, but otherwise unable to process plant matter. Even if the spell does change their gastric tracts to be compatible, the will still not be able to get enough sustenance; most plants simply do not provide enough nutrients. A few will survive, but, weakened by hunger, the vast majority will be killed by former herbivores.

(6) Due to competition among the suddenly-carnivorous plants, the rainforests will become barren wastelands, devoid of life. Those plants which remain will straggly, and thus not worth eating. Chances are, the rainforests will eventually become deserts.

(7) Humanity will probably die. To be blunt, we are a very specialized species; without modern technology we would be unable to cope. If modern technology is allowed, then we would probably survive; after all, we have developed weapons specifically for killing plants. Seriously, the 20th century made plant-killing into an art.

(8) The end result will be that the world will primarily be dominated by pigs (those things are invasive even in their home environments), plants, and maybe humanity if we have a high enough tech level.

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  • $\begingroup$ Nah, Humanity probably wouldn't die. We're the smartest, most adaptable, and most lethal species on the planet. Every other animal spends its life worrying if it'll stay alive; we spend our time worrying about killing entire other species on accident. Even before modern technology, we not only colonized pretty much every corner of the planet, but adapted whole species of animal and plant to our whims. Don't underestimate our ability to survive in terrible/strange/new situations. $\endgroup$
    – Kevin
    Commented Jan 9, 2020 at 18:22
  • $\begingroup$ @Kevin. Survive, probably, but with huge attrition $\endgroup$
    – Mad Physicist
    Commented Jan 9, 2020 at 19:59
  • $\begingroup$ @Kevin Yes and no. Yes, we are incredibly good at "accidentally" killing other species, but at the same time your average joe isn't very good at genocide. What I was saying is that those people who are good at genocide (i.e. the military) will be overwhelmed, causing "average joe" to die. $\endgroup$
    – In Hoc Signo
    Commented Jan 9, 2020 at 20:01
  • $\begingroup$ If plants now eat meat, who is producing our oxygen? $\endgroup$
    – Tezra
    Commented Jan 9, 2020 at 20:33
  • $\begingroup$ It's not the military. Humans in general don't tolerate threats to our survival. Take the wolves in North America. They used to number 2 million. Now they're endangered, and it was due to average joes. Humans in general will also figure out ways to survive immensely inhospitable conditions - not only using their ingenuity to survive, but mold the environment around them as they will. I agree with Mad Physicist - there'd be a lot of attrition, but it's actually pretty tough to kill off humans as a species. Underneath modern civilization is a lot of evolutionary pedigree. $\endgroup$
    – Kevin
    Commented Jan 9, 2020 at 20:40
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Predators can predate one another to get protein. So if the prey vannishes, there will be levels of fallback strategy.

Eat other weaker predators.

Eat remains that others can not eat. Your ex-hervibores would for example chew on old skin and bones until nutrition can be extracted.

Self-consumation, aka starving.

Eat plants again, as a emergency strategy.

Other strategys include various hibernations and hibernation of the offspring (triggered to hatch by repopulating prey).

Nature is something desperat, improvised and willing to use all options. As long as there is food, any food, it will be eaten.

One thing that would change though, is a disruption when it comes to parasites. A creature used to dwell on or in a herbivore, certainly would find its lifecycle disrupted.

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