There was a nuclear war in my world Anthropocene. It caused a mass extinction of numerous animals on the planet, so many I can’t cover them all completely, so I’ll tell you what animals are left:

  • Rats

  • Most insects/arachnids

  • Humans (ofc)

  • Canines

  • House cats

  • Pigeons

  • Livestock Animals (cattle, sheep, horses, goats, donkeys, pigs, chickens, llamas).

  • A new, genetically engineered creature called a Trihorner (reptile)

Those are basically it. The last surviving animals in the region my world is set. Plants come back as usual, so ignore that, but my question is: with these last surviving animals, could humanity or even nature itself build up a new, healthy ecosystem/food chain? Or all they all screwed?

Edit: All micro-stuff (tardigrades and algae) survived, and I don’t care about them for my question

When you give your answer and you believe something is lacking, give me a suggestion for an animal that should be added

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    $\begingroup$ How did humans, rats and livestock survive something so devastating that it wiped out the (incredibly tough) tardigrades? Your background is not plausible. $\endgroup$ – Mike Scott Jul 13 at 5:17
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    $\begingroup$ Could you add a bit more for vegetation? Even with that it's an uncertain answer, but somewhat answerable. $\endgroup$ – Trioxidane Jul 13 at 5:19
  • $\begingroup$ @Mike Scott you would assume more is left. If you're going to say tardigrades, you can also say fish, plankton, bacteria, virusses, etc. $\endgroup$ – Trioxidane Jul 13 at 5:21
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    $\begingroup$ You could just say "a huge mass extinction occurs wiping out all non-domesticated megafauna except humans". which should get around all the nitpicking. That would make it earths 6th mass extinction event. $\endgroup$ – John Jul 13 at 13:37
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    $\begingroup$ @SZCZERZOKŁYIt doesn’t matter if plants are self-pollinating; the insects survived, and relatively few plants rely on birds or mammals for pollination. Distribution of seeds may be a problem for some species, though. $\endgroup$ – Mike Scott Jul 13 at 13:46

Let's remember 2 things really quickly: as far as we know, ever single vertebrate in our world can be essentially traced back to a single ancestor and we had dozens of extinction events throughout the history of our planet. Right after these extinction events, we usually see a large amount of niches available due to the death of the previous occupants and a relatively small number of surviving species, which in cases can result in speciation booms which allow for new species to appear and fill in the gaps. However that's not quite the entire story, why? Because some species are capable of faster speciation than others, for example, an animal that has more offspring at a faster rate can undergo changes and select beneficial mutations much faster than a species which has 1 or 2 children every 2 years.

So let's have a simplified analysis about the remaining species regarding predation and temporarily assuming plant life is not a problem (because I'm no biologist and not all factors are known about your world, plus you seem to be intending to focus on the terrestrial environment):

  • The surviving arthropods seem to have no remaining predators other than one another, rats, chickens and an eventual pigeon unless humans too begin using them as a food source.

  • Rats, unless the dogs and humans start using them as food, only have to worry about chickens and pigs (a pig can and will eat a rat if it's hungry or if it stole the pig's food, a chicken can and will eat a rat period, both these farm animals are omnivores after all), however despite their ability to reproduce fast, it's hard to say if they'd undergo a speciation boom, since lifespan is also an important factor, and a rat will hardly live more than 4 years even in captivity. Rats may eventually eat one another in times of hunger as well.

  • Your cows, horses, llamas and donkeys superficially have little to worry about other than a large pack of hungry wolves and the now relatively uncontrolled (depending on how the remaining chickens and pigs are doing) rat population (and humans of course, though humans might actually help them survive in the long run by doing what we've done for centuries, save them from other predators so we may be the ones to eat them).

  • Sheep might be at a greater risk of being attacked by wild dogs, but seem to be on a relatively similar situation as the other large ruminating animals, assuming humans keep using them as livestock.

  • Goats might be similar to sheep, except they might actually be desired since they've shown to help controlling rat population in some cases.

  • Regarding pigs and chickens, apparently a mostly similar story, except they have the potential to act as predators (to rats and, if the pigs are starving, whatever they can catch).

  • Humans seem to have mostly the environment, the rats, an eventual pack of wild dogs and the trihorners to worry about and seem to be better off making their best to keep the livestock strategy going and plant whatever edible grains are left. Keeping dogs close by also seems like a good choice.

  • The dogs will likely be split into 2 groups:those who keep living with humans and those who go wild, trying to fill the niche of their wolf cousins. They'll likely predate mostly on sheep, some pigs and goats, as well as some chickens and an eventual rat, if they're hungry and it's easy enough to catch. I don't see too many attacks on larger livestock unless the dogs belong to breeds more optimized for hunting and are in large enough packs, and their causalities will likely be predominantly due to failed attacks, human/domestic dogs intervention and potential predation by a trihorner.

  • The trihorners: you didn't say much about them other than that they're genetically engineered, so I'll work with some assumptions and what was discussed in some of your previous questions: assuming a warm enough climate, your trihorners, assuming they're slightly larger than a komodo dragon, will likely compete for the position of Apex predator with wild dogs and humans, depending on how much these reptiles were engineered to resemble a living weapon. Assuming a deathclaw scenario (created to look like a salt water crocodile and a komodo dragon, both on steroids, somehow had a mutant baby which was feed nothing but protein shakes and more steroids) your Trihorn will easily occupy the niche of solitary yet powerful reptile and occupy the top of the food chain. Dogs might be a problem for the young and the elder, but a healthy adult Trihorn, similar to moose, will likely have no natural predators and will only need to watch out for humans trying to hunt it (or not, depending how much the "on steroids" factor is present in this creature).

  • The cats: we can easily assume they'll rely mostly on the rats, possibly arthropods they happened to hunt and pigeons for food, as well as maybe the young of some other remaining animals, such as chicks (and maybe baby pigeons as well). They'll likely be predated predominantly by wild packs of dogs and an eventual trihorner which manages to ambush them. The cats will also most likely be divided between those which adopted a more wild and less "human-dependent" lifestyle and those who still live with humans to some degree, aiding in further controlling the rat populations nearby zones of human occupation. They're also good candidates for speciation, being able to reproduce fast and having a decently long lifespan at around 10-13 years in the wild, further allowing natural selection to manifest itself on the wild cat population.

So overall, considering the animals only, the environment as is seems to be already able work out relatively fine (although the number of available potentialy predatorial species seem smaller than ideal), assuming plant life is doing well enough and the remaining animals are already present in necessary concentration. If not, we will have problems regarding overpopulation of some species followed by rapid decline in said species, though unless something like the rats runs completely beyond control, I think the environment can regulate itself enough with what's available. What I mean is that, while there are many niches which were left unoccupied, there are still some animals helping in population control,and seeing how mammals all seem to have come from a single ancestor, all vertebrates seem to have come from a single species of fish and we've had a fair amount of extinction events already, we can assume that the most likely scenario is for some of the species still present to slowly undergo specialization to fill in niches left by the extinct land vertebrates. Which species will undergo such speciation is hard to tell, but my bet lies on mostly the dogs, pigs and chickens, as they can all reproduce relatively fast and will all usually live more than half a decade (unlike rats, which reproduce incredibly quickly but don't live as long, though they too might give origin to new species).

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    $\begingroup$ @DT Cooper I did notice that, I also noticed you really decided to adopt the idea of genetically engineered creatures. If they're the size of a dog, then I assume they'll likely fit more of an ambush predator niche and feed mostly on chickens, rats and potentially some smaller pig or goat it manages to catch, if they're solitary. $\endgroup$ – ProjectApex Jul 13 at 15:17
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    $\begingroup$ Well, can I restate myself. They’re more the similar to the size of a really big dog, similar to the wolves in size $\endgroup$ – DT Cooper Jul 13 at 15:24
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    $\begingroup$ @DT Cooper well in that case it has good chances of securing the place as an Apex predator. The key here is that unless it's a social creature, your reptile needs the size, strength and defense to fend off even packs of animals on its own to have no problems securing its place at the very top of the food chain, like we can see in a way with brown bears (it also helps surviving, since you can scare off a pack of dogs and steal their kill like hyenas often do with cheetahs and other felines). $\endgroup$ – ProjectApex Jul 13 at 15:58
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    $\begingroup$ One very important thing to remember: speciation takes a VERY long time if you want to talk in re: human history timespans. Civilization is a few thousand years old. Speciation on that small a timeframe I can see happening, I guess, but not much. $\endgroup$ – Ton Day Jul 14 at 1:50
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    $\begingroup$ @Ton Day that's why I also said how lifespan and reproductive rates deeply affect it. Though It is true that the availability of new niches is known to allow for processes like faster speciation, these processes are fast by evolutionary standards (which consider 150.000 years a short period). $\endgroup$ – ProjectApex Jul 14 at 2:20

If I remember correctly, the common ancestor of mammals during the times of dinosaurs extinction was something small similar to a rodent.

It looks like you are in a similar situation: rats, lacking predators, will proliferate and occupy all available niches, then, to mitigate food competition, they will start specializing on a certain diet, which in the long run will separate them in new species.

It won't happen in a few centuries, but that's how it might go.

Pigs and chicken, among the livestock, are versatile enough to be able to start their own species trees, too, if they manage to escape human control.

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Predators are sorely lacking

Without predators keeping them in check, anything that rats can eat, they will eat into extinction. Then the rats will starve. Canines - i.e. wolves - are the big exception here. Basically any mostly wild area will be full of wolves, living off of mostly rats, or no animals at all.

People, who regard rats as pests, will keep the population in check but not be able to wipe them out (we never have before, have we?). Therefore human settlements will have a constant rat population. This means the area surrounding any settlement is either full of wolves, or a barren wasteland.

You're sure there's no birds?

Lot of kinds of birds live in human cities.

Many plants go extinct from the lack of codependent animals

Many many plants are adapted to take advantage of the existence of animals to spread. Burr seeds get caught on animal fur, then eventually fall off, the seeds spilling out potentially a great distance away from the source. Berries and fruit are designed (insofar as evolution designs anything) to be eaten, and then the seed survives its typical eater's digestive tract, and gets deposited elsewhere.

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  • $\begingroup$ I never knew what type of plants could survive. Also, what could I do about the rat problem $\endgroup$ – DT Cooper Jul 13 at 5:34
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    $\begingroup$ @DT Cooper it might be simpler that you think. Your story seemed to suggest decades, if not hundreds of years might have passed. Nature tries to find equilibrium. Disease, hunger and predation will regress to a certain equilibrium. If that fails, invent a disease that is rampant in the rat population, keeping them under control. $\endgroup$ – Trioxidane Jul 13 at 5:58
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    $\begingroup$ Rats are omnivores. They will eat each other long before everything else is gone. $\endgroup$ – Mary Jul 19 at 18:24
  • $\begingroup$ My understanding is that this is the most accepted theory of what happened to Easter Island now - rats arrived on board a ship and they wiped out the trees that were the basis of the ecosystem, and it all collapsed. $\endgroup$ – Ton Day Jul 19 at 19:41

Rats reproduce extremely quickly and can handle a wide range of environments. This allows to expand both geographically, and into a wide range of niches. Their fast reproduction allows them to evolve quickly into creatures similar to squirrels, mice, and other small mammals. Eventually some evolve into creatures similar to raccoons, possums, otters, and even primates.

Pigeons reproduce quickly, and without aerial predators, they are able to expand as well. Pigeons eat grass seeds supplemented by invertebrates. With less competition, they are able to diversify somewhat into more roles. Some evolve to eat fruit, others evolve to eat small rodents, and eventually into eating small birds. Eventually their descendants fill most of the roles currently filled by birds.

Additionally, pigeons are able to access islands that rats cannot. On these islands pigeons diversify to a greater extant. Large flightless pigeons, similar to dodo birds, are soon the main herbivores on remote islands. With time, they may diverge further, perhaps into something like Emus.

House cats protect human farms against rats, which means that they are given many opportunities to go feral. As they become more dependent on eating rodents for their food, their reliance on humans diminishes. Within a few decades bobcat-sized cats have spread across most continents. In time, some evolve into larger cats, similar to leopards or mountain lions.

Dog are more domesticated than cats, so they tend to stay closer to humans at first. Feral dogs hang around settlements, eating trash and the occasional rodent. In a few centuries dingos evolve from the feral dogs, and in the distant future dogs evolve into wolves.

Feral pigs become monsters, much as they are currently doing in North America. Unchecked, they spread and reshape the land.

The other domestic animals find their niches: mountain goats, mustangs, and so on. Chickens are a bit of a wild card. They could do well, but the rats and pigeons will probably get to most niches before they can. I see them spreading fairly widely but not dominant anywhere, into niches such as that held by the wild turkey now.

Trihorns, well, I don't know much about them. Reptiles tend to do well in warm areas, so maybe some evolve into something like crocodiles. Because of their urine chemistry, reptiles have an advantage over mammals in desert areas, so expect to see a bunch of lizard-like animals in deserts. Limbless and semi-limbless reptiles have evolved several times, so something like snakes will appear sooner or later. The crocodile-like trihorns make gradually become more and more adept at water life, turning into sea creatures in places with warm water.

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Let's approach this from a r/K Selection standpoint, mostly because there's a lot more detail that could be desired and a lot of gestation, etc. numbers to crunch if we did it the full predator-prey population cycle way.

  • Rat, Insects, Arachnids, Vegetation, & Micro-fauna for r-selected species.
  • Humans, Canines, House Cats, Livestock, Pigeons for K-selected species.
  • Trihorner (herbivore, omnivore, carnivore? r/K? I'm using it as our joker)

So K-selected species will tend to balance themselves as long as r-selected keeps their end of the deal and remains plentiful. Although it is certainly more stable if it forms a predation loop and/or predator-prey ratios are balanced. Humans can also certainly help dampen extinction-potential population swings by selective hunting or just fully integrate as part of the cycle (your interpretation), as was done in Yellowstone.

As for the r-selected:

  • I'm unaware of any place with vegetation that lacks insects. Even a highly specialized insect predator would have trouble wiping out a strong species if it was K-selected. If your insects are diverse then this is even less likely.
  • I severely doubt any place with insects would have arachnids dying out.
  • Vegetation and micro-fauna can have problems but they're generally fine on their own.
  • Rats... as pointed out elsewhere they're a potential problem. They're not generally guaranteed to self-balance since they're not K-selected. And they're able to dine on most, if not all, the other r-selected species. Luckily, if rats multiply by 4-7 every 3 weeks, then that's about 1 additional rat every 3 days. Given that there is 3 potential predators then any population explosion can be checked by them. In the event you got a mega-colony from being isolated in a predator-free resource-rich area, you'd still need an incursion large enough to be triple the local combined population of predators to be a major ecosystem threat (While obviously a different set of ecosystems, in a modern human city you might have barely any predators but plenty of resources. So rats are only resource bound. Our non-urban areas are more balanced related to rats but also supply resources to the urban areas. Cutting off the resources supplied by those more balanced areas you can get things like cannibalism). All of that said though the cat is probably your K-selected guardian of choice for this part of the ecosystem. You probably can't get rid of them, but if you're there to correct an ecosystem imbalance it's not a fight you won't win.

Next is the known concern of over-grazing from livestock killing themselves off. Which could wipe canines and humans out. But given that they are livestock humans managing them can probably prevent that.

Which only leaves the Trihorner to fit into the web. They could very well destroy it as an r-selected so let's make them K-selected if that's alright. If they are:

  • Herbivores our livestock have a problem with too much competition. We'll have to hunt them.
  • Insectivores (arachnids included) we're probably good.
  • Carnivores it'll depends on what they eat. Canines or pigeons makes things more stable. Rats relieves humans of some of the burden. Livestock is probably perfectly okay since we'll try to protect them but will fail from time-to-time. Cats would maybe be pretty bad. It depends on whether canines preferentially hunt cats over rats. Eating humans is not great but probably moderately okay. It replaces the Apex Predator with something with less intelligence to manage the population swings. But it does knock out an omnivore so as long as livestock isn't terribly self-sustainable in the wild the cycle should be okay.
  • Omnivores would just be bad, especially if they were an Apex Predator. If their size allowed them to hunt canines and rats preferentially, then no problem. The biggest problem is that they have no extinction clause in this herbivore-dominated ecosystem. Anytime you might have a typical K-selected balancing, predator-prey ratios won't allow it. Anytime the prey is almost wiped out they won't dial back from hunger and reproduction loss, because they can still eat plants and insects. The only check they would have is a Predator Explosion due to the abundance of themselves as Prey. But they would still strain every tier of Prey underneath them.
  • Apex Predator (see omnivore and carnivore)
  • "Other"-vore (fructivore, etc.), specialization makes this less likely to survive as it hinges on the survival of something else. The more specialized the less likely to survive (a "problem" that is, in general it'll be more efficient at what it does and flourish). However it also is highly unlikely to mess with the rest of the ecosystem, probably at most it would knock itself out with its Prey.

So will this food chain survive?

If the trihorner isn't there, then probably. With it actually present though it will need to meet some conditions. Maybe strict preferences in diet, maybe specialization in hunting a particular insect, maybe it eats all meat except human meat. That said, humans, as always, will have an amazingly large potential to upkeep or royally screw over the ecosystem.

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  • $\begingroup$ Maybe a little more "off-the-cuff" than my typical answer (although it might feel about the same given my usual citation and "step-by-step math" quotient). But it's not tagged science-based, so... "generally" feels more like "yeah, let's do that". $\endgroup$ – Black Jul 20 at 9:14

Plants (and fungi which you missed) are the most important part of the food chain as far as multi-cellular macro-scale organisms are concerned. You cannot claim such widespread destruction of the food chain if plants are still there. Many animals would die but with plants and insects around, the basis of the food chain itself would still be there. Omnivores, herbivores, and insectivores would be fine.

The food chain would be pruned, but not destroyed. It's just different than what it was before. It therefore wouldn't need to recover and should just regain its diversity in time as vacant ecological niches are re-occupied.

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I will try to be as brief as possible!

Insect farms could be made. Similar to these in China and south east Asia.

Same goes goes for dogs and cats, similar to the food in Vietnam and the two Koreas. Although feeding for the dogs and felines will be required. Therefore, these two would be a luxury food.

Genetically engineered plants will be a must if you want to re-establish settlements then civilization, similar to what the nomads did.

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  • $\begingroup$ What king of genetically engineered plants) $\endgroup$ – DT Cooper Jul 25 at 13:28
  • $\begingroup$ In a watery Environment, rice and soy would be the best food source. In a regular land environment, Corn and wheat are the top corps to plant. $\endgroup$ – Noor Jul 25 at 15:47
  • $\begingroup$ And of course hills could be used to plant genetically modified Apple trees, orange trees, and most importantly, Olive trees. Olive could be used to extract olive oil, which works well as a lubricant for organic use, and a good fuel for making torches, fire, and lubricating machines. $\endgroup$ – Noor Jul 25 at 16:00

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