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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:

Anyway, I’ve been extremely interested in the biology and environmental systems of my world, particularly the future of the animals that inhabit it. My question is, with these animals being the only survivors of the mass extinction, in what ways could they be likely to evolve over the next few hundred thousand to million years?

Ignore humans- I’ll deal with them myself. Your answers should be written in a way that completely negates humans as a factor-thank you

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    $\begingroup$ Who are the herbivores in this scenario? You cannot really have a food chain without herbivores... And what has happened to all the fish? (And, congratulations for the effortless almost organic advertising for Google's selfless services.) $\endgroup$ – AlexP Jul 27 at 18:24
  • $\begingroup$ What kind of timeframe are you interested in? For instance, short term of months/years where different population numbers will rise and fall as they move into various ecological roles, but remain mostly physically/genetically the same? Or long term of hundreds/thousands/more years where species could evolve different traits? $\endgroup$ – TheUndeadFish Jul 27 at 18:30
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    $\begingroup$ Yes, that's perfectly possible. Chicken, pigs, horses and goats (at least) are known to be ready to liberate themselves and live successfully on their own. But then they are no longer livestock... $\endgroup$ – AlexP Jul 27 at 18:41
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    $\begingroup$ @AlexP: Livestock and feral descendants of livestock $\endgroup$ – DT Cooper Jul 27 at 18:43
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    $\begingroup$ @TheUndeadFish: Can humans be ignored as a factor? I’d like humans to be ignored as a factor $\endgroup$ – DT Cooper Jul 27 at 19:37
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Lots of evolution to eat bugs

First off, I'm assuming that all plant life continues. There are some complicated relationships between plants and animals that would be disrupted, so you'd expect to see a lot of plants going extinct pretty quickly. (I sincerely hope that you plan to keep bees around or else it'll be a massacre.)

Rodolfo Penteado's answer is right in saying insects will become a major problem in the short and medium term. In the timeframe you're talking about, they could become a resource. An evolutionary niche that's not filled causes disruption only until other creatures fill it. A niche that big isn't going to last hundreds of thousands of years. There would be evolutionary pressure to start eating insects. And perhaps an even stronger pressure to adapt to life with lots of bugs. Humans can already live on a diet composed primarily of certain insects, so insects could become a big food source. Cats, pigeons, rats, and other of your surviving creatures could also adapt to eating insects without needing to make major physical changes.

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I'm sorry, but...

It still needs to occupy some ecological niches there.

Without their usual predators, insects increase their numbers on an absurd scale and form clouds of insects that make the now-feared locust clouds look like a gentle breeze. The imbalance caused by how much they are able to help reproduce with pollination is quickly lost by starving millions of tons of insects.

Humans will try to fight these pests with all sorts of pesticides but the numbers are really favorable for bugs. The desertification of the few areas where vegetation still exists after the nuclear winter leads to the massive death of most herbivores that humans use as livestock and makes the maintenance of pets unfeasible.

The few humans that remain in the following centuries become sterile due to severe nutritional deficiencies, eating exclusively recycled food. The Earth returns to the Precambrian period where all remaining life is single-celled or, at most, very primitive.

Another important point is in the flora. What will exist after this nuclear winter? Even after solves the problem with insects, having only herbivores like cattle without all sort of predators, except humans and their pets, will make the vegetation insufficient to feed an entire herbivorous fauna.

In addition to amphibians and other types of insentivorous animals necessary for a certain ecological balance, there are all kinds of moles, earthworms and other worms that can go through a situation like this very well.

There is also something interesting about plastic-eating microorganisms, which may even remove the marks that geologists intend to use as differentiators from the Anthropocene.

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    $\begingroup$ This seems a little bit extreme to me. There have been plenty of exictions that have resulted in all but a few species surviving. Sure the first few thousand years can be terrible but the remaining species have always adapted to fill all available niches. Sure bug clouds may be a huge problem at first, but they aren’t sustainable for long as food sources dwindle. Very quickly something would evolve to eat them as it’s free food. $\endgroup$ – Alex Jul 31 at 12:10
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If a nuclear holocaust has taken place in your world, lots of particulate matter will be ejected into the atmosphere. You’ve probably read before about how volcanos have a huge impact on the environment, to the extent that some have speculated that volcanic eruptions have been responsible for various major famines and cold periods over the course of human history. A nuclear bomb is much less powerful than a single volcanic eruption, but I could see an entire nuclear war exceeding the effects of a single volcano.

The ash and particulate matter in the atmosphere will significantly increase the albedo of your planet, causing the global temperature and sunlight to drop off rapidly. This is where your food chain will be dramatically affected. The frozen poles will expand slightly, enlarging the tundra biome (not exactly an icy wasteland, since the poles do host a decent amount of life, but nowhere near as much as closer to the equator).

Plants will become more sparse because of the decrease in sunlight. Plants and animals will go extinct because of the rapid decrease in global temperature, which will produce and extended global winter. Surviving plants and animals will gradually migrate towards the warmer equator to offset the temperature decrease. Cold-blooded ectotherms, including insects and reptiles, will have a really hard time keeping warm enough to survive, while mammals will slowly evolve to grow more hair.

Animals that feed on insects will have a hard time. Even if certain insect species survive the temperature drop by migrating, the small mammals or arachnids that eat them won’t be able to keep up, and they’ll lose a major food source. Mammalian herbivores will probably have the most secure niche, but because plant mass will also be decreased, there will be increased competition between them.

Does that answer your question?

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Take it From the Top

I think this question has some really specific nuances that can make it work. In order to arrive at the answer, I'm going to walk back from the question to arrive at a place where it becomes possible, and we will allow handwaving to make it plausible for your scenario. Since the question seems to focus on the Trihorners and Geckos (and since those species are the cool, new ones in this context) I'm going to focus on those, and fill in the gaps around them with the other pieces of the food-chain.

Strap in, this might take a while.

Large, Carnivorous, Active Reptiles

The two new species are large reptiles. As Franklin Pezzuti Dyer has already pointed out, the likely outcome of a nuclear war, nuclear winter, is very unfavorable to reptilian species due to the global temperature drop. Reptiles are ectothermic, so they require heat energy to be available outside of their bodies. However, there are two ways to deal with a lack of external heat: 1) Movement heat and 2) [Gigantothermy][1].

When a reptile moves it generates heat in its body due to friction and muscle activity. In a cold environment, physical activity can help offset a lack of heat. This, of course, comes at the cost of calories, so a more active reptile will need to consume more food. Reptiles do tend to be carnivorous or omnivorous in general, though, and if the insect population increases the reptile population can increase consumption in kind. Under this logic, a colder environment with enough available food will tend to produce replies that are more active and carnivorous.

A reptile's survival is still ultimately left to the whims of the climate, but that's where gigantothermy can help. You can describe an animal's size using both its volume and its surface area. As an animal's volume increases, its surface area increases proportionally less. This is known as the [square-cube law][2]. In practice, this means that larger animals have less surface area per volume than smaller ones, and thus are less affected by external temperature change. In mammals, this is not great, because we produce heat and need excess heat to be removed via our skin. Ectotherms, however, would find this very useful in colder climates, since larger versions would shed less heat than smaller ones over the same time frame, thus allowing them greater freedom in terms of the lowest temperature they could survive in. According to this principle, colder climates with enough food to go around might tend to produce larger reptiles.

Now we have explanations for the large, carnivorous, active reptiles, maybe we can assert that these reptiles come from specific current species. I first propose that the names be switched on the descriptions and that trihorners be the smaller version and geckos be the larger. The name trihorner evokes a sense of sharp, bony protrusions on the animal, and this seems to fit tortoises best as a precursor. Tortoises already grow quite large in general, and if they had to become more active and predatory to compete, the notion that they might adapt their bony shell into large protrusions is not too far-fetched. Add to that the fact that tortoises and turtles are generally omnivorous and I think you have a good match for this creature. Also, turtles are known to communicate with each other and learn better in groups, which supports the pack-hunting dynamic.

As for the gecko, which I propose to be the larger carnivorous animal, comodo dragons seem like the best precursor. They have claws, they are already quite large, and there's a bonus highly-deadly saliva weapon. They are a type of lizard that is very territorial and solitary and they dominate any ecosystem they live in. Also, comodo dragons are known to be able to produce offspring asexually from females, so if you were looking for a species that can go till the bitter end, look no further. As long as one female is alive, the species can continue. Imagine this creature, the size of a lion, crawling towards you at high speed because it needs more food.

Some Other Aspects Of Reptiles

Reptiles have an inefficient circulatory system as compared to most mammals, and when you couple that with the large body size what you get is a very short-ranged hunter. They won't be able to pursue their objective for long. This is fine for hunting animals that are probably dumb enough to get surprised easily or trapped, but humans would likely be able to outrun them easily if they could get any meaningful distance between them after the initial attack. Also, when energy is not needed, reptiles have a tendency to just sit and wait, using their excellent camouflage to hide from predators and prey alike while they conserve energy. They are also naturally danger-avoidant, so the smaller ones especially will try to run from a threat they are not ready for instead of attack unless running wouldn't work in the situation. That said, more aggressive reptiles wouldn't be hard to believe in this context.

Other Animals, what about them?

Literally all the animals are explainable due to humans keeping them while the rest of the world sorts itself out. Rats and bugs will hang out around humans and their settlements, functional or not, and the same is true with domesticated animals. The insects will likely breed out of control, as all of the answers point out, and this can be used as a food source for other populations. Also, it's worthwhile to note that radiation will, 99% of the time, just kill everything it touches. The idea that radiation will mutate things that survive is pretty ridiculous since those sorts of mutations will be cancerous and instead just kill everything. Have I mentioned that anything that gets heavily radiated will die? You can avoid this outcome by using fusion bombs instead of fission bombs, which produce way more blast power per level of radiation and should, therefore, leave behind way less overall radiation as a consequence. This also means a better short- and mid-term prognosis for this planet, as would not have to contend with the heavy fallout associated with fission bombs or "dirty" bombs.

Note: I know I need to add more links and information, but I'm going to save this now and come back to a little later. There will be further edits.

[1]: https://en.wikipedia.org/wiki/Gigantothermy#:~:text=Gigantothermy%20(sometimes%20called%20ectothermic%20homeothermy,virtue%20of%20their%20smaller%20surface [2]: https://en.wikipedia.org/wiki/Square%E2%80%93cube_law#:~:text=The%20square%E2%80%93cube%20law%20can,the%20cube%20of%20the%20multiplier.

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Rats would become even more common than they are now. As omnivores with prolific breeding rates, I would expect them to become the most populous. Capable of surviving on vegetation, and flesh.

Larger mammals would still consume more plants, but rats would effectively form the base of the chain, which would be highly reduced. Since rats can survive and would eat meat at any oppotunity, you'll see a decrease in the number of predators that would take the rats. Cats and rats would probably be locked in a cycle like the Hare and Canadian Lynx. Dogs might get by on the short term with cats, but only the largest of dogs would stand a chance with former farm animals, and the extinction of several farm animal species and dogs in the long term is a very real possibility.

Large animal populations tend to live on tight margins. For example, assuming the American Mastodon had the same survival as Asian Elephants, all prehistoric humans had to do was lower the female yearly survival rate by 2-3% to cause an irreversible death spiral that would plummet the population into extinction.

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Wait, no. I don’t wanna ignore the humans. Every species you just mentioned is basically the list of everything that survives human-encroachment-caused-extinction well (although the canines should have more coyotes and less wolves). Dogs, house cats, and livestock basically survive because humans make them survive. Rats, pigeons, feral dogs, and coyotes eat our waste. Insects are just generally almost impossible to kill (seriously there’s been 5 mass extinctions and 1 involved insects). The biggest challenge is that the 2 new species are reptiles. Reptiles do not do cold well (and they aren’t big enough to survive on size-based heat until they're at least the size of an elephant) and nuclear apocalypse tends to make things cold, so how do you make it warm instead? I would suggest having the humans react to nuclear war with massive habitat encroachment. If Russia’s gonna target Denver then Denver just moves further into the rockies with a wave of anthropogenic destruction. This all exasperates the 6th mass extinction and global warming plunging us into a nice reptile-friendly hothouse.

Now immediately after a mass extinction (especially one this big) the ecology gets to be wonky for a bit and whatever species just happened to survive takes over in sort of similar positions to what they held previously. The P-T extinction had these weirdos called Lystosaurus taking up 95% of fossil beds afterwards. Your real hole is in the herbivore category so it’s a similar position to them. I suspect moderately evolved escaped livestock will do the take-over. Alternatively, some canines, like coyotes, are pretty good at adapting and evolving and can take a quick trip into being mostly an herbivore. Or maybe it’s just rats everywhere, that’s fair and they’ll get huge. Around this phase your Geckos and wild canines (especially wolves, potentially excluding coyotes) are in way too similar of an ecological niche unless they’re in different areas and are going to get into a battle over resources and one will go extinct.

As the world recovers (takes a couple million years) from a mass extinction the animals left evolve rapidly and try to fill in the remaining holes. Explosions follow extinctions. This is a chaotic mess and expect every species left to take a shot at being everything else. Evolve every level of herbivores, carnivores, and scavengers for every environment from every one of the remaining animals. Then they compete for top dog, and generally the same group wins across most categories. The winner probably won’t be something anyone would have saw coming. Putting this together is like living in the Cretaceous Era, picking up an early mammal (it looks like a rat) and imagining every single mammal we have now. It will seem nuts, but on the bright side it’s a great excuse to make things nuts. (Note: This top dog will not be bugs unless you increase oxygen levels, they can’t get bigger)

There’s one last issue I haven’t addressed. The Trihorners and Geckos should not evolve at any point during this process. You haven’t saved any reptiles and big, specialized predators are very bad at surviving mass extinctions. You could save some reptiles (I’d recommend crocodilians since they’re pretty good at surviving extinctions) and let them evolve in the subsequent explosion since, as I mentioned, explosions make anything and everything but then they’d get into the reptile/canine wars again. But otherwise you probably need an artificial way to make/keep them alive. Maybe radiation or bored scientists. Hopefully you’ve got a nice way to make them from the start.

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    $\begingroup$ PS: The two reptiles were genetically engineered, not naturally occurring. They emerged after the nuclear winter had ended $\endgroup$ – DT Cooper Aug 1 at 5:06
  • $\begingroup$ Also, question: Since the Geckos and wolves take up the same niche, what should I do? $\endgroup$ – DT Cooper Aug 1 at 5:17
  • $\begingroup$ @DTCooper Either one of them starves to death from being out-competed or you need to keep them in different habitats. Maybe geckos can outcompete wolves in the plains but they're awful in the mountains so wolves stick to the mountains. Or they each get a continent. Or one of them finds another niche. $\endgroup$ – PoorCorrelation Aug 1 at 5:34
  • $\begingroup$ What other niche could work for the Geckos? $\endgroup$ – DT Cooper Aug 1 at 5:47
  • $\begingroup$ @DTCooper Well they're omnivores already so make them more herbivores (this could also make them the species that's absolutely everywhere). Or split your pack up and you can make them more adaptive omnivores that just eat whatever. $\endgroup$ – PoorCorrelation Aug 1 at 14:53

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