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The chicxulub asteroid impact caused climate change, and the climate change caused a mass extinction. But many land-based animal groups survived, crocodillians, lizards, snakes, turtles, frogs, salamanders, birds, and mammals, including primate ancestors.

How would a radiation-based mass extinction event affect current animal groups? Specifically, what would be the apex predators and/or herbivorous "apex species" (term for "animals/organisms with no [significant] natural predators" that encompasses herbivores) that survive?

Assumptions, limitations, details, etc:

  1. The scenario is akin to nuclear world war 3, all sides throwing all the nuke they have, etc.

  2. Assume exactly enough radiation is released in to the atmosphere to kill all humans through radiation related effects, but no more. Higher radiation concentrations are allowed in directly attacked areas, but even the most remote human populations are still killed by absolute minimum lethal doses.

  3. Assume nuclear winter is negligible. This is about radiation-induced, not climate-change-induced, extinction.

  4. Sea life and plant life can be disregarded (unless it directly affects a population of land-based animals that would otherwise survive), stick with land-based animals.

To summarize and reword the main question, for clarity: At this level of global radiation, many species would die off due to radiation poisoning many more would survive the radiation itself but their prey would not, so they would also die off. Food chains/webs would be massively disrupted. Which apex species(singular) or apex species(plural) would end up at the "top" of whatever food chains/webs were left after things stabilized?(because they can both survive the radiation, as well as have a food source that also survives the radiation)

EDIT: I used the term "Stabilized" to mean just that the "apex species" can live long enough to reproduce for 1-3 generations, depending on the length of their reproductive cycles. And the radiation itself is no longer having any significant effect on food chains and food webs. I'm not talking about geological or evolutionary time-scales.

EDIT 2: The "spirit" of the question is meant to be about animals with higher radiation tolerances than humans, either due to biological differences or (as pointed out in an answer) lifestyle differences. Not about just how difficult it might be to wipe out every last human on the planet with radiation alone (after all, that wouldn't leave anyone left for the story...). So the radiation levels involved should be considered to be lethal to unprotected human populations. Not necessarily those that made it to shelters that don't suffer direct nuclear strikes.

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    $\begingroup$ The total yield of all nuclear warheads that exist today is only a small fraction of the energy released by the chixulub impactor, and the radiation released by these detonations is likely to be negligible once distributed globally. You'd see an uptick in cancers for a few generations, but that's about it. $\endgroup$ – Arkenstein XII Jul 25 at 21:46
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    $\begingroup$ What general timescale of stability are you looking for? In the short term I'd expect small / quick / versatile carnivores with relatively short generations to thrive - eg, foxes. If you're thinking more on evolutionary timescales, they'll likely be replaced by larger predators once the latter have time to find their niche and generation timescales / bottlenecking become less of an issue - eg, wolves $\endgroup$ – Punintended Jul 25 at 21:46
  • $\begingroup$ @ArkensteinXII I understand that much of the causal scenario I vaguely described is not going to be an accurate representation of what would actually happen, hence the word "assume" in item 2 and many other items. The question is about the results if the radiation was there. $\endgroup$ – Dalila Jul 25 at 21:47
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    $\begingroup$ You're going to need cobalt salted bombs to have any chance of seriously disrupting the ecosystem through radiation. Humans like to think we can destroy the planet, but our ability to do actual real long-term damage is much smaller. $\endgroup$ – stix Jul 25 at 21:48
  • $\begingroup$ @Punintended I've edited the question to clarify that, but I'm talking more short term. For your example of foxes, I'm more interested in whether or not a fox can survive the radiation, and have enough prey that survives as well, than I am in what evolves from those foxes in a few thousand years. $\endgroup$ – Dalila Jul 25 at 21:54
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The capability to survive the event depends, in part, on the species capacity to heal its own genetic and cellular structure. Higher-order mammals that evolve quickly have the least ability to repair this kind of damage, since its why we evolve so quickly compared to creatures like crocodiles and cockroaches. Mammals have changed immensely while other species are nearly identical to their ancestors from eons ago.

From the graph below, 10 Gray is the radiation level that will kill mammalian cells. And some bacteria are susceptible at ~40 Gray.

(1 Gy (gray) == 100 rad)

enter image description here

And from this table, we can see the relationship between mammals and a few other species. enter image description here

Since you are specifying worldwide extinction of humans by lethal levels of radiation, most surface-dwelling mammals and birds are dead too but subsurface dwelling mammals like shrews and voles probably survive.

If the war started in the deep winter, then maybe hibernating bears survive when they leave their dens in the spring.

Lots of reptiles most likely survive, especially Tortoises, crocodiles, and alligators.

If the ocean life cycle survives then while their numbers would be attenuated I would think that Deep-sea monsters like giant squids and the big sharks would survive since the radiation will be attenuated by the depth of water. Same for deepwater fish. Shallow water fishies might succumb to radiation though.

On land, Croco-gators would be the most likely apex predator and possibly bears. I can't think of an apex-herbivore that would survive since the only one I can think of is the Elephant and I am pretty sure they are dead.

In the air, I think only bugs remain. Maybe bats since they are rodents and they have a slightly higher tolerance for radiation compared to humans.

In the sea, I think whales, sharks, and giant squids are alive.

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    $\begingroup$ +1. I very much like the idea of bears waking up from their hibernation and being like, "Hello? Where is everyone?" :P $\endgroup$ – Arkenstein XII Jul 25 at 22:47
  • $\begingroup$ Excellent points about hibernation and subsurface dwelling. When I asked the question I was only considering it from a perspective strictly of radiation resistance alone, I hadn't considered how big of a factor life styles be. About herbivores, some might group hippos and rhinos in with Elephants in Africa, and I'd put Bison in that category from North America too. A case could be made for Gorillas to be included as well. I'm not sure about wildlife on other continents. But I'm more interested in knowing what the biggest things are left, rather than current apex things that aren't. $\endgroup$ – Dalila Jul 26 at 13:22
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    $\begingroup$ I like the bear idea too, but if they do come out of hibernation into the aftermath of a global nuclear exchange, there's not going to be much for them to eat. Polar bears might do ok, actually, but I wouldn't want to be a grizzly. $\endgroup$ – Morris The Cat Jul 26 at 14:14
  • $\begingroup$ @Morris The Cat, Bears are om-nom-nom-nivors. I figured there'd be enough vegetation and bugs for some to survive. But, I agree that Polar bears might stand a better chance given the proximity to the ocean $\endgroup$ – EDL Jul 26 at 18:04
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Framing Challenge

You're getting a lot of pushback over the specification on radiation levels, but realistically (and most in what I think is the real spirit of your question), it's not actually radiation that's going to kill all the humans, it's starvation.

From a 2007 study on widespread global nuclear war:

"A global average surface cooling of −7 °C to −8 °C persists for years, and after a decade the cooling is still −4 °C (Fig. 2). Considering that the global average cooling at the depth of the last ice age 18,000 yr ago was about −5 °C, this would be a climate change unprecedented in speed and amplitude in the history of the human race. The temperature changes are largest over land … Cooling of more than −20 °C occurs over large areas of North America and of more than −30 °C over much of Eurasia, including all agricultural regions."

From an even more recent study:

"... the coldest average surface temperatures in the last 1000 years. We calculate summer enhancements in UV indices of 30–80% over Mid-Latitudes, suggesting widespread damage to human health, agriculture, and terrestrial and aquatic ecosystems. Killing frosts would reduce growing seasons by 10–40 days per year for 5 years."

So I think it's not so much a question of radiation tolerance (although that's certainly relevant), but also which species are most likely to be able to persist through the disruption of the food supply. This is what will get rid of all your large terrestrial herbivores for starters, and likewise anything that relies exclusively on them for food.

Even with that, you're going to have SOME small localized pockets of humanity that make it through, prepper communities in Idaho perhaps, or the North Koreans who have been preparing to get nuked for the last seventy years, but that's not really germane to your question anyway.

All of that said, I agree with @EDL's answer that crocodilians and sharks are going to be the most likely winners there not only for the reasons he specified, but also because they're able and willing to eat absolutely anything, and have a metabolism well suited to surviving prolonged famine. Certainly they've survived similar extinction events several times in the past.

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