In my story, all the fauna in the planet (basically Earth) has been instantly killed, leaving only flora and fungi. How long will it take before the effects start to show, and how long until all the CO2 disappears, (which would freeze up the planet)?

  • $\begingroup$ Do you count bacteria as part of flora? $\endgroup$
    – Alexander
    Feb 21, 2018 at 1:01
  • $\begingroup$ Does fitoplancton counts as animals? Does bacteria counts as animals? $\endgroup$
    – Sasha
    Feb 21, 2018 at 1:10
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    $\begingroup$ Title and text don't really match. Earth itself will survive for approximately another 5 billion years, until the sun explodes. Plants and other life, on the other hand, are much more fragile. $\endgroup$ Feb 21, 2018 at 1:19
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    $\begingroup$ @JakobLovern The Sun isn't going to "explode". It doesn't have the mass to go nova. $\endgroup$
    – user
    May 15, 2018 at 12:34
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    $\begingroup$ @Michael it's radius is going to grow to include the earth. Doesn't really matter if it explodes or not, Earth is gone. Explode was simply a convenient word to use. $\endgroup$ May 15, 2018 at 13:30

3 Answers 3


Billions of years

The death of the kingdom Animalia, while tragic, isn't anything new. Life managed to muddle along for some three billion years without any animals, and I imagine it would do fine again.

A variety of plants will go extinct immediately, of course, in particular, the currently dominant flowering plants will lose their evolutionary advantage of having evolved alongside the animals that pollinate them. But there are plenty of primitive and otherwise plants that have never in their evolutionary history had any need for animal-driven pollination, such as conifers, cycads, and ferns.

With plenty of plants, there will be plenty of food for the fungus to live on, so life will continue, and thrive. In another hundred million years, some other branch of life will colonize the niches vacated by the now-absent animals, and we will have swimming fish-like foram protists or walking and flying fungus or something.

In any case, life will continue quite happily until the sun bakes all the water off the Earth in some billion years, and that is plenty of time for some fungus civilization to raise up and surpass our intellectual achievements.

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    $\begingroup$ Orks orks orks orks. $\endgroup$
    – Cognisant
    Feb 21, 2018 at 7:04
  • $\begingroup$ Expanding on what @Cognisant says - Orks in the Warhammer 40K Universe, I believe, are actually borne of fungi. There's some interesting magicy stuff that happens, but you can get some ideas from their 'creation story' to improve. Additionally, there are real examples of plant-borne pathogens that take over the host body - perhaps in the event of mass-extinction, some unchecked foliage develop a more aggressive stance on growing... For that, look up "Zombie Ants". It's real, and creepy. $\endgroup$ Feb 21, 2018 at 15:44
  • $\begingroup$ @Cognisantand It is spelled "Orc". $\endgroup$ May 15, 2018 at 12:41
  • $\begingroup$ @DarthVader he means specifically the Orks in the WH40K universe, which are very different to Tolkien's Orcs $\endgroup$ May 15, 2018 at 13:05
  • $\begingroup$ @SydneySleeper Oh ok. $\endgroup$ May 15, 2018 at 13:08

What you're describing may well not happen.

The assumption here is that ALL CO2 in the atmosphere is caused by animal respiration and that's simply not the case. One additional source of CO2 that hasn't been considered is plants, for instance. They produce CO2 when they respire and use the O2 and Carbohydrates they produce through photosynthesis to produce their own life energy. Granted, they generate less than they produce, but it's a factor.

Secondly, I'm going to assume that by 'animals' you're not including the microbial life in the soil that breaks down dead organic matter so that plants can then absorb it through their roots for nutrition. That also respires, producing CO2.

Next, you have volcanoes. They produce a massive amount of CO2 and it's generally believed in the scientific community that the end of the 'snowball earth' era was caused by a fresh outbreak of volcanic activity around the earth.

As such, that's the variable that would need to be accounted for in order to provide more precise estimates as to whether or not the earth would indeed enter another snowball earth epoch. Certainly animal respiration would reduce CO2 production, and getting rid of humans and industrial activity would also reduce it, but you've still got all the CO2 in the oceans to consider (Ocean-Atmosphere exchanges) and the impact of volcanoes to factor in to your equation.

Take a look at this XKCD temperature timeline which shows over time how temperatures have changed, and then compare that against any estimates of animal biomass that you can find in literature, and I'm pretty sure you'll find that the correlation is not particularly tight.

Bottom line is that the affect of removing animals could easily be countered by fluctuations (periodic increases in) volcanic activity before you see any appreciable effects in the environment, in my estimation.

  • $\begingroup$ That's awesome! I've never even thought of volcanos, and I had no idea they were that powerful. I guess I was over thinking my original idea. $\endgroup$ Feb 21, 2018 at 1:13

If all the fauna on earth has been killed (vertebrates and invertebrates), there are no pollinators left. Which would kill off many plants. (That is, make them unable to reproduce, so once they die, there won't be any more of them.) The plants that are wind-pollinated will survive.

There will still be CO2: from fungi, from zooplankton. Plants also produce CO2. The earth would survive indefinitely. New species will evolve to fill up the emptied ecological niches.

  • $\begingroup$ That's a good point about the pollinators; plants have existed since before animals so I'm sure some species could evolve back to not needing insects like bees for their survival, but it would definitely cause quite a disruption in the initial post-animal era. Good Catch. $\endgroup$
    – Tim B II
    Feb 21, 2018 at 1:16
  • $\begingroup$ @Tim B II, to evolve, you need generations of the species gradually changing. If the fauna is killed instantly, there's no chance for the plant species relying on pollinators to evolve, since there won't be even a single new generation. $\endgroup$ Feb 21, 2018 at 1:19
  • $\begingroup$ I agree with that, but first you have the plants that don't require animal based pollination that will thrive due to a lack of competition, then you have those pollination plants that have pistils and stamens (or other equivalent 'organs') close enough to each other that wind might get them through. Naturally, the ones most likely to survive in this instance are those where wind or other alternatives are more likely to support their pollination practice, and hence in time evolution. Long bow to draw I grant you though. $\endgroup$
    – Tim B II
    Feb 21, 2018 at 1:23

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