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I was wondering what would happen to life, when an infamous epidemic plague would be able to infect and kill any imaginable plant, be it on land or in the waters.

Would the lack of oxygen eventually kill Earth's fauna? Or would a gradually fading out food production be the bottleneck?

(Edited as per comments, thank you!)

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  • $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site Herb. Right now your question looks really broad, as you post multiple questions into one post. Please try to stick with one at a time. And discussing how humans would react is likely opinion-based. I'd recommend to remove everything after the second question mark and keep it for later. After some feedback here you can ask new questions incorporating the feedback. Have fun! $\endgroup$ – Sec SE - clear Monica's name May 8 '17 at 11:40
  • $\begingroup$ I was a bit hasty and missed the hard science tag, I only just saw it after I wrote an answer. $\endgroup$ – Mormacil May 8 '17 at 11:46
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    $\begingroup$ Please note that this is mostly esoteric. Even if you really had a plague that infects and kills ALL plants (which is a concept defined by humans), and if humans wouldn't or couldn't react, and if the seeds including databases were destroyed and if this happened very quickly on land everywhere on mountains and also on the ocean floor within a very short period of time (it sounds like that) - you get a scenario that is not possible without some "higher power" behind it. I think what will happen to the fauna is common sense, the details don't matter under such unrealistic circumstances. $\endgroup$ – Raditz_35 May 8 '17 at 14:43
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Lack of oxygen won't be a concern, we got enough for millions of years. Sure it will run out eventually but not for a long time. So yes eventually it would kill Earth's fauna but not before evolution could make a dent in it.

Food supply will indeed be a much larger issue. Within days to weeks the majority of herbivores will start to die from malnurishment. Remember that it's generally the carnivores that do well without eating regularly. Plant matter is much harder to digest and generally requires a much larger portion of the day directly dedicated to eating.

So with a large portion of the herbivores dying there will be a surge in available food for carnivores. But this will quickly rot and then starvation will set in for our predators as well.

Within months the majority of species will have simply died off. The rest will likely quickly follow. Seas will become cesspools of disease and toxins. Humanity will be among those that die out. We can't survive on a diet of pure meat. Long term storage of food will remain for a long time if limited to a small group of consumers.

There are a couple ecosystems that don't use photosynthesis to supply all their energy. These get their energy from heat-loving bacteria that live on underwater volcanoes and get their energy there. These ecosystems are very deep in the ocean and include worms and blind crabs.

What humanity will do, there is no objective answer here. Will we die out? I see it likely if we can't regrow any plants ever.

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    $\begingroup$ and generally requires a much larger portion of the day directly dedicated to eating. Very true; my mom owns horses. Horses--as an animal--are designed to eat 23 hours a day (cows are probably similar). If their stomach ever gets full, they empty it. Exercise for the reader: how do you keep a diabetic horse from eating too much? $\endgroup$ – Draco18s May 8 '17 at 13:41
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Most of the oxygen is not produced by plants, but by ocean algae. What is produced by the rainforest is used locally too — and since all the animals there will die, it comes out even.

You didn’t indicate whether seeds and spores were affected, so I can’t contemplate a timeline of what happens next.

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    $\begingroup$ Aren't at least several algae species considered plants and thus dead? $\endgroup$ – Mormacil May 8 '17 at 11:57
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    $\begingroup$ There is no universally agreed upon definition. I use it to mean “not plants” and don’t consider being single-celled to be an overriding criteria. As for the exact quantitative value of the stastic in light of modern understanding of cell types, I don’t know. Pretty sure the vast majority of mass is cells that are not in the plant kingdom. $\endgroup$ – JDługosz May 8 '17 at 12:03
  • $\begingroup$ Now if his disease killed all Viridiplantae that would definitely cover “green algae”. I don’t know what portion of the oxygen producing life consists of that. $\endgroup$ – JDługosz May 8 '17 at 12:07
  • $\begingroup$ @Mormacil There are enough varieties of algae from enough different domains of life, that I imagine you would have to extinguish all of them to keep the remaining types from quickly multiplying to replace what was lost. $\endgroup$ – kingledion May 8 '17 at 18:49
  • $\begingroup$ Sure but I feel the OP wants everything but fauna to die and likely considers algae just plants, my guess anyway. $\endgroup$ – Mormacil May 8 '17 at 18:52
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Would the lack of oxygen eventually kill Earth's fauna? Or would a gradually fading out food production be the bottleneck?

The latter, oxygen would last a lot longer than food.

Your earliest ones would probably be herbivore mammals on land. These need to eat constantly just to survive. Some reptiles will last a long time, perhaps years and some insects will be among the last of all to die off because they feed on animals.

Sea life is an unknown, most fish won't last long, because a huge amount of fish live on algae, so eventually the food chain will break down in the sea. With scavengers like crabs and deep sea animals feasting for decades.

Life will still exist deep under the sea in places with ecosystems that are based on bacteria that process vent chemicals etc,. Unless you class those bacteria as plants.

All in all it wouldn't destroy life totally, and even some of the eventual extinctions might take 100 years or more to come about.

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As has been pointed out, food will give out long before the oxygen does.

However, there are a few microbes that get their energy from the chemicals released by the undersea vents. The ecosystems that revolve around them may survive. However, I am not postitive about that they can survive without the "snow" of organic particles that rain down from the higher layers of the ocean.

There is also a type of bacteria that actually gets its energy from radiation. So far, these are only found underground and are not part of any ecosystems that I am aware of.

So, almost all life on the planet would die but that has happened a number of times in the past. The survivors evolved, spread and multiplied to give us what we know. It is likely to happen again. However, if an intelligent species develops again, it is likely that they will be in a situation where the Sun would be near exhaustion.

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