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Let's say that Australia has been taken over by a hyper-aggressive fungus that grows on any biological life form larger than an ant. The stuff covers the entire continent, has subsumed every form of life on it, and is capable of a primitive form of photosynthesis in order to stay alive when other life runs out.

However, it can't expand outside of Australia, even if someone carved a sample of the stuff out, shipped it to another continent, and let it loose.

Why is this? What biological or chemical mechanism would stop a fungus from reproducing outside of that one landmass?

It evolved to not grow on ants because of the square-cube law; ants don't have much usable mass (volume) relative to the amount of fungus required to kill an ant and utilize its mass (surface area). Something like a human or an elephant has a much greater ratio of volume to surface area, meaning that the fungus gets more "bang for its buck".

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    $\begingroup$ "developed" a primitive form of photosynthesis? Is it sentient? And why is there a minimum size of lifeform it can grown on? (For that matter, there's enormous variation in size of ants in Australia.) $\endgroup$ Aug 19 '21 at 4:31
  • $\begingroup$ @KerrAvon2055 I'll change that to "capable of". Point is, it can survive without something to grow on. It evolved to not grow on ants because of the square-cube law; ants don't have much usable mass (volume) relative to the amount of fungus required to kill an ant and utilize its mass (surface area). $\endgroup$
    – KEY_ABRADE
    Aug 19 '21 at 4:37
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    $\begingroup$ Ants also have formic acid, which is poisonous to these fungi. $\endgroup$ Aug 19 '21 at 15:25
  • $\begingroup$ Because "Australia". $\endgroup$
    – RonJohn
    Aug 20 '21 at 7:19
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    $\begingroup$ ˙spɹɐʍʞɔɐq ƃuᴉlɹᴉʍs sʇǝlᴉoʇ ɹo puɐ ˙˙˙ sǝɯᴉʇ llɐ ʇɐ uʍoǝpᴉsdn ǝq oʇ ƃuᴉpǝǝu ɥʇᴉʍ op oʇ ƃuᴉɥʇǝɯos qoɹd $\endgroup$ Aug 20 '21 at 20:59
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Because it is not a colony of independent microscopic fungi. It is a single massive lifeform that looks like terrestrial fungus. The lifeform has a centralized nervous system which controls and maintains the life functions of the entire organism. When any part of it is separated from the whole, that portion dies for lack of connection to the centralized brain.

When a portion of the organism attaches itself to an ambulatory creature such as an elephant or human, it quickly infiltrates the pain and pleasure nerves, then uses intense sensations to train its host to periodically touch/connect with the main organism mass. This is done with a basic carrot and stick approach. Whenever the host is touching the main mass, it feels orgasmic bliss. Whenever it is not touching the main mass, utter agony.

...and it doesn't like salt water, so growing a tendril through the ocean to the adjacent land masses won't work.

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    $\begingroup$ Honestly, just the first paragraph alone was perfect. No further explanation needed. +1 $\endgroup$
    – Vladimir
    Aug 19 '21 at 8:27
  • $\begingroup$ I like this solution because it also provides for an "oh shit" plot point later. After it does take over all of Australia and people elsewhere begin to feel safe after awhile as they've realized it cannot leave due to being a single organism and adverse to sea water, one day, it produces a single spore. $\endgroup$ Aug 20 '21 at 15:25
  • $\begingroup$ @Shufflepants, ...or it finds its way into one of the undersea conduits or pipelines. $\endgroup$ Aug 20 '21 at 15:35
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    $\begingroup$ So it's not just all the wildlife in Australia that wants to kill you... Australia itself is semi-sentient and wants to kill you? I like it :p $\endgroup$
    – Patrice
    Aug 20 '21 at 15:57
  • $\begingroup$ "Because it is not a colony of independent microscopic fungi. It is a single massive lifeform that looks like terrestrial fungus.". That's actually a real thing. But I forgot what it is called. Youtuber Anton Petrov has a video about it, if you want to find it. $\endgroup$
    – wha7ever
    Aug 20 '21 at 17:08
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By choosing all of Australia you've cut off a couple of options, those being too hot/cold/wet/dry as Australia as a whole has a vary varied environment. If you limit it to the regions where hot and dry is the primary environment, and slow it down in wetter places, that would do a great deal to limit spread into other environments and entirely prevent it crossing the sea unaided.

This links into its ability to photosynthesise and its current large size, that being it has reached a point of having a critical minimum surface area. It's not that it can't expand outside Australia, it's just that the only place it could survive would be somewhere like the Empty Quarter and you'd need to move several truckloads of it there. The little samples that scientists tend to take could never survive.

You could potentially add to that the weakness in the ozone layer over the southern pole leads to higher than average UV levels in the area upon which it depends, meaning the dryer regions of Australia are the only place it could maintain its core, with tendrils into wetter areas dependent on its mycorrhizal network but with less dominance.

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  • $\begingroup$ I love the idea, maybe implicit in this answer, that the giant fungus sort of "evolved in place." When it first arose, and there wasn't so much of it, it must've had the ability to survive at small scales. Once it became huge, it could lose that ability at little cost—and it did. There may be a parallel with the evolution of passenger pigeons, which needed huge breeding populations to persist. Their ancestors must've been able to persist at smaller populations, like other doves. $\endgroup$
    – Vectornaut
    Aug 19 '21 at 20:47
  • $\begingroup$ If the giant fungus did evolve from ancestors that could survive at small scales, it might have the potential to regain that ability—especially if scientists unintentionally select for it by keeping samples in small containers. $\endgroup$
    – Vectornaut
    Aug 19 '21 at 21:02
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Could it be related to Australia's geology? Australia has the world's oldest rocks, and the fungus may be dependent on the particular zircon deposits found in the Kilbara: https://www.britannica.com/place/Australia/Geologic-history

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    $\begingroup$ This sounds extremely unlikely to me, as an Australian who knows some geology. The western half of Australia is indeed extremely old, but the eastern half is not. Sydney is Tertiary sandstone and shale, Melbourne is mostly basalt of similar age, Brisbane is… actually, I’m not sure, but it isn’t likely to be any older than Silurian. If it were dependent on Pilbara zircon, it wouldn’t get much outside WA. $\endgroup$
    – bradrn
    Aug 20 '21 at 4:33
  • $\begingroup$ You are right, of course. OP said that it covered all of Australia. $\endgroup$
    – AlDante
    Aug 22 '21 at 6:09
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The fungus evolved into such a potent form by consuming Australium from the soil:

Australium is a mysterious metal element that can adapt and transform itself into different states and forms, with invigorating health effects, therefore making it extremely valuable and sought-after. (...) Prolonged exposure to Australium increases both intelligence and virility.

Now the fungus needs Australium to live, and there is none to be found outside Australia.

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    $\begingroup$ It might be worth pointing out that Australium is a fictional substance from the Team Fortress video game series. $\endgroup$
    – nick012000
    Aug 20 '21 at 9:53
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It is not that it cannot survive outside Australia, it is that humanity can easily kill any new colonies.

It started in Australia and grew in a remote area for years before anybody discovered it. And even then it wasn't taken seriously at first. By the time it was, it was too late.

Sure, people could kill it off in one area, but it simply grew more in other areas. And fighting it is a very risky job. Make one mistake and it is time for another mercy killing.

In the end Australia was abandoned.

Fortunately salt water seems to stop it... so far.

Still, infected birds can land on nearby islands sometimes. They are met with EXTREME violence and never get a chance to establish themselves. Every surviving country in the world supports this with personnel, equipment and money.

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There exists a soil microbe that parasitizes and kills the fungus, but it isn't present in Australia. It also parasitizes some other organism as part of its normal life cycle, and this other organism isn't present in Australia or can't survive now that the fungus has completely taken over, so the microbe can't reproduce in Australia.

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what you are asking for is called endemic's, endemic plants, endemic animals.

https://en.wikipedia.org/wiki/Endemism

idk, probably not the best wiki article I have seen but has some examples.

endemism includes a variety of species with different degrees of selectivity of their environment, with maybe a lesser percentage of those are strictly dependant on specific conditions in the place they grow, which is hard to recreate elsewhere, maybe even nearly impossible atm.

So can it be - yes, there are examples of such in reality, of selectivity to a place.

But at the same time making it to be essentially a carnivore which can thrive on almost anything - maybe a bit tricky combination,imho - freedom of options on one side as a carnivore, but at the same time be constrained as a plant, but not to some small area of a specific ecosystem but to quite a big one like Australia which includes plenty of different stuff.

But is it still possible - maybe, there are examples in nature when subtle changes, like water temperature can have a drastic difference in the success of reproductive cycles, for some Jellyfishes as an example.

imho, a good chance to create such dependence is to constrain, pinch/constrain reproduction cycle in more than one stage and it would mean that there won't be a single magic factor X that explains everything, but rather a bunch of factors that create such behavior.

maybe a good layman explanation can be a specific combination of specific microbiological strains of bacterias and alike which can be inhibitors or a vital part of the procreation process. But that is just an option. if there are solutions (in math sense of the word) then it is probably more than one solution.

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Since fungus evolved not on Earth in environment with much lower salt level in water, it's cells have very low concentration of salt. So, this fungus cannot survive in salted water because osmisis.

Fungus cells has weak membranes, since it can dissolve and consume nearly anything. When fungus cells enters salted water, they start consume too much salt from seawater, because salt concentration is much lower in them. And when salt concentration is high in fungus cells, it causes poisoning or methabolism issues, that kills fungus.

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    $\begingroup$ Well, that doesn't solve the problem of "how can't it survive if someone physically takes the stuff out of Australia?" $\endgroup$
    – KEY_ABRADE
    Aug 19 '21 at 20:36
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The fungus now (and perhaps has always) is dependent on heat and lots of ultraviolet light. (Did it come from a planet where more ultraviolet light reaches the surface?)

While there are lots of places in the world that are plenty hot enough, all of them except Australia are protected by the ultraviolet-light-blocking ozone layer.

While there is a huge area near the south pole where the Antarctic ozone hole (and a smaller area near the north pole under the smaller Arctic ozone hole) lets in enough ultraviolet light to keep the fungus alive, all of that land and sea area (except Australia) is too cold for the fungus to grow.

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