Discussion of the distribution and abundance of living organisms and how these properties are affected by interactions between the organisms and their environment
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Question: What type of creature, large or small, could sustain it's population by feeding only on it's dead plus air and water? Living creature, but not plant life.
I'm researching for my science fiction novel, hence the silly sounding question.
If you specifically exclude plants, then what you describe is physically impossible and even the answer "detritivores" (although cunning) is wrong.
The reason is simple: living beings can only extract a fraction of the energy contained in their food, which explains that excrements still contain energy (otherwise dungbeetle would starve to death ).
This is the reason why one kilogram of grass will never become one kilogram of cow, which itself will not allow to produce one kilogram of John Smith. This is why food chains have the shape of a pyramid when you consider biomass (or energy): it requires a lot of rabbits to sustain a single fox.
This rule is also valid for detritivores/scavengers. The energy contained in the body of a dead hyenna is vastly insufficient to allow another hyenna that would feed from it to grow to the age of reproduction and breed.
However, plants do exactly what you describe, and deciduous trees continuously recycle their own dead leaves, but they do it for nutrients, not energy (theirs come from light). A plant population could (in theory) live forever by recycling its own organic matter and CO2, provided it has water and light.
To get out of your conundrum you would have to imagine an animal that is somehow able to extract energy from the air (which contains a heck of a lot of nitrogen, for example) but this is really SciFi...
Note that even assuming your creatures could extract 100% of the energy contained in their deads, it would still not be enough since digestion itself requires energy to take place.
air might contain a lot of Nitrogen, but not much energy. Organisms that can use the nitrogen in the air need in fact to put a lot of energy of their own to do this.
Addressing the original question, what you described is impossible, even for plants: you simply don't have enough energy. By feeding exclusively on its dead, the organism would need to have a 100% assimilation efficiency, which is IMPOSSIBLE. for most organisms, this efficiency is more toward 10%(ex: one pound of grass creates 0.1 pounds of cow which creates 0.01 pounds of John Smith). The efficiency is so small from a number of reasons: one was mentioned by DrD - digestion requires energy, for making enzymes etc. However the most important reason is a key concept of biochemistry: no chemical reaction has 100% efficiency. Some of the energy is always released as heat. For example, an average car only converts about 15% of the energy in gasoline to the moving of the car, the rest is released as heat. By comparison, cellular respiration converts about 38% of the energy in a glucose molecule to ATP(the chemical compound usable by living cells as energy). Thus, it would be absolutely impossible for ANY organism to just eat its dead, because every organism constantly loses energy to the outside world as heat, energy that eventually will run out. That is why every population needs an outside energy source.
OK, now what do organisms do for this outside energy source. There are three strategies: photosynthetic organisms(plants, algae, some bacteria) can harvest energy from the sun. Chemosynthetic organisms(some bacteria) use the energy in some chemical compounds in their surrounding which they oxidize in order to extract energy from them. Heterotrophic organisms(all animals, fungi, protozoans, some bacteria) practically eat food, in one way or another. But this food has to be from other sources, not (only) their siblings, from the chemical points of view described above.
Thus, your suggestion is impossible from all points of view. Energy needs to come from somewhere.
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
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