Is a food chain without plants plausible?

Question

Suppose there was a world on which evolution and natural selection took an odd turn and never produced an organism that could be considered a plant by our modern definition, but which still produces viable organisms for that world's ecosystem.

While there theoretically are alternatives to photosynthesis as the basis of a food chain, is it reasonable to believe that life will find a way to create a complete and self-sustaining food chain without plants to support it?

For extra credit, provide a brief depiction of how the food chain would work, including what the foundation organism(s) is and how it provides the necessary energy to the rest of the food chain.

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It occurs to me that oceanic food chains are dependent on phytoplankton, rather than plants. So, for the sake of argument, please restrict your answers to non-oceanic environments.

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To address the duplicate suggestion:

The question What kind of animal... does not solve this problem because that question begins with the assumption that life already exists and includes a means for humans to keep and use an animal as a primary food source. This question challenges the premise that any stable food chain can exist at all with the absence of plants. Additionally, no assumption is made here than the ecosystem would be anything akin to what is found on Earth or that would produce humans or allow them to survive.

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As it appears to be a persistent problem, the following is from the linked site and is the definition of a plant:

any living organism that typically synthesizes its food from inorganic substances, possesses cellulose cell walls, responds slowly and often permanently to a stimulus, lacks specialized sense organs and nervous system, and has no powers of locomotion.

Answer 1

If phytoplankton count as "not plants", and you want a non-oceanic environment, then the answer is quite straightforward: just take the phytoplankton out of the ocean.

You could have a planetary ecosystem based on photosynthetic scum that covers the ground, or algae which look a lot like plants but technically aren't.

You could also have an ecosystem based on chemosynthesis. The most well-known examples of these on Earth are also oceanic, based on around volcanic vents in the seafloor, where the base of the food chain is formed by chemosynthetic bacteria living as symbiotes inside sessile animals. But, there are non-oceanic, non-volcanic chemosynthetic ecosystems that exist in, e.g., cave systems as well.

On worlds with the right kind of atmosphere, you could also have food falling from the sky, rather than being produced on the ground. With a thick, dense atmosphere, the equivalent of phytoplankton could be floating aerostatically, forming photosynthetic clouds. Or, photochemical reactions in the upper atmosphere could produce energy-rich organic molecules that rain down on the surface, like tholins on Saturn's moon Titan, which can then be consumed by chemosynthetic organisms.

Answer 2

Two options, I think. The first is going to be sulfur as detailed in that link about photosynthetic alternatives and, really, much better explained there than I can do.

But, I'd much rather talk about animals using sunlight. And I'm not talking about sea slugs or salamanders (which eat plants and steal their genes). I'm talking about the oriental hornet. These crazy things trap light and generate electricity with it. What do they do with that electricity? Scientists aren't quite sure about that. They still primarily acquire their energy from food.

But, this opens up some intriguing possibilities. Plants rely on chlorophyll for photosynthesis, a process which rearranges CO2 and H2O into sugar O2 and H2O. As far as scientists can tell at this early stage of research, these wasps are bouncing light through specially structured tissues and a pigment called xanthopterin to generate electricity. Chlorophyll essentially does the same thing (one photon in, one electron out). Which means, maybe xanthopterin can take the place of chlorophyll in photosynthesis (which I will now call x-photosynthesis).

Imagine a world populated primarily by animals. Fungus? Sure. But mostly animals. Most of these animals are covered in bright yellow stripes containing xanthopterin.

The animals most heavily invested in xanthopterin only need water, vitamins, and minerals to survive, just like Earth plants. They're synthesizing all their own sugars via x-photosynthesis. They actually respirate very little; they need some CO2 for the beginning stage of photosynthesis (like Earth plants), but, to consume these sugars they basically have to do the same process in reverse (recombining the O2 and C and to re-produce CO2). This means they can re-use the same batch of CO2/C + O2. This is something that Earth plants already do, they just use a lot less O2 than they produce. But, our x-photosynthetic animals would use a lot more O2 than Earth plants, so it would be more closely balanced.

These animals who are all-in on xanthopterin are going to start looking very similar to plants, since they have similar needs: large surface area for sunlight collection (leaves), absorption of vitamins/minerals from dirt, lots and lots of water. Most will probably maintain their mobility, but some might put down permanent fleshy roots.

Some animals, however, would have much less investment in xanthopterin. These animals would prey upon the more strongly x-photosynthetic animals for their energy. Funny thing about our world: no plants means very few herbivores, just the ones that rely on fungus. Which means anything not consuming exclusively sunlight, water, and dirt is a predator.

Answer 3

Hydrothermal Vents are a real version of this

They support a plant-free chemosynthetic food web as illustrated below:

Just bacteria and animals.

More details here

So like the other answer, you just need to stick chemosynthesising bacteria on land, that's your bottom block.

Edit-- As Chris H points out in his comment, microbial weathering could be considered to be an example of this.

Answer 4

Yes. Interestingly, I had to look at a similar problem to this a couple of weeks ago, when I was trying to develop a plausible food supply for a city in a polar desert.

You would need something else to do the same job as plants - that is, take free energy from the world and convert it into chemical energy for everything else to eat. You could come up with a plant-analogue to do this kind of job - fungi are popular, though technically they're using chemical energy already stored elsewhere - as long as you have some energy source.

1. Guano. There are cave systems on earth with ecosystems based entirely on the droppings of bats. The bats leave the cave every night, feast on insects, then return to the caves and, ah...relieve themselves onto the cave floor. Technically this is still ultimately reliant upon photosynthesis, but if you're looking for a means to keep a cave ecosystem going, it's a good option.

2. Bacteria. There are a number of ecosystems on earth which rely upon chemosynthetic bacteria. The classic example is of course the black smokers in the deep ocean of earth, where bacteria feeding on the chemicals released from hot vents form the basis of a food chain. There are other places where similar food chains exist in caves etc.

3. Hydrocarbons. This is a fancy word for chemicals that are composed of carbon and hydrogen, and they're very good at storing energy. Indeed, you're most likely using that energy right now - coal and oil are both hydrocarbons. Hydrocarbons are formed readily in nature, and it's plausible that you could build up a food chain based upon them.

Ultimately, as long as there's some way to convert free energy into chemical energy, you can have life.

Answer 5

Assume a planet similar to Io, but larger, circling a 'Jupiter' that orbits in the habitable zone.

Because the planet is larger, it has a larger volume to surface ratio. Thus is has proportionally more tidal heat generation than it does blackbody radiation into space.

Now the planet also has a heavy CO2 atmosphere with a large greenhouse effect. There is also much hydrogen sulfide in the atmosphere released from volcanic activity. The atmosphere is high pressured and dense, like Venus'.

Io produces a plasma torus that contains many free sulfur and oxygen ions from its atmosphere escaping into space and then being ionized by Jupiter's massive radiation. Our planet does the same, and its larger gravity pulls many of the ions into its upper atmosphere.

In the cooler temperatures of the still-dense upper atmosphere, polyphenylene chains link themselves together with sulfur bonds in a replicable process. These chains develop 'cells' which consist of long strands barely more than a molecule thick which harvest the free floating sulfur and oxygen ions, combining them into sulfur dioxide and using the Gibbs free energy to create or break sulfur links. The long thin strands of PPS float in the heavy atmosphere, rising and sinking with thermals from the planets volcanic surface.

Eventually some strands learn a new trick: they can use the energy from sulfur dioxide emission to dissociate hydrogen sulfide. They then store the hydrogen and trap it in a 'chamber' made of PPS strands, thus providing powerful buoyancy in the dense atmosphere. Life is now able to expand into multi-'cellular' forms and diversify.

Answer 6

One can imagine a world in which there are bird-like predators with large wings covered in solar energy collectors far more efficient than those seen in plants (closer in efficiency to solar panels; 10-20%). These creatures take in plenty of energy, but need nutrients. So, they produce "fruit", highly energy-dense droppings that their prey feeds on. Their prey? Mice-like animals that consume the fruit but, since it has no nutrients, also has to "eat" dirt to gain the elements needed for life other than carbon, hydrogen and oxygen. The birds would mostly breathe in co2 and mostly breathe out o2, converting the sunlight and the carbon into sugars for the fruit. The mice would be the "animals", breathing in oxygen and out co2.

An interesting thought experiment.

Answer 7

First, I would consider the existence of life to be proof enough of self-sustained life (or else everything would have died, making the point moot), So I'm going to basically answer with what the food chain is.

The food chain is basically a group of animals linked by who eats who, with primary producers at the bottom (plants). Primary producer take energy from the world, and store it as energy (in the form of chemicals). Primary consumers (herbivores), eat the primary producers, making the primary energy theirs, and then secondary consumers (carnivores) would eat the primary consumers for THEIR energy. (with something like 20% conversion efficiency. varies from animal to animal. But the rate of energy loss will determine how many animals on the next tier. So unassuming a perfect line with 20% effecency, you would have 100 plants, 20 herbivores, and 4 carnivores)

So basically, life can't survive without a primary producer (on earth, we have plants that convert sunlight, and deep in the sea, bacteria that convert chemicals from deep sea vents). The primary producer doesn't have to be obvious though. If you can have an animal that can efficiently gather what its internal bacteria can convert with Chemosynthesis, than you don't need plants! (this is technically how it works in deep sea, but the energy conversion is so low, that the host animals might as well be plants) The only way around plant like things though is probably chugging lots of nutritious water (fish that literally inhale their food... will either need to keep moving or drift like alge)

Answer 8

The word plant refers to any member of the kingdom Plantae, which are sessile photosynthesising dendriform replicators from the planet Earth. There's no reason to suppose that replicators from other planets would be divided into the same categories, or even that the same names would be used in the scientific community if they are. Thus, if you find replicators elsewhere then the answer is yes, but trivially so.

Aside from that, there's nothing in the laws of physics that require the basis of an ecosystem be sessile photosynthesising dendriform replicators. The only things we can say with certainty are that the basis of the ecosystem will be replicators that depend on the largest source of available energy. On Earth, that's photosynthesis. Given the way that stars and planets are formed it does seem likely that ambient light from a star will be that source on the majority of planets, but that doesn't rule anything else out. Just keep in mind that the likelihood for life is going to be proportional to the overall amount of energy available, so whatever your alternate source is it'll have to be highly available.

Answer 9

An interesting option would be a slowly entrophying world where all plant-like energy storing organisms became extinct and the remaining food chain is slowly dying off.

In such environment all remaining animals are carnivores or scavengers that are constantly consuming each other. There can be fungus and other decomposers that nourish off small remaining of an animal but absolutely no organisms capable of photosynthesis or other means of storing/generating energy.

This means that while animals can survive on hunting each other, but the entire ecosystem will slowly diminish and collapse due to no energy coming in. Kind of a doomsday/apocalyptic scenario.

EDIT: re-reading the question, I can see I'm somewhat off topic. As there was never a plant like species, we can constitute that with a planet that had huge but finite stores of consumable nutrients which kept the animals fed and there was no real need for plant like species while it lasted.

Answer 10

You don't have to have "any organism a human from Earth would consider a plant", and you don't have to have anything that draws its energy from sunlight, but you absolutely must have autotrophic species, in the most general sense: drawing energy from some source other than eating other living things.

If your ecology consists entirely of creatures that get their energy from eating other creatures, then it is a perpetual motion machine, which is impossible.

Answer 11

Suppose there was a world [without] an organism that could be considered a plant by our modern definition

The definition you linked says:

multicellular organisms that typically produce their own food from inorganic matter by the process of photosynthesis and that have more or less rigid cell walls containing cellulose

Let's see what we get when we leave each individual part of the definition away:

• "multicellular" + "rigid cell walls": This seems quite simple. If you take a plant and keep everything but the cellular structure, you get a massive "blob" type of thing (note, an outer shell/hull is still permitted, just no internal boundaries). This blob would be filled by some more or less homogenous soup of bio-chemicals, just like the inside of a cell as we know it; just big instead of tiny.
• "from inorganic matter": This cannot be left away. At some stage you need something in your food chain which starts of with inorganic fuel, or you get an earnest hen-or-egg problem.
• "photosynthesis": Again, easy. Photosynthesis is just a convenient way to convert unlimited external energy into chemical energy. We know of others. For example, those ecosystems in the depths of oceans that live off of hot underwater geysers. You certainly can become creative here. Stuff that lives very close to lava (either on the surface or tunneling very far downwards).

An idea for a heat-based ecosystem:

• Your non-plants tunnel down until it gets so hot that they almost, just almost burn up; they still are able to convert all that heat energy into chemical energy to do the things plant-equivalents do (grow, make seeds, etc.). They grow upwards (by tunneling through and filling up the soil down there) until it gets too cold for them to grow anymore.
• The next step up are herbivores that also tunnel down until they live just above the non-plant area. They feed off the too-cold bits and pieces of the plants. They cannot kill the non-plants as they cannot get as far down as they before being harmed by the heat.
• And so on. Ever more animals live on top of the lower layers, and from then on everything basically works like with us.
• Eventually, the first earth animal stepped into air, and from there on, animal live as we know it started to develop. Of course, everything is still getting its food from tunnels that go ever further down, but since it's a long way down there, everything is basically a carnivore, and only on the lowest levels you find any herbivores.

Answer 12

You could use fungi as the base for your ecosystem, both in the microscopic scale (analogous to bacteria) and the macroscopic one. Those fungi could exploit an inorganic resource (like sulfur, CO2, silicon, etc) using heat as an alternative to solar light, or even in conjunction (for heat, not photosynthesis). This is similar to some, more complex, oceanic ecosystems, where organisms metabolize the inorganic compounds exhausted by volcanic fumes.

Answer 13

Why assume that the biology has to be earth-like? But to stick to that assumption, do we not have animals that feed only on other living animals? And do we not have animals that feed only on other dead animals?