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Could a world with no animals whatsoever (not even insects, no humans, etc) still have plant life? These plants do not have to include all the plants in our world, or even any of the same plants. I can think of these problems that would need to be addressed:

  • Respiration. Plants in our world consume carbon dioxide and produce oxygen. Eventually, the CO2 will run out, unless the world has some other method of recycling the atmosphere (other plants, maybe?)

  • Pollenation. Perhaps plants can exist that piggyback on the world's water cycle for pollenation? Maybe they place their pollen in fresh water and other plants extract the pollen-rich water via capillary action.

  • Fertilization. I got nothing (lol). Come to think of it, where would animals get the nutrients they excrete if not from the plants they eat in the first place?

Preferably, the world should be as Earth-like as possible.

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    $\begingroup$ I came across this story (self-contained garden that's needed basically no outside influence aside from the sun for ~50 years) today that details a very small system that thrives without the influence of animals. Not sure how to expand this to a planetary scale though. $\endgroup$ – SnoringFrog Jul 19 '16 at 16:57
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    $\begingroup$ Plants already respire. Photosynthesis produces sugar (simple energy storage), not metabolic energy. Plants still need to break down the sugars like animals, fungi, etc, do. $\endgroup$ – jaxad0127 Jul 19 '16 at 18:53
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    $\begingroup$ You are aware that Algae is at least 750 million years old while animals are only 580 million years old, right? $\endgroup$ – corsiKa Jul 19 '16 at 20:55
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    $\begingroup$ Are you considering life evolving from scratch on a world that never happens to have animals, or are you considering removing all animal life at some point in history? $\endgroup$ – trichoplax Jul 20 '16 at 11:13
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    $\begingroup$ Define more precisely what you mean by "plant" and "animal". The answer is going to come down to a discussion of those definitions. In particular, make it clear if you do or do not accept the existence of living things that are neither plant nor animal. $\endgroup$ – Mike Scott Jul 20 '16 at 13:20
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If you allow the third kingdom (after plants and animals), namely fungi (mushrooms and their many, many cousins), then yes.

Fungi will take care of the oxygen surplus, using it up and releasing CO2 for the plants to breathe.

For pollenation: There are plants that use the wind for this, and other plants reproduce through non-sexual reproduction - strawberry offshoots, old willow trees breaking apart and new ones growing from the parts, root networks sprouting new aboveground plant parts, potatos and onions creating tubers or child-bulbs underground...

Fertilization: See fungi. Plants die, Fungi (and bacteria) break apart the plant matter, rinse and repeat.

Animals actually take care of only a relatively small part of the "Plant matter to fertilizer and CO2" conversion, with fungi already doing the bulk of the work. Without our contribution to pollination and spread of seeds etc., we could actually consider us animals superfluous for the ecology as a whole ;)

Edit: Found a book (trilogy) I was thinking of when writing this answer - "Of man and manta", from Piers Anthony. Sci-Fi, features a planet with no animal life where fungi evolved into mobile, and IIRC sapient, beings.

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  • $\begingroup$ I was going to say wind and fire. But a nice addition of Fungi. $\endgroup$ – Jammin4CO Jul 19 '16 at 20:17
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Plants existed before animals ever evolved, and if all animals were to disappear, plants would continue to exist a million years from now. Just mostly different species.

Respiration. Plants produce both CO2 and O2. Without animals, there would be a higher concentration of O2. If you wanted, you could easily evolve plants that have internal processes that take in environmental oxygen at a higher rate than they use CO2, and perhaps use it to increase mobility and growth. Otherwise, it's likely that bacteria would take up any slack, assuming you just didn't re-evolve animals (Oxygen is a great energy source, if it doesn't kill you first).

Pollination. Insects and other animals are only one mechanism by which plants are pollinated. Wind dispersal is an older method, and one that most pine trees use effectively. The plants that cause most spring allergies rely on wind distribution of their pollen.

Soil fertility. In a compost pile, worms are famous for doing the work of breaking waste vegetation into new soil. But this happens even without animals. Instead, soil bacteria and fungus do the job. I wouldn't even suggest that the process needs to slow down.

Seed dispersal. There are lots of plants that rely on animals carrying their seeds, either as food (eaten or stored) or by burrs attached to fur, feathers, or skin. But there are many, many plants that rely on other strategies. Fluffy feathery seeds floating in the wind are common and maple trees with their helicopter seeds are two examples. Seeds in flood areas can use flooding both as a means of being carried away from their parent, and as a signal that it's time to germinate. More unusually, there are even plants whose seed pods explode (video), propelling the seeds many feet away.

Competition and predation. With or without animals, plants need to defend against predation and competition. Dodder and mistletoe are both parasitic plants that get their energy by tapping into the sap of host plants. They don't usually kill the host, but they can certainly weaken it. Strangler figs are a species that germinate on a host tree, then as they grow, they wrap around and choke the host to death. Oak leaves (and others) contain tannins that poison the soil at the tree base, making it harder for other plants to grow there. It's a violent world out there.

If you also eliminated fungi, many, many plants would struggle because they rely on fungus at their roots to increase their nutrient uptake. Without that, plants would probably be limited in size, and many existing species would die.

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  • $\begingroup$ One more for your list: Seed Dispersal. I'm pretty sure there's at least one kind of berry whose seeds are not viable until they've made a trip through a bird's gut. $\endgroup$ – Solomon Slow Jul 19 '16 at 22:09
  • $\begingroup$ @jameslarge, I'd originally considered it and decided not to write about it. But since I'm not the only one thinking about it, I added a section. $\endgroup$ – Karen Jul 20 '16 at 13:04
  • $\begingroup$ Remember that earthworms in North America were wiped out by the glaciers in the Ice Age. When Europeans came, there were no earthworms in most areas that had been glaciated. The settlers (re?)introduced them in to the environment. $\endgroup$ – Jon Custer Jul 20 '16 at 13:24
  • $\begingroup$ "Without animals, there would be a higher concentration of O2" - Not much. 78% of the atmosphere is nitrogen, 21% is oxygen. Carbon dioxide is 0.04% of the atmosphere. If plants consumed it all, oxygen would rise by that much, ie, practically nothing. $\endgroup$ – Luís Henrique Jul 20 '16 at 13:27
  • $\begingroup$ Trees evolved a geological age (the Carboniferous) before fungi evolved a means to digest dead wood. Back then dead wood built up to depths of hundreds of meters with more trees growing on top. These layers fossilised into thick coal strata.Today's peat bogs are something similar on a smaller scale. $\endgroup$ – nigel222 Jan 30 '17 at 17:44
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Many plant species shed branches and leaves as well as many which shrivel and whither away seasonally. Wild fires happen frequently, turning plant matter back into CO2. There are diseases which kill plants. if your planet had immortal plants it might be in trouble. As @Syndic said fungi do a great job of converting plants into CO2.

The simple fact is that the mass of CO2 in the atmosphere far exceeds that the Earth's plant biomass. You can expect this will be the same on your planet. Although its atmosphere would have a higher oxygen composition. That helps wild fires too.

Also, plants release CO2 at night when they're photosynthesizing. This helps maintain the balance. After all, night lasts almost as long as day.

In essence, the CO2 won't be used up. There are enough mechanisms to convert plant matter back into CO2.

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    $\begingroup$ +1 especailly for the wildfires - seems like it's easy to forget that large-sacle combustion is what's led to our current levels of CO2 in the first place! $\endgroup$ – fluffy Jul 19 '16 at 18:13
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The short answer is "yes", since plants came into existence some 3 billion years ago and were quite content until the Cambrian Explosion some 550MY ago.

Longer answer is that plants will most likely continue to inhabit the oceans for the vast majority of the time until they produce enough oxygen to create an ozone screen and can move onto land. Plants can propagate themselves without animals or insects, the simplest solution being to allow pollen to drift into the ocean or the wind in enough quantities that the corresponding pistels will be fertilized. In your world, there might be enough pollen being released to seriously cloud the waters and fill the air, since that is the only way for plants to carry out sexual reproduction.

Plants will also develop interesting ways to project seeds, ranging from dispersion through the air and water to developing "catapults" or other ways to throw seeds to needing fire to germinate (much like trees in the boreal forest). Other methods of reproduction seen in modern plants will also be developed, such as using rhizomes. Other ways to propagate may develop depending on the conditions of your world.

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    $\begingroup$ Interestingly, though, the first evidence of land plants post-dates the Cambrian. I have no idea whether there's a connection between the existence of animals and the existence of land plants: it's 20 million years from the first known land plant to the first known land animal, so if they really were the first that again indicates no dependence. You have an absolute slam-dunk for "plants" as asked. But since the questioner says "perhaps piggy-back on the water cycle for pollenation", they don't seem to have aquatic plants uppermost in their mind ;-) $\endgroup$ – Steve Jessop Jul 20 '16 at 1:25
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Without kingdom animalia you would still have kingdom fungi as well as domains prokaryota and archeae.

These are all perfectly capable of respiration (in fact plants themselves can do it too) and would expand to fill in the new niche of oxygen consumption. Fertilization is already done by microorganisms such as nitrogen-fixing bacteria so no problem there either.

Pollination will be an issue. Those plants that are pollinated by animals would likely go extinct. The other plants which are pollinated by wind or water will take over their ecological roles.

If animals were to vanish overnight, many ecosystems would undergo crises, but ultimately a rich flora (different though it may be from what we are accustomed to today) would persist on the planet.

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Pop-culture bad science reporting says Humans and Bananas have about 50% shared genes. Not easily verifiable but some similarities must exist.

Photosynthesis evolved about 3.5 Billion years ago.

Multicellular organisms only evolved about 800 million years ago.

About 450 Million years ago land plants appeared.

230 MYA dinosaurs.

So you've got huge opportunities for evolutionary branches.

Kingdom Plantae and the Evolutionary History of Plants would be worth investigating.

Eukaryota

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Could plants survive without animals? Sure. Plants can respire, keeping oxygen and CO2 levels stable. Fertilization is largely based off of nitrogen fixation, which certain plants (legumes) can do. While many modern plants did evolve to utilize animals for pollination there are countless other solutions (airborne spores or seeds being the most obvious).

However, evolution being what it is, there is a pretty good chance that in the absence of animals keeping their population down, plants would evolve to kill each other in order to acquire more resources, and many would learn to eat other plants. Many of these would probably lose the ability to photosynthesize and become something similar to fungi in our world.

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No one has addressed Fertilization yet; this is the topic I shall address.

Fertilizer is composed of vitamins, minerals, and nitrogen. Vitamins can be produced by plants and minerals aren't ever destroyed, so those two aren't a problem.

Manure, on the other hand, is composed of mostly nitrogen. Nitrogen is used to produce amino acids, which are used to produce protein - and you can't live without protein.

Most plants can't produce Nitrogen, but certain legumes (like beans and stuff) can "fix" (fancy word for produce from) nitrogen from the air (remember, the air is about 78% nitrogen). Lightning and certain symbiotic bacteria can as well.

And remember - nitrogen is never destroyed - it has to either decompose into the soil or into the air.

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