Questions on life without (most) animals

[Booper]

One of my favorite things to worldbuild are enviroments. I've got one that's a modified earth, one that has either lost almost all of its animals, or never evolved them to begin with. All other life is still around; plants, fungi, bacteria, plant-like animals such corals and tubeworms, protists, etc. And maybe a small handful of insects/bugs/stuff like that.

I'm wondering how a world like this could function. Animals are so important to earth's ecosystem that I'm not sure how to remove them without decimating everything else. Obviously there would be mass extinction without animals, but how can non-animal life keep going? Where does the carbon dioxide plants need to grow come from? How are plants kept from growing out of control and choking each other without herbivores eating them? Is pollination solely wind/water based or are there other options? Where do coral's ammonium come from if not fish?

Everytime I think of an answer to one question, two more pop up! So I've come here to ask for suggestions. How are we gonna keep this post-animals world going??? And how would life be different if animals never evolved in the first place???

[Mechalich]

One of my favorite things to worldbuild are enviroments. I've got one that's a modified earth, one that has either lost almost all of its animals, or never evolved them to begin with. All other life is still around; plants, fungi, bacteria, plant-like animals such corals and tubeworms, protists, etc. And maybe a small handful of insects/bugs/stuff like that.

In a hypothetical scenario of this nature you can't have any maybes. To even begin to narrow things down it is necessary to know exactly what clades remain operational and which ones do not.

I'm wondering how a world like this could function. Animals are so important to earth's ecosystem that I'm not sure how to remove them without decimating everything else. Obviously there would be mass extinction without animals, but how can non-animal life keep going? Where does the carbon dioxide plants need to grow come from? How are plants kept from growing out of control and choking each other without herbivores eating them? Is pollination solely wind/water based or are there other options? Where do coral's ammonium come from if not fish?

Animals are extremely important to Earth's current ecosystem. The planet had a living ecosystem that did just fine without them for 3 billion years of so (give or take a five hundred million years). However pre-animal, or simply early animal ecosystems looked very different. Our best understand example of an ecosystem without animals as currently known is the early Ediacaran Biota, which presents a world where ecosystems seem to have been dominated by bacterial mats and mostly sessile animals that may have survived by absorbing dissolved nutrients directly from the water column - a mode of life entirely impossible following the development of anything like zooplankton. Understanding of Ediacaran life is still limited and numerous competing, and controversial, hypotheses exist.

Regardless a world of this type - with only free-floating and sessile forms - is going to be fairly bland by comparison to any sort of modern ecosystem, because many modes of life currently extant will simply not be available at the macroscopic level.

[Palanan]

Originally Posted by Booper
And maybe a small handful of insects/bugs/stuff like that.

If you have even a small number of terrestrial arthropods, they should quickly diversify to take advantage of the great world of plant and fungal resources available to them.

Insects will be your primary grazers, and there’s no reason they won’t develop an abundance of herbivorous forms, as well as a diversity of predators that will keep the herbivores in check. There will also be a great diversity of arthropods that feed on and live within the fungi.

If you absolutely, positively don’t want insects to have a major role, then don’t forget that there are many fungi which are parasitic, both on other fungi and on plants. It’s possible that parasitic fungi could evolve to take advantage of plant resources, both living and nonliving, as well as parasitizing other fungi.

As for CO₂, note that fungi take in oxygen and release carbon dioxide much like animals, which—assuming enough fungi—could serve to keep the balance in the absence of vertebrates and arthropods.

Originally Posted by Booper
…plant-like animals such corals and tubeworms….

It may be difficult to explain these without any other complex animals. Might be simpler to allow some form of primitive fish in the oceans, to keep marine ecosystems as close as possible to our own, and reserve terrestrial habitats for plants, fungi and/or arthropods.

[sktarq]

first options....

Zooplankton, animal-like-bacteria (chemically), and fungi could well do a lot to cover your basic chemical needs. (Carbon/oxygen cycle, ammonia (I mean nitrogen fixing bacteria is the main way to get this anyway), etc)

also I would recommend you invest heavily in jellies, tube worms, polychaete worms, giant deep sea amoeba/sponges deep stuff, nematodes, maybe shellfish (including those hosting interesting bacteria) and possibly echinoderms (including ones with larval forms stabilized as adults perhaps)

getting a bit weirder you could have various bacteria that already have mobility (cilia, flagellum, etc) combine with mat formation and move as a group. probably not very fast but different than what we see on earth (possibly more like oozes we see in D&D I guess)

also look at slugs (both aquatic and terrestrial) for ideas on another potential animal form that would probably qualify as "okay".

while wind and water pollination would again dominate remember flowering plants are basically a Cretaceous development and there were plenty of plants before then.
If you do wish to push into the possible but unknown a plant that uses a similar hijack system to Ophiocordyceps type fungi...but instead infect say a species of large slug with a tiny seed. as it sprouts it changes the behavior of slug (in a similar chemical manner to the fungi) to go die somewhere ideally suited to a new sprouting plant of that species.
Also a lot of anti grazing/browsing adaptations would disappear. Thorns, many toxins, even the basic grass growth from the root instead of tip perhaps.

[Mechalich]

It's important to recognize that, when considering a simulation of this nature, invertebrates represent almost the entirety of animal biomass on Earth, especially terrestrial biomass (the largest component of vertebrate biomass is teleost fish). If you allow any of the major invertebrate phyla to exist you're simply going to get a world dominated by said phyla, a 'bug world' or 'slug world' or whatever. Those are interesting hypotheticals in their own right, but they are distinctly not a world without significant animals.

Additionally, all major animal phyla have existed, in some form, since the Cambrian Explosion (probably at least somewhat earlier in most cases, but fossils are limited) so if you want an animal-free world you have to halt animal evolution prior to that point. That leaves with only very simple forms and probably nothing suitable for terrestrial animal life.

Our best analogue for terrestrial life without significant animal-based herbivory comes from the Devonian period, for though land-based arthropods existed during this period their diversity appears to have been limited and, critically, there do not seem to have been any animals capable of digesting hard lignin-backed compounds (ie. wood) in the manner of termites, which may have caused early plant life to act as a carbon sink and accumulate massive amounts of carbon on land while reducing atmospheric CO2 and leading to global cooling, ultimately causing the End Devonian Extinctions, though this remains highly speculative.

Hypothetically, if this hypothesis is accurate, such a scenario would lead to a continual boom-bust cycle caused by cooling events that killed all the plants, an increase in weathering do to the now bare ground that liberates geologic carbon, warming that allows for renewed plant growth, and then loss of carbon and subsequent cooling again, possibly on a several million year time scale.

[sktarq]

Oh and you'll have to massivly boost the plant on plant war that we mostly ignore because it is chemically based.

Plenty of plants already use parasitism, soil toxins, messenger chemicals (and spoof ones), etc to attack, defend etc from other plants....

This may well grow as it would become a prime driver instead of having to compete with anti-herbivore defenses. So some may grow into have attack tendrils (in some ways similar to coral on coral wars)

Actually plant-on-plant parasitism could grow into a major or even dominant part of the nutrient cycle taking up a major part of the "herbivore" field. And if they are true parasites once they kill their host they themselves die. Leaving space a mass of nutrient breakdown components, etc.

Also I recommend thinking on a different time scale than you are used to....many good plant docs (from the BBC's the secret life of plants on) uses time compression to show how plants are highly active.

[Palanan]

Originally Posted by sktarq
Oh and you'll have to massivly boost the plant on plant war that we mostly ignore because it is chemically based.

I kept meaning to come back and mention this. Allelopathic interactions between plants can have major consequences at the individual, population and ecosystem scales, and I agree with sktarq that you should dial this up to about 37.

[Booper]

I've gotten a lot of interesting replies and for that I thank you all. This alternative earth is sounding bizarre and fascinating.

There's been a lot of discussion on the anthropod situation and I've decided not to include them. Since they'd eventually evolve out to fill the niches left by animals, at that point they functionally are animals. But an insect world is a cool idea for another story.

getting a bit weirder you could have various bacteria that already have mobility (cilia, flagellum, etc) combine with mat formation and move as a group. probably not very fast but different than what we see on earth (possibly more like oozes we see in D&D I guess)

I especially like this idea! It reminds me of modular (is that the right word?) organisms, like reproductive slime molds and on a much bigger scale, the man o' war.

such a scenario would lead to a continual boom-bust cycle caused by cooling events that killed all the plants, an increase in weathering do to the now bare ground that liberates geologic carbon, warming that allows for renewed plant growth, and then loss of carbon and subsequent cooling again, possibly on a several million year time scale.

Also a cool idea, no pun intended. I like the thought of a continuous cycle on such a massive scale. I'm no expert on evolution or paleontology though; how would life change between the cycles? Would plants look much the same or would they be vastly different each time?

ecosystems seem to have been dominated by bacterial mats and mostly sessile animals that may have survived by absorbing dissolved nutrients directly from the water column - a mode of life entirely impossible following the development of anything like zooplankton.

Could such things have a chance at returning in the absence of zooplankton? Could they survive on phytoplankton alone?

[Eldan]

Let's see. In order.

One of my favorite things to worldbuild are enviroments. I've got one that's a modified earth, one that has either lost almost all of its animals, or never evolved them to begin with. All other life is still around; plants, fungi, bacteria, plant-like animals such corals and tubeworms, protists, etc. And maybe a small handful of insects/bugs/stuff like that.

Well, most animals are "bugs", if you count arthropods anyway. The overwhelming majority, in fact. All animal species, to an order of magnitude approximation, is an insect. In fact, a third or so of all animal species are beetles. With numbers, it's far, far more skewed than that. So, really, if you say you "only" have insects, you still have a very similar planet. If you want there to be fewer insects, you need a factor controlling them. In addition to all the other factors that already exist controllign them, like predators, parasites and every existing plant out there having a dozen different toxins against insects, plus chemicals to confuse them, and attract predators.

But let's ignore that for now.

I'm wondering how a world like this could function. Animals are so important to earth's ecosystem that I'm not sure how to remove them without decimating everything else.

Not as important as you think, really.

Obviously there would be mass extinction without animals, but how can non-animal life keep going? Where does the carbon dioxide plants need to grow come from?

Plants, for starters. Plants digest carbohydrates too, so they produce CO2 as well. In fact, adult plants that don't grow much anymore, like mature trees, produce about as much carbon dioxide as they use up. So do fungi, and bacteria.

How are plants kept from growing out of control and choking each other without herbivores eating them?

Again, other plants. Plants poison each other all the time, for starters. Then they strangle each other, sterilize each other and suck each other's nutrients out. And poison the ground and the air and the water near them for other plants. It's constant chemical warfare. Then, of course, fungi. Fungi have no problem at all driving entire species to extinction in record time. Look at the American Chestnut, or the Gros Michel banana. Which you cant, because they are extinct.
And of course, insects.

Is pollination solely wind/water based or are there other options?

Assuming no insects? Self-pollination, cloning, mechanical force, wind and water, yeah. A lof of plants do fine without animal pollinators, I wouldn't worry about it. You wouldn't see flowers, though, those are entirely for the benefit of animals.

[Composer99]

IIRC most carbon dioxide that plants draw on comes from reservoirs such as the ocean and volcanism, so you don't have to worry about the lack of animals mucking things up, as long as you make sure the fluxes balance out over geologic time (which isn't really a problem for your typical RPG campaign).

Others have already noted some major differences. One thing I'd note that if you are completely cutting out animals, save perhaps for the Edicarian fauna, is that plants would not rely on reproductive strategies that make use of animals (pollination, eating fruit, that sort of thing). This doesn't mean the world wouldn't be covered with plants, necessarily, but they have to have other strategies. So you have to replace plants that rely on those strategies with ones that don't. That will result in a world that looks very different from ours, as far as the flora goes.

Also a cool idea, no pun intended. I like the thought of a continuous cycle on such a massive scale. I'm no expert on evolution or paleontology though; how would life change between the cycles? Would plants look much the same or would they be vastly different each time?

If memory serves, a typical animal species is likely to exist for 2-3 million years. Many genera of animal have lasted for a very long time with very little change, and I couldn't say how much species overturn there has been among those genera.

You could probably have flora overturn in a similar way: species changing over time, but possibly in the context of otherwise very stable genera. The moss found in Region X today might be different from the moss that grew there 5M years ago, but that's because today's temperate moss evolved from a subtropical moss that survived a glaciation, while the temperate zone moss of 5M years ago disappeared because the habitat was covered over with ice, and the subtropical moss slowly spread north as the ice retreated. Or perhaps the temperate moss spread south and hybridised, and it's the hybrid that moved back north after. However things shake out, what's in Region X today is still a moss.

[sktarq]

also I would have to ask...

what is the direction you want to take this simulation.

If it is for, say, a university project then higher fidelity, gas concentration questions, loam production (which is where most of mature plants carbon sink effects are...they turn air and water into earth) phytoplankton rot effects leading to boom bust oxygenation effects

if it is for a book or TTRPG higher dramatic material would be more values, rule-of-cool is weighed more heavily, etc.

[Draconi Redfir]

my best estimate would be a world with a large middle and upper canopy of plants that effectively cover the ground beneath them in perpetual darkness. most of these plants are probably either the type that spread as a single organism via the roots (i.e. the quaking aspen forest) or via dropped seeds, possibly with some glider mechanism to get some distance.


Beneath them though i'd think you'd have an excellent zone of darkness you can explore with fungi. no animals would mean no competition there, and you could potentially have large mushrooms or other fungi just dominating the forest floor like a second, miniature forest. This could also help provide the needed CO2 for the plants above them.

i could also see forest fires as being large and powerful here, providing even more CO2. perhaps there is one forest fire that circles around an entire area, consuming everything in it's path, leaving the land behind it barren and fertile to allow new plantlife to grow quickly, which it does until the firestorm loops back around and consumes the area again.