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  • In my world, there's an era when only some 20 visible (big enough to see with the naked eye; all other animal life forms are microscopic) animal types lived on the planet. Mammals such as wolves, livestock animals, rodents, small wild cats, as well as a few different types of fish, bird, insect, reptile, and arachnid species.
  • Most of the animal types link with each other in a food web and with the ecosystems on the planet.
  • Some of the creatures prefer certain climates over others.
  • There isn't that much variety in visible animal types (environment or mutational adaptations).

Could such an era be somewhat scientifically possible?

The weather and biome conditions resemble those of Earth.

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    $\begingroup$ Visible. How many is "a few", and how big does a species have to be to count as visible? On Earth, the vast majority of animal species which live on land are insects; bird species are fewer by more than an order of magnitude, and mammal species fewer by an order of magnitude again. $\endgroup$ – AlexP Apr 19 '18 at 2:15
  • $\begingroup$ To be counted as visable animals in my case, they would have to big enough to see with the naked eye (not needing a microscope), though in my particular case, the smallest animals would probably be some of the insects, fish, and rodents. Also by a few, I mean probably around 20 or so general species (not counting environment or mutational adaptations), including mammals such as wolves, livestock animals, rodents, small wild cats, as well as a few different types of fish, bird, insect, reptile, and arachnid species. Those are the types of animals that would need to be delt with. $\endgroup$ – BlueTangsRock Apr 19 '18 at 4:14
  • $\begingroup$ Do you want it to stay that way over "evolutionary" timescale? $\endgroup$ – Mołot Apr 19 '18 at 5:53
  • $\begingroup$ twenty is impossible, you could not build a global ecosystem with just a 100 varieties of insects much less a 20. $\endgroup$ – John Apr 19 '18 at 14:05
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    $\begingroup$ Frank Herbert goes pretty in depth in The Ecology of Dune, an appendix to the novel Dune, where there's only one species. A bit dry but kind of interesting. $\endgroup$ – MParm Apr 19 '18 at 19:52
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Yes this is relatively easy, you just need a mass extinction. Right after a mass extinction there are very few large organisms left, survivors tend to be small generalists, species diversity drops like a stone (no pun intended) their populations become huge but there has not been enough time for them to really diversify.

Note this does hinge on your definition of "few", there will still be millions of different macroscale species, however the vast majority will be insect or rat sized.

The bigger the extinction the fewer species you will have, after the KT extinction (dinosaur killer) the only terrestrial animals bigger than a dog were crocodiles. 75% of all animal species became extinct. You have a window of around a million years after the event to work with. The PT (permian triassic extinction was even worse, taking out 95% of all multicellular life on earth. It killed off so much life even the oxygen levels of the atmosphere noticeably changed just due to the massive loss of plant life.

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  • $\begingroup$ a mass extinction actually does happen before then, though the comments from my post about the survivability of life after that mass extinction I thought up, allows very basic life and moss to survive at best, though the time of that mass extinction and the era in the post are thousands of years of years apart, so the animals in this era would've evolved after the recovery era, would it still apply to your comment above? $\endgroup$ – BlueTangsRock Apr 19 '18 at 4:28
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    $\begingroup$ it takes millions of years to recover from a mass extinction, the more severe the longer it takes. It took on the order of tens of millions for the PT extinction. Your animals would have evolved very little (unnoticeably so morphologically) in just thousands of years. but again it depends on what you mean by few, even after the FT your still talking about millions of species of animals, just not any large ones. $\endgroup$ – John Apr 19 '18 at 4:42
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Yes, but it's difficult. I'm not sure if you ever played the game "foxes and rabbits" (rabbits consume X amount of grass, breed at rate Y and die at rate Z, foxes consume rabbits, breed at rate Y', die at rate Z'), but it was murder trying to get the relative starting populations right.

As was found by James Lovelock, in his research on the stability of ecosystems, the more variety the more stable it gets.

Other factors, such as having to maintain cycles and feedback loops, don't need to be considered as we're only looking at part of the system.

I'd recommend a predator and prey for each zone and a predator and prey that can cross two similar zones, with each predator able to eat from three different prey. It should make the system more stable and less sensitive to initial conditions.

Such a system could arise naturally and probably has done on multiple occasions, as it's not unusual to have single niche and multi-niche animals. To get a good feel for it, you really need to simulate it. That may not be necessary, but it cod give insight into relative numbers.

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  • $\begingroup$ Usually, the apex predators end up as the de facto managers of the ecosystem. $\endgroup$ – ohwilleke Apr 19 '18 at 7:17
  • $\begingroup$ Such a system could arise naturally, perhaps, in a small niche. For there to be only a few "visible-sized" species on the entire planet is another matter. $\endgroup$ – Robin Saunders Apr 19 '18 at 13:55
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Sure. That's what the Earth was like early in the Cambrian Period.

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

The Cambrian marked a profound change in life on Earth; prior to the Cambrian, the majority of living organisms on the whole were small, unicellular and simple; the Precambrian Charnia being exceptional. Complex, multicellular organisms gradually became more common in the millions of years immediately preceding the Cambrian, but it was not until this period that mineralized—hence readily fossilized—organisms became common.

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  • $\begingroup$ Multicellular life had barely evolved in the cambrian, there was no terrestrial life but there was a large diversity of life, also the climate was most definitely not like modern earth. $\endgroup$ – John Apr 19 '18 at 3:49
  • $\begingroup$ @John no mention in OP of the requirement that they be land animals. (Yes, those early multicellular creatures were animals: en.wikipedia.org/wiki/Animal#Evolutionary_origin) $\endgroup$ – RonJohn Apr 19 '18 at 4:58
  • $\begingroup$ I was more worried about the implication that there was not much variety in multicellular life, there was still a large variety just not a variety we would be familiar with. $\endgroup$ – John Apr 19 '18 at 13:15
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Yes. Just introduce small sentient beings. If they are like humans, they will extinguish all species larger than themselves quickly after entering an environment. Fossil evidence all over Earth shows megafauna vanishes quickly after humans arrive in a place. See “Timing and possible causes” in this article: https://en.m.wikipedia.org/wiki/Megafauna

We don’t like to be prey, and we find meat tastiest if we only have to hunt one to feed the tribe. If you introduce a highly sentient squirrel, I think you can plausibly extinguish larger animals.

Timely new study reported today (April 19) on this topic: http://www.wbur.org/npr/604031141/new-study-says-ancient-humans-hunted-big-mammals-to-extinction The NPR station summary opens with, "Over the past 125,000 years, the average size of mammals on the Earth has shrunk. And humans are to blame." And conclusion: "We still have lots of furry little mammals on the planet. But the pattern is clear: 11,000 years ago, the average mass of a non-human mammal in North America was about 200 pounds. Now it's about 15 pounds. And the researchers say they're getting even smaller."

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  • $\begingroup$ except we didn't do this on any continent save australia, sure we killed a lot of things and thinned the megafauna species numbers but we did not "extinguish a all species larger than ourselves". In some places we did did the opposite importing our own megafauna. $\endgroup$ – John Apr 19 '18 at 13:17
  • $\begingroup$ @John it's true that humans didn't have the effect the OP is looking for, but with some small tweaks to the metabolic requirements and societal structure of the sentient species, the effect would easily be possible. $\endgroup$ – Austin Weaver Apr 19 '18 at 13:57
  • $\begingroup$ not really the only place humans did not do this is africa, where we evolved, and this will have little impact on the majority of visible animals only on megafauna. $\endgroup$ – John Apr 19 '18 at 14:00
  • $\begingroup$ @John Data strongly correlates to us killing off megafauna in North America and Madagascar. Even in the oceans, we have massively depleted the leviathans. $\endgroup$ – SRM - Reinstate Monica Apr 19 '18 at 21:15
  • $\begingroup$ megafauna are only a tiny fraction of the visible life around, and we have only been successful in killing of some of the megafauna, and we were really trying there for a while. Its just not gong to give you thenmassive loss in diversity you need, humans are bad sure but we barely a drop in bucket compared to the last large rock that dropped on the planet and that was only a minor mass extinction. $\endgroup$ – John Apr 20 '18 at 4:33
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In principle yes, but evolution is still working.

Tho it is quite plausible to have a "small" number of animals in an ecosystem at a certain time (it needs a cause, either a big die-out event, or a planet that is newly conquered with a small stock of pioneering species), this state won't last.

Evolution will start forking the given species into a larger variety of species (as we can see in insular ecosystems, this can happen in relatively short timespans like 100k year to 1 Mio. years or less) and make the ecosystem richer over time.

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  • $\begingroup$ true, though it's just an era, eventually that era will end, and I'll expect that more species will be evolved and the specie count will rise, I'm just wondering if at that particular era if only a few visable (non-microsopic) species could create a working ecosystem for that amount of time. $\endgroup$ – BlueTangsRock Apr 20 '18 at 18:22

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