I want to have a group of space colonists get stranded on a planet where the only way they can get enough nourishment is to gather or cultivate edible plants.
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$\begingroup$ are large marine animals OK, it is easy to keep animals on land small much harder to keep the ones in the ocean small. $\endgroup$– JohnMay 4, 2019 at 17:35
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$\begingroup$ Quite a coincidence that the colonists found a planet with life what was chemically compatible with their own. Though it would stink to be one of the many colonists who must die of poisoning or malnutrition to determine which plants are safe and nutritious. Why can't the colonists eat the insect-sized animals? $\endgroup$– user535733May 5, 2019 at 2:10
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$\begingroup$ @uaer535733 Interestingly, there are a number of different primitive tests you can do to determine with a reasonable level of accuracy whether something is poisonous. You start by rubbing it on your skin to see if it reddens it (wait 30 mins), then doing the same for the juice. Then just tap it on your tongue and wait. If there's still nothing, put some in your mouth and chew it for a bit and wait again. If it passes these then there's a good chance it's edible (even without feeding it to a pig to test!). $\endgroup$– YnneadwraithMay 5, 2019 at 9:37
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4 Answers
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Sure.
All land vertebrates descend from bony fish. If those (or other equivalent) never evolve in your world, and land animals depend on a tracheal system or branchia to breath, they won't get much big.
How big they can get depends on the availability of oxygen in the atmosphere. Nowadays insects rarely grow larger than 10cm long, though some land arthropods can grow to be a meter long such as the coconut crab. During the carboniferous period, though, some dragonflies (meganeuras) could have wingspans of 70cm.
So low or normal O2 levels and no development of lungs ever will keep your land based fauna pretty small.
answered May 4, 2019 at 3:10
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1$\begingroup$ What about all the jungles, woodlands and so on mentioned in the question? While your answer is reasonable, I feel like a planet covered in diversified plant life would probably have a higher concentration of oxygen rather than a lower one. $\endgroup$ May 4, 2019 at 8:42
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$\begingroup$ @InquisitiveGeek our planet has diversified plant loge and has a normal concentration. $\endgroup$ May 4, 2019 at 9:57
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$\begingroup$ What I'm trying to say is that a planet that has well established and diversified plant life but a low oxygen concentration in the atmosphere is unlikely. And even with a concentration of about 20% animals with no lung get to be much bigger than the average modern day insect, especially since the question doesn't specifically talk about land animals. $\endgroup$ May 4, 2019 at 10:57
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3$\begingroup$ @Inquisitive Geek Having low O2 concentrations but a lot of flora producing oxygen is realistic and has happend on earth. Just let the environment be chemically reducing instead of being oxidised. On earth O2 production began 2.5 byr ago, yet it only started to be a substantial part of the atmospere about 0.85 byr ago. Landbound plants developed much earlier on this world and all the oxygen they produce is still absorbed by the oceans, seabeds and land surfaces. en.m.wikipedia.org/wiki/Great_Oxygenation_Event $\endgroup$ May 4, 2019 at 16:11
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3$\begingroup$ @TheDyingOfLight This. I've noticed a pattern in how people seem to treat planetary environments where something present-day earthlike is treated as stable, but anything modelled on earlier earth environments is viewed as transitory. In reality, all of it is transitory and you're only ever visiting a planet at a snapshot of its geological history. Perfectly valid for this planet to have started its journey to life later, and still be in the 'insects only' phase. Given the timescales for each epoch, that's probably more likely than having big vertibrates. $\endgroup$ May 5, 2019 at 9:42
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There are 2 distinct problems mentioned in your question:
Could a planet evolve to have [a diverse fauna] - but no animal bigger than an insect?
Probably yes.
Could a group of space colonist get stranded on a planet where the have to cultivate it's native fauna to survive?
Sure, but not on the same planet...
In spring the plants on this planet probably look similar to plants on earth. Some have flowers and rely on insects for pollineation, others have spores that are carried with the wind. But in autum, the seeds of almost all plants would be tiny enough to be carried by an insect or the wind.
Plants, especially trees, want to spread their seeds as far as possible to avoid competition for nutrients with their own seedlings. Without animals to carry seeds a long distance, there would be nothing like an apple, avocado, nuts or beans on this planet, because the energy required to grow such big fruits and seeds would be wasted.
Some plants could maybe form a symbiosis with an insect like a Scarabaeus that would roll their fruits some distance away, but ultimately those would be the exception and not enough to feed a group of people.
Another alternative are roots. On Earth we have potatoes and cassava that are the nutritional bases for some people. Add in carrots, beetroot, turnips and other edible roots, and you have a realistic chance of feeding your stranded astronauts ... in theory.
Realistically those roots would look like their ancient, scrawny forms before the beginning of human agriculture. A plant only needs to store anough nutrients in it's root to survive a period of lack of nutrition. No more than that. Our modern root vegetables are all bred and selected to store more nutrients than required, in order to grow big enough to feed humans.
On a planet where the evolution perfected plants to grow and spread only with the aid of small insects, humans wouldn't find enough fruits, seeds or roots to survive.
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1$\begingroup$ I could see some fruit like reproduction concepts develope near rivers and on islands. After all coconuts rely on the ocean to get somewhere else, not on animals. $\endgroup$ May 4, 2019 at 10:45
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1$\begingroup$ Thank you. Yes, I was thinking I'd probably need to stick with roots and such. Plus, I also plan to have the colonists raise what earth-native plants they can. But they still collect wild sources to complement what they grow. And when their crop yield is low, they need to collect more wild items. $\endgroup$ May 4, 2019 at 11:04
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1$\begingroup$ @BillBittner If they have seeds of crops native to earth, they should try interbreeding space crops as soon as possible. The first american settlers had a similar problem: their european crops could have fed them in Europe, but wouldn't grow and yield as much in their american settlements. On the other hand, introducing new plants with different pollen and self-defence chemicals could wipe out the local insects that are not adapted to those newcomers (again, compare to the native american population after settlement). $\endgroup$– ElmyMay 4, 2019 at 11:10
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1$\begingroup$ @BillBittner Sorry, but asking and answering more questions in commends is not a good fit for this site. Please consider posting a new question, but always keep in mind that you are the author. You make happen whatever you like to happen in your story. $\endgroup$– ElmyMay 4, 2019 at 11:24
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1$\begingroup$ Grains and nuts evolved high calorie seeds because said seeds either need to survive a long time before growing or they need a large head start due to short growing seasons, animals are not involved. $\endgroup$– JohnMay 4, 2019 at 13:14
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You know Army-ants?
Credits: Von Der ursprünglich hochladende Benutzer war Karmesinkoenig in der Wikipedia auf Deutsch - Selbst fotografiert, CC BY-SA 2.0 de, Link
Army ants are a good example for tiny creatures killing much larger ones.
Have your planet being inhabited by some aggressive ant-like insects frequently killing any large animals they find, so animals larger than the earths insects cant even evolve.
I aggree to Elmy that you most likely wont find large fruits on this planet, but:
Some plants could evolve large roots and tubers in order to store carbohydrates over long cold or dry periods.
maybe the planet has very long years with a very long and mild but dry and dark winter and a very long and dry summer, so the plants have to store lots of water and nutrients in their roots in order to survive this.
Credits:
photo by Scott Bauer, USDA ARS [Public domain].
see source for more information
roots and tubers could be eaten by your stranded spacetravellers,so they could find nurishment there.
The travellers only have to find a way to get rid of the ant-like killer insects.
For example, they could hide in their spaceships wreck.
Maybe they build or use a flamethrower to fight the insects when they go out of the wreck to find nurishment.
Or maybe they have some insecticide on their ship, for example against parasites like mosquitos or ticks, and it works for that alien insects too.
Or maybe the alien insects cant stand some desinfectant your travellers use frequently, and the travellers note that.
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1$\begingroup$ This is even part of the plot of Sid Meier's Alpha Centauri. +1. $\endgroup$ May 4, 2019 at 13:21
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Sure, just make the insects the biggest creatures on the planet
https://ghibli.fandom.com/wiki/Ohmu
That way you can ensure no creature bigger than an insect evolves, problem solved :P
Macroscopic Organisms Haven’t Yet Evolved
In all seriousness, you could simply state that creatures larger than insects simply haven’t evolved yet. Plants dominated the earth long before the first macroscopic organism walked upon it.
Whilst eventually larger organisms would evolve and prey upon the abundance of insects, at the time of the human’s arrival no natural predators of insects have yet evolved. A very simple yet effective solution to your problem.
Plants May Only Be Inedible to Insects
An issue may be that plants evolved edible fruit so that animals would swallow the seeds and spread them after the edible portion had been digested. Your humans may struggle to find edible food.
However, that being said, coffee plants are a perfect starting point. Coffee beans evolved to contain a lot of caffeine so that, when an insect ate them, they would be overloaded and suffer a heart attack. Humans on the other hand are so large that the lethal dosage of caffeine is far higher than what a coffee bean contains. If the plants in your world evolved to only defend against insects, when a substantially larger creature comes along, their defences are useless. The fruits, berries, leaves, roots, flowers and even bark of plants may contain lethal dosages of caffeine to insects but are harmless to humans. You could also state that fungi evolved a similar form of defence.
Whilst i use caffeine an example, there may be other substances that are harmful to insects but would not kill a human at those dosages. For instance, i could imagine plants evolving a way to produce salt to defend against mollusks like snails or slugs, perhaps certain sugars could also harm an insect if concentrated enough. This allows you to have a wide range of human-edible food in a world that should not have edible food due to a lack of larger organisms such as deer.
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$\begingroup$ Not at all certain that larger organisms would evolve. If nothing ever evolves an endoskeletal body plan with lungs, then the size of non-aquatic animals is limited by mechanics. $\endgroup$– jamesqfMay 4, 2019 at 16:54
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$\begingroup$ @jamesqf Perhaps, though both endoskeletal and exoskeletal organisms evolved from single celled organisms so there is nothing to say that larger organisms wouldn't eventually evolve. Also, look at the coconut crab, its body is roughly the size of a human torso. Under lower gravity and with the right conditions, perhaps a far larger version of that could evolve. $\endgroup$ May 4, 2019 at 17:35