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In my world humans have found a barren planet that is capable of hosting life. What species do they send down to establish a sustainable ecosystem? Just a few or thousands? How many species of plants vs animals? Is tons of variety needed or can just one species of tree colonize the entire planet?

The planet has an earthlike gravity of 0.95 g, high atmospheric humidity, a single atmospheric circulation cell, and a warm-temperate climate(spread all over the planet, to the 70th degree north and south). The surface composition is 70% water and 30% land. The atmospheric oxygen level is 35%.

A. Dean
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    Welcome to the Site! Please take some time to visit the welcome center. Your Question is interesting, but will likely get closed as too broad or Off Topic. You have 4 distinct questions here and very little background about your world so far. A good start for you would be to look at Terraforming on Wikipedia. develop your idea from there and if you have some more specific questions, ask them here. We are a creative bunch – Paul TIKI Jan 15 '18 at 20:45
  • You need to specify a stage of terraforming, desired results, and planetary conditions. As it is now the question is not answerable. – Olga Jan 15 '18 at 20:47
  • What kind of planet? – Mołot Jan 15 '18 at 21:37
  • Okay. The planet has an earthlike gravity, high atmospheric humidity, and fertile moist soil. It also only has a single atmospheric circulation cell. Desired result is a sustainable ecosystem. – A. Dean Jan 15 '18 at 21:51
  • (a) Please edit your question with the answers. It's actually quite a bit of work to read through comments to find them. (b) There is no single species that is keystone to an ecosystem. We need to know much, much more about your planet before we could even guess at a group of critters/insects/bacteria/etc. Ecosystems are remarkably complex and there's never just one species that makes them collapse. – JBH Jan 15 '18 at 22:59
  • JBH I have edited them in with more details – A. Dean Jan 16 '18 at 00:15
  • @JBH i have edited them in – A. Dean Jan 16 '18 at 10:17
  • How is there fertile soil if the planet is barren? – rek Jan 16 '18 at 20:25

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An ecology is a dense web of interaction and energy exchange, and to answer your question with any degree of rigour, we would need to understand the "start state" of the planet: mass, orbital [period, insolation (i.e. how much energy is available), details about the atmosphere and hydrosphere and much else besides.

Consider, for example if I threw Mars, Venus and Titan on the table. Each planet has wildly different conditions, and what would conceivably work on Mars will fail drastically on Venus or Titan.

In the most general case, what you will need is a broad and diverse "base" capable of collecting solar energy and converting it into feedstocks (much like plants convert solar energy, water and CO2 into Oxygen, starches and sugars). Already you see the issues; unless we know how much water, sunlight and atmospheric gasses are present, who do you calculate what is needed for the "base" of your ecology? The Atacama Desert of Chile, for example has plenty of sunlight, but has very limited water or fertile soil to support plant life, and it would take tens of thousands of years under these conditions to accumulate enough biomass to support any sort of large scale, vibrant ecology.

After the "base" is set, you can then use a general rule of thumb that each higher level of the ecology is 1/10 of the size of the lower level. A very simplified ecology would then be the amount of grasslands produces "x" biomass, which is fed on by .1X mass of herbivores, which is then preyed on by .01X mass of carnivores. The system is balanced by fungi and bacteria which break down the dead biomass and convert it back into organic matter in the soil, to restart the cycle. A more complete discussion is here.

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very simplified model

Of course if you really want to get into the world building, then you can roll up your sleeves and look at this diagram:

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http://www.pnas.org/content/100/4/1781/F1.expansion.html

So work out your starting assumptions (energy, water, atmosphere, temperature etc.) and then the rest can be worked out from there.

Thucydides
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  • From the terraforming point of view I would mostly care about the producers. The rest is gravy. – Censored to protect the guilty Jan 15 '18 at 21:25
  • For the "producers", might want to check out my old question How much land area do my land-based animals (herbivores) need for food? – user Jan 15 '18 at 21:30
  • "So work out your starting assumptions (energy, water, atmosphere, temperature etc.) and then the rest can be worked out from there." How would I go about doing that? – A. Dean Jan 15 '18 at 21:53
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    This is a great answer, and it underscores the problem with succumbing to the temptation of getting bogged down in the details. That complex chart is "The food web of Tuesday Lake in 1984." Let me say that again, it's ONLY the chart for ONE LAKE. Ecology is unbelievably complex. The Svalbard Global Seed Vault has nearly 1,000,000 types of seeds. You wouldn't need anywhere near all of them to colonize a new world, but you would need thousands (and eventually you'd want a lot more). Find a happy middleground with detail. – JBH Jan 16 '18 at 14:28