An island world where potatoes are the only land vegetation - can it support human life?

Question

NOTE I have re-edited the title to make it clearer. I have also added some detail in the paragraphs below that does not invalidate any of the existing answers.

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Let us suppose that a group of humans several thousand years ago were shipwrecked on a barren volcanic island. The only living things they brought with them were potatoes both for food and for seeding. These are ordinary potatoes as we know them. There are no other vegetables or animals on the island.

The original settlers had fishing equipment but over the centuries this has disintegrated. Growing potatoes has given them food plus fibres from the roots, stems and leaves. They can fashion simple tools from seashells and fish bones, etc.

There is no metal ore to be found so they are limited in what they can make. Nevertheless they are intelligent and resourceful and invent as much as they can using potato and sea products. There has been absolutely no contact with the outside word.

Is it possible for a population to survive for this long? Can they live on just fish, potatoes and the occasional sea bird? Can they build shelters from potatoes? Can they, given time, even build boats from potatoes?

Question

What are the limits to potato living?

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Please ask for any necessary clarifications.

Assumptions

1. The climate is generally moderate with the occasional severe storms.

2. The original settlers did bring some artefacts with them but nothing remains that is of practical use.

3. They have a constant supply of clean fresh water from a stream.

4. The ancestors brought fire with them and have faithfully kept it going all this time, keeping it alive with potato leaves and roots.

5. Potatoes grow best in loose, well-drained loam soil. Heavy clay soil retains too much moisture and tends to become hard as it dries, which can eventually slow water absorption and make it difficult for tubers to grow. Sandy soil drains too quickly, which can result in drought stress unless you water often. (Source).

6. The island is protected nearly all round by a reef. The fish are anything that might have been found in the healthier days of Australia's Great Barrier Reef.

7. The rocks are mainly volcanic.

8. They can make fibres from potatoes and/or their own hair.

Answer 1

There would be serious issues:

In a a world where potatoes are the only plant, you would have some really serious issues to overcome to make this realistic.

• If you think you can grow potatoes alone, you are mistaken. First, on an island without plants, you will have no soil. Even within a couple of years, places like Mt. St Helens and Krakatoa started growing plants, so the seeds would be somewhere unless this is an alien world. No other plants mean either the soil is unusable or that it has all washed away. An alien world without other organisms would have alien conditions, incompatible biology, or erosion. MINIMALLY there would need to be soil bacteria, insects, etc.
• Pollinators would make the whole thing a lot simpler, or else your people need to manually pollinate every single plant.
• Potatoes don't actually store that well. They have a lot of moisture and tend to rot. Modern storage allows for keeping them but most islands are not conducive to dry storage — dried potato slices? You would need cultures growing all the time for a steady food supply.
• There are a very limited number of nutrients in potatoes, so the potato is only there to provide bulk calories. You will need to have alternate sources of foods for almost every nutrient. MINIMALLY there had better be some good plants in the sea or really nutritious fish, as mentioned by Galactic. This means that need to be really lucky to get the right starting seaweed. Some of this can be worked around if you have microorganisms that are able to synthesize needed vitamins via fermentation. This is the kind of super-coincidence luck/genetic-engineered hand-waving you indicated you didn't want. (The upside of fermentation is your natives might get drunk "for their health" and kids need to drink potato beer, too).
• You have serious concerns about monoculture. Potatoes almost DEFINE the problems with monoculture. Any little thing that goes wrong with your plants could destroy the entire food chain for your islanders, recreating the Irish potato famine but without alternatives. I'm assuming you are starting with one or two varieties of "white" potatoes, but normal potatoes can mean any and all of 5,000 varieties, so if you allow many of these, the situation improves a BIT.
• Along with monoculture, having only one kind of plant means the soil gets depleted of the stuff potatoes need, and other organisms aren't there to restore them. Bacteria, fungi or insects that ate your potatoes would start to enrich in your soil, eating them after a short while. MINIMALLY fields would need to be left fallow for years between crops.
• From an evolutionary standpoint, your potato is likely to go wild (assuming a pollinator) and start mutating as a wild plant, trying to fill vacant niches. Potatoes are related to belladonna, and most of the species (including many potatoes) CAN, under the right conditions, be toxic (remember anyone ever telling you to not eat green potatoes?). Even wild mutants could still interbreed with your domestics, introducing dangerous mutants with poisonous variants.
• All parts of potatoes except the tuber are toxic (again, related to belladonna) but who knows, maybe some of these plants will end up producing nicotine (another potato relative) and introducing a new kind of crop. This limits the general usefulness of the plant in other applications due to its relative toxicity.

So, concentrating entirely on the actual potato, there are serious problems with such a scheme, and you need to think about adding some bees, grass, friendly microorganisms, and genetic diversity to your scenario before leaving the survival of your society to the humble potato.

Answer 2

Yes; they could.

The Inuit largely lived on only seafood and kelp. Seafood contains all the necessary nutrients for humans to survive. However, the Inuit diet contains little carbohydrates, so they were ketogenic. Your people, on the other hand, have access to potatoes, an good source of carbohydrates.

Fire would likely be done by burning potato leaves, as they are toxic and have little use.

The people of the civilization could build homes out of earth since the climate is mild and severe storms are rare. They can rebuild them every time a storm occurs.

Since the island has igneous rock, they could create spears to hunt by attaching rocks or shells to animal bones from fish.

Technological progression will likely be slow, as your people will spend most of their time hunting and farming potatoes rather than researching technology. However, given long enough, they would technologically progress.

Writing would likely be done through clay tablets much like Mesopotamian civilization:

In the event that buildings in your civilization collapse during a storm, the clay tablets will still be preserved.

Eventually, they could make bricks out of stone/clay that they could use to build more solid structures.

Items can be wrapped in cloth made from potato fibers.

Boats can be made out of solid granite (an igneous/volcanic rock). Your people can use oars made from bones and stones to navigate the ocean.

The civilization can then use selective breeding to breed potatoes containing more nutrients.

After that, the people on your island can invent bioplastics made from potatoes, a much more reliable material than stone.

Basic machinery such as the steam engine can have a stone base and working components made out of bioplastic.

Since the resources needed to make an electronic computer are unavailable on your island, your people will likely develop fluidic computers made out of bioplastic.

However, it will take a long time for these technologies to be developed, as people will have to hunt more frequently, since non-potato agriculture is not possible on your island.

Answer 3

Can you even grow potatoes here?

To answer this question we first need to explore the properties of un-pioneered volcanic soils and compare those to the needs of Potatoes.

As long as your soil includes a low aluminum andisol you will be fine. Andisol is an inorganic soil made of volcanic rock that is broken down by rain instead of pioneer plants. An island formed from basaltic lava would be best because the andisol will form better and quicker with a lower silicon content. Potatoes and other tubers grow particularly well in this kind of soil; so, your potato crops will not need any special treatment to pioneer this environment as long as the island is old enough for the lava to have broken down to andisol by the time you get there.

Can you live off of just potatoes?

More or less, yes. Adults have been reported to be able to live at least a year off of just potatoes; so, other foods are not necessary in a short term survival situation. That said, they are deficient in Vitamin D, Vitamin A, Calcium, and important lipids and proteins that you won't get enough of out of potatoes for long term survival. The good news is that fish are rich in all of these things; so, a diet of just potatoes and fish is adequate to maintain reasonably good health. You will probably also see a fair amount of seaweed consumption for variety if nothing else.

The big issue you will run into is that potato plants are not woody. They are not very good for making hot fires, or tools, etc; so, you may run into some serious issues when it comes to basic technologies. Without woody plants or large animals to hunt, human remains may become essential materials. Bones used as tool handles, skin used for leather, hair used for string, etc. The good news is that this is just a temporary condition. Tropical volcanic islands do not stay lifeless for very long, in the course of the few thousand years your people live here, seeds of various islander plants will wash ashore giving you other plants to work with over time like coconuts which will in turn give you a more proper wood to work with and fill out all your needs for a basic neolithic level civilization.