Dunbar’s Number for Plants

Human psychology evolved under the conditions present during the 40 thousand odd years of our time on Earth as modern humans. During that time humans lived exclusively in small groups of no more than 150 individuals, with larger groups tending to split once they grew beyond this size. In other species of social animals there exists a clear relationship between the size of the neocortex of the brain and the maximum size of a social group since maintaining stable relationships requires a minimum amount of brain power. This is known as Dunbar’s number and it represents the number of people an adult human can mentally keep track of in order to understand their roles in the group and to allow a reasonable tally of gifts shared in either direction. In groups below Dunbar’s number there is no need for formal systems of law or economics because people tend to spontaneously manage their collective behaviour and resources. Individuals who behave badly toward others quickly find themselves shunned by everyone, unlike the situation in a big city where a petty criminal can always find another stranger to exploit. In more recent times this number has proven to be the ideal maximum size of military units and small companies that lack strict hierarchical roles.

Similar dynamics come into play when considering the relationships between humans and plants. Every plant that we interact with places a cognitive demand upon us, meaning that there is an upper limit to the number of plants that we can have meaningful relationships with. This is true for both hunter-gatherers and settled farmers, though the relationships are somewhat simpler for hunter-gatherers and thus allow them to potentially form relationships with a wider array of plants. While a hunter-gatherer only needs to know when and where to collect a plant, how to harvest it, how to process and store it (and potentially how to manage the ecosystem to favour it), a farmer also needs to know how to propagate it (including long term seed storage with all its attendant pests and diseases), how to fertilise and/or irrigate, and in some cases how to breed it (including techniques like hand pollination and artificial selection). Just the simple act of growing a plant relies on a wealth of knowledge- what type of soil does it like and how to prepare it, when to sow (often based on feeling the weather more than reading a calendar), how to sow, what spacing, how to thin, how to manage pests and disease (though I usually skip this step), how to manage weed competition, and finally how to harvest, process and store seed so you can repeat the cycle. The nature of the plant influences how close our relationship needs to be. As for human relationships the amount of time and energy devoted to each individual is not evenly spread. Instead we tend to have an inner circle of five or so people (often blood relatives) who consume nearly half our energy, spanning out to a much larger circle of acquaintances who we rarely interact with. This is also reflected in our relationships with plants in our life, with certain core staple crops requiring a large amount of time, equipment and expertise, while semi-wild and tree species might require little more than harvesting every year and propagation once in a generation. Some specialised herbal remedies might only be needed once in a lifetime.

I manage the complexity of the many relationships I juggle with different plants by dropping each species into one of three categories: Reliable, Marginal and Experimental. When I first start growing a particular crop I usually plant just one promising variety to begin to get a general sense of its adaptation to my soil, climate and palate. These are the plants in the experimental category. For annuals with any potential for filling an empty niche I will usually try growing them on a small experimental scale for several years first. This allows me to evaluate their reliability since often a crop will grow well one year and then fail the next few years in a row. Those crops that show the ability to produce consistently get put into the reliable category. Those that produce inconsistently or have lower useful yields (or worse yet have another crop or three that does the same job but better) get put into the marginal category. Crops in the reliable class then become targets for collecting and evaluating as much genetic diversity as possible in order to improve both long term productivity and reliability. Crops in the marginal class may also attract this attention but usually only if they offer something unusual that other crops cannot do, such as producing at a time of year when nothing else similar is cropping or yielding a different type of nutrient such as fat.

Managing these diverse collections can consume a lot of time and energy if you are determined to keep everything labelled with its original name and details. If you start doing hand pollination it becomes even worse as every cross accumulates an even longer label detailing all the parents, and individual selected plants attract their own names as well. Instead of this I tend to only keep names on original highly diverse clones, and beyond that only record the seed parent of any crosses since this removes the need to worry about random natural pollination giving you an unexpected result. I figure at the end of the day every plant must succeed or fail based on its own natural attributes and a bunch of letters on a label won’t change any of that. As evaluation and breeding continues I prefer to settle on one main strain for each species so this removes even the need to label seed packets since I can recognise the species by the seed itself. That period of high diversity and complexity is transient, meaning energy is eventually freed up to spend on other species instead.

Looking at my current line-up of species after about a decade of trialling and evaluation shows the following categories, summarised below. You can see that the number of experimental vegetables is very low (just lab lab beans since while they grow fairly well I have not experimented enough with eating them to be confident they are that useful). With a mere 13 reliable vegetables I can have 3-6 different ones producing at most times of the year, providing sufficient fresh produce to add to our basic meals with minimal hassle since almost all the marginal performers have been discontinued. I might keep adding one new experimental crop each year just for novelty, but there aren’t many niches left open. Many tree crops are in the experimental category due to the longer times needed for them to mature and demonstrate their productivity and usefulness. The marginal staple crops are those that have been showed to be unreliable year to year, or have difficulties in processing that may be overcome in the future. The experimental staples require trialling with different cultivation approaches or require more extensive breeding and selection work before they can be properly evaluated. This list comes in at total 99 different species of plants which I regularly interact with. Each one requires a different set of skills to grow, harvest, propagate and process. I didn’t mention the several dozen herbal plants I am learning to grow and use, or the couple hundred ornamentals I grow as well that are a bit simpler to just plant and admire. That leaves enough breathing room before hitting Dunbar’s number of 150 to also include livestock demands, plus some time left over for learning handicrafts, and of course a little time left over for maintaining relationships with the actual humans in my life.

Staple crops1458
Tree crops17713

The take home message from all this is that while diversity has many benefits, it also comes at a cost. There is an optimum balance point between monoculture and infinite diversity and there are limits to how much one person can know about and care for. Don’t be afraid to try something new, but likewise don’t be afraid to ditch something that doesn’t reward your attention.

A 2018 pumpkin variety trial, focused on diverse C. moschata to find a variety that worked for me.
And the winner is C. moschata Seminole, an amazing hardy performer that sails through our humid droughts and produces storage pumpkins that last for almost a year. This is now my main line with a little diversity from a couple other types thrown into the mix.

2 thoughts on “Dunbar’s Number for Plants

  1. Interesting. I’ve often thought about putting together a database of the food plants I grow along the lines of yours, but never seem to get around to it. I’d also include information on ease of attack by pest species, nutrients supplied, seed production, quantity produced and ease of storage and ultimately attempt to come up with a list ranging from valuable (potatoes, asparagus) to seasonal treats (persimmons). Think I’ll get onto it. Thanks for the push start.


    1. Putting all that extra effort into documenting everything about every plant would probably shrink the total number of plants you could handle by 5-10 % (only half joking). A lot of traits like insect issues would vary so much year to year as to be not worth recording for me at least.


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