Synopsis: Total population of all species have already reached their limit.
In a world where the human population has been growing rapidly for generations, before researching population, I had assumed that humans, and probably many other species, were still “growing into” their environments.
The calculations as to how long is needed “grow into” an environment on Earth to the limit of what is possible are detailed in:
In summary, it turns out every species is capable of reproducing at a rate that would result in that species covering the entire surface of the planet within around just 10,000 years.
Given that on our over 4-billion-year-old planet, almost every species has been around longer than 10,000 years, it becomes clear that basically every species has had long enough to increase its population to its limit.
The only exception would be where circumstances have changed. For example, due to prolonged weather events or weather or climate change, recovering from natural disaster, or a change in other species such as a predator, or a new potential food source.
In summary, life on Earth is not, as at one time I had just assumed, still growing into our planet.
Which raises the question, how has the population of humans managed to keep growing, and even had a recent explosion of population growth?
What triggered this population explosion is covered elsewhere, but the question examined now for this page is, if the planet was already full, how is the population growth accommodated?
The conclusion is clear. We are not exempt from the rules, and even without climate change causing CO2 emissions and our pollution, the total biomass of not just humans but our food supplies, pets and gardens, and environment reducing urban landscapes, mean that the total biomass of the rest of the biosphere must contract. Lower numbers of in total of all other species is the inevitable consequence of increased human population size.
Rules of population on our Finite Planet
Rule 1: Every Organism, Even Humans, Has Had Time for Way Over 100 Population Doublings.
Even needed 1,000 years to double population, then could double its population 100 times in 100,000 years.
Considering how most species have been around for many millions of years and are able to double in population far more rapidly than once every thousand years, most species have had time for many, many population doublings.
What is being questioned here is the time for just 100 doublings.
How long does it take humans to double in population? During the 20th century humans doubled in population on average in less than 50 years. That would enable 100 doublings in 5,000 years.
The 20th century was fast population growth, but assuming even 1/20th of the 20 century population growth rate, the population would double in 1,000 years, so 100 doublings would then take 100,000 years. Given humans have been around for 300,000 years, we have had time do 100 doublings of population 3 times over, even if we assume only 1/20th of 20 century population growth rate.
Humans are one of the most recent species on Earth, and we have had more than enough time for 100 doublings of population.
Even pandas, famous for slow reproduction, have been shown to be able to increase population 17% in a decade. A 17% increase means 117 pandas for every 100 after 10 years. Since 1.17 to the power 4.5 is greater than 2, then pandas at that rate would double in population in 4.5 decades or 45 years, which is just a little faster than human population growth rate of the past century of one doubling every 50 years. Plus, pandas have also been around far more than 100,000 years.
Relative to length of time life has existed on Earth, 100 doublings of even the slowest reproducing species on earth can happen in a very small percentage of time relative to planet over 4 billion years old. While one land has had less then 400 million years, but that is still a long time, and even humans, one of the most recently evolved species, have had 300,000 years to grow our population.
Every organism can multiply, or they could never have reached their current population level, or recover population level in the event of catastrophe or disruption. But how long can multiplying continue?
Rule 2: 100, or Just 60, Doublings, Is Impossible And Would Cover Every cm2 Of The Planet.
Every organism has had time for 100 doublings in population. But how many do you get after 100 doublings?
The ‘wheat and chessboard problem‘ illustrates how large numbers grow by repeated doubling, also known as exponential growth.
The wheat and chessboard considers doubling 63 times, in 63 steps from step 1 to step 64, doubling each step. One grain of wheat on the first square (20=1)as the starting value, leads to 2 grains on the 2nd square (21=2), 4 on the 3rd (22=4), 8 on the 4th (23=8), all the way to 9,223,372,036,854,775,808 on the 64th and last square (263). So a single living organism would result in 9,223,372,036,854,775,808 organisms after 63 doublings.
Given the total land and ocean surface area of the Earth 510,064,472 km2, and each square kilometre is 1 million square meters, the 63 steps results in 18,082 organisms per square metre of the entire surface of the Earth, which for those who do not speak metric, is over 1,800 organisms per square foot.
So starting from two humans, 62 doublings would result in 18,000 humans for every square metre of the Earth. Oceans included.
The population growth rate for humans of the 20th century is simple, so it seems imaginable that it has been happening for a long time. If every couple has 5 children, which is below the historic average prior to the 20th century, and if 4 of those 5 children live to have their own children, then humans would double in population every generation, or a doubling approximately every 30 years. Just one child above replacement rate would result in a doubling every 50 years.
But a but a doubling in population every 50 years would result in 60 doublings in just 3,000 years producing more than 1,000 individuals for every square metre of the earth, which with animals the size of either pandas or homo sapiens, would more than completely cover the surface.
For perspective, humans doubling at this rate, would have resulted in 100 doublings during the time of Ancient Egypt (over 5,000 years with almost 30 centuries as the leading civilization).
Not very comfortable for humans, but possible for something very small or perhaps microscopic. Allowing the 100 doubling steps would generate 2,485,275,234,437,872 organisms per square metre ( over 25 quadrillion per square foot) or 2,485,275,234 organisms per square millimetre of the entire surface of the Earth.
So 100 doublings would overrun the earth, even with microscopic animals:
- 2.5 billion organisms for every square millimetre of the entire surface of the Earth, as a result of doubling 100 times.
- So 25 quadrillion organisms for every square foot of the entire surface of the Earth, as a result of doubling 100 times.
For larger animals such as humans, even filling the ‘chessboard’ is not required, as just 60 doublings would mean over 1,000 individuals per square millimetre of the entire land and ocean surface. So what have humans been doing all this time? How come it took humans so long to reach 1 billion? In fact, why is the Earth not just totally overrun?
The takeaway is that every living organism, even us recently evolved homo sapiens, have had far more than enough time to double in population 100 times, but never reach a population that would result form 100 doublings. Something limits population growth.
Rule 3: Population Is Not Controlled By Starvation, But By Birth-Rates Adapting.
At First, It would Seem Availability Of Resources Must Control Population.
So, every organism on Earth has had far more than sufficient time for 100 doublings of population, but no organism has reached the incredible population number that would result from 100 doublings.
So either organisms just stop increasing in population, or something makes them increasing.
At first it may seem logical it would be starvation, as critics of human population levels always predict starvation from overpopulation, but looking at nature, population stability is everywhere, and mass starvation is the exception, and rather than being normal, it is a sign of an environmental problem.
Overpopulation Damages The Environment Before Starvation Results.
Consider a paddock with too many sheep. The sheep will eat all the grass until there is none left before they die. The overpopulation first results in environmental damage before the sheep starve. Yet grazing animals in nature don’t grow in population until the point when they destabilise the population of their food supply, somehow population is constrained before this point. Consider a mouse plague. There are so many mice they damage the environment, but far more mice are able to exist for a period time. This shows what happens when there is overpopulation, and the mouse population is not normally limited by food supply.
Life In Balanced Population Is Everywhere.
Consider our close relatives in the wild, chimpanzees, bonobos and even gorillas. To our knowledge, none of these animals was experiencing significant population growth prior to their recent population decline due to habit loss. What stopped their population expanding, given that, like all species, their birth rates can achieve population growth?
There seems no evidence that starvation is the mechanism of population control, as we do not see a percentage of chimpanzees, bonobos or gorillas starving, and nor do we see their populations growing to level where they damage the environment. If starvation was the mechanism of population control it would be everywhere throughout nature.
This is explored in full in ‘optimal population‘, and the ‘whales‘ section provides an example of population reaching a plateaux with any observed starvation.
The conclusion is clear, animals tune their rate of reproduction to match what the environment can support. While bacteria in a petri dish may simply populate exponentially in total ignorance that they will exceed what is a viable population, more evolved species adjust their rate of reproduction in response to what is happening in the environment. Yes, this mechanism cannot handle all sudden environmental changes, which is why there are mice plagues, but it does adjust, which is why the plagues always end.
Every species can reproduce to quickly exceed environmental carrying capacity, but, instead of population growth to levels that will overtax and destroy the environment, almost all species can normally managed to control their own population. Without such a mechanism, overpopulation of species would result in environmental collapse, followed by mass starvation and they a population collapse before the cycle repeats. It may not be a coincidence than animals with many predators have many young, while apex predators do not.
All it takes is a simple nature walk to see that animals almost always exist in sustainable numbers, and their population is not controlled by a significant portion of their population starving, or having devastated their food supply through overpopulation.
This means every organism exists in sustainable numbers, without starvation acting as population control.
Rule 4: Population Stability Is Normal, But When There Is A Vacancy, It Is Quickly Filled.
The example of the humpback whales of the South Atlantic has already be used to illustrate how population growth stops once optimum population is reached, but this example also illustrates how quickly populations can recover following setback. While the whales were unable to increase reproduction sufficiently to stabilise population while whaling accelerated, the whales where able to return the population to the historic level previously maintained for thousands of year of around 30,000 from just 440 whales in within 70 years. This demonstrates clearly how when a ‘vacancy’ does arises, species can respond to fill the vacancy.
The times an organism would experience a vacancy and respond with population growth are:
- When a species first evolves, first reaches a new suitable environment, or evolves new traits enabling out competing other species.
- Following a major catastrophe or disruption that reduced the population below previous levels.
- In the event of changes to the environment that alter constraints, such as weather or climate events, or disruption of predators or competitors for resources.
Note that as all similar environments are not necessarily connected, an organism reach a new suitable environment, long after it first existed on the planet.
When population increases are observed, either a for a new species, or species new to the environment, or following catastrophes or other major disruptions, the population growth of what is for that species, effectively a new vacancy, or an opportunity to evict the current occupants.
Vacancies can be short term, such as weather events, long term such as ice ages and long term climate events, or the result of evolution as observed by Darwin, or evolution of technology such as stone tools, or farming.
Rule 5: Total life on Earth is not increasing, so increase of any one species requires a decrease in others.
On any planet, in the early days of life, species can simply colonise the planet, resulting in an increasing amount of total life. This “total life” is called the biosphere, and while the biosphere is increasing, there can be reoom for more life in total.
But, it turns out, the biosphere is not only no longer increasing, as explained in the timeline of the solar system paper, and total life on Earth has been in gradual decline for the past 540 million years.
The first land plants and animals appeared about the beginning of the Phanerozoic Era around 540 million years ago, when land first became inhabitable due to the atmosphere finally having enough oxygen to block harmful radiation and provide for respiration, creating a boost to the amount of the planet available for life to exist. But exponential growth means this new space was used up in what appears on geological times scales as ‘instantly’.
Without some other similar change that no one so far can foresee, from the point land became habitable. no increase in total life will again be possible.
If every environment is fully populated, and not expanding, then the only way to increase population is to outcompete other species. Outcompeting other species requires some form of evolution, either of genetics or behaviours.
Since the start of the Phanerozoic Era, the land joined the oceans in seeing a succession of life has replaced previous forms of life, with each species that dominated a niche reaching, and then remaining at the capacity of that niche, before eventually being replaced by an improved species.
Rule 6: Every species must find population stability at some point while limited to one finite planet.
Continuing to replace other species has a limit. Eventually there would be only one species.
Some species are automatically resource constrained from overpopulation. I suspect this applies to all plants, as a major resource, sunlight, cannot be ‘overconsumed’. However even a population of butterflies can reach a population level where their caterpillars consume all food in their environment, and as their food needs time to grow, this would leave no food for the next generation.
All organisms need to ensure they live sustainably, and for any organism that relies on existence of sufficient numbers of other organisms for food or coexistence, this means some mechanism to ensure they do not out compete the very organisms they rely upon.
What about humans? Are We exempt from these rules?
Human Population Growth, is it still in unconstrained growth?
It could appear that human population is still growing long after we should have reached our constraints.
Have humans managed to have broken these rules? The theory says we humans should have reached a stable population close to 300,000 years ago, at which point population growth would stop unless humans continued to evolved to become ‘fitter’ for existing or new environments.
Yet human population growth still continues, as was doubling almost every generation as recently as between in 1965-1972, and doubling every 50 years for most the 20th century.
This would seem to suggest humans have never reached their limit, and our population is still growing unconstrained.
But further exploration reveals this recent growth is not a continuation of historical trends. Homo Sapiens have existed for at least 300,000 years, which is sufficient for 6,000 doublings of population, yet if there were only 2 people 300,000 years ago, the population growth to 8 people billion now represents just 32 doublings in over 300,000 years. Just 32 doubling in that time is a population doubling at an average rate of less than once every 9,000 years.
To take 9,000 years to double the population requires an annual growth rate of around 0.008%. A rate close perfect population stability, yet we have seen from recent times, that there can be period of rapid population growth. It seems likely that growth is mostly effectively zero, with occasional burst of real growth, and we have just witnessed one of these bursts.
While, looking at the history of human population growth, as far back as back as we have any data, we have never before seen population growth anywhere near the level that was seen in the 20th century, we do as humans seem to keep raising the bar of what constitutes our optimum population level.
Even excluding the recent population explosion, human population growth seems to have extended far longer than the rules suggest, unless their has been an expansion of the environment, or evolution in some form.
The reality is there has been evolution. Humanity has evolved through knowledge and technology, without us evolving physically as individuals. Each step of evolution allows us to out compete more and more of the rest of nature.
Burst of Population Growth Through Technical Evolution.
Instead of a recent series of steps of biological evolution, humans have experienced technical evolution.
A list of some notable steps includes:
- A population expansion during the Palaeolithic age arounds 60-80,000 years ago, through “the emergence of newer, more advanced hunting technologies”, following further developments of language.
- Another growth phase at the start of the Neolithic age that from around 5,000BCE 7,000 years ago to 200BCE as farming spread to global adoption (see graph).
- The population explosion from around 1650 with the industrial age and the reductions in child mortality.
Note that even during periods of population stability, from 10,000BCE to 5,000BCE and from 200 BCE to 1600AD, there was still some population growth as humans managed to colonise more locations.
Ignorant Displacement: As Population Grows, The Displaced Go Unnoticed.
Our current society has evolved the technology to be ‘the fittest’ in almost any niches, that we can maintain a higher human population than ever before. We can also, per unit land, maintain a higher population of crops and livestock to feed us than ever before.
The downside is a history of not even seeing organisms displaced population increases are introduced.
In fact, historically even other humans displaced by humans have been repeatedly overlooked and/or underestimated. Despite that experts now believe between 10 and 16 million people lived above the Rio Grande in North America prior to Europeans arriving:
Few contemporaries agreed with Catlin’s lofty estimate [16 million] of the Indian population before contact with the white man. “Twaddle about imaginary millions,” scoffed one Smithsonian expert, reflecting the prevailing view that Indians were too incompetent to have ever reached large numbers. Alexis de Tocqueville’s cheery assertion that America before Columbus was an “empty continent… awaiting its inhabitants” was endorsed by no less than the U.S. Census Bureau, which in 1894 warned against accepting Indian “legends” as facts. “Investigation shows,” the bureau said, “that the aboriginal population within the present United States at the beginning of the Columbian period could not have exceeded much over 500,000.”How Many People Were Here Before Columbus?
Even if there were only 500,000 people before Columbus, the nature of exponential population growth tell us, that as people had been in North America for around 30,000 years, the continent would have been populated up to the level of environmental constraints. Any land mass with even 3,000 years occupation will reach the maximum population possible for that society. Yet to people from Europe, America was ‘an empty continent’. Not only did the new arrivals not understand or see that the continent would be fully populated with the current population, they even failed to recognise the size of that population.
The new arrivals failed to recognise that this ‘new world’ continent was fully populated, and that their arrival must displace those living there already. In the 30,000 years since people first arrived in America, culture in free trading European/Middle Eastern/Asian society had managed to evolve 1,000 or perhaps even 2,000 years further in terms of dominating more of the environment, increasing population density and as a result displacing other organisms. The population of many species would need to decline in order to accommodate the influence of European/Middle Eastern/Asian evolution of society.
The spread to new territory and the impact on life before that spread highlights the changes humans had over time to the environment of Europe/Middle East/Asia, displacing other species as advances made humans the most ‘fit’ for ever more niches within the environment.
Delusions Shattered And Questions Raised.
Calculating these numbers, has shattered some illusions I had previously held, but has also raised some interesting questions still to be answered. I had not thought through, that every increase in human population, and the population of the chosen foods of humans, inherently means a reduction in other populations. I had thought habitat destruction and decline of natural diversity as solely a consequence of poor management, rather than a mix of poor management and a natural unavoidable consequence of population growth.
The problem seems to be that I looked a the world through a filter which does not recognise what is different and being displaced, in the same way Europeans in the ‘New World’ did not recognise what they were displacing.
- Shattered Delusions:
- Both North America and Australia were fully populated prior to the arrival of Europeans.
- I had previously thought, as apparently did the European arrivals, that these continents were only sparsely populated prior to Europeans arriving. Realistically, any land mass continuous land mass occupied by humans for as long 3,000 years will be fully populated to the extent possible by that human society, as were North America and Australia.
- I had thought population levels have been growing because the Earth had never been populated to capacity.
- The reality is, Earth has been populated to capacity for the hundreds of thousands of years. Population increases result from changes to society that allow humans and their food to displace other species of life on Earth.
- The question that arises is, has the recent unprecedented population explosion stayed within the bounds of the population now supported by our changed society, or has the change to infant mortality created an ‘overshoot’ resulting in overpopulation and the environmental damage that follows.
- Both North America and Australia were fully populated prior to the arrival of Europeans.
- The questions raised:
- As already covered, has the population explosion resulted in overshoot? Yes.
- What does natural population constraint look like? This was explored in Optimum Population.
Conclusion: Humans Were Not Granted An Empty Planet.
There is the biblical line: “go forth an multiply”. To me it suggests humans being given an empty, and effectively infinite planet, ready to be colonised.
I personally grew up, not necessarily with the religious aspect, but with the concept of a planet that is effectively partially vacant, and growth of population of all living things was how nature worked.
Yet, long before the first human walked the Earth, there was already ‘no vacancy’. For humanity to even exist, we had to outcompete and displace other living things. But is it our mission to replace every living thing possible until it is just us and the food we farm?
If our mission is to perpetually deliver economic growth, as opposed to wealth per person, then yes, continual population growth is the simplest path to that mission.
However it may be that at some point, it feels like humanity is being ‘farmed’ to generate wealth for a small subset of people, at some point our farm will start to feel crowded to the point of existing like battery hens, rather than having our free range.
- 2022 April 4: fix typos.