Optimum Population Responses: nature’s birth control.

Introduction: Natural Optimum Population Responses.

The concept of a natural ‘Optimum’ population responses is that the population of many species is determined not only by resource constraints and predation, but also by natural responses to environment that trigger changes in the rate of reproduction.

There is a separate debate on what population of humanity would be optimum for us individual humans, but this is about a ‘natural optimum’ population, which from as a result of evolution, is not necessarily about individual happiness, but about sustainable survival of any given species.

As opposed to the model of the ‘bacteria in a petri dish’ that perpetually grows in numbers until all resources are consumed, as established in the section ‘noticeable population growth is not normal’ below, population growth of any species normally follows a similar growth pattern to the pattern the population of cells within an individual animal:

1. First cell(s) and then birth of an individual, or first individuals as the origin of a new species.
2. A growth phase, where number of cells increases as the animal increases in size, or with a species, as the population of the species increases.
3. Maturity at an optimum stable size, or for species, maturity at an optimum stable population determined by:
   • Resource constraint by external resources such as water or sunlight.
   • ‘Optimum’ population mechanisms that adjust the rate of reproduction according to numbers and conditions.

Obviously the ‘life span’ is on an entirely different timescale for species vs individuals, with species ‘born’ far less often, and able to ‘live’ for hundreds of millions of years.

The key point is that there are different rates of reproduction of cells in the ‘growth phase’ than in the stability phase, and this all occurs naturally in every species, with population growth of cells ending when an individual is at peak health.

That humanity does not appear to be following this pattern is discussed in why human population growth even before the explosion, but that pattern is followed by basically all other species.

When you look at the natural world, it is clear all other complex organisms on Earth have, and can maintain over the longer term, an ‘optimum population’ level for any environment, without the need for starvation, nor, outside of plagues with are the exception rather than the normal, populations rising to unsustainable levels and environmental collapse.

Other species seem to somehow, in normal circumstances, avoid overpopulation. Even when a rapid change of circumstances does result in overpopulation or plagues, the population level soon returns to stable levels.

The External factors of predation and resource constraint can both play a role in determining population numbers, and for some species, resource constraint alone may be the only mechanism required. However, for most species, numbers would not balance without additional mechanisms that vary the number of offspring is required to achieve population stability for any given environment.

Just as the rate cell reproduction responds to needs, so does the reproductive rate of most species, with mechanisms to target an optimum population, rather than a perpetual population growth. For example, despite that just 5,000 years of reproduction would be more than enough to carpet the planet with gorillas, the mountain gorillas of Gorillas in the Mist have existed in their limited environments for up to 8-12 million years without overpopulation despite having no predators.

In some animals, such as kangaroos, we can easily observe mechanisms which naturally control their rate of reproduction in order to maintain option population, but what is surprising, is how when looking at the data, it seems likely most advanced animals to some extent naturally control their population.

So, there are mechanism in nature that vary rates of reproduction, and more examples are outlined below.

The big question is whether humans also have such mechanisms, and whether what we are seeing with the current decline in human birth-rates is a natural response that is in fact natural and key to our survival as a species, or the coincidence of separate health problems all with the same outcome, that need to be feared.

Surprise: Noticeable Population Growth Is Not Normal.

I grew up with the belief population growth must be normal.

I grew up in a world where it seemed population growth was ‘normal’. In Australia, as with other ‘new world’ countries, the population was always growing, and ‘development’ to build new homes for an ever-increasing population. Even now, in 2021, governments at all levels plan only for perpetual growth, never considering the concept of a population target, or an end goal.

I assumed that the population of all living things just kept increasing. If the expectation was that the human population will just keep growing, why would that no apply to every species? Like the religious phrase, “go forth and multiply“.

Perhaps the writers of the words assumed also assumed that population growth must be normal. At that the words were written, with the population observed from the approximately 150 million of the time, on a world without clear know limits, commonly assumed at that time to be less than 5,000 years old, the maths could have looked to add up.

The rate of human population growth I have experience in Australia, is also far faster growth, than humans experience prior to the industrial revolution. If populations of wildlife grew at this rate, visits to national parks would see more and more wildlife

The rate of human population growth I have experience in Australia, is also far faster growth, than humans experience prior to the industrial revolution. If populations of wildlife grew at this rate, visits to national parks would see more and more wildlife each decade.

Of course, 2,000 years later, and with evidence the Earth is far older, it should have occurred to me that any reasonable rate of continued multiplying, would result in an impossible amount of life on Earth.

With what we know now, the maths for continuous population growth doesn’t add up.

Plus, even representations of the age of dinosaurs, depict a planet already teaming with life, well over 100 million years ago. It should have been quite obvious to me that if those images are all accurate, there cannot have been a significant increase in the total amount of life happening in that last 100 million years of time. In fact, even if I had it wrong and thought that instead of there being at least a similar total amount of life even back back then, I thought there was instead just one tenth (1/10th) as much total life back in the age of the dinosaurs as there is life now, I should a realised even an increase in the amount of life by a factor of 10x over 100 million years, is amazingly close to no population growth at all. In fact, that would be annual growth at a rate of less than 0.000021% per year. All this proves is the logic for either very gradual population change, or none at all. But what about actual data from research?

In fact, the science makes it clear that the total amount of life on Earth is falling.

Looking at research papers on the timeline of life on Earth reveals the consensus is that the greatest amount of life at one time, or ‘peak life’ on Earth, occurred very early in the ‘era of visible life’, the Phanerozoic era, about 500 million years ago, when complex life was abundant, and the higher CO₂ levels that were possible with the fainter Sun earlier in its lifetime, resulted in more efficient photosynthesis. Ever since, the population of life overall has been on a very, very gradual decline.

In summary, despite observed human populating growth, there is no for live overall on Earth, the amount of life is not increasing, and it is all about, as Darwin observed, survival of the fittest, with every species experiencing growth in population being offset by other species experiencing reduction in population.

The Full Planet Problem.

The second surprise was what logically followed on from the first: if the total amount of life on Earth is not increasing, then any increase in the population of one species, such as more humans, must correlate with a decrease in the total population of other species.

The limit of the population of humans, would be when the total life on the planet consists only of humans, and food for humans.

Why Can’t We Just Keep Growing After We Grow Up?

The Absence Of A Human Role Model.

Imagine a human growing up, without any adults as a reference as to their future size. Year after year this human gets larger. Why would this person assume that at a certain age they will stop growing? I can imagine it would be disturbing for such an individual, having experienced year after year of increased growth, to observe their growth decrease and eventually stop. In such a situation, would you wonder if you were dying? Or worry if something was wrong with you to stop you growing?

Or perhaps this. lone individual human, would notice that other animals tend to quickly grown to s specific size, and then exist at very much that same size for the rest of their life?

With Three Centuries of Abnormal Population Growth, Perspective Is Distorted.

Long term, average human population growth is negligible, and less than 0.05%.

Strangely, no one seems to look at nature and realise that no other living organism is experiencing long term substantial population growth. If it was normal for a population to keep increasing year after year, wouldn’t we expect every species on the planet to be increasing in population year after year?

I grew up in Australia, a “new world” country, that imagined itself as a “young country”. From the declaration of ‘Australia’ as a nation in 1901 through to 2000, the population grew by a factor of 5 from 3.8 million to 19 million, and everyone simply seems to assume that rate of growth should continue forever. Unlike a child that grows to maturity, there has never been any thought about the ‘young’ nation growth stopping once the country is an adult. Australia, and many other countries, just pictures growth continuing for ever, despite that nature all around us follows a different pattern.

Don’t We Notice Stability at “Optimum Population” in Nature?

Maybe the human growing in the absence of other humans, would see that other animals grow to an ‘adult’ size and stop growing, but would assume as a human, what happens with other animals will not happen with them? Humans have habit of assuming we are beyond the rest of nature, and it can seem that we have not learnt about population growth from observing other animals. We don’t expect the populations of other living things to just keep growing, and we don’t expect their lack of population growth to be result of significant starvation, yet it is common to assume that our own population will naturally just keep growing unless our population is constrained by starvation or some other problem.

When I walk though a national park, I have never even though about the fact that the number of lizards in that park has remained basically the same for millions years, because I assumed that like us humans have lately, all animals must always increase in population. Of course when you think about it, clearly it is impossible for these species to have been increasing in number for millions of year, but I never thought about it.

But I had also not thought about the fact that when I enter a national park, there is no abundance of animals dead from starvation as a consequence animals having too many young. When there are fires or other disasters naturalists all talk of populations recovering, but as humans we don’t think about how these populations reach an ‘optimum’ level and then remain at that level.

Somehow, we have animals all around us, most existing at or near their with ‘optimal population’, without it occurring to us to ask “do we have an optimal population?”

Society Blindness: We don’t even notice ‘optimum population’ in other societies .

Australia started as a country in 1901, with a belief it was that the country needed to grow, with population growth required to populate the what was considered an underpopulated land prior to Europeans arriving. But the blindness began with first contact between Australian aboriginal people and Europeans. It never even occurred to the Europeans that Australia was already fully populated.

Despite that it is now estimated there were between 300,000 and 1.25 million inhabitants, Australia was seen by the Europeans as effectively mostly uninhabited, or terra nullius when Europeans firs arrived. In fact, logic and some simple maths dictates that after over 50,000 years of being inhabited, Australia had to have reached a population stability, and thus was already fully populated. The same applies to North America, where again Europeans assumed native populations exempt from the continual exponential growth they have come to assume as normal for their own society, leading to false assumptions that land masses were “unoccupied”.

The 3 drivers of stable population.

‘Optimal Population’ vs Resource Constraint and/or Predation.

Despite our human experiences that can create either the impression that population is always growing, and perhaps that population growth is out of control, those studying other species are very familiar with the fact that population stability is the normal. Two main mechanisms are well know factors in ensuring population stability, and this sections looks at these two factors, and why they alone cannot deliver the population stability we actually observe, and how the mechanism of ‘optimum’ population are necessary to explain what we see in nature.

Resource Constraint: In some circumstances.

The Principal.

It sounds simple. The number of organisms is limited by available resources. In fact, this simple model does apply for some living organisms.

Recall the original “bacteria in a petri dish” by David Suzuki? This example highlights that:

• bacteria do demonstrate resource constraint,
• the limitations of relying on resource constraint for population control.

When Resource Control Does work for population control.

Consider grass in a paddock. The grass consumes CO₂, sunlight, water, and nutrients from the soil. For that paddock the CO2 supply is effectively infinite.

The sunlight and water supply are determined by sunlight and rain that reaches the paddock. If the grass is resource constrained by either water or sunlight, then grass will grow until it is resource constrained by either sunlight or water, and can continue at that amount of grass as long as resources continue to be available.

If the amount grass is constrained by soil nutrients, then the picture becomes more complex. For farmers, they can rotate crops in this situation, as growing grass that exhausts the supply of soil nutrients will be problematic.

What is clear, is that provided it is sunlight or water that limits population, then population control by resources control works.

Carrying capacity.

There is a well established concept in environmental science: carrying capacity. Carrying capacity is maximum amount/number of any species which can exist sustainably within an environment. Exceed carrying capacity, and the environment sustains damage, which then reduces carrying capacity.

In the above example the carrying capacity of the paddock for grass could be exceeded if the amount of grass is constrained by soil nutrients, but cannot be exceeded if the amount of grass is constrained by sunlight or water.

The problem with population control by resource constraint.

The problem with population control by resource constraint, is that for many species, resource constraint only limits population growth after the population exceeds carrying capacity. For example, the grass keeps growing even if consuming soil nutrients faster than soil nutrients are being produced.

Consider sheep left to breed and grow in the finite environment of the same large paddock. If there are too many sheep, the first consequence is that, rather than all excess sheep just dying, the grass that is their food is over grazed, damaging the grass and lowering the paddock carrying capacity for sheep. By the time sheep are staving, grass has been over eaten and the carrying capacity is now very low, and then the sheep population will fall far below the normal paddock carrying capacity for sheep. Once the sheep numbers are low, the grass can then start to recover. Now consider the options for what happens next.

1. If the sheep population then recovers quickly, the grass would never get to return to the level that provided maximum carrying capacity for grass, or for the sheep.
2. If the sheep population recovers slowly, the grass can reach its maximum carrying capacity before the growing population of sheep could grow to again have too many sheep, and the cycle would repeat.

Neither 1 or 2 is ideal for either the grass, or the sheep. Both sheep and grass will at least at times have sub optimal populations, that would make them at great risk of dying out in the event of some damaging environmental event. From an evolution perspective, the sheep are “less fit” then sheep who could limit their population at paddock carrying capacity for sheep.

Observation: Optimum population vs resource constraint.

Observation reveals most complex organisms have evolved population control that avoids the problems of relying on resource constraint. Even ignoring the fact that relying on resource constraint could devastate critical resources, we rarely see animals where the ongoing normal population control mechanism, is a significant number of deaths by starvation as a result of lack of resources. If population control was a result of only resource constraints at work, other than for a period after a population collapse, there would always be too many lions in a safari park and a given percentage would always be dying of starvation. When we went into a national park, we would see a percentage of dying starving lions, and there would be cycles populations collapsing and recovering as lions exceed carrying capacity and then damage their own food supply.

What we actually see is that, as with the kangaroos, animals seem to normally manage to reproduce in numbers such that their surviving offspring can be sustainably supported in their environment.

Starvation: The trough of a population on a roller coaster ride.

In the 60s and 70s, people were predicting overpopulation of humanity would lead to mass starvation. The birth-rates of the 60s and 70s did not continue, but in any event, environmental damage is the indicator for overpopulation, and starvation only follows environmental collapse. Consider a paddock with too many sheep. The sheep will not start to starve until they have eaten every single blade of grass, which makes the paddock a devastated environment for the sheep, and from that point, they starve. The grass will take a long time to recover, and until it does, the paddock will for a long to be unable to support even a far smaller population of sheep. In nature, even though there are plagues from population surges, we clearly do not see the pattern of animals reaching a population that would exhaust their food supply as a common pattern in nature. This same pattern of population falling prior to starvation seems to apply with any resources that can be exhausted by overpopulation. Starvation is most often of a sudden reduction of a resource, and most often renewing resources from outside the environment, such as water which is replaced by rain even following all water having being consumed, once food sources are totally depleted, they do not recover quickly.

Constraint by Predation: Only ever part of the answer.

Predation alone may not result in stable population.

One alternative to a species controlling its own population would be external control from predation. Certainly for many animals, a reduction in the number of predators will see population numbers increase, which suggests at least two possible explanations for this observation:

1. The species is has no population control mechanism other than predation.
2. The range of reproductive rates of the species has evolved to allow for predation.

The first explanation has three problems. Firstly, it is natural that the species would evolve before predators for their species exist, so they would have nothing to prevent catastrophic population growth prior to predators appearing. Secondly, while predation would slow population growth, there is no way it would automatically result in a stable population, as any time the population of the species grows, survival rate from predation would increase because predators cannot instantly increase in number, resulting in even faster population growth, and a decline in population would result in a higher rate of predation leading to extinction. Thirdly, this mechanism breaks down with apex predators, for whom predation provides no population control, and so apex predators would continue population growth until they wipe out all of their prey.

The second explanation seems far more likely, and species adjusted reproduction rates as predators evolve or enter their environment. Although in theory, the adjustment could come solely by evolution of the rate itself, evolving a mechanism that allowed some variation as rate of predation varies would clearly provide a more robust species.

The optimum population hypothesis: technical analysis.

There are really only two ways births can so precisely match deaths. Either:

1. Deaths track births so deaths end up occurring at the exact rate which will exactly balance the number of births, so all species end up with exactly two children who will reproduce, per female.
   • or
2. Births track deaths, so births end up happening at the exact rate to match deaths, and enable two surviving children per female.

Of course, humans have not been achieving this balance in recent centuries, but consider what would be required for humans. Avoiding the complication of changes in couples, this requires and average two surviving and “breeding” children per couple. Four surviving long term per grandparent, and 8 surviving long term per great grand parent etc.. Any more or less on average, and population is changing.

For option 1, deaths to track births, then for all couples on average, children must be dying before having their own children at a rate that results in only two children surviving per couple. This balance will work if infant deaths always rise as population increases. In the case the population will grow until the balance is achieved.

For option 2, births to track deaths, then the balance requires an optimum population mechanism, where species can vary their birth rate.

If the answer is always option 1, then a balanced population requires the rate of child death, or deaths before reaching being able to reproduce, must rise as the population increases, as population growth will not stop until child deaths have reached the target. So if each couple has 7 children, the population will grow until eventually, 5 children are on average dying per couple.

If the answer is at least sometimes option 2, then population growth will at least sometimes stabilise without the need for a rise in the rate of infant deaths. To be specific, if there are examples where a population grows and then reaches stability without the stability being a result of increased child deaths, then that is an example of an optimum population mechanism, where births reduced to balance deaths at a target population.

Examples: Natural Population Re-Stabilisation at ‘optimum’.

Real World Example: Humpback Whales Population Rebound.

As an example of an optimal population mechanism at work, consider humpback whales. The population of humpbacks in the south Atlantic ocean fell from an estimated 23,000 to 34,000 in 1830 to 440 by in the late 1950s, and has since recovered to an estimated 99% of their previous population.

This is profound. The population the whales reached after existing for over 1 million years, is almost the exact same population level their numbers returned to and have again stabilised at, within 70 years after the population had been decimated. Yet no rise in mortality of whale calves was observed.

To return to the population required 5.7 doublings of population, in less than 70 years, which is on average, one doubling every 12 years. Despite their ability to double the population once every 12 years, and having existed in the Earths oceans without any significant predators prior to whaling for well over 1 million years, their population long ago reached a specific level, and then remained for millennia at that level. If starting with just two whales, this ‘optimum’ population would be reached in just 15 doublings, which at the rate seen recently of a doubling every 12 years, means the whales had enough time to double in population 83,000 times, their population stopped doubling, after just 15 doublings, even if there were only 2 whales 1 million years ago. Clearly there is a normal whale population, and as these whales have not decimated their plankton food source, and there are not young whales continually dying of starvation or disease , the ‘optimal’ population is not a result of increased deaths, or whales or running out of food. Some natural process results in an whales growing in population up to an optimum number and then population stabilising, without increased child deaths, as also happens with elephants, lions, or many other animals.

Despite every animal on Earth having had more than enough time to overpopulation many times over, most animals reach an ‘optimum’ population level, at which point they only reproduce at a level that results in a stable population. At least for a large number of animals, this happen without an increased child death rate finally to balance the rate of births that grew their population until that point.

Observations On Population Stability.

A Species As A Living Organism.

The normal growth pattern for an individual is for each individual to experience a growth phase, until reaching an adult size, and then to remain at approximately the same size for the main phase of their life. Every individual animal is also a ‘colony of cells’, and continues making new cells for its entire life, but the population of cells reaches a point of stability at the end of the growth phase, and while new cells continue to be produced, for most of the individuals life there is a stable population of cells. From maturity the number of cells in the animal remains at a ‘ideal’ population: cell population stability. New cells are always being produced, but once mature, this happens only fast enough to continue the stable population. The average cell in a human is around 7 years old, even if the human is 70 years old. The same ‘person’ even though almost all the cells are new. When there is damage, there can be more rapid grown, just as when we lose skin cells, but once the damage is repaired, cancer aside, cell reproduction goes back to just the rate required for population stability.

The population grows to ‘ideal population’, and then cell production naturally drops to the level required to achieve population stability, just as cells populations do in an individual adult. If the population is temporality reduced, the population will recover and again stabilize.

The concept of optimal population, is that the entire species can also be considered as a living organism that grows the population of individuals until maturity when ‘ideal population’ is reached, and then there is population stability.

Did Elephant or other animal populations keep growing in the past?

I existed my entire life until recently, assuming that reproduction just blindly produced offspring in some fixed ratio relative to their parents, and only recently realised this is impossible as populations would either never grow, or perpetually grow unless deaths start increasing.

If we look at the populations of animals such as elephants, prior habitat destruction and poaching by humans, all evidence indicates populations historically were stable. This is the same from whales in the ocean pre-whaling, through to polar bears and penguins. For creatures such as penguins, it would be possible that

Plants and animals have flourished in natural habitats without perpetual population growth, and given the length of time these organisms have existed, if there was continual population growth, then every habitat would soon be overrun. The reality is we see that population growth is not ubiquitous in nature, and at least almost all species manage a stable population when the environment is stable. This suggests either, all species are resource constrained, or that there are natural mechanisms that control population growth.

It is clear that some mechanism keeps populations from perpetual growth, as when you do the maths, every creature on the planet has had sufficient time to reproduce to staggering numbers.

So what is it that constrains populations?

Two principles become relevant:

1. “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”
2. Occam’s razor

The first I believe is only half relevant, in that it is important to eliminate that which is no possible as an explanation. There are possible partial explanations that may apply for a percentage of species, such as predation could be a mechanism for for penguins, but does seems impossible as an explanation for elephants, polar bears, or whales, but are still worth examining even as a partial explanation.

Once the impossible is rules out, instead of just accepting the improbably, I prefer to look for evidence to support what remains, and choose the simplest answer that fits with that evidence.

The Elk in Yellowstone: elimination of predator example.

An interesting example is the experience the Elk in Yellowstone. At first it may seem that elk did not adjust to the removal of their predator, but in fact this actually provides a sound example of ‘optimal population’. Firstly, note the recovering wolf population could grow quickly back to previous levels, but stopped growing prior to exceeding the park carrying capacity for wolves.

More importantly, note that while the elk population grew, and elk displaced other species after their predator the wolves had been removed, it was not perpetual exponential growth until the elk began starving. Rather, the elk population stabilised after the initial population growth. With no reports of the same percentage of elks dying from some new cause as there were when elks were preyed on by wolves, it becomes clear, elks managed to lower their number of offspring, once the population reached a new optimum. The elk did not need a new mechanism in terms of a new predator, or more deaths from starvation for population stability at the new higher level. Previously, a percentage of elk would die before ending their reproductive years due to wolves, or the population would not have surged after the removal of wolves.

The new higher population of elk was however highly detrimental to other species. Although this new elk population level damaged the environment from our perspective, there was no reason elks needed to constrain their population at the original level, unless they wanted the same numbers of the other species to be able to live.

With humans, we while we had long overcome wolves as predators, it can be easy to overlook that disease continued to prey on our young and elderly in much the same manner as the wolves preyed on the elk.

Small living things like plague and smallpox, that meant, prior to 1800, the normal life expectancy of a human was between 30 and 40 years, not because no one lived to 70 or 80, but because 2 out 3 people died as children. Historically, just like elks, human had to reproduce in numbers that allowed for the attrition. Until the 20th century, most humans were lost to ‘predators’ prior to reaching old age. Humans have largely now overcome ‘predation’ from childhood disease, and as a result our population is projected to grow from 500 million in 1650 to 10 billion later this century. Growth from that factor should then stop due to a new lower birth rate, but with far more humans, and far fewer of many other species. In this case, we humans don’t want the old balance back, and that is the difference with the elks.

Predation does play a role, but as with humans where the predation was previously disease, removal of predation would normally result in a surge in population that displaces numbers of many other species.

Data suggests the elk population had reached a new stability, although to the detriment of many other species, just as humans will reach a new stability, but to the detriment of many other species. Had we been willing to live with more elk at the expense of other species, as we are with more humans at the expense of other species, it is likely the elk would have maintained a new stable population with a lower birth-rate that had adjusted to less predation.

Humanity: Continuous Growth, or an ‘Optimum Population’.

It Can Look like Resource Constrained Continuous Growth: But its not!

There is that David Suzuki model of population growth like bacteria in a petri dish, which looks a lot like t human population growth of 1960, or even 1980. Human population growth has been at 2% per year a rate of doubling every 34 years! Clearly unsustainable, unrestrained growth that will soon result in all resources being decimated!

But then, stepping back, doubling every 34 years would produce 64 doubling in just over 2,000 year, so we would expect at least 9,223,372,036,854,775,808 humans by now, even if there were only two humans just over 2,000 years ago! We should have at least 18,000 humans per square metre of the entire earth!

It turns out, through most of history, population growth was almost non-existent, and then we have a had a recent explosion.

This seems the exact opposite of what is expected. Instead of growth before reaching a ‘optimal population’, humanity had a boost of growth long after reaching ‘optimal population’.

Full Human Population History: The Reality Of Human Population Growth.

If we consider humans as a species that evolved around 300,000 years ago, we should have been able to reach our ‘optimum’ population within at most 10,000 years.

Yet, although normally very gradual, there has been long term human population growth. Discounting the recent population explosion since 1800 for the moment, to go from two humans to around 1 billion humans in 1800 CE, would be an annual growth rate of only 0.007% per annum. That is assuming continuous homogenous growth, which seems unlikely. But there is still a long term trend of very gradual growth, and that can’t happen with every species, or the world would be getting fuller and fuller, and it is not.

An increase in population of one species would normally occur following a step in evolution, and result a decline in the population of other species displaced by the new improved more evolved species. With humans, it is not the species that has continued to evolve, but the societies of people that have evolved.

Looking more closely, human societies have driven population increases due to changes:

• Increased range due to migration.
• Improved tools in the progression from Palaeolithic age to Neolithic age.
• The invention of farming.
• The rise of civilizations.
• Continual introduction of of new technology.
• Propagation of new technology societies.

So to put all of this together, the history of human population is linked to the evolution of not the human species, but instead evolution of human society. The pattern is that human populations:

• Are completely static for most of human history.
• Exhibit very slow growth are evolution of society increases range and slowly improves technologies.
• Has seen bursts of growth following major society evolution such as the introduction of farming.
• Saw unprecedented growth during the population explosion from around 1800 to year 2000.

Population And The Evolution of Human Societies.

The optimal population principle should see a ‘optimal population’ quickly achieved, followed by population stability until evolution results in a new species.

People have not really evolved, but society certainly has evolved. In many ways, rather than the species ‘homo sapiens’ being the organism, the organism is the society. Practices adopted by each society changes both the ability to compete with other species for resources, and ‘optimal population’ that can be sustainably supported within a given environment. The entire basis of ‘optimal population’ is that the control of population evolved to protect the organism that is the colony. Just as individuals stop growing at maturity, societies stop growing at maturity.

But when a society evolves into a new society, just as when an individual gives birth to offspring, the limitation of growth is reset, and the society grows to a new maturity, just as would an individual baby.

As the evolved society grows to a new ‘normal’ people can increase family sizes, lifting population levels. Still, in all the more recent human history where we have statistics, population levels were remarkably stable up until the recent population explosion. In fact, population levels have been so stable, that either it has just been an amazing coincidence, or humans also have some mechanism that has ensured births are in balance with deaths.

Historically, we can see over time the human population has grown at specific times, but the long term stability suggests long periods of population stability. As a ‘colony’ or society, there has been major evolution, sometimes gradually, and sometimes in great leaps. Population ‘normal’ does seem to increase incredibly gradually when society evolves gradually, and move ahead in leaps with breakthroughs to society like the introduction of farming.

Population Explosion: A Breakdown of ‘Optimal Population’.

Prior to around 1650, humanity had been a very close to population stability, with 10,000 years of average growth below 0.05%, there had been close to population stability, but everything soon changed.

An initial assessment could be that the industrial revolution must be an evolution of society that triggered a new, higher ‘optimal population’ for human beings, so in response the population exploded. Was it improved farming techniques able to support a larger population, reduced deaths from starvation, or triggered people to have more children?

Turns out, it was none of these, and was instead a reduction in infant mortality.

Reality is, it was not reduced deaths from starvation during the explosion, and instead, in countries, such as China and Bangladesh their population explosion caused famines. There is a link for the population boom causing starvation, and no link for a reduction in starvation causing a population boom.

Further, analysis of birth rates shows that birth rates declined during the population explosion. So it was not that people perceived there was an increased ‘optimum population’ and as a consequence, increased birth rates, as birth rates did not increase. Perhaps there were rare locations where birth rates rose somewhat for short periods, but if so, this was more than offset by almost all of the world experiencing reduced birth rates.

All evidence is that that main driver for increased population, was the reduction of infant mortality. The goal of almost eliminating infant mortality, is the reduction of suffering, and the resulting population explosion was a side effect, not a motivation. This means that the previous balance of population was broken by improved an improved medical system, and that the population explosion was not humanity adjusting to a new level of ‘optimal population’.

The End Of The Population Explosion: ‘Optimal Population’ Resumed?

With infant mortality largely eradicated, far less children are required for births to be in balance with deaths. It does seem that birth rates are adjusting downward, to restore ‘optimal population’, and generate birth rates that are again produce a population in balance.

What we have seen, is that when families needed 6.0 (six) children to maintain the population due mostly to infant mortality, families had 6.0 (six) children. Now, with infant mortality down to a level where only 2.3 children are required for a stable population, families globally are having only 2.3 children. Is this a new coincidence? Or is this evidence of a mechanism for ‘optimal population’ in humans?

How: The Mechanisms Of ‘Optimal Population’.

How Can Reproduction Rates Respond To Environment.

The proposal is that evolution has resulted in many living things having evolved the ability to regulated their population at a desired ‘optimal’ level.

Observations as above, suggest that most living things tune their rate of reproduction to produce a stable population level.

For most organisms, reproduction is controlled by instincts. Most reproduce only during specific time windows, and often in response to favourable conditions. Nature controlling reproduction ranges from the simplest organisms where all behaviour appear to be very direct simple response to stimuli, through to humans at the other extreme, when individuals can even make conscious decisions on whether to have children.

So how can environmental feed back control rate of reproduction? In practice, it requires a feedback mechanism, so that when the population is below the ‘optimal’ level, reproductive rate increases, and when the population exceeds the ‘optimal’ level, reproductive rate falls. So for this idea to be reality, there must be examples in nature of mechanisms to control reproductive rates.

Bodily Growth As An Example: Cell Reproduction Stabilises At Optimum Unless Repair Is Required.

Consider how an individual experiences growth. For the first few years, growth is rapid, and then during teenage years, we stop growing, and spend the entire rest of our lives without further growth. It is not as simple as we stop growing, because while it seems possible for brain cells to last our entire lives, even bone cells only last 20 years, many other cells only last days. Our bodies are a population of cells from a series of generations. Cell production is continuous, takes place at varying rates for different cell types, but once we are adults, manages to produce cells at just the right rate for a stable population.

Further, consider what happens with skin cells when the skin is damaged. New skin cells are produced at an accelerated rate. If the accelerated rate continued, there would be excess growth around the wound, but once the wound heals, the growth rate returns to optimum.

At the Cellular Level: Contact inhibition of proliferation.

Some mechanisms of population control at the cellular level are well known. Contact inhibition of proliferation, a clear and simple example of population control, is where the density of cells in a given region controls the speed at which cells reproduce, with signalling between cells playing a key role. Although we are still learning all the details of ‘contact inhibition’, including having learnt that actual contact is not required, it is clear that population density can directly inhibit population growth at a cellular level.

Just as too many organisms endangers the entire colony, too many cells also endangers the entire organism, and without a mechanism to stop cell reproduction, our bodies would have cancer like growths or actual cancers.

Contact inhibition of proliferation, that is, the phenomenon that cells stop proliferating upon contact formation has been described several decades ago (Fisher and Yeh, 1967), but the underlying mechanisms are only now emerging. Importantly, loss of contact inhibition is a hallmark of cancer.

Science Direct

As discussed in ‘life in the colonies‘, in fact we are all colonies of cells and what happens at a cellular level normally propagates even to the level of societies.

Mechanisms of Population Control in Animals: Kangaroos.

Today in Australia the population of kangaroos is a problem for farmers, however it should be considered that, despite kangaroos in Australia having had 24 million years to grow their population , the Europeans did not arrive to a country already overrun by kangaroos. While there are now more kangaroos than desired by farmers, that is partly as farmers raise not kangaroos, but sheep and cattle, and they all eat the same food.

In fact, the kangaroo population manages to reduce to a ‘drought optimum’ in response to drought, and return to ‘non-drought optimum’ when droughts end, without any assistance from humans:

Periods of extreme drought may delay the onset of maturity in female kangaroos and lead to suppression of their fertility cycles. At the same time most fertile females cease to breed. As a drought worsens, fewer and fewer females have joeys either at foot or in their pouches.

After two years of drought a population may include females aged three years or more which have never produced young, while none of the kangaroos in the area would be younger than two, the precise duration of the drought.

Australia’s Amazing Kangaroos and the Birth of Their Young

That droughts last several years in Australia allows kangaroos to provide a clear example of how large mammals can not only maintain an ‘optimum’ population, but even adjust to a ‘different optimum’, in this case for the duration of a drought, and then return to ‘regular optimum’ when the drought ends:

Following rainfall and growth of new herbage, kangaroos come into breeding condition almost immediately. However, it can take as long as eight years for kangaroos, even though prolific breeders, to reach their pre-drought numbers again.

Australia’s Amazing Kangaroos and the Birth of Their Young

Mechanisms of Population Control in Animals: Eels.

Eels are sexless from the time they hatch until they grow about 30 centimetres in length. Then some version of eel puberty kicks and they transform, becoming either male or female. And which way they go depends on the population density. In an area with a lot of eels, the young eels are more likely to become male. But in areas with fewer eels — like further upstream, which is harder to get to — eels are more likely to become female.

ABC.net.au: Eels can travel over land, climb walls and take down serious prey. They may be Australia’s most hardcore animal

Eel reproduction responds not only be changing the fertility of individuals in response to population levels, but even the gender of individuals in response to population levels.

Triggers for reproduction and Seasonal Reproduction.

With many species, reproduction does not just happen all the time as with the bacteria. Sophisticated animals, and even plants, reproduce in response to stimuli. It also logically follows that they reproduce not only at the optimum time, but also in the optimum number. Any animal that reproduces in numbers that would destroy the environment, must be able soon adjust, or they risk extinction from destruction of the environment. So many species using biological stimuli to control reproduction.

When Mechanisms Fail.

From ‘Optimal Population’ to Plagues and Population Explosions.

Population stability results from a balance between births and deaths. Deaths are mostly determined by environment, with the species in question needing to adjust births to match deaths, over which the species itself normally has no control. Disease, predation, and natural disasters can all impact deaths, and as adjusting births takes time, there can be near extinction events or population explosions and plagues, but over time, populations return to optimum, and the environment recovers.

Human Mechanisms Of Stability and ‘Optimal Population’.

Instinctive Mechanisms As In Nature.

We are not aware of any other species being aware that sex results in babies, which adds another dimension. We assume that kangaroos do not have the option to get together and debate “drought headed this way, maybe we all should agree to hold off having children”. Instead, the reduction in offspring is the result of instincts, as with seasonal reproduction, and other response to triggers from the environment.

Are Humans Beyond Nature?

Are we humans really that much different? Isn’t our desire to have children also driven by instinct? While following those instincts may lead to some of the greatest joys in life, it still makes sense that these are instincts, and the joy itself may also be instinct driving us to reproduce. Instincts, that in just over 100 years have result in our joy being fulfilled by giving birth to far less children than in all of previous history.

I can see several possible factors altering the number of children people have:

• The rate at which people feel the need to find a partner suitable for reproduction.
• The number of children people feel the instinct to want to have.
• Sperm counts in males.
• Conception rates in females.

What is the reason an increasing number of people choose to be child-free? Is the instinct to have children influenced by environment?

Theories of Environmental Contamination.

Perhaps there is more to the global decline in male sperm rates than some people have considered. The problem with many theories is that so many variables have changed during the time birth rates and fertility have declined, that from any one data set, many correlations appear. A popular theory is the decline is due to chemicals in the environment, but it has been shown that declining birth rates and fertility also occur in societies where candidate chemicals are absent. In the end, I am not the first to consider it could at least in part be due to a natural response to being above ‘optimum’ population, and so far there is not contradictory data for this hypothesis.

Human Freewill, or freedom of choice, but not over how compelling choices feel.

As a species, we are heavily invested in the belief that our decisions are a result of freewill rather than simply following nature. Many assume that decisions to have a number of children are fully the result result logical choices by potential parents, and clearly this is possible. However the consistency falling of birth rates falling across the globe, statistically, everyone just happens to be changing their choices in a similar way. Perhaps yes, it is in response to similar information, but that is where the line between how we act the same way to the same stimuli, and we are all making free choices becomes blurry.

Across the globe, there are common factors which seem to correlate with falls in birth rates birth rates:

• infant mortality rates have been low for two or more generations
• overcrowding is most common
• the presence of reasons to have doubts about future security and safety

Even in countries with propaganda to encourage more or less children, patterns of birth rates seem unaffected, despite propaganda often being effective for other issues.

In fact, propaganda to encourage more children is surprisingly common despite all evidence that, not only is there no evidence population growth makes people happier and wealthier, the correlations actually suggest the opposite.

In practice, while the most wealthy companies and individuals, can benefit financially from population growth even when the rest of us do not, the best path to population growth would be to create a better world.

The COVID-19 pandemic is serving as a modifier – but not in the way commentators and comedians suggested when lockdowns began.

Remember all the jokes about people being stuck at home leading to a baby boom? As the data rolls in, its clear that in many countries, the opposite has occurred. Most children these days are wanted or planned children, especially in the developed world. Deciding to have a baby is contingent on being optimistic about the future – and optimism is difficult to muster during a global pandemic. In fact, the Brookings Institute estimates that 300,000 babies were not born in the US as a result of economic insecurity related to the pandemic.

weforum: The role of COVID-19 in declining birthrates

Has the Human Explosion Exceeded ‘Optimal Population’?

The Birth-Rate Reduction “Missing Puzzle Piece”.

Global Analysis.

Researching the history of human population uncovers a puzzle, and optimal population mechanism finally offers a viable solution to that puzzle.

I have been exploring the state of global population since 2014, as outlined in the my population journey. Initially motived by the passionate sound of alarm by David Suzuki, I soon had my first surprise on learning that rather than follow a path towards annihilation as highlighted by David Suzuki, population growth rates had fallen towards a level of population stability.

I then learnt that the recent population explosion was an aberration, and the human population over time is normally remarkably stable. In the words of the great medical researcher and statistician Hans Rosling:

People in the past never lived in ecological balance with nature, they died in ecological balance with nature. It was utterly tragic!

Hans Rosling (see video, 19m)

The puzzle emerged: how did people manage to be born in appropriate numbers to match deaths to acheive that ecological balance.

Doing the maths, it becomes obvious that for any animal in nature, exponential growth is impossible, because the timescales are too long. Just 63 population doublings in population takes any species from 2 individuals, to 9,223,372,036,854,775,808 individuals, which for anything beyond microscopic life, would fully cover the surface of all land and oceans of the Earth.

On a planet billions of years old, every living thing has had more than enough time for way, way beyond 64 doublings, and no species exists in the numbers that would result from continual doubling in population at even a fraction of the rate that species can double in population. Clearly, exponential growth over any significant time is impossible, and all life has some mechanism for population stability.

Plants, and some other organisms are resource constrained, which makes overpopulation impossible for those organisms. But for a huge range of species, from the bacteria in David Suzuki’s petri dish, through to humanity, population growth beyond a sustainable level is not only possible, but inevitable without some mechanism limit reproduction to exist in “ecological balance with nature”.

The missing puzzle becomes: how is it possible that most organisms can exist in a state of population stability?

Continent Specific Data: Australia.

Most population data is from Europe or Asia, but these have not been the only populations of humans. Establishing what populations were in the Americas, but Australia can provide data for additional insights.

It is generally accepted that humans arrived in Australia and had over 50,000 years of generations from a common gene pool, and the Aboriginal Population reached, on the highest estimates, 1.25 million, which would represent average annual growth of 0.028%. Of course, the population would not have grown homogenously over 50,000 years, but had steps as the environment changed and the culture evolved, and thus, most significantly, clearly spend long periods with a stable population. As with other human populations, there is no evidence of starvation constraining population growth, and no evidence of predators, which makes this another clear example of humans having the ability to maintain a stable population, without either predation or loss of life through resource constraint. Plus the ability to increase population when circumstances are suitable. That is, the people were able to reproduce at an appropriate rate for the environment.

The Recent Population Explosion in Perspective.

To recap, humanity has just experienced the greatest population ever. The industrial revolution is often given credit for supporting this population explosion, but data clearly shows rather than a rise in birth rates in response to people being motivated to have more children, people had less children during the population explosion.

Perhaps the advances of the industrial revolution caused birth rates to fall more slowly as we adjusted to the ‘new normal’ of almost all children surviving, but the increase in population was all a result of birth rates not falling quickly enough to adjust to the new smaller number of children required. The medical advances were rapid, and resulted in saving lives of children already born that would have perished without these advances. It would be impossible for people to adjust the number of children they had in anticipation of medical advances.

Clearly, even if there was no basis for an increase in population, solving the problem of infant mortality would result in an huge lift in population whether desirable or not, as people adjust to the new ‘normal’ number of children.

Statistics on birth rates show we are again at ‘peak child’ and population stability has returned, but now we have an increased population, that, for the first time, was not increased by human birth rates in response to a readiness for a population increase.

Now we are faced with this hugely increased population, whether society is ready for it or not, and the result is that we are currently not able to exist sustainably. The definition of carrying capacity is the number of a species that can exist sustainably. Clearly, we are currently over carrying capacity, and thus we have overpopulation.

You would never trade sustainability for saving all those infants from death. Solving infant mortality was worth the price of resultant overpopulation. But now we are in a race to change our society so that the current population is sustainable, before the damage to the environment is too great.

The Environment Always Recovers From Plagues In Nature, So No Problem?

The recent human population explosion is unusual, as it was largely a result of improvements in medicine. Things getting out of balance is common, as environments change and species must adapt. Nature gets out of balance and can produce population explosions. But in nature, populations normalise, and the environment recovers. Every time. Locust plague, mouse plague, whatever, the population normalises, and the environment recovers.

On that basis, the human population should normalise to a level that ends the damage to the environment, and then the environment will recover. But there are two potential problems:

• Humans are now a global society, so this is global population explosion, and the environmental damage is global, possibly making recovery most challenging.
• The degree of environmental damage from overpopulation is exacerbated by an increase in human environmental footprint that occurred in parallel with the population explosion.
• With several aspects of society currently designed around continued growth, there will be resistance to ending the population explosion.

The reality is the problem of a global overpopulation of humans is complex, and there is already significant focus on solving some of the most pressing problems. However, after such a long period of a population explosion, there will be attempts, by those who see the ending the explosion as unwelcome change, to find ways to further increase population.

What is population is optimal anyway?

Recent human population events.

At first, from our perspective given the recent growth of the human population, it seems either the pattern of reaching a population plateau and then remaining there, does not apply to us humans. Either that we are still in our growth phase, as the human population has just undergone a population explosion, that has resulted in a population currently not existing sustainably.

However, closer inspection reveals that explosion was triggered by a decrease in child mortality, overcoming childhood diseases in a manner somewhat similar to having eliminated a predator.

However, the population was already growing, and would arguably have reached the current level anyway. Even David Attenborough declares ‘Population Growth Must Come to an End’, and not ‘We have overpopulation’.

Yet, nature does appear to be deciding otherwise, as birth rates in most countries are heading towards population contraction.

The human optimum appears flexible.

Evidence for natural ‘optimum’ population mechanisms in humans changing includes:

• Humanity existed in birth-rates in close to perfect balance with death rates for almost our entire 300,000 year existence.
• Humans’ reproduction rates have responded to the population explosion by falling to new, lower that previously recorded levels, as required to be in balance with the new lower rates of child mortality.

Despite very low population growth rates over the longer term, there has still been substantial, and sustainable, long-term growth of the human population over millennia. As humanity has demonstrated societal evolution, optimum population increases.

The answer to slower expansion of the human population lies in the fact that unlike the number of cells in a mature individual animal, there are circumstances that can trigger changes in the level of ‘optimum’ population of species:

• Temporary or permanent changes to the environment.
  • Or
• Evolution of the species, allowing the species to be more competitive with species, and increase in population at the expense of other species.

Both of these factors have played a role in the long-term expansion of the human population. While evolution of a species can only increase potential population as “less fit” variations should not continue, changes to the environment can increase or decrease potential. So, predicting future population requires insight in the future of evolution and the environment, but even with that knowledge, still requires an understanding of the principle of optimum population.

Where does it end: With only one species on Earth?

For us humans, our society can evolve even without us evolving as individuals. This evolution of society allows humans to enter new niches and new territory, and to increase our competitiveness, and the competitiveness of our food sources, enables to increase continue increasing competitiveness and displacement of other species.

Remember, the total amount of life on Earth is decreasing, and no innovation of humans so far has managed to reverse that very long-term trend.

Is the direction of nature like some “Highlander” style “in the end there can only be one“, with the percentage of life on Earth that is either human, or food for humans, continually increasing as a percentage until all else is squeezed out?

Conclusion, and Where Next?

I feel the evidence for ‘optimal population’ is compelling, not only in nature, but also in humans.

There are other explanations given for falling human birth-rates as the population explosion ends, but claims such as “The chief driver of this change is women’s education and access to contraception” do not correlate well with real world data, as birth-rates often behave in a similar manner even in countries where these factors are very different. Further, many of the factors affecting are fertility seem to be biological response, and are very hard to link to women’s education or access to contraception. Plus, the idea that humans happened to educate women, which coincidentally lowered birth rates, just at the time the problems form the population explosion were increasing, seems to much of an amazing coincidence.

Humans managed to adjust, from what was previously a high ‘predation’ rate on the young by disease, to this now much lower predation rate. It seems highly that humans have also in the past had to respond to changes in the rate of child mortality, and yet humans have also previously maintained stable population over long periods.

The unanswered question is whether we can depend human ‘optimal population’ mechanisms to bring our population back to a sustainable level, in our current environment.

The danger is that economists, and those who ‘farm humanity‘ have become so addicted to the population growth that occurred before we managed to adjust our birth-rates, that they will be highly motived to try and stop nature.

Even if we do return to a stable population, interim steps to reduce the environmental impact of population overshoot are still essential to minimise the suffering that can result environmental damage. It is not that understanding population mechanisms means we can ignore the problems, but rather, that we can better plan our response.

It could be possible to adjust our lifestyle such that very little population correction is needed to return to sustainability, but I do think the best outcome will result it there is a period of natural population correction to a lower level than we have today.

What we do not need, is economic greed of the few who would benefit, driving a push for further population increases with a total disregard for nature.

Updates.

• 2022 July 28: Added ‘technical analysis‘ and the, quite illustrative, ‘Elk‘ example.
• 2022 April 5: Significant restructure for a 2nd edition.
• 2021 September 22: Initial version.

Pending: Update for nature reserve, human history in Australia.