Environment sensitive reproduction & population: survival is about the right number of kids.

Synopsis: The state of the environment determines future population.

The goal is a study of birthrates in nature to better understand what may be happening with humanities own birthrates. The background is that humans have, like other species, almost always existed at the limit the environment can support, although, unlike other species, we humans have managed to keep raising our population limit throughout almost our entire recorded history, resulting in an experience of continued population growth and even a recent population explosion. In the early 21st century human population growth is slowing with continue slowing birthrates, and this is an examination of birthrates in other species.

The better the environment for a species, the more of the species the environment can sustain. Analysis of nature leads to the conclusion that many complex species even adjust their rate reproduction depending on environmental conditions to ensure they and their offspring have the best chance of survival in the conditions. The result is they adjust reproductive rates to push towards optimum population for the environment. Is this also what is happening with humanity?

What future population for a species would be optimum? The population of humanity that would be optimum is a separate debate, with global corporations, politicians, the wealthiest 1%, and the average person, all possibly reaching different answers. For most species, optimum population is the level of population that will have the greatest chance of survival. At least when in the wild, rather than in an egg farm or something.

At first it may seem that producing the most offspring possible would give a species the greatest chance of survival, and while that may work for some species, the fact that there are species that could technically reproduce all year yet limit having offspring to the right season makes it clear this is not the only strategy. Further, while some species have thousands of young at a time, others have evolved to be survive the fittest with only one or two young at a time. If the right strategy for survival was always maximum population growth, then only those species with maximum young as often as possible would be the only survivors.

In practice, high birthrates are for species where the young are born into environments where they experience high rates of mortality and the young have very low survival rates, and lower birthrates are normal for species where the young offspring have higher survival rates and are often cared for by the parents.

Even in within one individual species, the optimum number of offspring will vary depending on circumstances, because sometimes population growth is best and at other times even a reduction in population may be optimum.

The simplest population growth pattern would mimic the growth of the population of cells within most complex species:

1. First cell(s) at the conception 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 for individuals, or for species, at an optimum population.

The need to vary the balance between births and deaths.

The need for the growth phase is self-explanatory. While humans are a special case who can alter the upper limit of our population, for all other species the need to limit population becomes clear when considering we live on a planet that only provides for a finite amount of total life, and all species have had time to reach their maximum.

Plus, we know from monitoring populations in the wild that over the long-term populations are usually quite stable, although we also know from events such as plagues mean that even though there is not always stability, even surges in population quicky reach levels where further population growth becomes impossible, and stability returns. Even unconstrained population growth of a low birthrate species from the first individuals to spreading across the entire globe can be shown mathematically to be unable to continue for thousands of years before population reaches the limit at the maximum possible.

While all species tend to exhibit significant long-term population stability under normal conditions, there are many species where population also fluctuates significantly in the short-term due with seasons and even weather events such as the most recent rain. While such species perhaps even further highlight the need for population growth rates to change, as the closer the species genetically to humans the less evident the short-term fluctuations, there primary focus here is on species with patterns more like humans where populations are normally relatively stable throughout at least an entire year.

The need for growth phases and stability means both the rate cell reproduction changes between growth and maturity phases, and the reproductive rate of many species also change between the rate needed to establish the species initially and recover population levels after natural disasters and the rate when population stability or “full population” is reach. There are many observed examples of population stability, such as how 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.

The change between a growth phase and population stability means the ratio births to deaths must change. Either births decrease or deaths increase to slow from a growth phase to population stability.

In theory the slowing could simply result from increased deaths, through starvation due to lack of resources or increased factors such as predation, but in practice it turns out that it is typical for species that invest their resources in caring for their young to be able to reduce their birthrates to avoid increased starvation from exceeding the population resources can support.

While even for those species that do use reduction of birthrates to avoid overpopulation the strategy will not aways succeed when aspects of the environment such as climate or a factors affecting predators also change, but avoiding unnecessary deaths improves species survival chances and in any animal where parents raise the young from delayed births in early marsupial mammals including kangaroos and other mammals such as bats, through to species that adjust the number of births to avoid risks to both parents and offspring that would result from population increases when resources are limited. The more the data is examined, and examples are found, and it seems to follow that species are mostly divided between two extremes:

• Those with very high numbers of offspring produced in far greater numbers than would be expected to reach maturity on the basis that the environment will determine the number that survive.
• Those where parents produce and then care for a far smaller number of offspring with parents investing resources to achieve a high survival rate.

Given the adapting birthrates is clearly possible, and can give a species a survival advantage, then it is very likely to have become a key ingredient for survival as “the fittest”.

When I began this research, I wondered why, given every species is capable of producing enough young to grow their population but almost all species spend most of the time at the maximum optimum population there is not clear evidence of more deaths through starvation in the wild. It turns out, that where there are mass deaths, these mostly occur while individuals at their most vulnerable while very young and often less noticeable. The amount early of death in the cycle of life becomes astounding when contemplating that the population stability that so predominant in nature requires an average of only 2 offspring of every female to reach the level of maturity where they produce their own full quote of offspring.

What about humans?

The historical human survival rate of at least two children from 6 to 7 births per woman average is remarkably high relative to other species, and the 21st century reality that now only just of 2 births per woman is required for population stability astounding even though now something we take for granted.

While we humans have managed a whole new step beyond all other species in terms of our ability to evolve, and this has resulted in a gradual long-term population growth that other species do not experience, as documented in why human population growth even before the explosion, we are still part of the group of species that care for and are heavily invested in the survival of our young.

With human birthrates having fallen from pre-industrial era rates of around 7, to rates in the early 21st century in most developed nations well below 2.0, clearly humans do also vary their birthrates. While many other explanations have been put forward, many people overlook the possibility of an at least partially instinctive adjustment in response to the either real or perceived rate of reproduction the global environment will sustainably support.

This returns to the question of would be the optimum human population of a fully populated Earth? There may be other factors that play a role in birthrates, but it could simply be that people believe that, on balance, the safely sustainable number is lower than where population is headed, and birthrates will continue to fall until people believe we are headed for our optimum population.

Surprise: Population growth is not the normal in nature.

Survival of the fittest doesn’t select for maximum offspring.

A common source of confusion is to confuse the ideal number of offspring for an individual with the ideal number for the entire species, and these two numbers are not the same. While for one single individual, having more offspring that survive to become adults and themselves reproduce helps their genes succeed, too many offspring for the species will mean competition for insufficient resources will produce generations undernourished and vulnerable endangering the species. Overall, for the species, the goal is neither too few nor too many, but the right number of offspring for the conditions.

I recall discussions on homosexuality in humans with where this confusion arises:

If homosexuality has persisted in the population, it’s likely there’s some kind of benefit that it confers to individuals that can compensate for its huge cost, i.e. no reproduction.

Homosexuality in nature explained

The underlying assumption is that anything limiting reproduction is a huge cost, yet the reality that some individuals having zero offspring may be what is required in the situation to have the appropriate number of offspring. Surely, if everything to maximise reproduction was beneficial in evolution, given there are genes linked to increased rate of having twins, and humans seem to have characteristics that can support feeding two children, then genes for twins would have taken over the human population long ago.

Further, it is clear from nature that it is possible for species to even have thousands of babies per litter, but that this only evolves in species with very low survival rates of those babies. If maximum reproduction rates were an advantage for all species, why have not species simply evolved with maximum possible rates of reproduction? The clear message from observing nature is for different species there can be very different optimum rates of reproduction, and it is achieving this best rate of reproduction for the environment that is selected by evolution. If humans needed higher rates of reproduction, then not only would there be no homosexuals, but there would also be far more multiple births. Clearly, however we are, is what has worked best so far for what we perceive as the environment.

I grew up assuming population growth must be normal, but it isn’t.

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 rapidly, with ‘development’ rushing to build new homes for an ever-increasing population as one of the major industries and influences in politics. 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.

In that environment I assumed that the population of all living things must just keep increasing. If the expectation was that the human population will just keep growing indefinitely, why would that not 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.

Simple maths shows continuous population growth just doesn’t add up.

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 thinking was at least a similar total amount of life 100 million years ago, I had thought that instead there was 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 that there is at most close to zero growth in the total amount of life on Earth. 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. This era of species with individuals large enough to see began only just over 500 million years ago, when complex life quickly becoming abundant. The higher CO₂ levels that were possible with the fainter Sun of that earlier time, enabled a higher rate of photosynthesis, which is the engine of complex life. Ever since the “Garden of Eden” of the start of the Phanerozoic era, the total population of life overall has been on a very, very gradual decline.

In summary, despite observed human populating growth, there is no growth for overall life 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 Limitation: birthrates for initial growth then stability.

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.

As species emerge, they must initially grow their population, but give the time period of initial growth is usually quite brief, the species must then maintain population stability or become vulnerable.

Yes, it is different for humans, as it could be argued that the progress of accessorised evolution keeps raising the limit for the population of humans and could potentially do so until all life on the planet consists only of humans, their pets and their food and organisms required for to support the life of humans, pets and their food. Obviously, there are questions on just how extensive that group of support organisms would be. It could also be argued reaching that point, even if it is possible, comes with undesirable risks and costs. Again, a discussion for elsewhere.

Why don’t we get we just can’t keep growing and must become an adult species?

The Absence of a species we see as a 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 grow to a specific size, and then exist at very much that same size for the rest of their life?

Or three centuries of abnormal population growth, has distorted our perspective?

Long term, average human population growth is negligible, at less than 0.05% per annum.

Strangely, few people seem to look at nature and realise how other living organisms aren’t also 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 an “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”.

Paths to a population level: variations in births and deaths.

The need to vary the balance between births and deaths.

Every species has to be able to grow in population, and then normally maintain a level of population stability. For a population to be able to grow, the rate of births must exceed the rate of deaths. Then, for population stability, either the rates of births must decrease, or the rate of deaths must increase.

To achieve growth and then stability, the simplest population growth pattern would mimic the growth of the population of cells within most complex species:

1. First cell(s) at the conception 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 for individuals, or for species, at an optimum population.

Interestingly, just as the individuals that make up the population of a species are continually being replaced, so are the cells within individuals, with most cells in humans being less than 10 years old even in individuals who are 80 years old.

Within species, 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 stability in the population of cells ending being reached at maturity and maintained while an individual is at peak health.

Both the population of cells within one individual, and the entire population of a species can experience the needs for growth spurts to recover from damage to the respective populations, although for more species, the capability for such growth spurts after maturity is more limited.

While humans can regrow, nails and some skin etc, and examples like lizards can regrow tails, entire species typically need to be able to recover from natural disasters reducing their population to as little as 5% of previous levels. While some individual plants can recover from being reduced to only 5% of their size, most individuals are unable to regrow the same percentage of their body as the species is able to regrow after a natural disaster. But while the extent differs, the principle is the same.

Another difference is that 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 and unlike with a complex organism, there is no reason for the entire species to enter old age, and there is typically a greater need to achieve regrowth for an entire species.

It is well established and widely accepted that within many complex organisms, the population of cells stops growing at maturity by genetic programming to slow the rate of “births” of new cells at maturity, and only raise rates again to repair damage. Clearly, as opposed to the model of the ‘bacteria in a petri dish’ model where reproduction simply increases numbers until all resources are consumed, the rates of production of cells within individuals responds to triggers to vary in rate depending on needs at the time.

What is not as well established or widely accepted is that in some cases entire species reach population stability by varying their birthrates. It is often assumed that a fixed birthrate is “hardwired” into all species and population limits are determined solely by deathrates increasing as populations approach limits.

It is easy to assume that predators limit the population of species preyed on by predators, and the predators will begin to starve if their population exceeds its limit. The balance of nature. If this was always true, then death rates alone could keep populations at their limits. But closer examination reveals that in many cases the balance simply cannot always be maintained without species also adapting their birthrates to the environment.

Resource Constraints limiting population through increased deaths.

The principle.

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 populations can continue to grow until resources the population growth is constrained by resources.
• the limitations of relying on resource constraint for population control, as once the limit is reached, the population collapses.

Clearly, if this simple principle alone applied throughout nature, the world would see populations almost everywhere growing to peaks and then collapsing. The reality is more nuanced.

Child mortality: When resource constraints alone can be sufficient for population control.

Consider grass in a paddock. The grass consumes CO₂, sunlight, water, and nutrients from the soil. For that paddock the CO₂ is a tap into the planet scale supply and us effectively infinite, but sunlight, water and nutrients limit growth. The grass can cover the paddock to a certain depth, but without infinite resources the grass cannot grow indefinitely within the confined space.

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 of 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 total population, then population control by resources control must apply. But how? It is very simple when considering the grass as a single organism the grows as far as possible, but if the grass is considered as a population of plants that reaches a population limit, then there are births and deaths within the population. For population growth to stop, either births decrease, or deaths increase. For births to decrease, the plants need to stop producing fertilised seeds, yet healthy plants may still continue to seed at the regular rate. If “births” continue at the normal ratio, then there would need to be a substantial increase in the number of dead plants if the amount of grass is no longer increasing, yet again, in a healthy grassy field, there is often no evidence of extra dead plants.

In these cases, it turns out that very few young seedlings survive in the dense growth of grass. The death rate does increase, but it is effectively high child mortality, rather than an increased death rate of mature plants. Like most species, it turns out the grass is most vulnerable when young.

Constraint by Predation: Only ever part of the answer.

One alternative suggestion for 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 only 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 risk of at least local 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.

While predation plays a key role in maintaining the balance, predation alone cannot provide the shift in population growth rate between a growth phase and a stability phase.

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 “population” of grass stops growing at the carrying capacity of the paddock. The “population” of grass may be increased though fertilisation if the amount of grass is constrained by soil nutrients and may be increased though irrigation if the constraint is water, but if the constraint is CO₂ or sunlight the intervention would become more complex. The good news for the grass is that availability of all of these resources is not destroyed by overpopulation of the grass.

The stability problem with population control by increased deaths due to resource constraint.

The first problem with population control by deaths due to resource constraint, is that for many species, resource constraint does not prevent population growth beyond carrying capacity. This happens when a population beyond carrying capacity can be supported for a time, even though the increased population is not sustainable.

For example, the grass keeps producing seeds even when the environment will no longer support more grass but provided the environment results in less seeds taking root and growing to maturity leaving insufficient resources for all, the system works. If the constraint for the grass is not water or sunlight but the rate at which nutrients in the soil are replace, then the grass could exist unsustainably for a time by consuming the reserve of nutrients, resulting in depletion of nutrients below normal level and reducing the carrying capacity, resulting in a die back of grass. This will still only be a temporary problem for the grass, provided the environment damage is only to the level of nutrient, and does not result in the elimination of some species critical to the supply of the nutrient. It is this situation when population control through increased death rates comes too late.

Consider sheep left to breed and grow in the finite environment of the same large paddock. If the sheep population becomes too large, the first consequence is that, rather than all excess sheep just dying, the grass that is their food is over grazed, damaging the grass to the level where it can no longer seed and lowering the paddock carrying capacity for sheep to level sustained by other food sources. By the time sheep are starving, grass may have been eliminated. Unless grass from outside the paddock can trigger regrowth of grass, then the sheep population will never be able to recover. If eventually new seeds do arrive, the grass could grow again while sheep numbers are low. Considering the rate of growth will be proportional to the “population” the grass can reach, then even if the environmental damage is not permanent, it may still not be ideal, for example:

1. If the sheep population recovers before the grass becomes fully established, the grass would never get to return to the level that provided growth of grass and thus maximum carrying capacity for sheep.
2. If the sheep population recovers slowly, the grass can exceed the stable level when kept in check by sheep, allowing the sheep population to rise beyond carrying capacity again 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” than sheep who could reduce their birth rates when the level of grass begins to fall. Reducing birth rates can take years to have an impact which means this requires a larger scale than just one paddock of grass and sheep, but it does raise the question, are their species that can reduce birthrates depending on the supply of food or even the future supply of food, the answer is a very clear “yes”.

The even greater risk from reliance on increased deaths: it requires more resources.

It stands to reason that having more children than the number that can possibly result in consuming additional resources in prolonging the life of those extra children.

If the sheep are raised only for wool and kept in a fixed sized fully stocked paddock then their population cannot grow, so births beyond the number required to sustain the population cannot be sustained. While on a farm such sheep can be sold, where the paddock is replaced with, for example, the environment of an island without people or predators, those excess lambs would have to die, or the environment would collapse.

In practice, sheep the birthrates of sheep do depend on the environment:

Have the flock in ideal body condition before breeding season. I know that I touched on body condition above, but here we are going deeper, we are kind of topping off their nutrition.

It’s best if you could have the ewes on an increasing level of nutrition for the few weeks before breeding season to increase the likelihood of multiple births.

This is called flushing, and it simply means giving the flock some additional calories right before breeding to get them to ovulate more eggs when they come into heat.

Flushing works because the ewe’s body will restrict lamb numbers due to lack of nutrition or body condition at breeding, which makes sense since the ewe needs plenty of feed to carry and raise the lambs.

How Many Lambs Do Sheep Have? (3 factors determine this)

Sheep reduce their fertility rates whenever resources being to become scarce which saves both mother an children from being at risk from lack of nutrition. As sheep are mammals and not only care for their young but provide them with milk, even the parents health would suffer from having too many offspring.

Reducing birthrates to avoid the costs of increased death rates.

Producing excess young that are only going to then die means there will always be an excess population leaving less than ideal resources for the entire population. It is not only the elderly who face the increased rates of death, as unless individuals die before the reproduce their own offspring, the population keeps growing and resources keep becoming ever scarcer. While the younger the excess population die the lower the impact on resources per individual, with species that care for the young, producing and raising the extra young comes at a cost.

Overall, an increased death rate comes at a cost to the “fitness to survive” of the species, which means if mechanism to reduce births in response to the environment can evolve within a species, that trait will become necessary to survive.

Two clear mechanisms are to reduce how frequently young are produced, and to reduce the number of young produced.

A very obvious form environmental conditions reducing births: Embryonic diapause.

More than 130 species of mammal can pause their pregnancies. The pause can last anywhere between a couple of days and 11 months. In most species (except some bats, who do it a little later) this happens when the embryo is a tiny ball of about 80 cells, before it attaches to the uterus.

It’s not just a single group of mammals, either. Various species seem to have developed the ability as needed to reproduce more successfully. Most carnivores can pause their pregnancies, including all bears and most seals, but so can many rodents, deer, armadillos, and anteaters.

Some Animals Can Literally Pause Their Pregnancies. Here’s Why

This process is called embryonic diapause, and can range from, as the article suggests, can simply shifting the timing of births to align better with the seasons and having no impact on total offspring produced, to delaying births by 11 months and effectively skipping a year’s reproduction, and reproducing the total offspring during the life of the parent.

Environment dependant “litter sizes”.

Another strategy, as shown above to apply with domestic sheep but that can be less obvious to detect in the wild is varying the number of young:

Cotton rats are small rodents common throughout the U.S. They are kind of a “weedy” species. Their litter size is highly variable. They have lots of young when resources are plentiful and smaller litters when resources are scarce.

Why are octuplet births so rare in humans?

Observation reveals most complex organisms have evolved population mechanisms to avoid the problems of waiting until individuals are dying from 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 due to lack of resources. If deaths were the only mechanism of population control, there would always be, for example, 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 find ways reproduce in numbers such that their surviving offspring can be sustainably supported in their environment.

Examples: Changes in birthrate towards stability at ‘full population’.

Real World Example: Humpback Whales Population Rebound.

As an example of reproductive rates delivering optimal “full population”, 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 happens without an increased child death rate finally to balance the rate of births that grew their population until that point.

Rodents, investment in the young and variable litter sizes.

While only so far scratching the surface of available research material, I have already found example of both species that vary “litter size” depending on scarcity or abundance of resources, as well as research into the significant resources parents in some species need to be available to raise their young.

Cotton rats are small rodents common throughout the U.S. They are kind of a “weedy” species. Their litter size is highly variable. They have lots of young when resources are plentiful and smaller litters when resources are scarce.

Scientific American article “Why are octuplet births so rare” by Brendan Borrell quoting Robert Sikes.

However, excessive cost of a larger litter may reduce a mother’s future reproductive potential or result in smaller-than-normal young at weaning. Because lactation is the most energetically demanding segment of reproduction in mammals, assessment of its cost provides an accurate gauge of short-term reproductive investment. If high proximate reproductive costs are correlated with decreases in lifetime reproductive potential or fitness, understanding the tradeoffs between costs and benefits of lactation for different-sized litters should lead to a better understanding of mammalian reproductive patterns.

Costs of Lactation and Optimal Litter Size in Northern Grasshopper Mice

The main takeaway of the above research is that for each environment there is an ideal litter size which will largely be determined the availability or resources and the level of competition for those resources.

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.

w 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.

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

Changing rate of reproduction by gender changes: 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.

Delaying fertilisation for optimum birth rates: Bats.

Bats are one of the most plentiful mammal groups around, accounting for 20% of known species (second only to rodents). But there’s still so much we don’t know about them, thanks in part to their nocturnal and secluded lifestyles. There have been hints that bat reproduction has its own quirks relative to other mammals, though.

In some species, for instance, female bats are capable of storing sperm collected before they undergo hibernation, allowing them to get pregnant after they emerge from their slumber in the spring. Other species can delay the development of their fertilized embryos until the conditions for a successful pregnancy are better, such as having more food available.

The authors of a new study, published Monday in the journal Current Biology, say their curiosity was initially sparked by a distinctive attribute found among male serotine bats (Eptesicus serotinus): their huge junk.

These Bats Seem to Use Their Huge Junk to Have Weird Sex

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.

How: Mechanisms and triggers for varying rates of reproduction.

What from the environment can trigger a change in birthrates?

A simple mechanism for a species to be triggered to alter birthrates is abundance or scarcity of resources which impacts diet and nutrition levels. This is assumed to be the trigger for the examples with sheep and rats. This trigger stabilise population by birthrates varying as populations grow and lower the available resources per individual, working towards an optimum population.

Another mechanism is weather and climate, which definitely applies to some examples of embryonic diapause it is unclear whether other examples, such as Kangaroos, are responding to the weather itself or the consequences of the weather.

The risk from predators, rivals of the same specie or any other threats could be another factor. Whether this would trigger a species to produce more or less offspring would depend on the exact circumstances.

Then there is the survival rate of previous offspring. Parents could either be caught up caring for previous offspring that survive or encouraged by their survival.

A fifth potential mechanism is population density itself. In response to the question “Do battery hens produce less eggs?”, I received the following answer from Microsoft Co-pilot:

Yes, battery hens do tend to produce fewer eggs compared to other systems of egg production. Let me explain further:

1. Battery Cages:
   • Battery hens are kept in small cages as part of standard egg production. These cages are often cramped, preventing hens from performing their natural behaviors.
   • Each hen has limited space, sometimes less than an A4-sized piece of paper, which restricts their movement and prevents natural instincts like wing flapping, grooming, and foraging.
   • The lack of nesting opportunities can lead to frustration and behavioral issues.
   • Chronic pain from standing on sloping wire floors and extreme inactivity can also affect their well-being.
   • Battery cages are being phased out in many countries due to welfare concerns¹.
2. Egg Production:
   • Battery hens are artificially lit to maximize laying activity. They lay roughly an egg per day until they are sent to the slaughterhouse.
   • However, the stress and confinement in battery cages may impact their overall productivity.
   • Factors like light exposure (16 hours of daylight during summer) play a role in egg-laying capability².

In summary, while battery hens are forced to produce eggs continuously, their living conditions can affect their overall egg production¹³. 🐔🥚

Learn more 1voiceless.org.au 2thehumaneleague.org 3sentientmedia.org 4foodrepublic.com 5abc.net.au

Microsoft Co-pilot response to “do battery hens produce less eggs?”

This is a potential source of a clear answer, with a lower egg production corresponding with conditions that would in the wild lower the rate of reproduction. The claim is made that having a limited space per individual does contribute towards reduced egg production, but it remains unclear if this has actually been proven.

The list of identified potential triggers include:

• Abundance of resources.
• Weather and or climate.
• Predators and other environmental threats.
• Survival rates of previous offspring.
• Overcrowding.

Biology to implement the birthrate changes?

With humans, any of these triggers could result in people making a conscious decision to have less children, but it is assumed that with most species, the actual change in birthrates result from biology. Of course, what happens in nature could also happen in humans, it is just that with humans there is an additional possible mechanism.

The possible biological methods include:

1. Variation in rates and timing of ovulation.
2. Delaying births through embryonic diapause.
3. Lower rates of egg fertilisation.
4. Variation in sperm counts.
5. Changes to the gender balance.
6. Changes to mating behaviour and the strength of the impulse to mate in a manner to produce offspring.

The least clear evidence is the for variation in sperm counts, as we have little historical data for species other than humans and farm animals, and what data shows is that studies show sperm counts in humans have declined in environments where counts have not declined in farm animals:

In some countries, sperm counts in normal human semen seem to have declined over
the last 50 years. If this decline is real and due to environmental factors, falls might also be
seen in sperm numbers in the semen of farm animals. Sperm counts are available for bull,
boar and ram from the early 1930s, obtained using techniques similar to those used for
human semen. Data have been obtained from the literature between 1932 and 1995 from
137 studies involving bulls, 76 involving boars and 130 involving rams. All were normal
adult animals, from which semen was collected regularly but at a frequency which would
not be likely to cause a fall in sperm counts. The references were obtained systematically
from Animal Breeding Abstracts, and where possible the original articles were consulted to
obtain mean values for each study; where the original reference was not easily obtainable,
values were taken from the abstract. The bull data showed no correlation of sperm count
with year of publication (r2 = 0.000), for the boars there was a slight but non-significant
positive correlation (r2 = 0.041), and for the sheep there was a slight, but significant, rise
in sperm counts with time (r2 = 0.124 for sperm counts and 0.126 for total sperm per
ejaculate; not all authors gave both values). It would appear that, if the fall in human
sperm counts is real, then it must be due to something which is not affecting farm
animals.

BP Setchell: Sperm counts in semen of farm animals 1932±1995

We simply have no explanation for the fall in sperm rates in humans, and while this being a natural mechasim to reduce human birthrates is the only possible explanation I have seen that does not conflict with the data, in part that is because there is a lack of data.

Examples of mechanisms from nature.

The Bodily Growth Example: Cell Reproduction Stabilises at optimum until repairs are needed.

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.

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.

How could environment & other factors impact human birth rates.

For a moment ignoring the question of what is impacting human birthrates and instead focusing on what possibly could impact human birthrates.

The extra human element: instinct vs the ability to choose whether to a child.

We have no evidence that any other species is aware that sex results in babies, and this awareness adds another dimension to birthrates with humans. For example, 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”, which suggests the response is purely based on instinct. The picture is arguably more complex for us humans, as we can take steps to deliberately and knowingly change the number of children we have.

There is no known seasonal pattern for when to have babies or other very obvious signals that the natural environment has an impact on humans, but well-known events such as a surge in post-war babies does provide evidence that people do vary their birthrates in response to the overall environment. The fall in birthrates from so early in the population explosion is very clear evidence that people also reduce the number of children they have when survival rates climb, and people have even confirmed this behaviour.

Governments even form policies based on the assumption that changing the economic environment can change the number of children people have. While these policies can sometimes be declared a failure, it is clear they have some impact, which is enough to prove that people can vary the number of children they have depending on the environment.

Instinct vs human freewill: Difficult to separate?

Humans may have freewill and the ability to choose, but instinct may play a key role in how compelling we find each choice.

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 logical choices by potential parents, and clearly this is possible. However, given the consistency falling of birth rates falling across the globe, statistically, then if this is a matter of choice, 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.

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.

Choice can be difficult to separate from instinct. We feel free to love who we choose when if you consider the percentage of parents who love their children there does not seem to be a lot of choice in that case. Everything around having children is likely to be driven us responding positively to things evolution determined result in survival as the fittest. We can’t rule out that evolution has made it part of our nature change our choices in response to various conditions, but many of these choices could also be purely explained by logic.

The choices driven either by logic or instinct we may face would include choices to:

• seek partners or remain single.
• raise children at all or have none.
• feel compelled to have either more or less children.

Example of things that may affect which choices we make could include:

• comfort in the environment and the supply of food.
• feeling of safety and security and freedom from conflict.
• survival rates of both children in society and our own children across generations.
• confidence in the future.
• perceptions of overcrowding and population density.

This is probably not an exhaustive list, but hopefully conveys the idea. While all of these factors are listed here on the basis that they may impact our thinking, it is also possible that even our thinking is driven by biological responses affecting the release of chemicals and hormones that motivate us towards certain behaviours. Which leads to the biological aspect.

Environmental Contamination.

There is some potentially questionable data on a global decline in male sperm rates, with degradation of the environment due to human activity as a leading proposed cause.

No one has found an actual environmental damage possible cause that matches with the available data, and the suggestion is that farm animals are not showing the same decline as humans from the same environment, and data on declining birth rates and fertility also occur in societies where candidate chemicals are absent. This then suggests food that only humans consume, or technology that only humans use.

Overall, there seems to be a far higher correlation with birthrates and the number of children people choose to have than with a decline in sperm rates. Given the fall in birthrates began early in the industrial revolution and the rate is still falling globally as recently as 2020, it is very hard to find a increase in the presence of something physical in the environment that could correspond with a global decrease in sperm counts sufficiently well to play a significant role in the fall in birthrates. Couples failing to have the number of children they desire is clearly part of the picture for the fall in birthrates, but it is not even clear how prevalent conceiving was for some couples in the past when there was no available program to solve such problems, limiting the number who would present with problems.

We can’t rule out chemical in the environment having effects on fertility even beyond sperm counts, but we just do not have proof yet. Realistically, we already have sufficient reasons to work harder on protecting the environment, but it is far more likely on evidence so far that lack of confidence in sustainability and the future environment is having a greater impact on birthrates than a decline in sperm counts due to chemicals in the environment.

In the end, I am not the first to consider any fall in sperm rates being possibly due to a natural response to being above ‘optimum’ population, and so far, there is no contradictory data for this hypothesis.

Human biological environmental responses potentially affecting reproductive rates.

Humans and children: How many and how often?

Humans normally produce one child at a time, and with normally around 1 year required between children. Despite the misleading nature of historical life expectancy numbers, all evidence is humans who manage to live a full human lifespan live long enough to potentially parent 20 children. Yes, historically most of those children would die, and even those that lived to become adults frequently did not live that full human lifespan, but these numbers are the potential. Further, we know that historically 6 to 7 children have been the average when the population was quite stable, and it is seen as likely the human population was able to quite quickly recover from events such as Toba bottleneck at a rate which would require a slightly higher average number of children.

The key points are that with multiple births being rare the on average there is less than 1.05 children per pregnancy, which means the main fertility change for humans is to either get pregnancy at all, or to not get pregnant. This is significantly different scenario from many species where there is huge scope to vary reproductive rate just by changing the how many of children per fertilization. The list compiled from nature in general is:

The possible biological methods include:

1. Variation in rates and timing of ovulation.
2. Delaying births through embryonic diapause.
3. Lower rates of egg fertilisation.
4. Variation in sperm counts.
5. Changes to the gender balance.
6. Changes to mating behaviour and the strength of the impulse to mate in a manner to produce offspring.

Biology to implement the birthrate changes?

Neither 1 nor 2 appear to be relevant to humans. Lower rates of egg fertilisation with humans would mean greater difficulty becoming pregnant, and while this could be happening as part of a biological response to lower birthrates, this is not assumed to be the cause of lower rates of conception in humans. Number 4, the variation in sperm counts has possibly been observed and while that this, is real, could potentially be a natural response providing perhaps the only answer so far not in conflict with available data, but although under discussion it is not yet widely accepted as a cause. Still a lot of questions, and very little clear data on any real impact on birthrates in humans. Number 5, changes to the gender balance, could be possible in humans as change to the balance of sexuality, but there is no clear data on whether there is any change in the ratio of homosexuality although it could also impact birthrates. This brings us to number 5: “Changes to mating behaviour and the strength of the impulse to mate in a manner to produce offspring“. Clearly this could apply and could possibly be a major factor in the fall in human birthrates.

Genetic Change?

It seems possible that the prevalence of genes leading to lower rates of reproduction is increasing, but there is definitely no sign of any new mutations or genetic change in humans that would result in lower birthrates.

The evolutionary principle of “survival of the fittest” would mean that any increase in the occurrence of genes leading to lower birthrates would be driven by such genes providing a survival advantage. No possible survival advantage has been found, and remember that in order to have been responsible for the fall in birthrates within one generation of the start of the increase in child survival rates though reduced infant mortality, the survival advantage would have needed at least initially unrelated to the upcoming need for reduced children in response to upcoming improvements in health care.

Realistically, no genetic explanation makes sense at this time, but perhaps new data will one day appear.

Human Population & Birthrates How did we get here!

Gradual growth recap: What happened before the start of the falling birthrates?

We don’t really have accurate data prior to the industrial revolution, but the best calculations are that births per woman have been at around 6 to 7 from at the least the beginning of civilization and most likely even before that.

What we do know is that there was close to zero population growth prior to the start of civilization, and until around 3,000 BCE when population growth accelerated and then maintained a quite consistent rate until the start of the industrial revolution.

As explained in more detail in “Why population growth even before the explosion?“, there were minor variations in the rate of population growth, which show as bumps on the graph but for the almost 7,000 years from 5,000 BCE until at least around 1650 CE, the underlying trend was for growth of around 0.066% per annum.

This 0.066% rate is far too low to represent unconstrainted population of the human species which demonstrated as high as 30 times faster growth for a period within the 20th century, and the logical explanation is the progression from bronze age to iron age and beyond continually raised the limit of human population over the period, and the population was continually able to grow as the limit increased.

How was population so stable historically?

en 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 achieve 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.

The wheat and chessboard problem 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 even 63 doublings.

Population On A Finite World: A Zero-Sum Game With No Vacancies.

Considering humans have proved they can double the global population in just over 30 years, clearly to have less than 10 million people on the planet at the start of civilization, humans population had been for the previous 300,000 years been kept to a population limit by births very closely ,matching deaths.

Industial revolution: Population growth with falling birthrates, and faster falling child mortalities.

The same advances in science that kicked off the industrial revolution, also led to improvements in medicine.

As can be seen from the graph of child mortality rates from 1800 in the USA, which is around the date medicine in the USA reached parity with Europe, child deaths began to fall.

As it is people who die prior to having their own children that slow population growth, this quickly began to have an impact on population.

Around 1800 CE is also the date population growth reach a new level, although population growth rate red line was highest from around 1920 for the century until 2020 with a peak in 1970. Global birthrates from 1920 to 2020 halved and were already significantly lower at the population growth peak in around 1970.

How did the population rise as birthrates fell? From the graph of child mortality in the USA above, child deaths fell almost 200 per 1,000 in 1920 to near zero in 2020 and were already under 30 per 1,000 people at the peak population growth time in around 1970.

The conclusion is that unlike the previous period when advances in civilization drove population increases, in this case it was medicine by reducing child mortality rates that drove population increases and advances in civilization raced to try and keep pace.

If the historical average birthrate of 7 births per woman had continued throughout the entire 20th century and beyond to 2022, without some new source of deaths the global population would in 2022 be at over 25 billion.

Just imagine, every city with 3 times the population and producing 3x the emissions and needing 3x the global food production. Just finding 3x the water for many cities would become impossible.

The population explosion.

Why we can’t have population stability with reduced child mortality without lower birthrates.

To live in population stability with a birthrate just 2x the births per 1,000 people of that of today, since stability requires deaths to equal births, if would require 2x the deaths per 1,000 people. As 12.5 deaths per 1,000 allows for people on average to live 80 years, it at first sounds life people that just people having an average life expectancy of 40 years would balance things out. The problem is, a population of people typically dying at 40 having the same number of births of a population dying at 80 still requires reducing the birthrate of people in their fertile years. Unless children die in order to raise the number of deaths and reduce life expectancy, just as was the reality of the year 1900, the maths would not add up. There is no way to in practice reduce child mortality and have population stability without less births.

2020 and beyond: So why are human birthrates falling ending the population?

Clearly, if birthrates had not fallen humanity would have either had a huge population problem by now, or a new wave of deaths.

There are many theories that I label as “coincidence theories” in that these propose that the fall in birthrates was caused by changes in society unrelated to humanity developing the need for birthrates to fall. While many such theories correlate well with a subset of data from a period within the fall in birthrates, I have not seen any of these theories that still correlate when applied to more data.

So far, the best theory is the simplest. People reduced their birthrates in response the environment at a time when failing to reduce birthrates would have resulted in an existential risk.

This requires that either we have evolved an instinctive response to detect when we need to reduce our birthrates, or evolved to reduce birthrates when logic tells us more children makes no sense.

How the environment does and does not lower human birthrates.

Falling confidence that the environment is appropriate for having children.

The clear conclusion is that we cannot rule out our early 21st century environment generating a lesser desire to have children than was triggered by the less populated environment that did not cause people we were living beyond sustainability of 100 or 200 years earlier.

Similarly, Instead, the reduction in offspring is the result of instincts, as with seasonal reproduction, and other response to triggers from the environment.

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

Government and media campaigns so far have limited impact.

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, but the correlations also 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.

Continent Specific population balance example: 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.

Updates.

• 2024 Feb 26 th : Further complete revision for 3rd edition.
• 2023 November 21st : The story of the bats with large penises and population regulation.
• 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.