Our Environment: Part 6 – Human Population Growth

Following the textbook I used when I was teaching environmental science there were 6.7 billion humans on the planet in 2011.  I remember when we hit 6 billion.  There was a lot made of it in the media.  Currently (2025) we are at 8 billion.  The textbook I used was published in 2011 – which means we have seen a growth of 2 billion people over 14 years – that’s a net gain of 143 million each year – 12 million each month – 400,000 a day – 17,000 an hour – 275 each minute – a net gain of 4 humans every second.  Our population growth rate is astounding.

The calculation used to measure population change over time is relatively simple.

Population change = (births + immigration) – (deaths + emigration)

The natural history of creatures on our planet follows the same basic principle.  There is a correlation between the number of offspring produced and the amount of parental care provided.  The goal of each parent of any species is to have at least one of your offspring reach sexual maturity.  Those who give very little parental care – trees, grasses, sea turtles, jellyfish, etc. – will produce large numbers of offspring knowing that over 90% will not make it – many will not make it past the first few days.  Those who produce few offspring – songbirds, manatees, and gorillas – provide parental care to assure at least one makes it.  Some will provide parental care for a few days, or a few weeks, others will provide it for a few years.  BUT even those who show parental care typically produce 2-3 offspring knowing that a couple will not survive.

This is a piece of the population control story.  It does not benefit any species to have an overpopulation problem.  I had to take an oral examination to complete my graduate degree.  You are in a chair facing a long table full of professors who are asking you all sorts of questions.  One question was “who is a rhinoceros’ greatest competitor?”  Being very nervous I was bumbling and fumbling in my brain trying to figure WHO would be their biggest problem… and then – it came to me… another rhinoceros.  They want the same space, the same resources, and the same mates as you.  Competition begins day one.  Over population is a problem for everyone.  Other things that effect the number of newborns from reaching sexual maturity are predators and disease.  In some species even the parents (typically the father) may be a predator – and will kill their own offspring (polar bears).

Placing humans in this model – we will see that typically there is only one offspring produced.  Parents must do all they can to help this offspring reach sexual maturity.  For Homo sapiens females reach sexual maturity between the ages of 8 and 13, 9 and 14 for males.  In many cultures this is when the parent’s job is over and the child either enters an apprenticeship or gets married.  Most cultures do not recognize “adulthood” until the age of 18.  So… your job as a parent is to make sure they reach that age.

For most of our history environmental factors played a role in controlling population growth – namely disease.  200+ years ago, many children did not survive childbirth or died early from diseases.  But things have changed.

It took from the time humans arrived on the planet to 1927 for us to reach 2 billion humans.  It took less than 50 years to add the next 2 billion (1974), and only 25 years at add the next 2 billion (1999).  The textbook I used stated we will reach 7 billion by 2012 – we reached it in 2011.  They are expecting 9.7 billion by the year 2050.

What happened?  What changed to create such a large exponential growth in the human population?  The answer lies on both sides of the population change equation – more births are making it to puberty and less people are dying.  Modern medicine has done miracles in improving overall human health on both ends – and this is wonderful news, but it does come with a cost.  As with any species population – there is only so much space and resources.  When the population of a species begins to stress this – many creatures will cease (or slow) reproduction.  Another answer we see is dispersal – members of the population will move to new locations and build new populations reducing the stress on the resources.  As we mentioned in a previous article – they will spread across the landscape until they reach a barrier that stops them from going further.  But in nature the role of predators and disease has not gone away as it has with humans.  The current growth rate for humans is 1.1% but has been slowly declining since the 1960s.

This brings up the question of carrying capacity (K) – the maximum number of individuals a population’s space and resources can support.  As we mentioned, when K is reached in the natural world species either slow reproduction, disperse, or both.  But what IS the carrying capacity for humans on this planet?  The textbook I used mentioned there was no consensus on this.  Some were saying as low as 2 billion – which we know was not accurate, though some would say some parts of the world are already in a population crisis.  Others say it may be as high as 30 billion.  A quick search on the internet finds the following…

1. The AI response states somewhere between 9-10 billion.
2. The Australian Academy of Science states it could be between 500 million and 1 trillion.
3. The Population Connection states it is between 500 million and 1 sextillion (21 zeros).

Bottom line – we do not know – it depends on which model you use and how you view/term a functioning population.  When I was teaching the class, most models stated somewhere between 10-15 billion.

As we mentioned, when you are reaching carrying capacity – most will either slow/stop reproduction or disperse.  In our next article, we will look at how humans might address this problem.

References

Miller, G.T., Spoolman, S.E. 2011. Living in the Environment. Brooks/Cole Cengage Learning. Belmont CA. pp. 674.

Puberty and Precocious Puberty. Eunice Kennedy Shiver Institute of Child Health and Human Development. https://www.nichd.nih.gov/health/topics/puberty.

Population. United Nations. https://www.un.org/en/global-issues/population.

Human Population Growth. Libretexts. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.04%3A_Human_Population_Growth/45.4A%3A_Human_Population_Growth.

How Many People Can the Earth Actually Support? Australian Academy of Science. https://www.science.org.au/curious/earth-environment/how-many-people-can-earth-actually-support.

What is the Carrying Capacity of Earth? The Population Connection. https://populationconnection.org/blog/carrying-capacity-earth/.