We live on a planet with finite resources, and all animals, including humans, are forever tied to the availability of resources, including food, water, oil, space, and more. Overpopulation has detrimental impacts on all species since the overpopulated species put extra stress on resource availability. A species becomes overpopulated when its population size exceeds its ecosystem's carrying capacity (denoted by "K"). Unsustainable population growth occurs due to many factors, including decreased mortality, increased birth rates, the removal of natural predators, migration, and more. In nature, overpopulation is regulated by limiting factors (e.g., the amount of food available) contributing to its carrying capacity. This is why overpopulation in the natural world is rare and short-lived when it does occur. A species that overpopulates experiences the consequences of these limiting factors, such as starvation, increased predation and spread of disease, and more. Thus, sometimes population control is required.
Carrying capacity: The largest population an ecosystem can sustain with the resources available (e.g., food, water, habitat).
Limiting factors: These are the abiotic and biotic factors that keep populations in check. These factors can be density-dependent (e.g., food, water, disease) and density-independent (e.g., volcanic eruptions, wildfires).
Different Strategies for Population Growth
Before we go directly into discussing population control, we first need to look at the two main population growth strategies. These are referred to as "K-selected" and "r-selected".
Remember that "K" refers to a population's carrying capacity and "r" refers to a population's growth rate.
The populations of K-selected species are limited by their carrying capacity. In contrast, r-selected species are limited by environmental factors that affect their population's growth rate, such as temperature and moisture level. In general, K-selected species tend to be large and long-lived, with fewer offspring, while r-selected species are small, short-lived and have numerous offspring. Please see the table below for a comparison between the two types, along with some examples.
K-selected species | r-selected species |
Regulated by carrying capacity | Regulated by environmental factors |
Larger-sized | Smaller-sized |
Long-lived | Short-lived |
Few offspring | Numerous offspring |
Humans and other primates, elephants, and whales. | Frogs, toads, spiders, insects, and bacteria. |
You may wonder, "do all animals fit neatly into these two categories?" Of course, the answer is "no". These are merely two opposing extremes of population growth strategies, and many species lie either in between or include elements of both.
Take crocodiles and turtles, for example- both are large and can be very long-lived. Still, both also produce numerous offspring, giving them elements of both K-selected and r-selected strategies.
In the case of these two groups, both experience very high hatchling mortality rates, so having more offspring benefits survival.
Population Control Theory
We often see population control methods being used to keep populations of certain wildlife species at manageable sizes.
Population control refers to the maintenance of any living organism's population at a specific size through artificial means.
These populations often become unmanageable in size due to the removal of a natural limiting factor, such as a natural predator. Several different methods can be used to control wildlife populations.
Methods Used to Control Population
In non-human animals, populations are usually controlled through the aforementioned limiting factors. However, in some cases, humans have modified the environment to such an extent that other methods are needed.
In many parts of the United States, deer species no longer have any natural predators. Mountain lions (Puma concolor), a significant predator of deer, have been eradicated from all of their historic range in the eastern U.S. (barring one small remnant population in Florida), leaving deer living east of the Mississippi River without any major predators.
Humans can implement several methods to control the deer population, including the following three.
Hunting / Culling
Deer hunting is a popular past time in many parts of the U.S. Hunting and culling are methods of population control that have been employed for many species worldwide:
some of which are overpopulated due to the removal of predators,
some of which are non-native/invasive,
others not overpopulated but deemed too common for human comfort (e.g., some large predators).
Hunting and culling can effectively mitigate overpopulation, but they fail to address the underlying cause.
In many cases, the underlying cause of overpopulation is removing one or more critical predator species.
It may seem shocking, but did you know that wolves once roamed most of the English countryside? Did you know that wolves, grizzly bears, AND jaguars once roamed much of the U.S.? Or that saltwater crocodiles and Indochinese tigers once inhabited the jungles of Thailand?
All of these predators were eradicated from much of their range by humans. These eradications also had unexpected consequences, such as the expansion in the range of coyotes (Canis latrans) and black bears (Ursus americanus) due to a lack of competition from the larger, more dominant predators that were formerly present.
Reintroduction of Predators
Another effective form of population control involves the reintroduction of these predators.
In Yellowstone National Park, for example, the reintroduction of the grey wolf (Canis lupus) has had many positive impacts on the surrounding ecosystem, including effectively controlling prey species populations.
Wolves have long been persecuted by humans and currently exist in only a fraction of their historical range worldwide. The wolves are a significant predator of elk (Cervus Canadensis), which had become overpopulated in the wolves' absence. Since the reintroduction of wolves, elk populations are now under control. This, in turn, resulted in a cascading effect on the ecosystem. With elk populations no longer decimating willows along river banks, beavers (Castor canadensis) have been able to build more dams and have access to more food. This is a fine example of the vital role apex predators play in ecosystems and how they can be utilised to bring ecosystems back into balance.
There are ongoing discussions about the reintroduction of wolves into the United Kingdom, but, as of now, nothing is planned.
Habitat Management
The proper management of wildlife habitat can promote the natural population equilibrium of the wildlife present. The protection and management of habitat can allow predators to return to areas of formerly marginal habitat where they may have been eradicated or significantly reduced, allowing them to regulate the populations of prey species.
Humans can manage wildlife habitat by actively removing invasive animal and plant species, adding native plants and animals, and creating specific habitats that native species may use, such as piles of native brush and vegetation debris. This may include creating shelters for specific native species using native vegetation, such as cavities in trees and perching branches. Lastly, the habitat can be protected from the intrusion of livestock and other non-native species through fencing and better regulation of the human presence within the habitat.
Sterilisation / Neutering
Rendering animals unable to breed is another potentially effective way to control populations. Feral domestic animals, particularly cats and dogs, can breed unsustainably and wreak havoc on natural ecosystems. Feral cats, in particular, are voracious predators, and in areas where feral cats are numerous, wildlife populations suffer immensely. One humane way to curb the population of wild pets is through capturing, neutering, and releasing them.
Regarding feral cats, this practice is known as Trap-Neuter-Return (TNR).
When controlling the human population, things are much more complex for various reasons. Some methods can mitigate the negative impacts of global human population growth. We will go over these in the next section.
Human Overpopulation
Unlike other animals, humans have been able to extend their carrying capacity through the use of artificial technology. The creation of agriculture, in particular, has allowed human and domestic livestock populations to grow beyond their expected natural maximum sizes.
The human population has more than doubled over the past 50 years, from 3.84 billion in 1972 to 8 billion in 2022, and is expected to reach 10 billion by 2050.
As you can imagine, this puts massive pressure on the Earth's natural resources and ecosystems. An unsustainably expanding human population has resulted in widespread habitat destruction to make way for agriculture, aquaculture, cattle farming, and housing to sustain such a large population. So what do we do about overpopulation?
Global Population Control
Given the significant negative impact that unsustainable human population growth has had and continues to have on the environment and the human quality of life in many countries, several methods of mitigating human population growth have been proposed.
Increased Access to Contraception and Family Planning Globally
On a global scale, nearly half of all pregnancies are unintended or unplanned. Increasing sexual education, access to contraception (including vasectomy), and family planning opportunities could significantly reduce the number of unwanted pregnancies.
This is important in both developing and developed countries for different reasons.
While population growth has slowed in many developed countries, lifestyles have become much less sustainable, resulting in a more significant carbon footprint per person than in developing countries. On the flip side, population growth continues to increase in many developing countries, putting further pressure on already threatened ecosystems and facilitating the spread of disease and increased poverty.
With a population of 160 million people living in less than 150,000 square kilometres, Bangladesh is one of the most densely populated countries on Earth. The country subsequently suffers from extreme resource pressures and severe poverty. In Bangladesh, around half of all pregnancies are unintended. Empowering the population with better education, access to contraception, and family planning could help countries like Bangladesh relieve ecosystem pressure and decrease pollution levels.
One-child Policy
A more controversial form of human population control is implementing a one-child policy.
China famously implemented a one-child policy for 35 years, from 1980 to 2015, in an effort to control overpopulation.
While theoretically effective, in practice, one-child policies can be tough to enforce and lead to human rights abuses, imbalanced sex ratios, and general discontent throughout a population. Some scholars claim that the one-child policy effectively curbed the country's population growth in China. In contrast, others argue that the reductions were due to increased education and economic development.
Wealth Redistribution
Another way to potentially curb human population growth is the redistribution of wealth. This is because birth rates tend to be lower in wealthier nations with better education and access to contraceptives.
With fewer people living in poverty, more people would be able to pursue an education and fewer unintended births.
Impact of Human Population Control on Biodiversity
By far, the most significant current threat to the planet's biodiversity is unsustainable human activity. Major industries are destroying large swaths of natural habitat, exacerbating climate change, and driving species to the brink of extinction. Such industries include:
Palm oil
Cattle farming
Sand mining
Coal mining
All of these industries exist to fuel the needs of an unsustainable human population. In addition, housing developments and farmland continue to encroach more and more into previously undisturbed ecosystems, resulting in further loss of biodiversity and increased human-wildlife conflict. If the human population curbs its growth and becomes more sustainable, biodiversity would likely rebound significantly.
Human Population Control's Effect on Climate Change
Specific industries have had a disproportionate effect on anthropogenic climate change. These industries include:
Coal mining
The automobile industry
Oil drilling
Cattle farming
These are all significant culprits of increased greenhouse gas emissions, and all of these industries exist to sustain an unsustainable population. A smaller, more sustainable human population combined with more sustainable fuels and technologies would render most of these problems inconsequential.
Population Control and Biodiversity - Key takeaways
Population control refers to the maintenance of any living organism's population at a specific size through artificial means.
In non-human animals, populations are usually controlled through limiting factors. However, in some cases, humans have modified the environment to such an extent that other methods are needed.
Control of wildlife populations includes hunting/culling, reintroducing predators, and sterilisation/neutering.
The human population has more than doubled over the past 50 years, from 3.84 billion in 1972 to 8 billion in 2022, and is expected to reach 10 billion by 2050.
Methods to control the human population include increased access to contraception, family planning, wealth redistribution, and one-child policies.
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