Have you ever wondered why populations of organisms in nature do not continue to grow and expand endlessly? Perhaps you have wondered what factors prevent populations of organisms from growing exponentially. In the following article, we will discuss how populations are regulated, which factors are involved, and how it relates to us as humans.
Definition of population regulation in ecology
Every living organism on Earth (including humans) has limits to how large its population grows. Infinite growth is not possible on a planet with finite resources and all populations will eventually be regulated. This article covers the mechanisms of population regulation. So what is the definition of population regulation in ecology?
Population regulation refers to the ecological processes (biotic and abiotic factors) by which the growth of populations is limited due to the effects on birth and death rates.
The ecological factors that limit population growth are known as limiting factors. There are two different kinds of limiting factors - density-dependent and density-independent limiting factors. In addition, there are two kinds of population regulation - top-down regulation and bottom-up regulation.
Density-dependent limiting factors in population regulation
Density-dependent limiting factors impact a population’s per capita rate of growth based on the population’s density. These factors will generally cause the growth rate to drop as the population gets larger. Density-dependent limiting factors usually cause populations to reach a maximum level (called the population’s carrying capacity).
At this point, the population size will level off and usually, but not always, become stable. This is known as logistic growth. When a population's growth rate remains constant, no matter its size, it will continue to grow larger at an exponential rate. This is known as exponential growth.
This is very rare, and when it does occur, it will usually be quickly corrected by the density-dependent limiting factors in the environment.
Figure 1: The logistic growth model. Source: Wiki Commons
Logistic growth occurs when density-dependent limiting factors cause population growth to gradually slow before reaching a maximum level at which growth will level off and become stable.
Exponential growth occurs when a population's growth rate remains constant, no matter the size, exceeding its carrying capacity.
Most density-dependent limiting factors are biotic. These factors can include intra - and interspecific competition, increased spreading of disease, and parasitism. In prey species, higher population densities may also result in higher predation rates. Individuals from a population that has reached carrying capacity may also wander out in search of new habitat that is not yet at capacity.
Biotic: Biotic factors are those that involve or are produced by living organisms.
Abiotic: Abiotic factors are those that do not involve and are not produced by living organisms.
By the early 1970s, saltwater crocodile (Crocodylus porosus) populations in Australia’s Northern Territory were nearing extinction, with only a few thousand individuals remaining. Thanks to protective efforts, over the following decades, the population recovered to the point that most rivers are believed to have reached carrying capacity, with crocodile populations nearly reaching pre-exploitation levels, leveling off, and becoming stable.
In this case, the density-dependent limiting factors include competition (e.g., finite prey availability and territoriality) and habitat limitations (e.g., breeding habitat and climatic restrictions), which prevent the crocodile population from continued expansion. In many of these rivers, this has resulted in smaller, less dominant males wandering out into the ocean and into suboptimal areas in search of new habitats, oftentimes bringing them into conflict with humans.
Population cycles
Populations experiencing density-dependent limiting factors often experience instability at carrying capacity, even without the effects of density-independent limiting factors. These populations may experience cycles of growth followed by a reduction in size in oscillating patterns called cyclical oscillations. Under specific circumstances, usually involving multiple species, these oscillations are driven by density-dependent limiting factors such as predation and resource abundance.
Density-independent factors in population regulation
Density-independent factors impact the per capita population growth rate regardless of the population’s density. Since these factors do not depend upon the population’s size, their impact does not amount to the “correction” that density-dependent factors bring to a population. In other words, density-independent factors can be potentially catastrophic to smaller populations, particularly populations of a species with a limited geographic range.
Density-independent factors can be abiotic, and perhaps the best example of a density-independent factor would be a natural disaster, such as a forest fire. A natural disaster may kill a significant fraction of the population living in the area, regardless of how large that population was, to begin with.
If the population is limited to only a small area, a single natural disaster could even push a species to extinction.
For example, black bear (Ursus americanus) populations are known to be affected by wildfires by way of decreased cub survival.
Top-down population regulation
Top-down population regulation refers to situations where species at higher trophic levels (e.g., apex predators at the top of the food chain) control the populations of species at lower trophic levels (e.g., prey). Due to this, it is also called "predator-controlled" regulation. Typically, the population size and density of the apex predator at the top of the food chain is much lower than that of its prey, which is usually quite abundant.
Occasionally this may not be the case, as is seen with the American alligator (Alligator mississippiensis) and caiman species, which are apex predators and are often very abundant.
For example, mountain lions (Puma concolor) may control mule deer (Odocoileus hemionus) populations, but the mule deer may control the populations of certain plant species.
Bottom-up population regulation
Bottom-up population regulation is dependent on the resources of an ecosystem. Since all of the higher trophic levels are dependent on the continued presence of those below them, when those resources at the lower level are diminished or absent, all trophic levels are affected.
For example, if vegetation experiences a mass die-off, this may result in a decline in the mule deer population due to starvation. This, in turn, may also result in a reduction in the mountain lion population due to a lack of prey.
Population regulation in humans
Fifty years ago, in 1972, the human population consisted of around 3.9 billion people. Today that number has grown to over 7.9 billion.
Thus, the human population has more than doubled, growing more in the last half-century than in the entirety of human existence (at least 200,000 years).
This exponential growth is largely due to better technology, food availability, medicine, and more, which have allowed humans to artificially increase their carrying capacity.
However, this exponential cannot persist indefinitely, as the methods used to increase our carrying capacity are being outpaced by many density-dependent limiting factors.
For humans, these limiting factors include widespread resource depletion (food, water, gas), climate change, increased spread of disease, and pollution.
Figure 3: Human population growth since 1800, including future high and low projections. Source: Wiki Commons
Indeed, the consequences of these density-dependent factors have always been present, but their impact will continue to be amplified as the population further exceeds the natural carrying capacity. Density-independent factors also affect human populations, with some notable recent examples including earthquakes, hurricanes, and tsunamis that cause large-scale damage to infrastructure and high mortality. To counteract this growth and mitigate consequences, artificial methods of human population regulation have been proposed, including increased access to contraception, family planning, and increased education.
Population Regulation - Key takeaways
- Population regulation refers to the ecological processes (biotic and abiotic factors) by which the growth of populations is limited, due to the effects on birth and death rates.
- There are two different kinds of limiting factors- density-dependent and density-independent limiting factors. Density-dependent limiting factors impact a population’s per capita rate of growth based on the population’s density. Density-independent factors impact the per capita population growth rate regardless of the population’s density.
- There are two kinds of population regulation: top-down and bottom-up. Top-down population regulation refers to situations where species at higher trophic levels control the populations of species at lower trophic levels. Bottom-up population regulation is dependent on the resources of an ecosystem.
- Populations may experience cycles of growth followed by a reduction in size in oscillating patterns, called cyclical oscillations.
Explanations 
Exams 
Magazine 
