Population Regulation

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?

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.

Figure 1: The logistic growth model. Source: Wiki Commons

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.

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.

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.

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.

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.

However, this exponential cannot persist indefinitely, as the methods used to increase our carrying capacity are being outpaced by many density-dependent limiting factors.

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.