Population Ecology: Definition, Characteristics, Theory & Examples

Ecologists study how organisms interact with their environments on earth. Population ecology is a more specialized field of study of how and why the populations of those organisms change over time.

As the human population grows in the 21st century, the information gleaned from population ecology can assist with planning. It can also help with efforts to preserve other species.

In population biology, the term population refers to a group of members of a species living in the same area.

The definition of population ecology is the study of how various factors affect population growth, rates of survival and reproduction, and risk of extinction.

Ecologists use various terms when understanding and discussing populations of organisms. A population is all of one kind of species residing in a particular location. Population size represents the total number of individuals in a habitat. Population density refers to how many individuals reside in a particular area.

Population Size is represented by the letter N, and it equals the total number of individuals in a population. The larger a population is, the greater its generic variation and therefore its potential for long-term survival. Increased population size can, however, lead to other issues, such as overuse of resources leading to a population crash.

Population Density refers to the number of individuals in a particular area. A low-density area would have more organisms spread out. High-density areas would have more individuals living closer together, leading to greater resource competition.

Population Dispersion: Yields helpful information about how species interact with each other. Researchers can learn more about populations by studying they way they are distributed or dispersed.

Population distribution describes how individuals of a species are spread out, whether they live in close proximity to each other or far apart, or clustered into groups.

• Uniform dispersion refers to organisms that live in a specific territory. One example would be penguins. Penguins live in territories, and within those territories the birds space themselves out relatively uniformly. 
• Random dispersion refers to the spread of individuals such as wind-dispersed seeds, which fall randomly after traveling.
• Clustered or clumped dispersion refers to a straight drop of seeds to the ground, rather than being carried, or to groups of animals living together, such as herds or schools. Schools of fish exhibit this manner of dispersion.

Quadrat method: Ideally, population size could be determined by counting every individual in a habitat. This is highly impractical in many cases, if not impossible, so ecologists often have to extrapolate such information.

In the case of very small organisms, slow movers, plants or other non-mobile organisms, scientists scan use what is called a quadrat (not "quadrant"; note the spelling). A quadrat entails marking off same-sized squares inside a habitat. Often string and wood are used. Then, researchers can more easily count the individuals within the quadrat.

Different quadrats can be placed in different areas so that researchers get random samples. The data collected from counting the individuals in the quadrats is then used to extrapolate population size.

Mark and recapture: Obviously a quadrat would not work for animals that move a round a great deal. So to determine the population size of more mobile organisms, scientists use a method called mark and recapture.

In this scenario, individual animals are captured and then marked with a tag, band, paint or something similar. The animal is released back into its environment. Then at a later date, another set of animals is captured, and that set may include those already marked, as well as unmarked animals.

The result of capturing both marked and unmarked animals gives researchers a ratio to use, and from that, they can calculate estimated population size.

An example of this method is that of the California condor, in which individuals were captured and tagged to follow the population size of this threatened species. This method is not ideal due to various factors, so more modern methods include radio tracking of animals.

**Thomas Malthus**, who published an essay that described population's relationship to natural resources, formed the earliest theory of population ecology. Charles Darwin expanded on this with his "survival of the fittest" concepts.

In its history, ecology relied upon the concepts of other fields of study. One scientist, Alfred James Lotka, changed the course of science when he came up with the beginnings of population ecology. Lotka sought the formation of a new field of "physical biology" in which he incorporated a systems approach to studying the relationship between organisms and their environment.

Biostatistician Raymond Pearl took note of Lotka's work and collaborated with him to discuss predator-prey interactions.

Vito Volterra, an Italian mathematician, began analyzing predator-prey relationships in the 1920s. This would lead to what were called Lotka-Volterra equations that served as a springboard for mathematical population ecology.

Australian entomologist A.J. Nicholson led the early fields of study regarding density-dependent mortality factors. H.G. Andrewartha and L.C. Birch would go on to describe how populations are affected by abiotic factors. Lotka's systems approach to ecology still influences the field to this day.

_Population growth reflects the change in the number of individuals over a period of time. Population growth rate is affected by birth and death rates, which in turn are related to resources in their environment or outside factors such as climate and disasters. Decreased resources will lead to a decreased population growth. Logistic growth_ refers to population growth when resources are limited.

When a population size encounters unlimited resources, it tends to grow very quickly. This is called exponential growth. Bacteria, for example, will grow exponentially when given access to unlimited nutrients. However, such growth cannot be sustained indefinitely.

**Carrying capacity:** Because the real world does not offer unlimited resources, the number of individuals in a growing population eventually will reach a point when resources become scarcer. Then the growth rate will slow and level off.

Once a population reaches this leveling-off point, it is considered the greatest population the environment can sustain. The term for this phenomenon is carrying capacity. The letter K represents carrying capacity.

Growth, birth and death rate: For human population growth, researchers have long used demography to study population changes over time. Such changes result from birth rates and death rates.

Larger populations, for example, would lead to higher birth rates just because of more potential mates. However, this can also lead to higher death rates from competition and other variables such as disease.

Populations remain stable when birth and death rates are equal. When birth rates are greater than death rates, the population increases. When death rates outpace birth rates, the population goes down. This example does not, however, take immigration and emigration into account.

Life expectancy also plays a role in demography. When individuals live longer, they also affect resources, health, and other factors.

**Limiting factors:** Ecologists study factors that limit population growth. This helps them understand the changes populations undergo. It also helps them predict potential futures for the populations.

Resources in the environment are examples of limiting factors. For example, plants need a certain amount of water, nutrients and sunlight in an area. Animals require food, water, shelter, access to mates and safe areas for nesting.

Density-dependent population regulation: When population ecologists discuss the growth of a population, it is through the lens of factors that are density-dependent or density-independent.

Density-dependent population regulation describes a scenario in which a population's density affects its growth rate and mortality. Density-dependent regulation tends to be more biotic.

For example, competition within and between species for resources, diseases, predation and waste buildup all represent density-dependent factors. The density of available prey would also affect the population of predators, causing them to move or potentially starve.

Density-independent population regulation: In contrast, density-independent population regulation refers to natural (physical or chemical) factors that affect mortality rates. In other words, mortality is influenced without density being taken into account.

These factors tend to be catastrophic, such as natural disasters (e.g., wildfires and earthquakes). Pollution, however, is a manmade density-independent factor that affects many species. Climate crisis is another example.

Population cycles: Populations rise and fall in a cyclic manner depending on the resources and competition in the environment. An example would be harbor seals, affected by pollution and overfishing. Decreased prey for the seals leads to increased death of seals. If the number of births were to increase, that population size would remain stable. But if their deaths outpaced births, the population would decrease.

As climate change continues to impact natural populations, the use of population biology models becomes more important. The many facets of population ecology help scientists better understand how organisms interact, and aid in strategies for species management, conservation and protection.