A population is a group of individuals from the same species that live in the same geographic region. Members of the population share resources, are affected by comparable environmental variables, and are more inclined to engage and breed with one another.

The limits and size of populations are frequently used to characterize them (the number of individuals living within those boundaries).

Ecologists often begin an investigation of a population by setting limits that are appropriate to the creature being studied and the questions being addressed. The borders of a population can be natural, as in the case of an island or a lake, or they might be arbitrarily established by an investigator, such as a certain county in Minnesota.

• The term population refers to a group of individuals of a single species living in the same general area.

  

The attached image shows the survivorship curve for female Belding’s ground squirrels.

The logarithmic scale on the y-axis allows the number of survivors to be visible across the entire range (2–1,000 individuals) on the graph.

The number of people alive at the start of each year is the outcome.

The attached image shows the results of plotting these numbers vs age for female Belding's ground squirrels. The plot's almost straight line shows a relatively consistent rate of death.

The image depicts only one of several survival patterns found in natural populations. Despite their diversity, survivorship curves may be divided into three categories.

For sexual creatures such as birds and mammals, reproductive output is generally assessed as the average number of female offspring produced by females in a specific age group.

For certain species, the number of offspring produced by each female may be directly counted; alternatively, molecular techniques can be employed (as shown in the attached image).

Researchers tallied the offspring of Belding's ground squirrels, which began reproducing at the age of one year. The reproductive production of squirrels peaks at 4–5 years of age and then steadily declines in older females (as shown in the image).

The reproduction rates of different ages vary greatly amongst species. For example, squirrels have a litter of two to six young animals.

Population density, dispersion, and demography are influenced by biotic and abiotic variables. The number of people per unit area or volume represents the interaction of births, deaths, immigration, and emigration.

Individual dispersion is influenced by environmental and social variables. Populations grow as a result of births and immigration and decline as a result of deaths and emigration.

Life tables and survival curves highlight particular demographic patterns. The exponential model of population expansion explains population increase in an idealized, limitless setting. If immigration and emigration are excluded, the per capita growth rate of a population equals the birth rate minus the mortality rate

When resources are plentiful, the exponential growth equation dN/dt = rN describes population expansion, where r is the intrinsic rate of rising and N is the population size. The logistic model illustrates how a population slows down when it approaches its carrying capacity.

In every population, exponential growth cannot be sustained.

A more realistic population model constrains growth by taking into account carrying capacity (K), or the maximum population size that the ecosystem can support.

Growth slows as population size approaches carrying capacity, according to the logistic growth equation dN/dt = rN (K - N)/K.

Although the logistic model only fits a few real populations exactly, it is useful for projecting potential growth. Natural selection produces life history characteristics.

Life history characteristics are evolutionary consequences that are reflected in organism development, physiology, and behavior.

Creatures that reproduce once and die are known as big-bang organisms or semelparous organisms.

Iteroparous creatures have progeny on a regular basis.

Life history characteristics like brood size, maturity age, and parental caring are examples of trade-offs between competing needs for time, energy, and nutrition. K-selection and r-selection are two hypothetical life history patterns.

Population growth is regulated by density-dependent variables (pp. 1200–1205).

With increasing density, mortality rates rise and birth rates decline under density-dependent population management. A density-independent birth or death rate is one that does not fluctuate with density.

Population growth is slowed by density-dependent variations in birth and death rates, which can finally stabilize a population around its carrying capacity. Intraspecific competition for limited food or space, increased predation, illness, intrinsic physiological variables, and toxic chemical accumulation are examples of density-dependent limiting factors.

Because changing environmental circumstances disturb them on a regular basis, al

The human population is no longer rising exponentially, but rather rapidly.

The global human population has increased rapidly since around 1650, but in the last 50 years, the pace of expansion has been cut in half.

Differences in age structure demonstrate that while some countries' populations are quickly expanding, others' populations are steady or falling. Infant mortality rates and life expectancy at birth vary greatly among nations.

The ecological footprint is the total amount of land and water required to generate all of the resources that a person or group of people use and absorb all of their waste. It is one indicator of how close we are to exceeding Earth's carrying capacity.