Big Picture

Populations are groups of the same species. Populations make up communities and have many factors that can influence their population size, population density, and population distribution. A population’s rate of growth reflects how healthy the group of organisms is. The equation r=(b+i)-(d+e) displays the factors that impact r (the growth rate).

Key Terms

Population: Refers to a group of organisms of the same species that lives in the same area.

Population Size: The number of of individuals in a population.

Population Density: The individuals that make up the population size per unit area or volume.

Population Distribution: How the individuals in a population are distributed throughout their habitat.

Age-Sex Structure: The number of individuals of each sex and age in the population.

Population Pyramid: A bar graph that represents age-sex structure.

Survivorship Curve: A line graph that represents the number of individuals alive at each age.

Population Growth Rate (r): How fast a population changes in size over time.

Immigration: When part of a population joins another population elsewhere.

Emigration: When part of a population leaves to go elsewhere.

Dispersal: When offspring move away from their parents.

Migration: The regular movement of individuals or populations every year during certain seasons.

Exponential Growth: Pattern of population growth in which a population starts out growing slowly but grows faster as population size increases.

Logistic Growth: Pattern of population growth in which growth slows and population size levels off as the population approaches the carrying capacity.

Carrying Capacity: Represented by the variable (K), carrying capacity is the largest population size a niche can support without being harmed.

K-Selected Species: Species that live in a stable environment, in which their population growth is controlled by density-dependent factors.

r-Selected Species: Species that live in an unstable environment, in which their population is uncontrolled and rapid.

Size, Density, & Distribution

Populations make up communities and act as a unit of natural selection and evolution. Its health can be measured by how large it is and how fast it is growing.

  • Population size influences the chances of a species surviving or on the brink of being extinct. Very small populations generally are at the greatest risk of extinction.
  • Population density reflects how crowded the population is. A large population density means there is a lot of individuals living in the area.
  • Generally individuals in a population are not spread out evenly in the area. The population distribution looks to see how the individuals are spread throughout their habitat. Organisms may be:
population distribution
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0

Population Growth Rate

The population growth rate (r) gives us an idea on how fast a population changes over time. The rate can be calculated with this equation:


r = population growth rate
b = birth rate
i = immigration rate
d = death rate 
e = emigration rate

  • A population growth rate is most affected by the birth rate (b) and the death rate (d).
  • Immigration rate (i) also reflects individuals entering the population from somewhere else.
  • Emigration rate (e) reflects individuals leaving the population to go somewhere else
  • If the population growth rate is positive, the population is increasing. If the rate is negative, the population is decreasing.

Think of immigration as coming in, emigration as exiting.


Population Cont.

Population Growth

There are other factors that can affect population size and growth:

  • Dispersal prevents competition with the parents for resources.
  • Migration changes population size in a regular pattern. The purpose of migration is generally to find food, mates, or other resources.

Population Growth

Population growth reflects the changes in the size of the population over time.

  • The age-sex structure will influence population growth. Younger people are more likely to reproduce, so a large number of younger people may result in a large number of births, increasing the population. Older people have a higher rate of dying, so a large number of older people may result in a decrease in population.
  • The age-sex structure can be shown in a population pyramid.

Survivorship curves show how deaths affect populations. You may see one of these three types of curves:

  • Type I: Parents provide care to a few off-spring, which allows them to grow into adulthood and reproduce. Type I usually describes mammals, such as humans.
  • Type II: Parents produce a moderate number of offspring and provide some care, but deaths occur more uniformly throughout life than type I. This describes asexual species and animals like birds.
  • Type III: Parents produce a large number of offspring but provide little care. This results with most of the offspring dying at a relatively young age. Type III is typical of plants, invertebrates, and fish.
Figure. Survivorship curves for Type l (humans), Type ll (songbirds), and type lll (frogs).
population pyramid.
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0
Survivorship curves
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0



Population Cont.

Patterns of Population Growth

Patterns of Population Growth
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0

Types of population growth pattern:

  • Exponential growth(curve A in the graph): When a population’s initial growth is slow, but the population size increases as the growth rate increases. The larger the population, the faster it grows. On a line graph, exponential growth forms a J-shape.
  • The lower part of the exponential growth curve represents r-selected species, whose population growth is uncontrolled and rapid.
  • Logistic growth(curve B in the graph):When a population begins to grow exponentially, but as the population encounters factors that limit its growth, it starts to level off. On a line graph, logistic growth forms a S shape. The carrying capacity (K) limits the amount a population can grow, so the curve will start to level off near the carrying capacity.
  • Logistic growth describes K-selected species. K-selected species are generally close or at the carrying capacity.