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Study Guides
Page 1 of 2
v1.1.11.2012
This guide was created by Maxine Tsang and Jin Yu. To learn more about the
student authors, visit http://www.ck12.org/about/about-us/team/interns.
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=(b+i)-(d+e)
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 refects individuals entering
the population from somewhere else.
Emigration rate (e) refects 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.
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 fnd
food, mates, or other resources.
Populations are groups of the same species. Populations make up communities and have many factors that can infuence
their population size, population density, and population distribution. A populations rate of growth refects 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
Size, Density, & Distribution Population Growth Rate
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 uncon-
trolled and rapid.
Big Picture
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 infuences 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 refects 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
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0
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Types of population growth pattern:
Exponential growth (curve A in the graph): When a
populations 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, ex-
ponential 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 carry-
ing capacity.
Patterns of Population Growth
Population Growth
Population growth refects the changes in the
size of the population over time.
The age-sex structure will infuence pop-
ulation growth. Younger people are more
likely to reproduce, so a large number of
younger people may result in a large num-
ber 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 num-
ber 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 re-
sults with most of the offspring dying at a
relatively young age. Type III is typical of
plants, invertebrates, and fsh.
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0
Figure. Survivorship curves for Type l (humans), Type ll
(songbirds), and type lll (frogs).
POPULATION CONT.
