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Vocab for unit 3: populations
◦Population Density
◦Age Structure
◦Exponential growth
◦Logistic Growth
◦ Carrying capacity
◦ Limiting Factor
◦ Density-dependent limiting
◦ Density-independent limiting
◦ Demography
◦ Demographic transition
Describing Populations
◦We describe populations by
looking at:
◦Geographic range
◦Population density
◦Growth rate
◦Age structure
Geographic Range
◦The area inhabited
by a population
Example: the range
of the emperor
penguin is the coast
of Antarctica
Population density
◦The number of individuals
per unit area
◦ Example: if you have 375
dogs in 15 square
kilometers the population
density of dogs would be
25 dogs per 1 square
Growth rate
◦The population growth rate is the rate at which the
number of individuals in a population increases in a
given time period as a fraction of the
initial population.
◦ Example: Global human population growth amounts to
around 75 million annually, or 1.1% per year.
◦Determines if a population increases, decreases, or
stays the same
◦ Factors: Birthrate, death rate, immigration, emigration
Age Structure
◦The numbers of
males and
females in each
age group
How do populations increase?
How do populations decrease?
◦Populations stay the same when
growth factors (birthrate &
immigration) equal decrease factors
(death rate & emigration)
Plicker Questions (multiple choice)
◦The movement of individuals into an area is
◦All other things being equal, the size of a
population will decrease if…
2 Types of Growth
Exponential Growth
◦ Population growth where the
size of each generation is
larger than the generation
before it
◦ With exponential growth the
larger a population gets, the
faster it grows
◦ Under ideal conditions with
unlimited resources, a
population will grow
Logistic Growth
◦ The various growth phases
that most populations go
◦ Phase 1: exponential growth
◦ Phase 2: growth slows down
◦ Phase 3: growth stops (at the
carrying capacity)
Carrying Capacity
◦The maximum number of individuals of a
particular species that a particular
environment can support.
◦Once a population reaches the carrying
capacity of its environment the size of the
population starts to stabilize.
Plicker Questions
◦ In the presence of unlimited resources and in the
absence of disease and predation, what would
probably happen to a bacterial population?
◦ Which of the following refers to when a population’s
birthrate equals its death rate?
Limiting Factors
◦ A limiting factor is something
that controls the growth of a
◦ Limiting factors determine the
carrying capacity of an
environment for a species
Can be
limited by
Two Types of Limiting Factors
Density Dependent Limiting
◦Happen only when
population density
reaches certain levels
◦Does not typically
impact small scattered
Density Independent Limiting
◦Affect all populations in
similar ways, regardless of
population size and
Density Dependent Limiting Factors
◦ Competition: when populations are crowded individuals
compete for resources
◦ Predation: predator prey relationships
◦ Herbivory: herbivores are PREDATORS to plants.
◦ Parasitism / Disease: the denser the population, the easier it is for
parasites and disease to spread
◦ Stress from overcrowding: Some species fight amongst
themselves if they’re overcrowded – fighting can lead to high
levels of stress which weakens the individuals ability to fight
Density Independent Limiting Factors
◦Unusual weather: hurricanes, droughts, floods, …
◦Natural Disasters: wildfire, earthquakes, ….
Remember that not all limiting factors will limit all
populations. For example, a wildfire might not be
a limiting factor for fish in the ocean
Plicker Questions
◦ Which of the following is not an example of a density-dependent limiting
◦ Which of the following is not an example of a density-independent limiting
◦ What could cause a population to sharply decline?
◦ What would increase competition in a population?