Population Biology
How Study Populations?
Geographic Range
Density and Distribution
Growth rate
Age structure
Geographic Range
• The area inhabited by a
population
• Can vary greatly depending
on the species
 (bacteria on a rotting
pumpkin vs. cod in the western
Atlantic ocean from Greenland
to North Carolina
Density and Distribution
• Population density refers to the
number of individuals per unit area
• Distribution refers to how individuals
in a population are spaced out across
the range of the population:
 random, uniform or clumps.
Random
Uniform
Clumped
Growth Rate
• Determines whether the size of the population increases,
decreases, or stays the same.
 can be positive, negative or zero
Age Structure
• The number of males and females of each age a population
contains
• Important because most animals do not reproduce until a
certain age
Population Growth
Factors that effect population growth
1. Birth Rate and Death Rate
In a given period of time:
• Birth rate > death rate  population increases
• Birth rate = death rate  population stays the same
• Birth rate < death rate  population decreases
2. Emigration and Immigration
• Immigration – movement of individuals into an area occupied
by an existing populations
• Emigration – movement of individuals out of an area
Exponential vs. Logistic Growth
Would you take the money?
Exponential Growth
• Under ideal conditions with
UNLIMITED resources, a population
can and will grow exponentially.
• Exponential growth is when a
population increases exponentially
Xn (often doubling)
Logistic Growth
• Resources are NOT unlimited so a population can not grow
exponentially forever
• Logistic growth occurs when a populations’ growth slows and then
stops, following a period of exponential growth
Carrying Capacity
The maximum number of
individuals of a particular
species in a particular
environment can support
(without degrading it for
future generations)
* Population will
hover around the
CC
Limiting Factors
A factor that controls the growth of a population
Examples:
Competition
Predation
Parasitism and disease
Weather
Natural Disaster
Nutrients
Acting separately or together, limiting factors determine
the carrying capacity of an environment for a species
Limiting factors are either Density Dependent or Density Independent
Density Dependent Factor
operate strongly when population density (number of organisms per unit
area) reaches a certain level. Examples Include:
1. Competition - overcrowding lead to
competition over essential
resources….there more there are, the
sooner they use up the resources
2. Predation and Herbivory –
Predator population increases, prey
decreases and vice versa. Think of
herbivores as “predators” of plants
3. Parasitism and Disease– The
more organisms in a given area, the
easier it is to spread disease
4. Stress from overcrowding – organisms can fight within species if too
crowded, and cause females to neglect or kill their young, decrease birth
rates, and increase emigration and death rates
Density Independent Factor
affect all populations in similar ways, regardless of population size and
density. Examples Include:
Weather, hurricanes, droughts, floods wildfires, etc.
However, some of these independent factors work with density.
Example: Isle Royale (Moose & cold winter)