Describing Populations
What is a population?
Members of a species that live in the same area at the
same time
Determining Population
Population size
Population density
Differing densities
The number of individual
organisms present in a
given population at a
given time
The number of
individuals within a
population per unit area
High population densities:
can be susceptible to
higher predation due to
vulnerability, can spread
diseases easier, can suffer
from lack of space and
resources due to increased
competition
Low population densities:
animals have more space
and resources, harder to
find mates
Population Distribution
Random
Uniform
Clumped
Organisms arranged
in no particular
pattern
Organisms evenly
distributed
Organisms arrange
themselves
according to
available resources
Population Changes
Immigration
Emigration
Migration
Arrival of individuals
from outside a given
area
Departure of
individuals from
outside a given area
Seasonal movement
into and out of an
area
Calculating Population Growth Rate
• (individuals added) – (individuals subtracted)
• Or
• (birth rate + immigration) – (death rate + emigration)
• Example: if birth rate is 18/1000 and death rate is 10/1000;
immigration is 5/1000 and emigration is 7/1000, then:
(18+5) – (10+7) = 6. Multiplied by 100 to get a percent = 0.6%
How Populations Grow
Exponential Growth
• When a population undergoes
a fixed percent of growth per
year due to the right
conditions
• A “J” shaped curve on a graph
Logistic Growth
• Shows initial growth as
exponential but is then
stopped or slowed by limiting
factors
• Limiting factors: Determine
carrying capacity; which is the
largest population an
environment can support
How populations grow cont.
• If birth rate exceeds death rate the population will increase
• If birth and death rate remain the same then population stays the
same
• If death rate exceeds birth rate the population will decrease
Limiting Factors for Biotic Potential
Density-dependent factor
Limiting factors that are
influenced by density:
• Predation
• Disease
• Competition
• Food
• Water
• Space
Density-independent factor
Limiting factors whose influence
is not affected by density
• Climate change
• Natural disasters
• Cause a dramatic decrease in
population size
Biotic Potential
• An organisms maximum ability to produce offspring in ideal
conditions
2 types:
1. High biotic potential: cockroaches having 100s of babies
2. Low biotic potential: Orangutans, don’t reach sexual maturity
for 10yrs and only have one baby every 8 years