POPULATION BIOLOGY

Review: Population is a group of 1 species
living in the same place at the same time.

A species is defined by reproduction (with each
other, producing fertile offspring)
WHAT LIMITS A POPULATIONS GROWTH?
Answer: Limiting Factors
1. Density Dependent – increases as the
population increases.
EXAMPLES OF DENSITY DEPENDENT
DISEASE SPREADS FASTER
MAD COW DISEASE
COMPETITION FOR FOOD
COMPETITION FOR WATER
PARASITES SPREAD EASIER
EXAMPLES OF DENSITY DEPENDENT

Diseases, competition, parasites, lack of food

All of these things increase pressure on the
population as the population gets bigger.
WHAT LIMITS A POPULATIONS GROWTH?
Answer: Limiting Factors
1. Density Dependent
2. Density Independent – affects the population
regardless of the population size.
EXAMPLES OF DENSITY INDEPENDENT
STORMS
FLOODS
DROUGHT
POLLUTION
HABITAT DESTRUCTION
EXAMPLES OF DENSITY INDEPENDENT

Storms, floods, drought, habitat destruction,
natural disasters, pollution

All of these things affect a population of 10
individuals or 10 million individuals. The size
does not matter!
CARRYING CAPACITY
Definition: Number of organisms in the
population (remember: just 1 species) that the
environment can support.


Populations may be over or under the
CARRYING CAPACITY.

If a population is under the Carrying Capacity,
Then the population will increase
 (more births and survival because there are
enough resources like food, water, space)


If a population is over the Carrying Capacity,
Then the population will decrease
 (more deaths because of lack of food, water,
space,…)

POPULATION GROWTH CURVE POSSIBILITIES
J-Curve

Exponential Growth

Starts small

Rapid increase
POPULATION GROWTH CURVE POSSIBILITIES
S- Curve

Population growth
adjusts to amount of
resources.
POPULATION GROWTH CURVE POSSIBILITIES
Fluctuating Curve

Population
oscillates above
and under carrying
capacity.

Ex. Lynx and
snowshoe hare
ORGANISM INTERACTIONS THAT AFFECT
POPULATION SIZE

Predator- Prey – prey pop. Goes up, then
predator pop. Goes up, prey goes down, then
predator pop. Goes down.

Competition - btwn organisms for food, space,
light, water, mates,…
ORGANISM INTERACTIONS THAT AFFECT
POPULATION SIZE
Crowding and Stress  Stress symptoms in a population:

 Aggression
 Decrease
in parental care
 Decrease in fertility
 Decreased resistance to disease
All of these things can lead to population decline!
HUMANS ARE ANIMALS TOO!
We are not exempt from limiting factors like
Natural disasters, disease, lack of food, water or
space, overcrowding, and competition.
HUMANS ARE ANIMALS TOO!
Humans have been able to exceed their carrying
capacity by overcoming some limiting factors.
Ex. Medicine for diseases
Eliminating competition (wars, hunting,…)
Increasing food production
Every second: 4-5 babies are born somewhere on
Earth. Also in every second, 2 people die.
Net growth: 2.5 people every second.
9,000 people every hour.
214,000 people per day.
HUMAN POPULATION
Vocabulary Alert!
Demography: study of human populations
Population Growth Rate= birth rate – death rate

In the U.S.: birth rates
are down, but death
rates are down as well.

People are having fewer
children but life
expectancy increases
every year.
Vocabulary Alert!
Age Structure: number of individuals at each age
group.
Histogram (or Population Pyramid): a graph of the
age structure of a population (usually a
country)
EXAMPLES OF HISTOGRAMS
HISTOGRAMS CAN HELP DEMOGRAPHERS
PREDICT WHAT THE POPULATION WILL LOOK
LIKE IN THE FUTURE.

If a pyramid shape – the population will grow.

If a rectangle shape – the population is steady.

If an inverted pyramid – the population will
shrink.
Vocabulary Alert!
Mobility – movement of animals that affects
population size and structure.
Immigration – movement of individuals INTO a
population.
Emigration –individuals EXIT a population.