Population Ecology

What is Population?

A group of the same species

that lives in one area

5 Characteristics of Population

1. Geographic Distribution

2. Density

3. Dispersion

4. Growth Rate

5. Age Structure

Geographic Distribution

The range of the population

Describes the area that is inhabited

by the population

Such as:

The mold on a piece of bread

The migration area of whales

Population Density

The number of individuals per unit area

# of individuals

Area (units2)

Variation in density depends on:

The species

The ecosystem

= population density

Practice Problem

If scientists sampling a population of

deer counted 200 individuals in an

area of 10 square kilometers, what is

the density of this deer population per

square kilometer?

A: 20 deer per square kilometer

2000 U.S. population

density in persons

per sq. mile

(contiguous U.S.

only). Averaged on

a per-county basis.

Legend, light to dark

(white to dark blue):

0-1 (white)

1-4 (yellow)

5-9 (yellow-green)

10-24 (green)

25-49 (teal)

50-99 (dark teal)

100-249 (blue)

250-66,995 (violet)

Dispersion

The spatial distribution of individuals

within the population

Three types:

Clumped Dispersion

Even Dispersion

Random Dispersion

Clumped Dispersion

Individuals are clustered together

Occurs:

When resources (food, water, living

spaces) are clumped together

Species have a certain social behavior

For example: herding animals, flocks of

birds, schools of fish, hives of bees

Even Dispersion (a.k.a Uniform Dispersion)

Individuals are separated by a fairly

even distance.

Occurs:

As a result of social interaction

Individuals trying to get as far

away from each other as possible.

Limited resources, competition,

nesting

Random Dispersion

Location of one individual is

independent of the location of the

other individuals

Examples:

Seed dispersal by the wind or animals

Growth Rate

The amount by which a population’s size

changes in a given time.

A measure of the speed of reproduction

Factors affecting population size

1. The number of births.

2. The number of deaths.

3. Migration

The movement of individuals into

and out of a population

Immigration

Emigration

Types of Migration

Immigration

The movement of individuals into an area

Emigration

The movement of individuals out of an area

Population Dynamics

Population are dynamic, meaning that they:

Change in size and composition over time!

Considerations:1. Birth rate: the number of births occurring in a given

period of time

2. Death rate or mortality rate: the number of deaths in a given period of time

3. Life expectancy: the length of time an individual is expected to live

If more individuals are being born than die in a given period of time, the population size will ___________.

If birthrate is equal to death rate, the population size will ______________.

If the death rate is greater than the birthrate, the population will ______.

increase

remain stable

decrease

Studying Population Density

Questions to be answered:

Why is the birth rate unusually high

(or low)?

Why are more individuals dying than

normal?

Is there a reason for an unusually high

immigration or emigration?

Factors affecting population

Carrying Capacity (K)

the maximum number of organisms a

given area can support

Carrying Capacity

The actual size of the population is

usually higher or lower than the carrying

capacity

Below capacity

Birth rate increases

Above capacity

Population will die or starve

Factors Limiting PopulationLimiting Factor – the resource that runs

out first Helps to determine carrying capacity

Examples include:

1. Competition

2. Predation

3. Parasitism and disease

4. Drought and other climate extremes

5. Human disturbances

Types of Limiting Factors

1. Density-Dependent Factors

Only when the population density reaches a certain level.

2. Density-Independent Factors

Unusual weather or natural disasters

Floods and fires

Human activities

Clear-cutting a forest or damning a river

What limits population growth?

R-StrategistsThink “R” for “reproduction”

Population usually NOT near carrying capacity

J-curve growth (high growth rate)

Exponential Growth

Highly affected by abiotic factors like weather

Small body size and short life span

Have many offspring in a short time

R-strategist make lots of babies!

Here’s what the graph looks like!

K-StrategistsThink “K” for “carrying capacity”

Population lives near its carrying capacity

S-Curve Growth

Logistic Growth

Highly affected by biotic factors like competition

Usually large body size

Have few offspring, but live a long time

Kangaroos are K-Strategists!

Here’s what the graph looks like!

Age-Structure Diagrams

The growth of a

population depends on:

how many people make

up the given population.

The future growth of a population is predicted using:“age-structure diagrams.”

An “age-structure diagram” shows:the population of a country broken down by gender and age group.

Each bar in the diagram represents individuals within a 5-

year range. Males are shown to the left of the center line.

Females are shown to the right of the center line.

Look at the age structure of population A.

There are many more _________ than ______ groups. In each higher age group, there are fewer and fewer individuals.It can reasonably be predicted that population A will experience:rapid growth in the future as the large number of children reach adulthood.

children older

Human Population Growth

For most of human existence, the population grew very slowly. There were many limiting factors that kept the human population low:1. Food was not always

readily available.2. Diseases were rampant.

About 500 years ago, the human population began to

grow at a staggering rate. Reasons for this include:

1. The beginning of agriculture and industry made life much easier and much safer.

2. Food is available on a regular basis.

3. Goods can be shipped around the world.

4. Improved sanitation and living conditions eliminated the high levels of diseases.

5. Death rate dropped while birth rate increased.

As a result, the human

population is experiencing

____________________.

The human population

________ continue to grow in

this manner.

The resources on Earth are

________.

exponential growth

cannot

limited

The questions to be answered are: When will we reach the limit of these

resources? How large will the population get? Will the planet be able to support

this huge human population?

Demography is the scientific study:of human populations.

Some countries today have a much higher growth rate than other countries. Birthrates, death rates, and the age structure of a population help predict why some countries have a higher growth rate than other countries.