Population TrendsWhy do populations grow and shrink?

Populations A population is the sum of one species

living in a set area at the same time. For example, if we measured the number

of one specific species of bass in a lake in Iowa, we would say that is a population of bass.

Note that all adults in a single population form a reproductive group – they can breed within their own population.

Populations All populations have several traits that

can be used to describe them: Size, the total number of organisms in the

population Density, the number of organisms

divided by the total area or volume Dispersion, the placement of organisms

within the region

Population Growth A population’s growth can basically be

calculated by subtracting the number of deaths from the number of births. This gives you the net birth rate.

If the growth rate is: Positive: population is increasing Zero: population is static Negative: population is decreasing

Population Growth Population growth can also be described

in terms of the rate of growth. Biotic potential is the fastest rate that

any given population can grow. The upper limit for this is the reproductive potential, the maximum number of offspring that each individual organism can produce.

Population Growth Reproductive potential differs from

species to species. Some bacteria have a reproductive

potential of as many as 19 million offspring!

However, a pair of bowhead whales might take centuries to produce that many descendants!

Population Growth Reproductive

potential is determined by: Number of

offspring produced at one time

Rate of reproduction

Age of viable reproduction

Population Growth For instance, small organisms like

bacteria have very short generation times and produce offspring early in life. As such, they have a very high reproductive potential.

By contrast, larger organisms have long generation times and only produce offspring if sexually mature. They have a much lower reproductive potential.

Exit Ticket1. How do we determine a population’s

growth rate?2. Assume a population’s growth rate is

virtually zero. What would happen if new organisms migrated into the region?

3. Would pandas have a high or low reproductive potential?

Exponential Growth When a population’s growth rate is

approaching the reproductive potential, we say the population is achieving exponential growth.

Exponential growth requires sufficient access to resources to support all offspring. It also requires the absence of predators or competitors in the local ecosystem.

Exponential Growth

Exponential Growth As a population grows, it begins to face

increasing pressures on population growth. These pressures will eventually suppress future growth.

The carrying capacity is the theoretical limit on the number of organisms that can be supported in one area. Birth and death rates are virtually equal.

Exponential Growth

Limiting Factors Limiting factors are those that oppose

future population growth. There are two types of limiting factors: Density-dependent factors, which

cause more deaths as a population increases in density

Density-independent factors, which affect all populations equally, regardless of density

Limiting FactorsDensity-dependent

Scarcity of resources

Disease Predation

Density-independent

Natural disasters Adverse weather Climate change

Bell Ringer1. Estimate the

carrying capacity of deer.

2. List ONE reason that the deer population declined from month five to eight.

3. Predict what would happen if the number of deer exceeded the carrying capacity for a significant time.

Importance of Population Studies Population studies are important for

monitoring the health of various populations.

Through population studies, we can anticipate whether a given population is likely to grow or decline in the future. In doing so, we can monitor an ecosystem for new threats, such as disease or invasive species.

Importance of Population Studies Population studies are of particular

value in looking at the present and future growth of the human population.

For instance, we use age-structure diagrams to identify the potential for future local population growth.

Importance of Population Studies

The bottom of the graph shows the proportion of young people, and the top shows the proportion of older people.

Importance of Population Studies

Future population growth is predicted by a large number of young people.

On the other hand, a small base means that there will be fewer births.

Importance of Population Studies

Seeing a large percentage of old people indicates that people live a relatively long time.

In contrast, if few people live to see old age, then death rates are likely high.

Importance of Population Studies

Importance of Population Studies

Exit Ticket Looking at this graph:1. Are birth rates high or low?2. Are death rates high or low?3. Predict whether future growth will be

positive or negative.

Bell Ringer

1. The U.S. experiences slow, but steady population growth.

2. Nigeria is a developing country experiencing rapid population growth.

3. Germany is a developed country experiencing negative population growth.

A B C D

Match each description to the correct age structure.

Importance of Population Studies A survivorship

curve shows the percentage of offspring that remain after a given time frame.

It is very useful for looking at mortality.

Importance of Population Studies The mortality

rate shows the rate of death over time.

Often, this is not a static value – note the parabolas to the right!

Importance of Population Studies

Importance of Population Studies Type I

survivorship shows most individuals live to long age, and shows a low mortality rate.

Death is only common at advanced ages.

Importance of Population Studies Type II

survivorship shows a constant mortality rate.

As such, death occurs at the same rate for all ages.

Importance of Population Studies Type III

survivorship relatively few individuals live to adulthood.

The mortality rate is high starting from birth. To offset this, a large number of offspring are born.

Importance of Population Studies

Exit Ticket1. Which curve do

you believe best describes the U.S.? Why?

2. Name TWO countries that would show a Type III curve.

Fertility Rates Officially, the fertility rate is the number

of babies born each year per 1000 women. The total fertility rate is the average

number of babies a woman will give birth to in her lifetime.

The total fertility rate is compared to the replacement level, the average number of children each parent must have to maintain the population.

Natural

Time

Stage 1 Stage 2 Stage 3 Stage 4

increase

Birth rate

Death rate

Note: Natural increase is produced from the excess of births over deaths.

Lesson Plan: The Demographic Transition, Activity One

Demographic Transition

Demographic Transition

Sources: B.R. Mitchell, European Historical Statistics 1750-1970 (1976): table B6; Council of Europe, Recent Demographic Developments in Europe 2001 (2001): tables T3.1 and T4.1; CELADE, Boletin demografico 69 (2002): tables 4 and 7; Francisco Alba-Hernandez, La poblacion de Mexico (1976): 14; and UN Population Division, World Population Prospects: The 2002 Revision (2003): 326.

0

10

20

30

40

50

60

1775 1800 1825 1850 1875 1900 1925 1950 1975 2000

SwedenBirth Rate

Death Rate

MexicoBirth Rate

Death Rate

Births/Deaths per 1,000

1950

Lesson Plan: The Demographic Transition, Activity One