Populations

Honors Biology – Chapter 5

What is a Population?

A group of the same species living in a given area

It is populations of organisms that interact in ecosystems

Example:

An Introduction – Sea Otters of the Pacific Northwest

Life in kelp forest community

An Introduction – Sea Otters of the Pacific Northwest

Sea urchins as food Sea urchins eat kelp

Sea otters nearly eliminated due to hunting

Sea urchin populations rise

Kelp forests decline Restoring balance: Sea

otters declared endangered – protected from hunting

Predator/Prey Population Relationships

Hare / Lynx Hare population goes up

Lynx population goes up Lots of food (hares) available

Causes hare population to go down More lynx = more predation on hares

Causes lynx population to go down Hare population decrease means less food

for lynx and they die

Hare/Lynx

4 Characteristics of Populations

Geographic distribution Range Area inhabited by a population

Density Number of individuals per unit area

Growth rate Age structure

Population Growth

3 factors affect population size Number of births

natality Number of deaths

mortality Number of individuals that enter or leave a

population Immigration - enter Emigration - leave

Population Growth

A population will increase or decrease in size depending on how many individuals are added to it or removed from it

Growth occurs if More individuals are born than die If birth rate equals death rate the

population will stay near the same size

Movement into or out of a population

May be caused by Young animals mature and leave the area Shortage of food in an area

Exponential Growth

Individuals in a population reproduce at a constant rate Initially, growth is slow Overtime the population

becomes larger and large until it approaches an infinitely large size

Ideal conditions with unlimited resources required

Bacteria

Fastest rates of reproduction among living things

20 minutes for population to double

Why don’t they take over the Earth? NOT unlimited

resources

Logistic Growth

Growth pattern that occurs when a population’s growth slows or stops following a period of exponential growth.

Things that slow a population’s growth Natality (birth rate) decreases Mortality (death rate) increases Rate of immigration decreases Rate of emigration increases

Logistic Growth

Also, when birth and death rates are the same, the population growth will slow or even stop for a time

Even when a population stops “growing”, the population is still rising and falling somewhat – though the ups and downs average out to the same population size.

Logistic Growth Curve

Slow growth Exponential growth Growth levels off Carrying Capacity

K Dotted line The maximum

number of individuals that an environment can support

Limits to Growth

Limiting Factors Factors that cause population growth to

decrease Competition Predation Parasitism Drought Human disturbance

There are TWO types of limiting factors

Density Dependent Density Independent

Density Dependent Limiting Factors

Depends on the size of a population Usually greatest impact when the

population is large and dense Include

Competition Predation Parasitism Disease

Density Independent Limiting Factors

Affects all populations (large or small) in similar ways. Unusual weather Natural disasters Human activities

Like Habitat Destruction

Human Population Growth

Like other populations of organisms, the human population tends to increase with time.

Human Population Growth Curve Initially slow

Food scarce Life harsh

500 years ago More rapid growth

Currently EXPONENTIAL Why?

Agriculture and industry advances Food supply increased Improved sanitation Medicine

Human Population Growth Curve

Demography

The study of human populations Examines human populations and tries

to predict how they will change over time.

Demographic Transition

Population growth in some countries has slowed U.S. Japan Much of Europe

Why? Hypothesis: These countries have

completed the “demographic transition

Demographic Transition

A dramatic change in birth and death rates What happens

Initially both birth and death rates high Advances in nutrition, etc. lower death rate Population increases rapidly Societies modernize – increase education, etc. Families have fewer children Birthrate falls to meet the death rate Population growth stops

Populations that have NOT completed the demographic transition Most countries

India China

Age Structure Diagrams

Shows how many people of different ages and genders are in a population

Useful in predicting future population growth

Age Structure Diagram

Predicting Future Population Growth

Factors to consider Age structure of each country Prevalence of life-threatening diseases

Predictions for 2050 9 billion people on Earth

Effects of Human Population Growth

Some scientists suggest if human population growth rate does not become LESS than 0 (deaths outnumber births) Serious damage to environment Serious damage to global economy

Some economists say that technology advances and changes in societies can control negative impacts