Ecology: Populations. Characteristics of Populations 1.Geographic distribution 2.Density 3.Growth Rate 4.Age Structure.

Ecology:Populations

Characteristics of Populations

1. Geographic distribution2. Density3. Growth Rate4. Age Structure

Geographic Distribution• Also called Range• Describes the area inhabited by a

population

Population Density

• The number of individuals per unit area; can vary tremendously depending on the species & its ecosystem

How many Cacti are there per square kilometer in this desert?

Population Growth

Three factors that effect population growth

1. Number of births2. Number of deaths3. Number of individuals that enter or

leave the population

Examples

• What happens if the number of births are greater than deaths?

• What happens if the number of deaths are greater than the number of births?

• Which situation will allow for population growth?

Movement within populations

• Immigration - The movement of individuals INTO an area

• Emigration - The movement of individuals OUT OF an area

Which of these will allow a population to grow?

Exponential Growth• Occurs when the individuals in a

population reproduce at a constant rate• Under ideal conditions with unlimited

resources, a population will grow exponentially

J-shaped curve

Logistic Growth• Occurs when a population’s growth

slows or stops following a period of exponential growth

• This happens as resources become less available causing the growth of a population to slow or stop.

S-shaped curve

X – number of individualst – timeK – carrying capacity

Carrying Capacity - The largest number of individuals that a given environment can support

K

Limiting Factors

• A factor that causes population growth to decrease such as predation, competition, climate extremes or human disturbances

Two Types of Limiting Factors1.Density-dependent2.Density-independent

Density-dependent

• Limiting only when the population density reaches a certain level

• Biotic factors include competition, predation, parasitism, disease

• Operates most strongly when a population becomes very large and dense

Competion

• When organisms are crowded this causes more competition

• Examples: space, food, water, sunlight

• This competition can be between different species which may lead to evolution to occupy different niches

Predation

• Predator-prey relationships are one of the best mechanisms of population control

Parasitism and disease

• Parasites take nourishment at the expense of the host

• Usually weakens but can cause death• Similar to predation

Density-Independent

• Affect all populations in similar ways regardless of the population size

• Abiotic factors include natural disasters, seasonal cycles, unusual weather

• Can lead to a characteristic crash• Most populations can adapt to some

change• Some changes can effect entire

populations with major upsets leading to long-term declines

Human Population Growth

• Size of human population tends to increase with time

• Human population growth was very slow due to harsh conditions and limiting factors

• High death rates for years

Why did population growth increase?

• Life got easier with improvements in industry and agriculture

• Safer food supply• Improved sanitation• Improved health care• Decrease in death rate while birth

rate had no change• Exponential Growth

Demography

• Scientific study of human populations• Birthrates, death rates, and the age

structure help predict countries growth rates

Demographic Transition

• The hypothesis as to why population growth has slowed dramatically in countries such as US, Japan, and Europe

• A dramatic change in birth and death rates

• Historically human societies have had both high birth and death rates

Age Structure Diagrams

• Help predict future population growth• Bar graph of the number of people in

each age group in the population

• United States has a slow but steady growth rate

• What is the shape of Rwanda’s age structure diagram?

• Indicates that there more young children than teenagers and more teenagers than adults.

• Predicts a population that will double in approx. 30 years

Future Population Growth

• Estimated human population by 2050 will be more than 9 billion.

• Human population is still growing exponentially.

• Will it level out to logistic growth?

• How might that happen?