Biology

Chapter

5

Characteristics of Populations

• Geographic Distribution – the area inhabited by a population

• Population Density – number of individuals per unit area

• Growth Rate – an increase or decrease of the number of individuals in a population over time

Population Growth/Decline

• A population can grow when its birthrate is greater than its death rate

• Immigration – movement of individuals into an area

• Populations can increase

• Animals maybe searching for mates or food

• Emigration – movement of individuals out of an area

• Populations can decrease

• Animals leave to find mates or food

• Exponential Growth

• Under ideal conditions with unlimited resources a population will grow exponentially resulting in a population explosion

• Exponential growth is characterized by a J-shaped curve

• In nature exponential growth does not continue in a population for very long

• As resources become less available, the growth of a population slows or stops

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Exponential Growth

Exponential Growth

• Logistic Growth

• Occurs when a population’s growth slows or stops following a period of exponential growth

• Logistic growth is characterized by a S-shaped curve

Copyright Pearson Prentice HallFig. 46.6, p. 818

initial carrying capacity

new carrying capacity

• What prevents the world from being overrun with all kinds of living things?

• Carrying Capacity – the number of organisms of one species that an environment can support

• When a population reaches the carrying capacity of its environment, its growth levels off

Limiting Factors

• Limiting Factor – a factor that causes population growth to decrease

• Density-Dependent Factors – a limiting factor that depends on population size

• These factors operate most strongly when a population is large and dense

• As populations increase so do these factors

• Example: competition, predation, parasitism, disease

• Density-Independent Factor – a limiting factor that affects all populations regardless of the population size

• Example: natural disasters, storms, floods, drought, pollution

• Demography – the study of human population growth

• For most of human existence, the population grew slowly

• Limiting factors kept population sizes low

• About 500 years age, the human population began growing more rapidly

• Life was made easier and safer by advances in agriculture and industry

• Death rates were dramatically reduced due to improved sanitation, medicine, and healthcare, while birthrates remained high

• With these advances, the human population experienced exponential growth

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Historical OverviewHuman Population Growth

• Age Structure

• Age-Structure Diagrams – show the population of a country broken down by gender and age group

• Population growth depends, in part, on how many people of different ages make up a given population

• Demographers can predict future growth using age-structure diagrams

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Patterns of Population Growth

Rapid

Growth

Slow

Growth

Zero

Growth

Negative

Growth

• How can you tell if a population is growing?

• If the birth rate is more than the death rate

• Does it make a difference to population growth if the largest proportion of the population is in one age group?

• If a large percent of the population is teenagers and children there is rapid growth

• If the percent of people in each category is equal the population is stable