Lesson Overview

1. Lesson Overview 5.1 How Populations Grow

2. How To Describe Populations • Population: group of single species in a given area • Populations are described in 4 ways: • Geographic range • Density & Distribution • Growth Rate • Age structure

3. Geographic Range - area inhabited by a population • May vary enormously in size, depending on the species. • EX: bacterial population in a rotting pumpkin-small range cod population in western Atlantic- range that stretches from Greenland down to North Carolina.

4. Density and Distribution • Population density- number of individuals per unit area • Different species- different densities, in the same environment. • Ex: A population of ducks in a pond may have a low density, while fish and other animals in the same pond community may have higher densities. • Distribution- spacing of individuals in a population —random, uniform, clumps.

5. Growth Rate • growth rate- whether the population size increases, decreases, or stays the same.

6. Age Structure- number of males and females of each age • Most plants and animals cannot reproduce until they reach a certain age. • Also, among animals, only females can produce offspring.

7. Population Growth • Factors affecting population size: • birth rate • death rate • immigration • emigration

8. Population Growth • A population will increase or decrease in size depending on how many individuals are added to it (immigration) or removed from it (emigration)

9. Exponential Growth • Under ideal conditions with unlimited resources (food, space, protection, disease free), a population will grow exponentially. • In exponential growth, the larger a population gets, the faster it grows. The size of each generation of offspring will be larger than the generation before it.

10. Exponential Growth • Under ideal conditions with unlimited resources, a population will grow exponentially. • Is this realistic?

11. Logistic Growth • Logistic growth occurs when a population’s growth slows and then stops, following a period of exponential growth. • Natural populations don’t grow exponentially for long. • Sooner or later, something stops exponential growth. What happens?

12. The Logistic Growth Curve • This curve has an S-shape that represents what is called logistic growth. • Logistic growth occurs when a population’s growth slows and then stops, following a period of exponential growth. • Many familiar plant and animal populations follow a logistic growth curve.

13. The Logistic Growth Curve • Population growth may slow for several reasons: • a. birthrate decreases or the death rate increases—or if births fall and deaths rise together. • b. rate of immigration decreases, rate of emigration increases, or both.

14. Carrying Capacity: maximum number of individuals of a particular species that a particular environment can support. Once a population reaches the carrying capacity of its environment, a variety of factors act to stabilize it at that size.

15. Demography- scientific study of human populations • In the United States: nearly equal numbers of people in each age group • Guatemala- more young children than teenagers, and many more teenagers than adults