Populations

1. Populations Chapter 5, Sections 1 & 2 Pg. 118 - 128

2. Remember a population is… A group of the same species in the same area at the same time. The boundary of a population can be physical or arbitrary for the purpose of study. Populations have 4 important properties that interact to define the population: Size Density Dispersion Growth rate

3. Population Size and Density Population Size The total number of individuals in a population Can be difficult to measure, so many times sizes are estimates, usually done by sampling Population density How crowded the population is (on average) Expressed as the number of individuals per unit of area or volume Density = # of individuals area population occupies

4. Population Dispersion The spatial distribution of individuals in the population. Dependent on the area being measured, resources available, or social interactions. Three main types of dispersion: Clumped – Individuals are clustered Even – Individuals are consistently spaced Random – Individual location is independent of other individuals

5. Population Dispersion (cont’d.) What types of dispersion are shown in the pictures below? What might be some reasons for each type of dispersion?

6. Population Dispersion (cont’d.) Population Dispersion and Population Density are NOT the same thing! Population Density measures the average # of individuals per unit of space and considers the total area the population uses. Population Dispersion describes how organisms are arranged in their space. It is possible to have a clumped dispersion, but a low population density.

7. Growth Rate Population size can change and is dependent on 4 factors: Birth Death Immigration Emigration When calculating growth rate, scientists assume immigration and emigration cancel out so: Growth rate = birth rate – death rate

8. Exponential Growth Model Population grows slowly at first and then rapidly. Creates a J-shaped curve. Population continues to grow infinitely. Birth rate of the population remains constant, no matter how big it grows.

9. Exponential Growth Model (cont’d.) Predictions The larger the population gets the faster it grows. Occurs only rarely, under specific conditions, and for short periods of time. Occurs only when limiting factors are absent. Limiting factors – Factors that cause population growth to decrease. Ex: Disease, lack of food, space and other resources

10. Logistic Growth Model Population begins by growing exponentially, but eventually growth decreases and stops. Creates an S-shaped curve. Population growth decreases as it reaches carrying capacity (K). Carrying Capacity – The largest number of individuals an environment can sustain.

11. Logistic Growth Model (cont’d.) Predictions Birth and death rates do not remain constant. Population growth increases when the population is small (high birth rate, low death rate). Population growth decreases as the population reaches carrying capacity (low birth rate, high death rate). At the carrying capacity birth rates and death rates are equal and growth stops. Carrying capacity is not constant, it fluctuates with the environment.

12. Population Regulation There are 2 types of limiting factors: Density Independent – reduce the population by the same amount, no matter the size. Ex: weather, floods, fires Density Dependent – Have a greater effect on larger populations. Ex: food, shelter