1. Measuring Populations A single pair of elephants can increase to a population of 19 million individuals within 750 years! Why haven’t they increased their numbers?

2. Population Growth Rate The amount by which a population’s size changes in a given time Whether a population grows, shrinks, or remains the same size depends on: Birth Death Immigration Emigration

3. Population Growth Rate Immigration – movement of individuals into a population Emigration – movement of individuals out of the population Immigration & birth add to a population Emigration & death subtract from a population Assume immigration = emigration

4. Population Size Demographers divide large populations into groups of 1,000 and to present data per capita, meaning per individual Birth rates, death rates, and growth rates for large populations are usually expressed per capita

5. Population Size Example: If there are 52 births and 14 deaths per 1000 individuals per year: Birth Rate = 52/1000 = 0.052 births per capita per yr Death Rate = 14/1000 = 0.014 deaths per capita per yr Growth rate can be calculated by: Birth rate – Death Rate = Growth Rate

6. Population Size Calculating per capita growth: 0.052 births per capita – 0.014 deaths per capita = 0.038 growth per capita A positive growth rate means population is growing; negative means it’s shrinking

7. Population Size To find the number of new individuals that will be added to the population in a year, just multiply the per capita growth rate by the number of individuals in the population Ex] Population = 50,000 Growth = 0.038 per capita 0.038 x 50,000 = 1900

8. The Exponential Model At a steady positive growth rate, the population will add a larger number of individuals with each generation A pattern of increase in number due to a steady growth rate is exponential growth

9. The Exponential Model A graph of the population size over time for exponential growth makes a J- shaped curve Population size grows slow when small, but increases as individuals join the population

10. The Exponential Model Leads us to predict that population size will increase indefinitely and by a greater number with each time period Do you think this trend will continue? What will the graph look like in the future?

11. Applying the Exponential Model This model matches observed patterns of growth of real populations, but only under a certain number of conditions and for a limited period of time Example] Bacteria can grow exponentially if provided with an abundance of food and space and if waste is removed

12. Applying the Exponential Model This doesn’t apply to most populations because resources aren’t unlimited and harmful waste accumulates Any factor, such as space, that restrains the growth of a population is called a limiting factor All populations are limited by their environment

13. Applying the Exponential Model As a population grows, competition intensifies for resources Thus, each individual’s ability to fight off disease, grow, and reproduce decreases This results in a decreasing birth rate and increasing death rate

14. The Logistic Model Builds on the exponential model but accounts for the influence of limiting factors Carrying capacity (K) is the number of individuals the environment can support over a long period of time

15. The Logistic Model The graph of this model looks like a stretched-out letter S When population is small, birth rates are high and death rates are low, so looks like exponential growth As size approaches K, the growth rate slows At K, the birth rate = death rate and growth stops

16. Contains some assumptions K is constant and doesn’t fluctuate with environmental changes Reality is, it does. Ex] It is greater when prey is abundant and lower when it is scarce.

17. The logistic and exponential models are not universal representations of real populations – but – they are an important tool that scientists use to explain population growth and regulation.

18. True or False: Carrying capacity is the number of individuals the environment can support for an extended period of time. True!!!

19. True or False: Population growth can be predicted using only birth- and death-rate statistics. False – Immigration and emigration rates are important factors to consider!!!

20. How many new individuals will there be next year in a population of 85,000 people if there are 98 births and 75 deaths per thousand people? 0.098 births per capita – 0.075 deaths per capita = 0.023 growth per capita 0.023 x 85,000 = 1955 new individuals