How many are there?

 Collection of all the organisms in an area AND the non-living parts of the environment

 The total number of one species living in a defined region

 Several populatio ns in the same area that interact with each other

Populations

 How can we figure out how many individuals are in each population?  A census is a count of a population

 True census – actual count  Sample census – estimate of population  A sample census is made by marking off small areas or plots and counting in the areas

Name of Population DucksCattailsWater Lilies Actual Count Sample Count

 Each member of a population is counted one by one Name of Population DucksCattailsWater Lilies Actual Count 6204 Sample Census

Why?

Mark off a small area or plot. Count the population in one plot. Multiply by the total number of plots to get an estimate for the whole area.

Name of Population DucksCattailsWater Lilies Actual Count 6204 Sample Count 1 x10 = 103 x 10 = 301 x 10 = 10

Why was the last census not very accurate? What could we do to make it more accurate? Where have you already done a sample census?

Examples:  There are 270 people per square mile in Pennsylvania  There are 25 deer per square mile in PA

 Why is a sample a good way to count a population?  What are some disadvantages to using a sample?  What could you do to sample a population of animals that do not stay in one place?

 A way to count organisms that move or are hard to see.  Catch, mark, let go, catch again.  Use a formula to estimate the number.

 Is Mark and Recapture a true census or a sample census?  For the following animals, what method would be most appropriate for getting a census?