Chapter 22: Populations & Communities Section 2: Studying Populations

Guide For Reading How do ecologists determine the size of a population? What causes populations to change in size? What factors limit population growth?

Population Density The number of individuals in a specific area Population density can be calculated by the following equation: Population Density = Number of Individuals/Unit Area

Guide For Reading: How do ecologists determine the size of a population? Some methods of determining the size of a population are: –Direct Observations –Indirect Observations –Sampling –Mark – and – Recapture Studies

Direct Observation Determine the size of a population by counting its members one by one. Examples: Count all the trees in a forest Count all the elephants in a valley in Kenya

Indirect Observation If the members of a population are small or hard to find, it may be easier to observe their tracks or other signs rather than themselves Examples: –Counting entrance holes –Counting the number of nests

Sampling Used when the population is to large or to spread out. Take a small area, count up the population, and make an estimate based on the total area

Estimate: An approximation of a number, based on reasonable assumptions.

Mark-and-Recapture Studies Used when an animal hides or nests very well Animals are first captured, marked, and released into the environment. Then they are recaptured and counted

Figure 6: Inferring -What is the purpose of the silver band in the hawk’s leg? The band identifies the hawk as one that was captured previously Since the hawk has already been included in the population, it does not have to be counted again

Checkpoint (page 697) When is sampling used to estimate a population? Sampling is used when it is not possible to count every member of a population because: – there are too many, –they are too spread out –not all the members of the population can be found

Guide For Reading What causes populations to change in size? Populations can change in size when new members enter the population or when members leave the population.

Birth Rate & Death Rate Birth Rate: Number of births in a population in a certain amount of time. –Example: A population of 1,000 snow geese produces 1,400 goslings in a year. The birth rate of the population would be 1,400 goslings per year. Death Rate: The number of deaths in a population in a certain amount of time –Example: 500 geese die in a year. The death rate would be 500 geese per year.

Immigration & Emigration Immigration: Moving into a population. Emigration: Leaving a population. –Reasons: A portion of the population gets cut off of the rest because of a shortage of food. If they permanently become separated from the population, they are no longer apart of the population.

Checkpoint (page 698) Name two ways individuals can join a population. Individuals can join a population by being born into it or by immigrating into it.

Figure 8: Interpreting Graphs In what year did the rabbit population reach its highest point? What was the size of the population in that year? Year 4 About 850,000 rabbits

Limiting Factor An environmental factor that prevents a population from increasing

Guide For Reading: What factors limit population growth? Some limiting factors for populations are food, space, and weather conditions

Carrying Capacity The largest population that an environment can support