1. Studying Populations

2. Studying Populations
When studying a bald eagle population, you might ask yourself the following questions?
How has the population changed over time?
Is the number of bald eagles more, less, or the same as it was 50 years ago?
To answer these questions you must first determine the present size of the bald eagle population.

3. Population Density Population Density
Number of individuals in a specific area.
Ex. You counted 50 monarch butterflies in a garden measuring 10 square meters
Population Density = 50 butterflies/10 square meters
Population Density = 5 butterflies per square meter

4. Determining Population Size
Direct Observation
Ex. Count all the bald eagles living in an area.
Indirect Observation
Count number of organisms by observing their tracks or other signs.
Ex. Count entrance holes for number of swallow families. Average is 4 swallows per nest. So…multiply number of holes times 4
Sampling
Estimate the numbers of organisms in a large area
Ex. Count number of organisms in a small area and then multiply to find the number in a large area.

5. Determining Population Size
Mark & Recapture
Some animals captured, marked, and released into the environment.
Another group is captured, ecologists count the marked animals in this group
Using mathematical formula, the ecologists can estimate the total population of those organisms in the area
Ex. Catching field mice

6. Changes in Population
Populations can change in size when new members enter the population or when members leave the population.
Ex. Births and deaths
Birth Rate
Number of births in a population in a certain amount of time.
Ex. 1,000 snow geese produces 1,400 goslings per year

7. Changes in Population Size
Death Rate
Number of deaths in a population in a certain amount of time.
Ex geese die per year
If birth rate > death rate, population size increases
If death rate > birth rate, population size decreases

8. Changes in Population Size
Immigration
Moving into a population.
Emigration
Leaving a population
Graphing Changes in Populations

9. Factors that Affect Populations
Limiting Factor
An factor that limits how many organisms can live in an environment.
Carrying Capacity
The largest population that an environment can support.
Food
Suppose a giraffe needs to eat 10 kg of leaves each day. Trees in an area can provide 100 kg of leaves each day.
Area could support 5 giraffes but not 15 giraffes
No matter how much shelter, water, and other resources there might be, the population will not grow much higher than 10 giraffes

10. Limiting Factors Space Weather
Ex. Plants: young plants can’t grow under big trees
Weather
Ex. Severe weather, temperature, amount of rainfall

11. Summary Questions List four ways of determining population size.
How are birth and death rates related to population size?
List three limiting factors for populations. Choose one and explain how this factor can limit population growth.
Explain why it is often necessary for ecologists to estimate the size of a population.
A field measures 50 meters by 90 meters. In 1 m2, you count 3 grasshoppers. Estimate the total population of grasshoppers in the field. What method did you use to make your estimate.