Chapter 30

Population and Communities

• Objectives

• 1. Relate the importance and methods of counting populations.

• 2. Discuss why populations change in size.

• 3. Explain how limiting factors affect a population.

Population Size and Arrangement

• A change in size of one population will cause a change in size of another population

• For this reason scientists want to know of any increases or decreases of any population they study.

• Finding the size is not always easy

• Animals can be marked with ear tags, or transmitters and birds with leg bands.

• Trees are counted by marking them with paint or ribbons.

• Populations are not spread out evenly but are clumped.

• Bring clumped is beneficial because species can help each other.

• Animals live in groups for protection. Plants are protected from high winds by growing together.

Population Changes

Population can change for several reasons.

1. If the number of births goes up and the number of deaths go down, then the population will increase

2. The population will decrease if there are more deaths than births.

3. Animals can move from place to place. Moving of animals out of an area is called emigration. This would cause a decrease in population.

4. When animals move out of one area and into another, it is called immigration. This causes an increase in population.

Limits of Population Size

• Any condition that keeps the size of a population from increasing is called a limiting factor.

• Lack of light, space, water, or food can be limiting factors.

• As the population increases and shelter, food or water is used up, there becomes a struggle for survival among the animals. This is called competition.

• Because of the lack of these items, population growth slows down by decreasing the number of births and increasing the number of deaths.

• Plants that grow close together compete for minerals, water, space and sunlight.

• A decrease in these factors causes plant growth to slow. This is why gardeners pull weeds or thin out new seedlings.