Populations

• Are made up of individuals of the same species that live in a particular area.

• Show characteristics that help ecologists predict the future of that population– The characteristics include

population size, density, distribution, age structure, and sex ratio.

Population Size

• Describes the number of individuals that are present in a population at a given time.

• May increase, decrease, undergo cyclical change, or remain the same over time.

Population Size

• Example – Golden toad of Monteverde– Scientists reported• 1500 as of 1987• 10 in 1988• 1 in 1989• 0 in 1990

Describing Populations

• A population is – Described by four factors• Distribution• Density • Age structure• Sex ratio

Describing Populations

• Density– Refers to the number of individuals per unit area. – Of different populations may differ even in the

same environment.• Distribution – Refers to how population individuals are spread

out across a population’s geographic area.– A population distribution may be random,

uniform, or concentrated in clumps.

Describing Populations

Describing Populations

• Age structure– This refers to the number of males and females of

each age a population contains. – Is relevant because most plants and animals must

reach a certain age to reproduce.• Sex ratios– Refers to the proportion of females and males.– Is an important characteristic for populations of

organisms that reproduce sexually.

Describing Populations

Populations Assignment

• Answer the questions 1-4 of Assessment 4.2 on page 109. WRITE THE QUESTIONS AND ANSWER IN YOUR NOTEBOOK.

III. Population Growth

A. A population will increase or decrease depending on how many individuals are added to it or removed from it.

III. Population Growth

B. There are four factors that can affect population size.1. Birthrate and death rate

a. A population will (1.)Grow if the birthrate > death rate.

(2.) Shrink if the death rate > birthrate.

(3.) Stay the same if the birthrate equals the death rate.

III. Population Growth

2. Immigration and emigrationa. A population will

(1.) Grow if individuals move into a population’s geographic range.(2.) Shrink if individuals move out of a population’s geographic range.

IV. Exponential growth

A. A population will grow exponentially under ideal conditions and unlimited resources.

B. Occurs when1. The size of each generation of offspring is greater than the generation before it.2. Organisms reproduce rapidly, slowly, or move into a new environment.

IV. Exponential growth

Quick Quiz

Which of the following is NOT a condition

for a population to reach exponential

growth?A.) Presence of unlimited

resourcesB.) Absence of predation and

diseaseC.) Movement of individuals out

of the population

V. Logistic Growth

A. Occurs when a population grows slowly and stops, following a period of exponential growth.

B. All populations’ experience phases of growth (logistic growth curve)

V. Logistic Growth

V. Logistic Growth

C. Carrying capacity is a factor that stabilizes a population’s size

1. The carrying capacity is the maximum number of individuals that an environment may support.

How Populations Grow

• Assignment: Answer the questions of Assessment 5.1 on page 135 of your M&L Biology textbook with complete sentences. Write the questions and answers in your science notebooks.