populations

population size growth and structure

Population size is the number of individuals in a population. Population size influences the chances of a species surviving or going extinct. If a species populations become very small, the species may be at risk of going extinct.

population density and distribution

Another sign of a species state of health is the density of its populations. Population density is the average number of individuals in a population for a given area. Density is a measure of how crowded or spread out the individuals in a population are on average. For example, a population of 100 deer that live in an area of 10 square kilometers has a population density of 10 deer per square kilometer. Population density is an average measure. Often, individuals in a population are not spread out evenly. Instead, they may live in clumps or some other pattern. How individuals in a population are distributed, or spread throughout their area, is called population distribution. You can see different patterns of population distribution in Figure 23.3. Different patterns characterize different species and types of environments, as you can read in the figure.

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early population growth

The earliest members of the human species evolved around 200,000 years ago in Africa. Early humans lived in small populations of nomadic hunters and gatherers. Human beings remained in Africa until about 40,000 years ago. After that, they spread throughout Europe, Asia, and Australia. By 10,000 years ago, the first human beings colonized the Americas. During this long period of time, the total number of human beings increased very slowly. Birth rates were fairly high but so were death rates, producing low rates of population growth. Human beings invented agriculture about 10,000 years ago. This provided a bigger, more dependable food supply. It also allowed people to settle down in villages and cities for the first time. Birth rates went up because there was more food and settled life had other advantages. Death rates also rose because of crowded living conditions and diseases that spread from domestic animals. Because the higher birth rates were matched by higher death rates, the human population continued to grow very slowly.

the human population

Human beings have been called the most successful weed species on Earth. Like garden weeds, populations of human beings grow quickly and disperse rapidly. Human beings have colonized almost every terrestrial part of the planet. Overall, the human population has had a pattern of exponential growth, as you can see in Figure 23.6. The early human population grew very slowly. However, as the population grew larger, it started to grow more rapidly.

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recent population growth

Just as they did in Europe and North America, death rates have fallen throughout the world. No country today remains in Stage 1 of the demographic transition. However, birth rates are still high in many of the poorest countries of the world. These populations seem to be stuck in Stage 2 or 3 of the demographic transition. They have high population growth rates because low death rates are not matched by equally low birth rates. Whether these populations will ever enter Stage 4 and attain very low rates of population growth is uncertain.

demographic transition

Major changes in the human population first began in the 1700s. These changes occurred mainly in Europe, North America, and a few other places that became industrialized. First death rates fell. Then, somewhat later, birth rates also fell. These changes in death and birth rates affected the rate of population growth and are referred to as the demographic transition. The graph in Figure 23.7 shows the stages in which the demographic transition occurred. You can learn more about the stages by watching this video: http://education-portal.com/academy/lesson/what-is-d In Stage 1, both birth and death rates were high so population growth was slow. In Stage 2, death rates fell while birth rates remained high. Why did death rates fall? There were several reasons, including new scientific knowledge of the causes of disease. Water supplies were cleaned up and sewage was disposed of more safely. Better farming techniques and machines increased the food supply and the distribution of food. For all these reasons, death rates fell, especially in children. Birth rates, on the other hand, remained high. This resulted in faster population growth. Before long, birth rates also started to fall. People started having fewer children because large families became too expensive. For example, with better farming machines, farm families no longer needed as many children to work in the fields. Laws were also passed that required children to go to school. They could no longer work and help support the family. Having many children became too costly. Eventually, birth rates fell to match death rates (Stage 4). As a result, population growth slowed down.

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population growth

Whether its populations are growing or shrinking in size may be another indicator of a species health. Individuals may be added to a population through births and the migration of individuals into the population. Individuals may be lost from a population through deaths and the migration of individuals out of the population. The population growth rate is how quickly a population changes in size over time. The rate of growth of a population may be positive or negative. A positive growth rate means that the population is increasing in size because more people are being added than lost. A negative growth rate means that the population is decreasing in size because more people are being lost than added. Populations may show different patterns of growth. The growth pattern depends partly on the conditions under which a population lives. Two common growth patterns are exponential growth and logistic growth. Both are represented in Figure 23.4. With exponential growth, the population starts out growing slowly. As population size increases, the growth rate also increases. The larger the population becomes, the more quickly it grows. This type of growth generally occurs only when a population is living under ideal conditions. However, it cant continue for very long. With logistic growth, the population starts out growing slowly, and then the rate of growth increasesbut only to a point. The rate of growth tapers off as the population size approaches its carrying capacity. Carrying capacity is the largest population size that can be supported in an area without harming the environment. This type of growth characterizes many populations.

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population age and sex structure

Another way of describing a population is its age-sex structure. This refers to the numbers of individuals of each sex and age in the population. The age-sex structure of a population may influence the population growth rate. This is because only individuals of certain ages are able to reproduce, and because individuals of certain ages may be more likely to die. For example, if there are many individuals of reproductive age, there are likely to be many births, causing the population to grow rapidly. The age-sex structure of a population is often represented with a special bar graph called a population pyramid. You can see an example of a population pyramid in Figure 23.5. The graph in the figure actually has a pyramid shape because the bars become narrower from younger to older ages. However, this is not always the case. In some populations, for example, there may be more people at older than younger ages, resulting in a top-heavy population pyramid. Learn more about population pyramids and what you can learn from them, watch this TED video: http://w MEDIA Click image to the left or use the URL below. URL:

future population growth

As of 2014, there were more than 7 billion human beings on planet Earth. That number is increasing rapidly. More than 200,000 people are added to the human population each day! At this rate, the human population will pass 9 billion by 2050. Many experts think that the human population has reached its carrying capacity. It has already harmed the environ- ment. An even larger human population may cause severe environmental problems. It could also lead to devastating outbreaks of disease, starvation, and war. To solve these problems, two approaches may be needed: Slow down human population growth so there are fewer people. Distribute Earths resources more fairly so that everyone has enough. Hopefully, we will act before its too late. Otherwise, the planet may be ruined for future generations of human beings and other species.

instructional diagrams

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questions

The total human population reached 7 billion people by the year

a. 1000.

b. 1700.

c. 1914.

-->  d. 2014.

The demographic transition is a model based on what actually occurred in

a. the poorest nations of the world.

-->  b. Western Europe and North America.

c. all human populations worldwide.

d. none of the above

The human species evolved in Africa. Human beings first left Africa about

a. 200,000 years ago.

b. 100,000 years ago.

-->  c. 40,000 years ago.

d. 10,000 years ago.

At its current rate of growth, the human population will surpass 9 billion by the year

a. 2020.

b. 2030.

c. 2040.

-->  d. 2050.

Factors that influence the rate of growth of a population include

a. births.

b. deaths.

c. migration.

-->  d. all of the above

The most common pattern of population distribution is a random pattern.

a. true

-->  b. false

Only births and deaths change the size of a population.

a. true

-->  b. false

A pattern of exponential population growth generally cannot continue for very long.

-->  a. true

b. false

The age-sex structure of a population may influence the populations growth rate.

-->  a. true

b. false

The size and growth rate of its populations influences the chances of a species surviving.

-->  a. true

b. false

A random pattern of population distribution is typical of species in which individuals compete for

a. true

-->  b. false

The human population started growing very rapidly about 10,000 years ago.

a. true

-->  b. false

A population pyramid is a graphic representation of the age-sex structure of a population.

-->  a. true

b. false

Many countries today remain in stage 1 of the demographic transition.

a. true

-->  b. false

The human population is predicted to exceed 9 billion by the year 2050.

-->  a. true

b. false

Overall, the human population has had a pattern of exponential growth.

-->  a. true

b. false

Major changes in human population growth rates first began in the 1700s.

-->  a. true

b. false

__way in which individuals in a population are spread over their area

a. carrying capacity

b. age-sex structure

c. population density

d. exponential growth

e. population growth rate

-->  f. population distribution

g. logistic growth

__pattern of population growth in which the rate of growth keeps increasing as the population gets

a. carrying capacity

b. age-sex structure

c. population density

-->  d. exponential growth

e. population growth rate

f. population distribution

g. logistic growth

__how quickly population size changes over time

a. carrying capacity

b. age-sex structure

c. population density

d. exponential growth

-->  e. population growth rate

f. population distribution

g. logistic growth

__largest population size that can be supported in an area without harming the environment

-->  a. carrying capacity

b. age-sex structure

c. population density

d. exponential growth

e. population growth rate

f. population distribution

g. logistic growth

__average number of individuals in a population for a given area

a. carrying capacity

b. age-sex structure

-->  c. population density

d. exponential growth

e. population growth rate

f. population distribution

g. logistic growth

__numbers of individuals of each age and sex in a population

a. carrying capacity

-->  b. age-sex structure

c. population density

d. exponential growth

e. population growth rate

f. population distribution

g. logistic growth

__pattern of population growth in which the rate of growth slows as the population nears the carrying

a. carrying capacity

b. age-sex structure

c. population density

d. exponential growth

e. population growth rate

f. population distribution

-->  g. logistic growth

The population is the unit of

a. natural selection.

b. adaptation.

c. microevolution.

-->  d. all of the above

If a population of 80 foxes lives in an area of 8 square kilometers, what is the fox population density?

a. 80 foxes per square kilometer

b. 64 foxes per square kilometer

-->  c. 10 foxes per square kilometer

d. 8 foxes per square kilometer

In which pattern of population distribution are organisms evenly spaced over the area they occupy?

a. random

b. clumped

-->  c. uniform

d. none of the above

What does a positive population growth rate mean?

a. Fewer people are being added than lost.

-->  b. The population is increasing in size.

c. The population has reached its carrying capacity.

d. two of the above

The human species first evolved about

a. 10,000 years ago.

b. 40,000 years ago.

c. 100,000 years ago.

-->  d. 200,000 years ago.

The demographic transition

-->  a. began in the 1700s.

b. occurred first in Africa.

c. happened in two stages.

d. included an increase in the birth rate.

About how many human beings presently live on Earth?

a. 7 million

b. 1 billion

c. 5 billion

-->  d. 7 billion

diagram questions

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