Chapter 7: Populations: Characteristics and Issues
Population Characteristics
1) Natality: the number of individuals added to the population through reproduction over a
particular time period.
Or birth rate: The number of individuals born per 1000 individuals per year)
Two ways of reproduction: Asexual and sexual.
2) Mortality (death rate): the number of deaths in a population over a particular time
period.
Or death rate: the number of people who die per 1000 individuals per year.
Or survivorship curve:
(i) Another way to view mortality
(ii) Show the proportion of individuals likely to survive to a specific age
(iii) Three types: (a)Species with high mortality among their young
(b) Species with mortality evenly spread over all age groups
(c)Species with high survival rate among their young with high mortality
among their old
For the size of a population to grow: birth rate > the death rate. (Fig. 7.1)
3) Population growth rate = birth rate – death rate
4) Sex ratio: relative numbers of males and females
a. No meaning for earth worms and most plants as both kinds of sex organs in the same
body.
b. Number of females ultimately determines the number of offspring produced in the
population.
c. In most species, the ratio is 1:1.
d. Exceptions: many game animals, social insects (bees, ants, wasps)
5) Age distribution: the number of individuals of each age in the population.
Usually greatly influences the population growth rate. (Fig. 7.3)
Ex. Human population: several types of age distribution: (Fig. 7.3)
Rapid growth (Kenya); Slow growth (U.S.A.); Negative growth (Italy).
(iv) Population density and spatial distribution:
(a) Factors that affect the population density: soil type, quality of habitat, availability
of water.
(b) Population density: the number of organisms per unit area.
Cities vs. rural areas; high competition in high population density area.
(c) Dispersal: The movement from densely populated locations to new areas.
Relieves the overcrowded condition in the home area and at the same time
increase the population in the places to which they migrate.
(d) Emigration: Movement out of an area that was once one’s place of residence.
Ex. Birds emigrate from a place without water to a place with plenty of water.
(e) Immigration: Movement into an area where one has not previously resided, which
may introduce characteristics that were not in the population originally.
Ex. Europeans immigrated to North America: brought diseases that were foreign
to the Native Americans.
A Population Growth Curve
1) Biotic potential (or inherent reproductive capacity): the biological ability for each
species to produce offspring.
(i) Large differences between species:
a. apple trees: produce thousands of offspring.
b. pigs or geese: produce 10-12 young per year.
c. bears or elephants: produce one young every two to three years.
(ii) Because most species have high biotic potential, there is a natural tendency for
populations to increase.
(iii) A typical population growth curve consists four phases: (Fig. 7.5)
a. Lag phase: the population grows very slowly because there are few births.
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b. Log phase (or the exponential growth phase): the population begins to
increase at an accelerating rate as long as the birth rate >> the death rate.
c. Deceleration phase: population birth rate slows down to equal to death rate.
d. Stable equilibrium phase: population stops growing when birth rate = death
rate.
Factors That Limit Population Size
1) Limiting factors: factors that prevent unlimited population growth.
2) Environmental resistance: Collective term for all limiting factors that act on a population.
3) Extrinsic limiting factors: the factors that come from outside the population.
Ex. Predators, loss of food source, lack of sunlight, accidents of nature.
4) Intrinsic limiting factors: the factors that come from within the population.
Ex. Change in behavior results in lower birth rate and higher death rate:
Rats in crowed living condition  common abnormal social behavior  decrease in
litter size  fewer litters per year produced  mothers ignore their young  adults
kill many young.
Ex. A series of severe winters results in lower birth rate and higher death rate:
Whitetailed deer.
5) Density-dependent limiting factors: Those limiting factors become more severe as the
size of the population increases because
a. The predators have more chance to catch prays.
b. Disease epidemics are more common as easy to spread parasites.
6) Density-independent limiting factors: Those limiting factors are not affected by
population size.
a. Usually they are the accidental or occasional extrinsic factors (rain or snow storms) in
nature.
b. Usually the organisms most likely to be killed by density-independent factors are small,
short-lived organisms that can reproduce very quickly.
Categories of Limiting Factors
1) The availability of raw materials: Raw materials come in many forms.
Ex. Plants need N and Mg to make chlorophyll. Applying fertilizers is to prevent certain
raw materials becoming a limiting factor.
2) The availability if energy:
Ex. Sunlight for plants: small plants in the shades of trees  not receiving enough
sunlight  plants do not grow well.
3) The accumulation of waste products
Ex. Not a limiting factor for plants as producing few wastes but is a limiting factor for
bacteria (growth curve: lagexponentialdecelerationstabledeath).
4) Interactions among organisms
Ex. Rats in crowed living condition  common abnormal social behavior  decrease in
litter size  fewer litters per year produced  mothers ignore their young  adults kill
many young.
Carrying Capacity:
1) The maximum sustainable population for an area.
2) Determined by a set of limiting factors: O2 supply, food supply, disease, predators, space.
(Fig. 7.7)
3) A flexible number: Aquatic ecosystem, nutrient level fluctuates with changes in current.
Reproductive Strategies and Population Fluctuations
1) Two strategies: K and r.
2) K-strategists: Large organisms that have relatively long lives, produce few offspring, and
typically have populations that stabilize at the carrying capacity.
Ex. Deer, lions, humans, etc.
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3) r-strategists: Small organisms that have short life spans, produce a large number of
offspring, and do not reach a carrying capacity.
Comparison of life history characteristics of typical r- and K-strategists
Characteristic
r-Strategist
K-Strategist
Environmental stability
Unstable
Stable
Size of organism
Small
Large
Length of life
Short, most die before
Long, most live to reproduce
reproducing
Number of offspring
Large number produced, no
Small number reproduced,
parental care
parental care provided
Primary limiting factors
Density-independent limiting
Density-dependent limiting
factors
factors
Population growth pattern
Exponential growth followed
Exponential growth followed
by a population crash
by a stable equilibrium stage
at the carrying capacity
Examples
Protozoa, mosquitoes, annual
Alligators, humans, redwood
plants
trees
Population Cycles:
1) Many animals in the northern regions of the world show distinct population cycle.
2) Meaning: Periods of relatively large populations followed by periods of small populations.
Human Population Growth: (Fig. 7.10)
1) Show a long lag phase followed by a sharply rising exponential growth phase that is still rapidly
increasing.
2) Major reason for population increasing: human species has lowered its death rate.
a. Sharing technologies, improving sanitary facilities, increasing food production or controlling
disease, developing water treatment systems (engineers), giving nutrition advice
(physicians).
b. Doubling time = 70  annual rate of increase (%) (Fig. 7.11)
Ex. At a 2% rate of annual increase, human population will double in 35 years.
Human Population Characteristics and Implications
1) Economic development
a. Two segments of the world based on the state of economic development of the countries:
b. More-developed countries: Have a per capita income > US$ 10,000; including all of
Europe, Canada, the United States, Australia, New Zealand, Japan, about 1.2 billion
people.
c. Less-developed countries: Have a per capita income < US$ 5,000; including the remaining
countries of the world: majority of Asia, Latin American, African citizens; about 5.3 billions.
2) Measuring the environmental impact of a population
a. Human population growth: (i) tied to economic development
(ii) a contributing factor to nearly all environmental problems.
Ex.
Famine (food production can’t keep up with population growth
Environmental degradation (erosion, desertification)by poor agricultural practices
Water pollution by human and industrial waste
etc.
b. Several factors interact to determine the impact of a society on the resources of its country:
Ex.
Impact = Population x Affluence x Damage due to technology
I=PxAxT
c. Population density:
(i) the number of people per unit of land area
(ii) relates the size of the population to the resources available.
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 Low population density has less impact on the resources (Amazon Basin)
 Countries with abundant resources can sustain higher population densities than
resources-poor countries.
d. Ecological footprint: The area of the Earth’s productive land and water required to supply
the resources that an individual demands as well as to absorb the wastes that the individual
produces.
 Most of the more-developed countries, Japan, have a much larger ecological footprint
that requires import most of the materials it needs.
 Increasing human population increases environmental degradation.
 Much of the population growth will occur in the less-developed areas of the world.
Factors that influence human Population growth
1) Biological factors:
a. Demography: The study of human populations, their characteristics, and their changes.
b. Predictable:
if birthrate > death rate, then population grows rapidly. Ex. Mexico, Indonesia.
If birthrate = death rate, the population grows slowly. Ex. Japan, France
If birthrate < death rate, the population declines. Ex. Russia, Germany (political upheaval)
c. Total fertility rate: the number of children born per woman in her lifetime.
d. Replacement fertility: The number of children per woman needed just to replace the parents.
e. Zero population growth: The stabilized growth stage of human population during which births
equal deaths and equilibrium is reached.
f. Age distribution: The comparative percentages of different age groups within a population.
2) Social Factors:
a. Economically developed countries have low fertility rates and low rates of population growth.
b. Less-developed countries have high fertility rates and high rates of population growth.
c. Factors influence the number of children a couple would like to have: religion, tradition, social
and economic issues.
d. Culture and traditions:
 Major social factors that determine the family size are the status and desires of women
in the culture.
 In male-dominated cultures, the traditional role of women is to marry and raise children.
 Of the role is coupled with strong religious input.
 Typically, little value is placed on educating women, and early marriage is encouraged,
resulting in high fertility rate.
 In these cultures, women are totally dependent on their husbands and children in old
age.
e. Attitudes toward birth control:
 Women with higher education obtain higher-paying jobs and are financially independent
and marry later and have fewer children.
 Better-educated women are more likely to have access to and use birth control.
 The desire of women to limit the size of their families is the key to solve the population
problem, regardless of religion and previous historical birthrate. (Catholic: Spain and Italy:
1.3 vs. Mexico: 2.6)
 The average total fertility rate in the developed countries is 1.6; while the average in the
less-developed countries is 2.6.
 Why do women in less-developed countries desire more children?
Without government program of social security parents are more secure in old age if
They have several children to contribute to their needs when they can no longer work.
3) Economic Factors:
a. In less-developed countries, the economic benefits of children are extremely important:
Young children can be given jobs that contribute to family economy.
Young children can protect livestock from predators, gather firewood, cook, or carry water.
b. In developed countries, large number of children is an economic drain.
Young children are prevented by law from working.
Young children must be sent to school at great expense.
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4) Political Factors:
a. Government population policy:
Developed countries, Hungry, Sweden, and several other European countries with
extremely low population growth rate provide paid maternity leave for mothers during the
early months of a child’s life and the guarantee of a job when the mother returns to work.
Developing countries with extremely high population growth rate, China (total fertility rate:
1.6) and India (total fertility rate 3.0), provide information on maternal and child health and on
birth control and free or low-cost access to contraceptives as part of their population control
effort.
b. Immigration:
To prevent a shortage of working-age citizens in the near future in the low birthrate
countries like several European countries, Japan, and China is to encourage immigration
from other parts of the world.
 Developed countries are under tremendous pressure to accept immigrants because their
standard of living is a tremendous magnet for refugees or people who seek a better life than
is possible where they currently live.
 US experiences 1/3 population increase each year due to immigration.
 Canada encourages immigrants and has set a goal of accepting 300,000 new immigrants
each year.
Population Growth Rates and Standard of living
a. One important economic measure of standard of living is the average purchasing power
per person.
b. One index of purchasing power is the gross national income (GNI).
c. GNI is an index that measures the total goods and services generated within a country as
well as income earned by citizens of the country who are living in other countries.
d. As the price of goods and services vary from one country to another, a true comparison of
purchasing power requires some adjustments.
e. GNI PPP (gross national income purchasing power parity)
Hunger, Food Production, and Environmental Degradation
a. As human population increases, the demand for food rises.
b. People in less-developed countries must grow their own food and have very little money to
purchase additional food. Typically, the farmers have little surplus.
1) Environmental Impacts of Food Production:
Human population increases only if the population of other kinds of plants and animals
decreases.
 When humans need food, human convert lands to artificially maintained agricultural
ecosystem that destroy the mix of animals and plants.
 If the agricultural ecosystem is mismanaged, the region’s total productivity may fall below
that of the original.
Ex. Desertification in Africa, destruction of tropical rainforest
2) Human energy pyramid:
 People in less-developed countries usually feed at lower tropic levels (plants) because
converting less-concentrated carbohydrates of plants to more nutritionally valuable animal
protein and fat is an expensive process.
 People in developed countries usually eat more meat and other animal protein and suffer
from over nutrition by eating too much.
 If the grains used to feed animals were eaten by people, less grains would be grown and
the environmental impact would be reduced.
3) Economics and politics of hunger:
 In countries with food shortage, their agricultural land is already being exploited to its limit.
Food in greatest need: sub-Saharan Africa region.
 Food exporters: USA, Canada, Australia, Argentina, New Zealand, the European Union.
 India and China: enough food to support their people and have no food left for export.
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 Improved plant varieties, irrigation and improved agricultural methods have dramatically
increase food production.
4) Solving the problems:
 Provide humanitarian aids for countries that are unable to produce or buy enough food for their
people.
 Emergency food program causes people migrate from rural (agricultural) area to cities and
become dependent on food aid and stop working to raise their own food.
 The reason for them to stop working to raise their own food is they have to leave their fields to
go to the food distribution centers.
 Now the food distribution is at local villages and support projects that provide incentives to
improve local agricultural economy.
The Demographic Transition Concept
1) The demographic transition model: Has four stages that is based on the historical, social,
and economic development of Europe and North America. (Fig 7.16)
 Stage 1: Countries have a stable population with high birthrate and high death rate. Death
rates often vary because of famine and epidemic diseases.
 Stage 2: Population increase (high birthrate, low death rate) due to improved economic and
social conditions (control of disease and increased food availability)
 Stage 3: technological development begins and population continues to increase, birthrate
drops because people desire small families and use contraceptive.
 Stage 4: High technology is developed and the population is stable due to the balance of
birthrate and death rate. The population now has low birthrate and low death rate.
2) Applying the model:
 Does model apply to the less-developed countries?
 Does the less-developed countries have unexploited lands to expand population?
 Do the less-developed countries have ability to accumulate needed capital to develop
economically?
 Time element:
(i) Industrialization probably can not occur fast enough to have a significant impact on
population.
(ii) As long as people in less-developed countries are poor, there is a strong incentive to have
large numbers of children. Children are a form of social security because they can take care of
their elderly parents.
(iii) Only people in developed countries can save money for their old ages. They can choose to
have children, who are expensive to raise, or to invest money to some other way.
The U.S. Population Picture
 Total fertility rate in U.S. is low (2.0), similar to other developed countries.
 Two factors that influence U.S. population growth: (i) age structure of the population, (ii)
immigration policy.
 Birthrate in postwar baby boom area (1947-61) > today’s => population grows. Now population
age as younger people limit the size of their families.
 Both legal and illegal immigration significantly influence future population growth trends. Thus,
even with total fertility rate pf 2.0 children per woman, the population is still growing by about
1.1% per year. About 0.6% is the result of natural increase (i.e. birthrate – death rate).
 Census Bureau projects immigration will count 50% of population growth by 2050.
 Most immigration policy is the result of political decisions rather than decisions that
relate to population policy or a concern about the rate at which the U.S. population is
growing.
What Does the Future Hold?
Humans are subject to the same basic four limiting factors that other species face:
We cannot increase beyond our ability to acquire raw materials and energy and safely dispose
our wastes.
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1) Available raw materials:
Amount of food (limiting factor, malnutrition)
Technology (irrigation water, genetic research, antibiotics); other resources.
 About one billion (1/3 of world population) people suffer from a lack of adequate food.
2) Available energy: Problems similar to raw materials:
Depending on fossil fuels to raise food
Modify environment
Move from place to place
 When energy prices increase, much population in the world is in jeopardy because
incomes are not sufficient to pay for the increased cost for energy and other essentials.
 Increasing the efficiency of energy usage and the development of more efficient of solar
and wind energy conversion systems can reduce the dependency of fossil fuels.
3) Waste disposal: Most pollution is the waste product of human activity.
 Lack of adequate sewage treatment and safe drinking water causes large number of
deaths each year.
 It makes sense to control pollution and to clean our environment.
4) Interaction with other organisms:
 When we convert land to meet our needs, we displace other species from their habitats.
 Many displaced organisms cannot compete with us successfully and must migrate or
become extinct.
 Many organisms provide services that we enjoy without thinking about them:
Forest trees release water and moderate temperature changes
Bees and other insects pollinate crops
Insect predators eat pests
Decomposers recycle dead organisms
5) Social factors influence human population:
Human survival depends on interaction and cooperation with other humans.
Why population continues to increase, why do large numbers of people continue to live in
poverty, suffer from preventable diseases, and endure malnutrition when the current
technology and medical knowledge are available to control human population growth and to
improve the health of the people of the world?
Answers:
(i) Humans have the freedom of choice and frequently do not do what is considered “best”
from an unemotional, uninvolved, biological point of view.
(ii) People make decisions based on history, social situations, ethical and religious
considerations, and personal desires.
(iii) Biggest obstacles to controlling human population are not biological but the province of
philosophers, theologians, politicians, and sociologists.
Ultimate Size Limitation
 Human population is subject to the same biological constraints as are other species of
organisms.
 No one knows what the ultimate human population size will be or what is the most potent
limiting factors will be.
 Most agree that we are approaching the maximum sustainable human population.
 If human population continues to grow, eventually the amount of agricultural land available will
not be able to satisfy the demand for food.
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