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Unit 3 Notes
Chapters 8, 9, 10
POPULATIONS
A population is all the members of a species living in the same place at the same time. It refers to the
group in general and to the size of the population – the number of individuals it contains. A species is
all individuals that are capable of interbreeding and producing fertile offspring.
Populations may be described in terms of size, density and dispersion. These properties can be used
to describe populations and to predict changes within them. Population sizes can be determined by
factors such as births, death, immigration and emigration. r = (b - d) + (i - e)
Population density is a measurement of the number of
individuals living in a specified habitat.
Population dispersion refers to the relative distribution of its individuals within an area. Dispersion
can occur in patterns:
2
3
1
1. Clumped - individuals aggregate in patches;
influenced by resource availability & behavior
2. Uniform - individuals are evenly distributed; influenced
by social interactions such as territoriality
3. Random - the position of each individual is
independent of other individuals
Populations usually stay about the same size from year to year because various factors kill many
individuals before they can reproduce. A species’ biotic potential is the fastest rate at which its
population can reproduce. This rate is limited by an organism’s reproductive potential which is the
maximum number of offspring that each member of the population can produce. Reproductive
potential increases when individuals produce more offspring at a time, reproduce more often, and
reproduce earlier in life. Reproducing early shortens generation time, the average time it takes a
member of the population to reach the age when it reproduces.
Population Growth Patterns:
Exponential growth – occurs when populations have plenty of
food, space and have little or no competition or predators.
Populations rapidly increase due to an abundance of resources.
 This growth is not sustainable – short lived in nature. Natural
conditions are neither ideal nor constant; populations cannot
grow forever and rarely grow at their reproductive potential.
Resources are used up or the environment changes.
Logistic growth – populations are limited by environmental
factors and tend to attain equilibrium in size.
Carrying capacity of a particular species is the maximum
population that the ecosystem can support indefinitely. A
population may increase beyond this number, but it cannot stay at
this increased size due to resource availability.
 Because ecosystems change, carrying capacity is difficult to calculate exactly --- it’s a theoretical
limit.
 A species reaches it carrying capacity when it consumes a natural resource at the same rate at
which the ecosystem produces the resources. The limited resource determines the carrying
capacity for a species at a particular time.
Limiting Factors – Factors that prevent a population from reaching its biotic potential. Population
size can be limited in ways that may or may not depend on the density of the population.
1. Density Dependant Factors - Factors that have an increasing effect as the population increase.
As organisms crowd together, these factors cause more damage and spread faster in larger
populations.
 resource availability, predation, disease, competition, parasitism, stress due to overcrowding
2. Density Independent Factors – Factors that effect any population, regardless of density; NOT
influenced by population size.
 weather, natural disasters, human activities, adaptations & behaviors of organisms
Species Interactions
Species interactions are based on whether each species causes benefit or harm to the other species
in a given relationship. Each interaction affects evolution, persistence of a species and the overall
diversity of life. Organisms have evolved together and therefore adjusted to one another.
An organism’s niche and habitat are important factors in these interactions. The niche is the unique
role of a species within an ecosystem. A niche includes the species’ habitat, the environmental
factors necessary for the species’ survival and all the species interactions with other organisms.
Competition – A relationship in which different individuals or populations attempt to use the same
limited resources. Each individual has less access to the resource & is harmed by the competition.
 Members of the same species must compete with each other because they require the same
resource – they occupy the same niche. When members of different species compete, their niches
overlap, which means that both species use some of the same resources in a habitat.
 Competitive Exclusive Principle - Two species that have exactly the same requirements cannot
coexist in exactly the same habitat. Otherwise, competition will occur.
 Indirect competition – Species compete even if they never come into direct contact with each
other. (i.e. Insects feeding on same plant at different times.)
 Niche restriction occurs when each species uses less of the niche than it is capable of using. It is
observed in closely related species that use the same limited resource within a habitat.
Predation – The act of one organism (predator) feeding on another organism (prey). Most organisms
are vulnerable to predation, so there are strong selective pressures for adaptations that serve as
defenses against predators. Adaptations that can
enhance survival of prey include camouflage,
warning coloration, mimicry and protective
covering.
 Some predators eat only specific types of prey.
In this close relationship, the sizes of each
population tend to increase and decrease in
linked patterns. However, many predators will
feed on whichever type of prey is easiest to
capture.
Symbiosis – A relationship in which two species live in close association. These species may evolve
adaptations that reduce the harm or improve the benefit of the relationship.
 Parasitism – An organism that lives in or on another organism (host) and feeds on it. Parasites
usually do not kill their hosts, but weakens or develops a disease.
 Mutualism – Two species provide a benefit and depend on each other for survival.
 Commensalism – A relationship in which one species benefits and the other species is neither
harmed nor helped.
HUMAN POPULATION
The human population of Earth grew faster in the 20th century than it ever has before. This rapid
growth has led to environmental problems around the globe. Demography is the study of the
characteristics of populations, especially human populations. Demographers study:
 The historical size and makeup of the populations of countries to make comparisons and
predictions.
 The properties that affect population growth, such as economics and social structure.
Countries with similar population trends are often grouped into two general categories: developed and
developing countries.
 Developed countries have higher average incomes, slower population growth, diverse
industrial economies, and stronger social support systems. Developed countries makes up
15% of the world’s population and controls 80% of the world’s wealth.
 Developing countries have lower average incomes, simple and agriculture-based economics,
and rapid population growth.
The Human Population over TIME
The human population underwent exponential growth in the 1800s, meaning that the population
growth rates increased during each decade. These increases were mostly due to increases in food
production and improvements in hygiene that came with the industrial and scientific revolution.
However, it is unlikely that the Earth can sustain this growth for much longer.
Forecasting Population Size
Demographers can make many predictions based on age structure – the distribution of ages in a
specific population at a certain time. Age structure can be graphed in a population pyramid, a type of
double sided bar graph.
Survivorship
Demographers can use survivorship to predict population trends. Survivorship is the percentage of
newborn individuals in a population that are likely to survive to any give age. To predict survivorship,
a demographer studies a group of people born at the same time and notes when each person dies.
o Type I curve represents developed
countries
because most people live to be very old.
o Type II curve has a similar death rate at all
ages.
o Type III curve represents the pattern in very
poor
human populations in which many children
die.
o Both Type I and Type III may result in
populations that remain the same size or
grow
slowly.
Fertility Rates
Demographers calculate the fertility rate which is
number of births each year per 1,000 women of
childbearing age (usually 15 to 44). Total fertility
the
rate is
also calculated – the average number of children a woman gives birth to in her lifetime.
In 1972, the fertility rate dropped
below replacement level for the first
time in U.S. history. Replacement
level is the average number of
children each parent must have in
order to “replace” themselves. This
number is slightly more than 2
because not all children born will
survive and reproduce.
Migration
The migration of individuals causes populations to increase. Migration is any movement of
individuals or populations from one location to another. Movement into an area is immigration and
movement out of an area is emigration. The
populations of many developed countries
might be decreasing if not for immigration.
This graph represents the population of
the U.S. as it continues to grow.
Declining Death Rates – Life
Expectancy
The dramatic increase in Earth’s human population in the last 200 years has happened because
death rates have declined more rapidly than birth rates. Death rates have declined mainly because
more people now have access to adequate food, clean water, and safe sewage disposal. The
discovery of vaccines in the 20th century also contributed to the declining death rates.
The average number of years members of a
population are likely to live is their life expectancy.
Life expectancy is most affected by infant mortality,
the death rate of infants less than a year old.
In 1900, worldwide life expectancy was about 40
years and the infant mortality rate was very high. By
2000, the rate of infant mortality was less than 1/3 of
the rate in 1900.
Expensive medical care is not needed to prevent
infant deaths. Infant health is more affected by the
parents’ access to education, food, fuel, and clean
water. If basic needs are met, most children will
have a good chance of surviving. New threats to life
expectancy arise as populations become denser
such as contagious diseases.
Demographic Transition
In most developed countries, populations have stopped growing. The demographic transition is a
model that describes how economic and social changes affect population growth rates. The model is
based on observation of the histories of many developed countries.
In the first stage of the demographic transition, a society is in a preindustrial condition. The birth
rate and the death rate are both at high levels and the population size is stable. Most of the world
was in this condition until about 1700.
In the second stage, a population explosion occurs. Death rates decline as hygiene, nutrition, and
education improve. But, birth rates remain high, so the population grows very fast. During this
stage, the population can double in less than 30 years.
In the third stage, population growth slows because birth rate decreases. As the birth rate
becomes close to the death rate, the population size stabilizes. However, the population is much
larger than before the demographic transition.
In the fourth stage, the birth rate drops below replacement level, so the size of the population
begins to decrease. It has taken from one to three generations for the demographic transition to
occur.
Women and Fertility – The factors most clearly related to a decline in birth rates are increasing
education and economic independence for women. In the demographic transition model, the lower
death rate of the second stage is usually the result of increased levels of education. Educated women
find that they do not need to bear as many children to ensure that some will survive and learn family
planning techniques.
Women are able to contribute to their family’s increasing prosperity while spending less energy
bearing and caring for children. As countries modernize, parents are more likely to work away
from home. If parents must pay for child care, children may become a financial burden rather than
an asset. All of these reasons contribute to lower birth rates in both developed and developing
countries.
Today, the total fertility rate in developed countries is about 1.6 children per woman, while in
developing countries the rate is about 3.1 children per woman.
Changing Population Trends
Some countries have followed the model of the demographic transition – they have reached large and
stable population sizes and have increased life expectancies. But throughout history, and currently,
population that have high rates of growth create environmental problems. A rapidly growing
population uses resources at an increased rate and can overwhelm the infrastructure of a community.
Infrastructure is the basic facilities and services that support a community. The symptoms of
overwhelming population growth include suburban sprawl, overcrowded schools, polluted rivers,
barren land and inadequate housing. Problems of rapid growth include:
o Use resources faster than the environment can renew them. Vegetation, water and land are
the resources most critically affected by rapid growth.
o Shortage of fuelwood – In poor countries, deadwood is the main fuel source to boil water and
cook. As populations grow, deadwood does not accumulate fast enough and people cut down
trees. Without enough fuelwood, many people suffer from disease and malnutrition.
o In places that lack infrastructure, local water supply is used for drinking, washing and sewage
disposal. Water supply becomes a breeding ground for organisms that cause disease.
o Growing populations may have a shortage of arable land, which is land used to grow crops.
Populations make trade-offs between land uses for agriculture, housing or natural habitats.
o Much of the world’s population is undergoing urbanization, the movement of people from rural
areas to cities. This suburban sprawl leads to inadequate infrastructure and reduction of land
for farms and wildlife habitat.
A Diverse World Not every country in the world is progressing through each stage of demographic transition according
to the model. For example, some countries have modern industries, but incomes remain low. Due to
variety amongst countries, the international community has begun to focus on the least developed
countries. These countries show few signs of development and in some cases have increasing death
rates, while birth rates remain high. These identified countries by the United Nations are given priority
for foreign aid and development programs to address their population and environmental problems.
90% of population growth is occurring in third-world countries.
In 1994, the United Nations
held the International
Conference on Population and
Development (ICPD) to debate
the relationships between
populations, development and
environment. ICPD goals
include provide universal
access to family planning,
reduce infant mortality,
increase life expectancy, and
universal access to education.
BIODIVERSITY
Biological diversity refers to the number of different species in a given area. Biodiversity can be
studied and described at three levels.
 Species diversity – refers to the number of different species in an area.
 Ecosystem diversity – refers to the variety of habitats, communities and ecological processes
within and between ecosystems.
 Genetic diversity – refers to all the different genes contained within all members of a
population. A gene is a piece of DNA that codes for a specific trait that can be inherited by an
organism’s offspring.
BENEFITS of biodiversity –
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Species play an important role in an ecosystem because species are either dependent on or
depended upon by at least one other species. When on species disappears from an ecosystem, a
strand in a food web is removed. Keystone species are critical to the functioning of an
ecosystem. (i.e. The loss of sea otter populations led to the loss of the kelp beds along the U.S.
Pacific coast and how the recovery of otter populations led to the recovery of the kelp
populations.)
Genetic diversity within populations increases the chance of survival during environmental
change. When a population shrinks, its genetic diversity decreases producing a bottleneck. Even
if the population increases again, its genetic diversity will be reduced and the population is more
likely to inherit genetic diseases.
A variety of organisms on Earth have been used for food, clothing, shelter and medicine. About ¼
of the drugs prescribed in the U.S. are derived from plants and the majority of antibiotics are
derived from fungi.
Biodiversity preservation is our ethical responsibility and some regions earn the majority of their
income from ecotourism, a form of tourism that supports the conservation and sustainable
development of ecologically unique areas.
Biodiversity at RISK –
About 65 million years ago, a series of changes in the Earth’s climate and ecosystems caused the
extinction of about half the species of Earth. It takes many years for biodiversity to rebound after a
mass extinction, the extinction of many species in a short period of time. Large populations that adapt
easily too many habitats are not likely to become extinct (i.e. rats & cockroaches). Species with small
populations in limited areas can easily become extinct. At risk species include those that migrate,
those that need large or special habitats and those that are exploited by humans.
o Endangered species – A species that is likely to become extinct if protective measures
are not taken immediately.
o Threatened species – A species that has a declining population and that is likely to
become endangered if it is not protected.
1. Habitat Destruction and Fragmentation – As human populations grow, we use more land to
build homes and harvest resources. Humans destroy and fragment the habitats of other species. It
is estimated that habitat loss causes almost 75% of the extinction now occurring.
2. Invasive Exotic Species – An exotic species is not native to a particular region and can threaten
native species that have no natural defenses against them.
3. Excessive Hunting, Harvesting and Poaching – These activities have caused several
extinctions. Many countries now have laws to regulate hunting, fishing, harvesting and trade of
wildlife.
4. Pollution – Chemicals used by humans are making their way into food webs around the world.
The long-term effects may not be clear until after many years of use (i.e. DDT & bald eagle).
CRITICAL areas of Biodiversity –
Some parts of the world contain a greater diversity of species than others. An important feature is that
they have a large portion of endemic species ---- species that are native to and found only within a
limited area. Ecologist often use the numbers of endemic species of plants as an indicator of overall
biodiversity, because plants from the bases of ecosystems on land.
o The remaining tropical rain forests cover less than 7% of the Earth’s land. Biologist
estimate that over ½ of the world’s species live in these forests. Unknown numbers of
species are disappearing as tropical forests are cleared for farming or cattle grazing.
o Coral reefs occupy a small fraction of the marine environment yet contain the majority of
the biodiversity. Reefs provide a source of food, tourism, protect the coast from waves and
are sources of new chemicals. Nearly 60% of Earth’s reefs are threatened by human
activities such as overfishing and pollution. Similar threats affect other coastal ecosystems
which are travel routes for many migrating species and are linked to ecosystems on land.
o When islands form, it is colonized by a limited number of species and may evolve into
distinct new species.
o Biodiversity hotspots have high numbers of endemic species and are the most threatened
areas on Earth. Most hotspots have lost 70% of their original natural vegetation. The U.S.
hotspots include the Florida Everglades, the Californian coastal region, Hawaii, the
Midwestern prairies and the forest of Pacific Northwest.
FUTURE of Biodiversity –
The development of new approaches to conservation and sensitivity to human needs can slow the
endangerment or extinction of species.
1. Saving one species at a time - Wildlife experts may attempt to restore the population of species
through captive-breeding programs. Germ plasm, any form of genetic material, can be stored in
banks for future use in research or species-recovery efforts. Zoos, wildlife parks, aquariums and
botanical gardens help preserve some species. Ultimately, saving a few individuals does little to
preserve a species.
2. Preserving habitats and ecosystems – The most effective way to save species is to protect the
habitats. Most conservationists give priority to protecting entire ecosystems rather than individual
species. Strategies include identify areas of native habitats that can be preserved, restored, and
linked into large networks and promote sustainable practices of harvested products.
3. Legal Protection – In 1973, the U.S. Congress passed The Endangered Species Act designed to
protect plant and animal species in danger of extinction. Habitat conservation plans have
attempted to protect species across large areas of land through trade-offs or agreements.
4. International Cooperation – The International Union for the Conservation of Nature and Natural
Resources (IUCN) facilitates efforts to protect species and habitats. The IUCN publishes a list of
species in danger of extinction, advise governments on ways to manage their natural resources
and work with World Wildlife Fund to sponsor conservation projects.
o The Convention on International Trade in Endangered Species (CITES) treaty was the first
effective effort to stop the slaughter of African elephants. In 1989, member of CITES
proposed a worldwide ban on all trade of ivory.
o The United Nations Conference on Environment and Development (Earth Summit) was
created to tackle environmental issues worldwide. The international agreement, Biodiversity
Treaty, was created to preserve biodiversity and ensure the sustainable and fair use of
genetic resources in all countries.
o Many private organizations encourage the sustainable use of resources and supports
wildlife protection. The Nature Conservancy helped purchase millions of hectares of habitat
preservation in 29 countries. Conservation International helps identify biodiversity hotspots
and develop ecosystem conservation projects in partnership with other organizations and
local people. Greenpeace International organizes direct and confrontational actions to
counter environmental threats.