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ECOLOGY NOTES - CHAPTERS 5
POPULATIONS
Population Characteristics
1. Population – groups of individuals that belong to the same species and live in the same
area
2. Four important characteristic of populations:
a. Geographic Distribution – the area inhabited by a population
b. Density – the number of individuals per unit area
c. growth rate
d. Age structure – the number of people in different age groups in a population;
used to predict future population growth
Population Growth
1. 3 factors affecting population size
a. number of Births
b. number of Deaths
c. number of individuals immigrating or emigrating
1. Immigration – movement of individuals into an area
2. Emigration – movement of individuals out of a population
2. Populations can grow when there are more births than deaths. They can shrink when
deaths outnumber births. If these rates are equal, population size tends to remain the
same.
Exponential Growth
1. A type of population growth that occurs when individuals in a population reproduce at a
constant rate
2. This type of growth results in population size growing slowly at first, but then the
population becomes larger and larger until it approaches an infinitely large size.
3. Under ideal conditions (no predation or disease) and unlimited resources, a population
will grow exponentially.
4. This type of growth does not occur for very long.
Logistic Growth
1. a type of population growth that occurs when a population’s growth slows or stops
following a period of exponential growth
2. Can be caused by:
a. lack of resources
b. decreased birthrate or decreased immigration
c. increased death rate or increased emigration
d. equal birth and death rates or immigration and emigration rates
3. Carrying capacity – the largest number of individuals that a given environment can
support
E. Limiting Factors
1. factors that causes population growth to decrease.
2. Examples are: competition, predation, parasitism, disease, drought, climate extremes,
human disturbances
3. Density-Dependent Factors
a. limiting factors that depends on a population’s size
b. mostly affects populations that are large and dense
c. includes: competition, predation, parasitism and disease
4. Density-Independent Factors
a. limiting factors that affect all populations a similar way, no matter the size
b. includes: unusual weather, natural disasters, seasonal cycles, human activities
(damming rivers, clear-cutting forests, etc).
c. population size often decrease rapidly in response to these factors
F. Human Population Growth
1. Demography – the scientific study of human populations
2. human population tends to increase with time
3. About 500 years ago, the human population began growing exponentially because of
improved medicine, sanitation, agriculture, energy use and technology.
4. Demographic Transition
a. a dramatic change in birth and death rates
b. completed in the U.S., Japan and much of Europe
c. Underdeveloped countries often have high death rates and high birthrates.
Improved technology, sanitation, etc. results in more children surviving into
adulthood and more adults living to old age. These changes lower the death rate
and begin the transition.
d. In the U.S. from 1790-1910, births greatly exceeded deaths and the population
increased rapidly. Many parts of South America, Africa and Asia are still in
this phase.
e. As societies modernize, the birthrate falls and population growth slows. The
demographic transition is complete when population growth stops (birthrate =
death rate).
5. Current predictions suggest that by the year 2025, the world’s population will be 7.8
billion and by 2050, it may be more than 9 billion people.
ECOLOGY NOTES - CHAPTERS 6
HUMANS IN THE BIOSPHERE
A. Human Activities Affecting the Biosphere
1. hunting and gathering
2. agriculture
3. industry
4. urban development
B. Environmental Resources
1. 2 types:
a. Renewable – resources that can be renewed (trees, water) regenerated if alive,
replenished if non living but are not necessarily unlimited.
b. Non renewable – resources that cannot be replenished (fossil fuels, coal, oil,
natural gas)
2. Destruction of natural resources
a. increased soil erosion by over-plowing land.
b. desetification – turning once productive areas into deserts by farming,
overgrazing & draught.
c. deforestation – clearing and losing forests.
d. Smog – mixture of chemicals in atmosphere due to autos and factories.
e. Acid rain – drops of nitric and sulfuric acids resulting from the acidic gases
produced by burning of fossil fuels
f. Pollution – when harmful materials enter the biosphere through the land, air or
water
C. Biodiversity
1. the sum total of the genetically based variety of all organisms in the biosphere
2. Ecosystem diversity – includes all the habitats, communities and ecological processes in
the living world
3. Species diversity – the number of different species in the biosphere
4. Genetic diversity - sum total of all the genetic information carried by all organisms
living on Earth
5. Biodiversity is one of Earth’s greatest natural resources. Many different species have
provided humans with medicine, foods, industrial products, etc.
6. Human Threats to Biodiversity
a. altering and Fragmenting habitats – spilts ecosystem into pieces
b. hunting species to extinction
c. introducing toxic compounds into food webs; results in biological
magnification – concentrations of toxic substances increase in organisms at a
higher trophic level in a food chain or food web (e.g. is the pesticide DDT – non
biodegradable, can not be eliminated from bodies)
d. bringing foreign species into new environments
1. Invasive species – new species introduced into a habitat who reproduce
quickly
2. examples are Zebra mussels, nutrias
D. Global Changes
1. Ozone layer depletion
a. ozone layer – portion of the atmosphere 20-50 kilometers above the Earth’s
surface that protects Earth from the sun’s harmful ultraviolet radiation
b. holes in the ozone are found over Antarctica and the Arctic
2. Global warming
a. increase in the average temperature of the biosphere
b. Over the past 120 years, global temperatures have risen 0.5 degrees Celsius.
c. The 1990’s were the hottest decade ever recorded.
d. Caused by human activity that adds carbon dioxide and methane to the atmosphere,
resulting in the global atmospheric greenhouse retaining more heat.