CP BIOLOGY
POPULATION NOTES
CHARACTERISTICS OF POPULATIONS (3)
 Geographic Distribution = area inhabited by a population, can vary greatly in
size
 Population Density = number of individuals per unit area, can also vary greatly
 Population Growth = the increase or decrease in size of a population (number
of individuals making up population)
o affected by three factors
 number of births (increases population)
 number of deaths (decreases population)
 number individuals entering (immigration) or leaving (emigration)
o population grows when birthrate greater than death rate
o population stays same if birthrate = death rate
o population decreases when birthrate is less than death rate
o immigration leads to population growth while emigration can lead to
population shrinking
TYPES OF POPULATION GROWTH
 Exponential Growth
o occurs when population has abundant space and food
o individuals in population reproduce at a constant rate
o produces graph called J curve
o population becomes larger and larger until it approaches an infinitely
large size – would occur under ideal conditions with unlimited
resources
 Logistic Growth
o more natural population growth
o population grows exponential at first, but as resources become less
available, then growth slows down or stops
o produces graph called S curve
o carrying capacity = the largest number of individuals in a population
that the environment can support
CP BIOLOGY, POPULATION NOTES, page 2
LIMITS TO POPULATION GROWTH
 Limiting Factors
o factors that cause population growth to decrease
o examples: competition, predation, parasitism and disease, drought and
climate extremes, human disturbances
 Density-Dependent Factors
o depends on the population size
o becomes factor only when population reaches a certain size (usually
when large and dense)
o include: competition, predation, parasitism and disease
 competition = compete for food, water, space, sunlight, and
other essentials of life
 intraspecific – between members of same species
 interspecific – between members of different species
 predation
 predator-prey relationship
 one of best-known mechanisms of population control
 cycles – prey increases population so predator population
will begin to increase, because of predation, prey
population decreases and so does predator population,
with fewer predators, then prey population begins to
increase again
 parasitism and disease – parasites similar to predators, can
cause hosts to weaken which can result in disease or death
 Density-Independent Factors
o affect all populations regardless of population size
o examples: unusual weather, natural disasters, seasonal cycles,
o environmental changes always occurring, populations can adapt to
certain amount of change
o populations grow or shrink in response to such changes
o major upsets in an ecosystem can result in long-term population
declines
CP BIOLOGY, POPULATION NOTES, page 3
HUMAN POPULATION GROWTH
 human population has increased over time
 about 500 years ago, human population began growing more rapidly
o agriculture and industry made life easier and safer
o world’s food supply – more reliable
o goods could be shipped around world
o improved sanitation, medicine, and health care dramatically reduced
death rate and increased life spans
o birthrates remained high
 demography = study of human populations
 birthrates, death rates and age structure help predict why some countries
have high growth rates while other countries grow more slowly
 demographic transition = dramatic change in birth and death rates due to
modernization
o modernization leads to lower death rates due to longevity
o birthrates remain high, so population increases rapidly
o further modernization results in families having fewer children
o birthrate falls, population growth slows
o transition complete when birthrate = death rate and population growth
stops
o has only occurred in a few countries – U.S., Europe, Japan
 most of world’s population growth contributed to by only 10 countries which
include China and India
 age-structure diagrams = graph showing the numbers of people in different
age groups in population
 current world population projections
o 2025 – 7.8 billion people
o 2050 – 9 billion+ people