3 Properties of
• Population Size
• Population Density
• Population Dispersion
• Population-groups of organisms that
belong to the same species and live in a
particular area at one time
• Population size-number of individuals a
• Sometimes there are too many to count
so a sampling is used.
• Scientists count a number of organisms
in a certain area and multiply the area.
• Population density-measures how
crowded a population is
• The number is always expressed
as the number of individuals per
unit of area or volume
• Some areas are densely populated
and others are sparsely populated
• Dispersion is the spatial distribution
of individuals within a population.
• There are three types of dispersion.
• Clumped dispersion occurs because
resources and living space is clumped
or because of behavior, herding
• Even dispersion is the result of social
behavior and organisms stay as far
away from each other as possible
• Random dispersion results from wind
distribution of seeds so plants usually
have a random dispersal
Types of Dispersion
• All populations are dynamic,
meaning they change in size and
composition over time.
• Birth rate-number of births over
• Death rate (mortality rate)-
number of deaths over time
• Life expectancy- how long on
average an individual is expected
• Age structure-distribution of
individuals among different ages in
• Different countries have different
• We can use graphs to compare age
Population Growth Rate
• Growth rate- the amount by
which a population’s size
changes over time
• Immigration-individuals moving
into a population
• Emigration-individuals moving
out of a population
Exponential Growth Model
• Exponential Model- population
increases rapidly after only a few
generations; the larger the
population gets, the faster it grows
• Limiting factor-a factor that
restrains or stops the growth of a
• Limiting factors are available
resources, space, waste
accumulation, population density
Logistic Growth Model
• Logistic model-builds on the
exponential model but adds the
• Carrying capacity (K)- the number
of individuals the environment can
support over a long period of time
• Once carrying capacity is reached,
the population remains constant
• Density-independent factors-
weather, flood, fires; these reduce
the population regardless of size
• Density-dependent factors- food,
nesting sites, illness; these occur
as a result of population size
Perils of small populations-
• The rapidly growing human
population has caused extreme
reductions in the populations of some
other species and subspecies.
• Fewer than 200 Siberian tigers
remain in the wild due to over
hunting and habitat destruction
• The California condor is down to 9
• Fewer individuals means inbreeding
or mating with relatives.
• This mean the babies will be more
likely to have defects or diseases.
and consumes another
•Prey-is captured and
consumed for food
Predators, Prey and Natural
• A predator’s survival depends on
its ability to capture food, but a
prey’s survival depends on its
ability to avoid being captured.
• Deception is important in
• In a defense called mimicry, a
harmless species resembles a
poisonous or distasteful species.
• The harmless mimic is protected
because it is often mistaken to
be its dangerous look-alike
National Geographic’s Collection of Mimicry and Camouflage
• Animals that eat plants are called
• Through natural selection, plants
have evolved adaptations that
protect them from being eaten.
• Physical defenses, such as sharp
thorns, spines, sticky hairs, and
tough leaves, can make a plant
more difficult to eat.
• Plants have also evolved a range of
• They synthesize chemicals from
products of their metabolism, called
secondary compounds, that are
poisonous, irritating, or bad tasting.
• Examples are the tobacco plant and
poison ivy and poison oak.
• Many medicines are derived from
• Parasitism- a species interaction that
resembles predation in that one individual
is harmed while the other benefits
• Parasite- feeds on another individual,
• Host-fed upon by another organism
• Ectoparasites-external, ticks, fleas, lice
• Endoparasites-internal, worms, protists
• Competition results from niche overlap.
• Competition is the use of the same
limited resource by two or more
• Competitive exclusion is when one
species is eliminated from a community
due to competition.
• One species uses a resource more
efficiently and has a reproductive
advantage over the other.
• Competition is the most intense
between similar species using the same
Mutualism and Commensalism
• Mutualism- a cooperative
relationship in which both
• Some mutualistic relationships
are so close that neither partner
could live without the other.
• Pollination is a major mutualistic
relationship that benefits the
Mutualism and Commensalism
• Commensalism- a relationship
between organisms in which one
organism benefits and one
organism is unaffected.
• One example is birds eating
insects and lizards that are flushed
out by buffalo.
• The buffalo is not harmed and
does not benefit but the birds
clearly benefit from the buffalo.
CommensalismThese are mainly
could change to
mutualism if the