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Populations
318-5 Explain various ways in which
natural populations are kept in
equilibrium, and relate this
equilibrium to the resource limits of
an ecosystem
318-3 Explain why ecosystems with
similar characteristics can exist in
different geographical locations
• Changes in populations of one part of a
food web affect populations in other parts of
the web (e.g. sea otters)
• Can any population of organisms keep
growing forever?
• What might limit population size?
Carrying Capacity
• The largest population of a species that an
environment can support is called the
carrying capacity of the environment for
this species.
Four Factors that Determine
Carrying Capacity:
1. Energy and materials – any organism is
limited by energy, water, carbon and other
elements that are available
2. Food Chains – The population size at any
trophic level is limited by the population
size ( or biomass) at the levels below it.
3. Competition – Each organism has the
same needs as other organisms of its
species and it shares needs with other
organisms in different species. This creates
competition for food and resources.
Intraspecific competition – competition
among members of the same species.
Interspecific competition – competition
among different species.
4. Density – Different species have different
needs for space. The need for space
determines an organism’s population
density. Population density is how many
individuals can live in an area at one time.
If the population density increases beyond a
suitable level for a particular species, it
produces conditions that tend to limit further
population growth.
Factors that increase in significance as a
population grows are called densitydependent factors.
Density-independent factors are factors that
kills off some of a species, but the effect on
the population size is not related to how
many individuals there are in a population.
S-Curve and J-Curve
• The J-curve shows that if a population had
unlimited food and supplies it would grow
forever.
• The s-curve shows that if a population has
limited supplies (like most do), then the
population tends to level off as it reaches
the carrying capacity.
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P. 290
#1, 2, 3, 4
P. 291
Graphing Population Change
Complete all of it
Questions
1. Give one example for each of the following types of limiting
factors.
a. density-dependent
b. density-independent
2. What is meant by the term “carrying capacity”?
3. Give an example of
a. interspecific competition
b. intraspecific competition
4. How might overcrowding lead to a reduction in population size?
5. THINKING CRITICALLY – How might an increase in the
population of plants in an area lea to an increase in the population of
hawks?
6. THINKING CRITICALLY – How might an increase in the
population of hawks in an area lead to an increase in the population
of plants?