Population Ecology
Population: A group of organisms
that belong to the same species
that live in the same place at the
same time.
Properties of a Population I
 Boundaries:
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May be imposed by an environmental
factor
May be arbitrarily chosen to simplify a
population study.
 Different than properties of an individual:
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Individuals are born, reproduce, and die. A
population study focuses on HOW MANY
individuals are born, how many die, etc…
Properties of a Population II:
Population Size
 The number of individuals it contains
 Important, but can be hard to measure directly
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If the population is small and the organisms are immobile,
then the organisms can be counted.
Usually individuals are too abundant, too widespread, or
too mobile to be counted and the number must be
estimated using the random sampling method.
 Suppose
we want to know the number of sunflowers in a field.
 The ecologist counts the number in a 10 km squared area and
finds in that small patch there are 1,000 sunflowers.
 Assuming the field in 100 km squared, it is reasonable to
assume that there are 10,000 sunflowers overall. (1,000
sunflowers x 10)
Properties of a Population III:
Population Density
 Measures how crowded a population is.
 Expressed as the number of individuals per unit of area or
volume. (example: 30 people per square kilometer)
Properties of a Population IV:
Dispersion
 The spatial distribution of individuals within
a population.
 Clumped: individuals are clustered
together
 Often occur when resources are
clumped
 Also occurs due to an individual’s
social behavior (herds, flocks)
 Even: individuals are separated by a
fairly constant distance.
 Due to social interactionsorganisms want to get as far away
from each other as possible.
 Random: each individual’s location is
independent of the other individual’s
locations.
 Usually due to wind dispersal.
Population Dynamics I
 Change in size and composition over
time.
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Birth rate: number of births in a given time
Death, or mortality, rate: number of deaths
in a given period of time
Life expectancy: how long on average an
individual is expected to live.
Population Dynamics II:
Patterns of Mortality
 Tend to conform to
one of three patterns.
 Curves are call
survivorship curves:
show the likelihood
of survival at different
ages throughout the
lifetime of an
organism.
Population Growth Rate
 The amount by which a population size
changes over time.
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Birth
Death
Immigration
Emigration
 For
simplicity, can assume immigration =
emigration
 Birth rate – death rate = growth rate
Exponential Model
 Population that increases
rapidly after only a few
generations: the larger the
population gets, the faster it
grows.
 J-shaped curve- grows
slowly when the population
is small, speed up as more
individuals are added.
 Limitations- populations
grow exponentially under
rare conditions and for short
periods of time.

In reality- limiting factors
restrain the growth of a
population.
Logistic Model
 Accounts for the
influence of limiting
factors
 Carrying capacity (K): the
number of individuals the
environment can support
over a long period of
time.
 Assumes that carrying
capacity is constant
Population Regulation
 Two kinds of limiting factors
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Density-independent (weather, floods,
fires): reduce the population by the same
proportion regardless of a population’s
size.
Density-dependent (include resource
limitations): shortages of food, triggered by
increasing population density.
 All
populations fluctuate in size
Questions: Answer the following questions in on a separate
sheet of paper. Be sure to write out the question and answer
the question in complete sentences.
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1. Explain how two populations can be the same size but have different densities.
2. Explain two difficulties and ecologist might have in counting a population of migratory
birds. Develop and explain a method for estimating the size of such a population.
3. According to the exponential model, how do birth and death rate change with
population size?
4. Describe two differences between the exponential model and the logistic model.
5. Write a brief paragraph supporting the assumption that immigration and emigration
are insignificant when studying global human population.