population ecology notes

Ecologists study life at
many levels, from
individual organism to
the entire biosphere.
Ecology: “eco” means
“ology”means study of.
Levels of organization
Cells – Tissue-Organs –
Organ systems Organisms
Population – a group of
individuals of the same
species that live in the
same area at a given time
Example: a group of
white-tailed deer in an
oak-hickory forest in
southeastern PA
Community - groups of
living organisms that
interact with one
another in a particular
Example: deer, mice,
bushes and snakes in the
forest community
Ecosystem: a community
together with its
physical environment
Biosphere: all parts of
the earth that hosts
life, with all of its
organisms and
Ecosystems include both
biotic and abiotic
factors. Biotic factors
are parts of the
ecosystem that are
Abiotic factors are
parts of the ecosystem
that are non-living.
Biotic factors:
Organisms depend on
resources provided by
their habitat for
The specific
environment in which an
organism lives is its
A habitat provides an
organism with resources.
A resource is anything an
organism needs to
Describing Populations….
The OVERALL health of
a population can often be
monitored by tracking
how its size changes.
Population size describes
the number of individual
organisms present in a
given population at a
given time.
Population Ecology
Deals with the # of
individuals of a particular
species that are found in an
area and how and why those
numbers change or remain
fixed over time.
Population ecologists study…
How populations respond to its
Competition for resources,
predations, disease and other
environmental pressures
Determine population size
by sampling. Instead of
counting every individual,
ecologists count every
organism in a small area
than estimate the number
of individuals in the larger
Population density
describes the number of
individuals within a
population per unit area.
For example: 1500 golden
toads per 4 sq. kilometers
In general, larger animals
have lower population
density because they
need more resources.
High population density
can make it easier to
find mates and group
together. However, it
also leads to more
competition for
Overcrowded organisms
can also lead to increase
of disease and vulnerable
to predators.
Age structure diagrams
show the number of
males & females in
different age groups
within a population.
Age structure describes
the relative number of
organisms of each age
with in a population.
Age structure diagrams
are a visual tool to show
the age structure of
Stable – approximately the
same number of individuals at
each age grouping, mix of
young and old
Growing – mostly young
Declining – the largest
portion of population is
A population’s growth rate is
determined by births,
deaths, immigration and
All things being equal,
when a population
birthrate is greater than
its death rate, population
size will increase.
When death rate is
greater than its birth
rate, population size will
In addition to births &
deaths, population size can
also change by individuals
moving in & out.
Immigration – movement
of individuals into a
Emigration – movement
of individuals into a
Birthrate – number of
live births per 1000 in a
given year
Death rate – number of
deaths per 1000 in a
given year
Growth rate (r) = birth
rate (b)+ I – death rate
(d) + E
Population growth is
typically reported as a
net change per 1000
Exponential growth is
when a population
increases by a fixed
percentage each year.
Exponential growth
occurs in nature only
when the starting
population is small & the
environmental conditions
are ideal.
This type of growth
usually does not last long.
Most populations are
constrained by limiting
Limiting factors are
characteristics of the
environment that limit
population growth.
Limiting factors
determine a population’s
carrying capacity.
Example: New population
is introduced…mold
growing on bread.
Logistic Growth describes
how a population’s initial
exponential increase is
slowed & finally stopped
by limiting factors.
Carrying capacities are
not fixed. Carrying
capacity is the largest
population size a given
environments can
sustainably support.
Examples of limiting factors:
1) Food
2) Disease
3) Predators
4) Space
5) Water
Limiting factors have more
of an effect in dense
populations & other limiting
factors affect all
populations in the same
factors that limits the
population size or density
Disease (example: HIV)
factor: affect ALL
populations, regardless of
size of the population
Volcanic eruption
Biotic potential is the
maximum ability to
produce offspring in
ideal conditions.
Biotic potential is based on…
• Age at which reproduction
takes place
• % of life span that an
organism can reproduce
• Number of offspring in
each reproduction period
• Generally larger animals
have a lower biotic
potential than
microscope organisms.
Rapid life history
(example: mosquitoes)
Rapid reproduction
High number of offspring
Small body size
Small body size
Organisms mature
Reproduce early
Organisms have a short
life span
Slow life history (example:
Slow reproduction
Low number of offspring
Large body size
Organisms mature slowly
Reproduce late
Organisms have a long
life span
Organisms tend to have
parental care