Population Ecology
Study of
Populations in
relation to their
environment.
 Environmental
influences on

Pop. Density
 Distribution
 Age structure
 size

Population:
Individuals of a
single species
that occupy the
same general
area.
Important
Characteristics
1. Density
2. Dispersion
Density
Number of individuals per unit
area or volume.
Ex:
Diatoms - 5 million/m3
 Trees - 5,000/km2
 Deer - 4/km2

Dispersion
The pattern of spacing among
individuals within the boundary of the
population.
How do you count a
population?
1. actual count
2. Random plots, then extrapolate
3. Indirect indicators: number of nests;
burrows; tracks; droppings
4. MARK & RECAPTURE METHOD
Mark and Recapture Method
 N= Population estimate
 Trap, capture, mark, release.
 Later-recapture, determine proportion of those recaptured
that were marked.
N= # marked in first catch X Total # in 2nd catch
Number of marked recaptures
Assumptions:
1. the proportion of marked animals in the 2nd trapping is equivalent to the proportion of marked
animals in the total population
2. Those marked mix uniformly within the pop.
Population densities not
static
Birth
Death
Immigration
Emmigration
Dispersal patterns
3 general patterns
Know these!!
1. Clumped Pattern
Most common pattern
Results form patchy environmental conditions-food,
nutrient availability.

Ex: Mushrooms under rotting log.
Groups enhance predation
Safety. May increase chances for survival.
Ex:



Schooling behavior
Flocks of birds
humans
2. Uniform Dispersion
Individuals are evenly spaced
Results from direct antagonistic
interactions
Plants secrete chemicals-inhibit nearby
germination & competition (alleopathy)

territoriality
3. Random Dispersion
Spacing varies unpredictably
Absence of strong attractions or
repulsions between individuals.
RARE pattern in nature
Example: windblown seeds land
randomly & germinate.
Demography
Study of vital statistics of a population &
how they change over time.
Add
Lose
2 most important features
 Age
structure-relative numbers of
individuals of each age group in a
population
 Sex ratios
1. Life Tables
How long, on average, are
individuals of a given age expected
to live.
Age-specific summary of survival
patterns.
Follow the fate of a cohort (indiv.
Of same age) from birth to death.
Do males or females have a higher death
rate? Who lives longer?
2. Survivorship Curves
Graphic representation of data in Life
Tables.
Plot of the proportion or numbers in a
cohort still alive at each age.
3 general Curve Types: ( be able to recognize
and describe traits; members exhibiting each;
example organism)
 Type
I—humans, elephants
 Type II-squirrel
 Type III-oysters, clam
Draw and label in your notes
Type I
Low early deaths.
Steep decline in death rates among
those older
Produce few offspring, but provide good
care.
Ex:
 Humans
 Other large mammals
Type II
Intermediate
Constant death rate over life
span.
Ex:
 Annual plants
 small mammals ( grey
squirrel), lizards
Type III
High death rates for the young.

( sharp dip in curve initially)
Few live to adulthood
Associated with:
 Produce
large numbers of offspring-but
provide little or no care.
Ex: Oysters

Variations
Curve
type may change
between young and adults.
 Ex:
Nestlings - Type III
Adult Birds- Type II
 Stair-step
curve
 Invertebrates—crabs;
high mortality
during molt, followed by low mortality
Life-History Traits
Life History: Timing of reproduction and
death.
Highly diverse, but do show patterns
Determines how populations grow.
Results from Natural Selection.
Darwinian “fitness”
Darwinian “fitness”
Survive AND Reproduce
Reproduction and
survival—Life History
Traits
1. # of reproductive episodes
# offspring per reproductive episode
Age at first reproduction
2 Common Reproductive
Patterns
1. Semelparity: “big-bang” reproduction

1 reproduction event

Salmon, agave ( desert plants) , annual plants
2. Iteroparity
Fewer offspring at a time
 Over many reproductive seasons

Population Studies
& Reproductive
Rates
Focusing on females and female
offspring.
2 Kinds of Reproductive Strategies
identified
R
 K

Life History Selection
1. "r" Selected species
2. "k" Selected species
“r" – Selected Species (density
independent selection)
Increase fitness by producing as many
offspring as possible.
Do this by one of these strategies:
Early maturation
 Many reproductive events
 Many offspring in one reproductive event


“Big bang” reproduction (semelparity) (pink salmon,
agaves) is one time reproduction.
r-Selected Result
Maximize reproduction so that at least a
few offspring survive to the next
generation.
Most offspring die
(Type III curve).
“K" – Selected Species
(density dependent
selection)
Increase fitness by having most offspring
survive.
Maintain populations at or near “K”
Do this by:
High parental care
 Late maturation
 Few reproduction events
 Few offspring.

K- Selected –Results..
Maximize survivorship of each offspring
.
Few offspring, but most survive (Type I
curve).
What is the strategy
For a weed?
For Garden Pests?
POULATION GOWTH
Zero population growth:
Per capita birth + Per capita death=0
Exponential Growth
Produces a “J-shaped”
growth curve.
Ideal conditions
unlimited resources.
Example:


Introduce into new or
unfilled environment
Rebounding population
Logistic Growth-
Population growth
related to Life History
Traits
S-shaped” growth curve.

Characteristic of “k" species.
Common when resources are limited.
Populations are limited by space, food. That
limit is called the CARRYING CAPACITY
The graph shows a logistic population curve.
At what level do the deer reach their
CARRYING CAPACITY?
What Limits Population Size?
Density-dependent factors: limited resourcesspace, food, water, air

related to population size
Density-independent factors: random
occurrences that can limit population earthquake, bad weather.

not affected by population size
Is disease density dependent, or density
independent?
Additional Comments
Populations often overshoot “K” ( go
beyond) , then drop back to or below
“K”.
Regular Population
Cycles
Cyclic changes in N
over time.
Often seen in
predator/prey
cycles.
Ex: “Boom & Bust
Cycle” of Snowshoe
Hare -& Lynx
Predators kill and consume other organisms. Carnivores prey
on animals, herbivores consume plants.
Predators usually limit the prey population, although in
extreme cases they can drive the prey to extinction.
Why predators rarely kill and eat all the prey:



1.Prey species often evolve protective mechanisms such as
camouflage, poisons, spines, or large size to deter predation.
2.Prey species often have refuges where the predators cannot reach
them.
3.Switch its prey as the prey species becomes lower in abundance:
Know how to read a
graph for population
growth, where K is;
where the
overshoot “K” is
Age Structure Diagrams
Show the percent of a population in
different age categories .
Method to get data similar to a Life
Table, but at one point in time.
HUMAN POPULATION
GROWTH
Grown since history
1A.D. 300 million
Since 1650 ( 500 million), growing
exponentionally.
Over 6 billion today
Exponential Growth
Produces age structures that are a
triangle or pyramid shape.
Logistic Growth
Produces age structures that have even
sizes between most age categories.
Declining Populations
Produce age structures with a narrow
base and wider middles.
Reveal current and future
growth trends.
Ecological Footprint
Model to predict “K”
Relative to The Available Ecologic
capacity
Land usage requirements to meet our
varied human needs.
Summary
Know density and dispersion patterns.
Know Life Tables and survivorship curves ( and
representative organism for the 3 types).
Know what “K” is
Be able to contrast and compare “r” and “k”
strategies ( Suggest making a chart-include Logistic
Curve, Exponential growth curve, characteristics of
populations).
When/how density independent, and dependent
growth factors affect pop size.
Know how (match) Age-structure pyramids relate to
either Rapid, Slow, or Zero Pop. Growth; their
shapes & what the shape means.
Summary
Know exponential and logistic growth
curves.Relate them to PGR