Population Ecology
Populations
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Population
ecology is the
study of
populations in
relation to the
environment
A population is a
group of
individuals of the
same species that
live in the same
area
Population Ecology: Vocabulary
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To describe populations, we have to talk
about how they are situated
Density:
 The number of individuals per unit
area/volume
 Example: 47 elephants/km2
Dispersion:
 The pattern of spacing among individuals
in a population
 Clumped
 Uniform
 Random
Uniform
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Environmental
conditions are
uniform
Causes
COMPETITION or
antagonism
between
organisms
(territoriality)
Clumping
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Most common
Reproductive
patterns favor
clumping
Social behaviors
lead to clumping
Optimal density is
usually
intermediate
(medium)
Random
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No competition
No tendency to
group/clump
Conditions are
uniform
Rarely happens!
Estimating Population Size
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Rarely, it is possible to count the number of
species in a population
Usually, a statistical method is needed to
determine population size
The mark-recapture method can be used
to estimate the size of a population
Capture, mark, release
Recapture and count
Equation:
N = Number marked x Total catch 2nd time
Number of marked recaptures
Factors That Influence Population Size

There are 3 major factors that influence
population size:
1. the number of births
2. the number of deaths
3. the number of individuals that enter or
leave a population
- immigration:
individuals entering an existing population
- emigration:
individuals leaving an existing population
DEMOGRAPHICS

In looking at populations, biologists must
also look at demographics (the vital
statistics of a population and how they
change over time)
 Birth rates
 Death rates
 Life tables
 Survivorship curves
 Reproduction rates
 Growth rates
 Age structures
DEMOGRAPHICS
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One tool in demographics is to
estimate the life span of organisms
To do this they often create a life
table, age summary of a population
 A cohort (group of individuals of
the same age) are studied to see
what percentage of the population
dies
 This can determine life expectancy,
survivorship at each age category,
or male vs. female survivorship
Survivorship Curves


A way to represent a life table is a
survivorship curve, a plot of the
proportion of individuals that
survive at each age group
When discussing survivorship
curves, there are 3 general
categories
Survivorship Curves
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Type I- live to old age
& die (most large
mammals)
Type II- constant
mortality rate (rodents,
lizards, hydra)
Type III- high mortality
at young age, but if
they survive they live a
long life (sea turtles).
DEMOGRAPHICS
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Another important aspect of demographics
is reproductive rates
 For obvious reasons, biologists who
study reproductive rates only focus on
females
 Age of fertility
 Number of offspring for each age
group
 Time of year
 Spawning cycles
Reproductive tables summarize this data
Life Histories
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Life history are traits that affect an
organism’s schedule of reproduction and
survival
Clutch size:
 Number of offspring produced at each
reproductive episode
Semelparity
 A life history in which an organism
spends most of its energy in growth and
development, expend their energy in one
large reproductive effort, and then die
 Many insects, annual plants, salmon, etc.
Life Histories

Iteroparity
 A life history pattern in which
organisms produce fewer offspring
at a time over a span of many
seasons
 Example: humans, panda bears,
etc.
Life Histories (Semelparity vs. Iteroparity)

Many factors contribute to the life history of
an organism
 Finite resources
 Reproduction vs. survival
 Number and size of offspring
 Paternal investment in offspring
POPULATION GROWTH MODELS
Different models of how populations
grow
Formulas off your Cheat Sheet
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Rate: dY/dt
 Generic expression for change of some
variable (Y) over time (t)
Population Growth: dN/dt = (B – D)
 The change in population size (N) over
time (t) is the same as the birth rate (B)
minus the death rate (D)
Exponential Growth: dN/dt =rmax •N
 The change in population size over time
is equal to the growth rate (rmax)
multiplied with the current population
size
Formulas off your Cheat Sheet

Logistic Growth:
 dN/dt =rmax •N • [(K-N)/K]
 The growth rate over time is the same as
exponential growth adjusted for carrying
capacity (K)
Types of Population Growth (start
here)
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One of the biological imperatives is
to reproduce and pass on genetic
material to succeeding generations.
Yet population growth is controlled
by the environment and limited
resources
This causes different patterns of
population growth
Patterns of Population Growth

Exponential Growth:
 Occurs in ideal
conditions with
unlimited resources
 J shaped curve
 Example:
 1 bacterium
(reproducing every
20 minutes) could
produce enough
bacteria to form a 1foot layer over the
entire surface of the
Earth in 36 hours
Exponential Growth

Exponential growth is a useful model
when studying populations that are
introduced into a new, unfilled,
environment
 Recovery after a catastrophe
 Exponential Growth:
 dN/dt =rmax •N
 rmax is the maximum rate of
population growth for the
species
Patterns of Population Growth
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
Exponential growth
cannot continue
indefinitely
It is characteristic of
populations who are
entering a new
environment OR
those whose
numbers are
rebounding from a
catastrophic events
Patterns of Population Growth

Logistic Growth:
 Pattern of population
growth which takes
into account the
effect of population
density on population
growth
 Occurs when
resources become
more scarce
 Characterized by an
S-shaped curve
Patterns of Population Growth

dN/dt =rmax •N • [(K-N)/K]

Carrying capacity (K):
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
The maximum number of individuals that a
particular environment can support over a
long period of time
Determined by such limiting factors as
crowding and food resources
Graph levels off at carrying capacity
K-selected populations (equilibrial
populations) live near or at the carrying
capacity
Carrying Capacity
K-strategists (Life history)
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Density stays near carrying
capacity.
Large, slow growing organisms
Small population sizes
Long life span; slow maturation
Few young/small clutch size
Reproduce late in life
Parental care
Most large mammals; endangered
species
r- strategists (Life history)
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Grow exponentially when environmental
conditions allow; when conditions
worsen, population size plummets.
Short life span
Reproduce early in life
Many offspring/large clutch size
Usually small in size
Little or no parental care
Bacteria, some plants, insects
Environmental Factors
Abiotic and biotic influences on
population size
Limiting Factors
 There
are a number of factors
that limit the size of
populations:
Density-dependent limiting
factors (depends on the size of
the population)
 Density-independent limiting
factors (does not matter the size
of the population)

Density-Dependent Limiting Factors

The effect of density-dependent
limiting factors intensifies as the
population increases
 Intraspecific competition
 Food, space, etc.
 Territoriality
 Predation
 Waste build up
 Disease (if caused by
pathogen/contagious)
Density-Independent Limiting Factors

The occurrence and severity of
density-independent limiting
factors are unrelated to population
size
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Climate
Disease (if not caused by pathogen/not
contagious)
Pollution
The Interaction of Limiting Factors
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Density-dependent and densityindependent limiting factors often
work together to regulate the size of
a population
 Deer in snowy winter
 Starve from lack of food
(density-dependent)
 Severity of winter/depth of snow
determines access to food
(density-independent)
Population Dynamics

Population dynamics is the study
of the environmental factors that
cause variations in the population
size


Looks at fluctuations in population over
time to examine stability
Immigration and emigration also affect
population

Metapopulations are when you have
several interconnected populations
Boom-and-bust cycle

Another phenomena that affects population are
predator-prey relationships.
 Each population is interdependent and
causes a boom-and-bust cycle
 The prey population increases which
causes the predator population to
increase
 The prey are over hunted and their
population crashes
 This causes the predator population to
crash
 Now, with fewer predators, the prey
population can again increase (recovery
gives a geometric increase)
Boom-and-Bust Cycles
Human Population Growth
Increase in Human Population
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Agricultural Revolution Major period of population
growth began when
humans started to
cultivate crops and
domesticate animals
Industrial Revolution –
Improved food production
and distribution
Health Care – germ theory
lead to improved hygiene,
better waste removal and
water treatment
Decrease in Human Population
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Plague – disease that
greatly reduces the size
of population (Black
Plague in 1300’s reduced
the population in
England by 50%)
Famine –a severe food
shortage causing
starvation and death
(Potato Famine of
1840’s/China 18701890)
War – death by combat,
disease, cut off from
food supply (Germany
1618-1648/WWI/WWII)
Human Population Growth
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The human population is unlike any
other organism
Since about 1650, we have
remained in an exponential
population increase
Population increases by about
201,000 people/day worldwide
Human Population Growth
Human Population Growth

Even though there is a tremendous
increase in human population, it is not
evenly distributed around the globe
 Regional areas have different
population trends
 Some regions have stable regional
human populations (birth rate is
the same as death rate)
 Other regions show incredible
growth rates
Industrialized Nations
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An emerging nation usually has a
very high birth rate, but also a high
death rate (disease, lack of modern
medical treatment, famine)
An industrialized nation usually has
a low death rate, but also a low
death rate

Moving from an emerging nation to an
industrialized nation is known as a
demographic transition
Human Population Growth
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In the 1950s, mortality rates began
to rapidly drop (advances in
medicine and sanitation)
Yet, the birth rates have not always
dropped

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Has caused a huge increase in
population in some nations
About 80% of the world’s population
lives in emerging nations
Age Structure
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
One way to determine human
population growth is to look at the
nations’ age structure, relative
number of individuals at each age
By looking at the age structure of a
population, you can determine the
population growth
Age-Structure Diagrams
Human Population Growth
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Implications of exponential human
population growth:
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Lack of food supplies
Lack of space
Lack of natural resources (metals, fossil fuels, etc)
Lack of sites for waste disposal
Ecologists cannot agree on a carrying
capacity for Earth (2 – 40 billion)
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
Are we going to reach carrying capacity
through individual choices and/or government
programs?
OR
Is Earth’s population going to “level off” as a
result of mass deaths?