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Populations
Wednesday, August 24th, 2016
Populations
= Individuals of a species that live in
the same place at the same time
 Are dynamic – constantly changing
 Evolution occurs at this level

Population Dynamics
 Population
dynamics refers to changes
in a population over time
 Population dynamics includes four
variables:
 Age Distribution
 Size
 Density
 Dispersion
1. Age structure
 The
age structure of a population is
usually shown graphically
 The population is usually divided up into
prereproductives, reproductives and
postreproductives
 The age structure of a population
dictates whether is will grow, shrink, or
stay the same size
Age structure diagrams

Pyramid
= + growth

Vertical edges
= 0 growth

Inverted
Pyramid
= - growth
2. Population growth

Populations show two types of growth
Exponential
Logistic
Exponential growth
As early as Darwin,
scientists have realized that
populations have the ability
to grow exponentially
 All populations have this
ability, although not all
populations realized this
type of growth

 J-shaped
curve
 The
graph at right
shows what
exponential growth
looks like
 Exponential
growth is growth that
is independent of
population density
Doubling time/ Rule of 70
 Doubling
time is the amount of
time that is takes for a population
to double in size when growing
exponentially
 It
is calculated as
Doubling Time = 70 / percent increase
 Ex.
A rabbit population has an r value of 1.5,
so the percent increase = 150%
D.T. = 70/150 = .46, or 5.5 months
Do all species enjoy exponential growth?
NO!
 The
exponential growth of most
populations ends at some point
 Two general outcomes can be
observed….
 1.
Populations
increase so rapidly
that they over shoot
the pop size that
the environment can
support, and the
pop size crashes
 ex.
reindeer
 2.
Populations
increase to some
level, and then
maintain that
stable level
ex.
sheep
What limits population growth?
 Density-independent
factors:
 affect
populations randomly (without respect to
density)
 ex.
Hurricanes, tornadoes, fire, drought, floods
 poor
regulators of populations
 Density-dependent
 affect
 ex.
factors:
populations when densities are high
Disease, competition, predation, parasitism
 good
regulators of populations
Population Regulation/Logistic Growth
Most populations grow exponentially until
the effect of density- dependent factors
increases and limits population growth
 S-shaped growth curve (logistic growth)

1.
The population experiences
exponential growth.
2. Population size (and density)
increases, the growth rate decreases.
3. The population approaches the
carrying capacity, K, the number of
individuals that the environment can
support
3. Population Density
 Population
density (or ecological population density)
is the amount of individuals in a population per unit
habitat area
 Some
species exist in high densities
 Some
species exist in low densities ex. Mountain lions
 Density
depends upon
 social/population
 mating
 time
structure
relationships
of year
ex. mice
4. Population Dispersion
 Population
dispersion is the spatial pattern
of distribution
 There are three main classifications
clumped:
ex.
individuals are lumped into groups
Flocking birds or herbivore herds
due
to resources that are clumped or social
interactions
most
common
 Uniform:
Individuals are regularly spaced in the
environment
ex.
Creosote bush
due to antagonism between individuals, or do to regular
spacing of resources
rare because resources are rarely evenly spaced
 Random:
Individuals are randomly dispersed in the
environment
ex.
Dandelions
due to random distribution of resources in the
environment, and neither positive nor negative
interaction between individuals
rare because these conditions are rarely met
Species Distribution
Related
to THREE things:
Fundamental
Niche (abiotic conditions
that it can tolerate)
Ability
to disperse to a new area
Interactions with other species
(symbiosis)
Interactions Among Organisms
 Symbiosis
An
intimate relationship between members of
2 or more species
Participants
may be benefited, harmed or
unaffected by the relationship
Results
Types:
of co-evolution
Competition, Predation, Mutualism,
Commensalism
Competition
 Interaction
among organisms that vie for
the same resource in an ecosystem
Intraspecific
Competition
between individuals in a population
Interspecific
Competition
species
between individuals in 2 different
Limiting Resources
 Any
environmental resource that, because it is
scarce or at unfavorable levels, restricts the
ecological niche of an organism
Predation
 Predation-
the use of one species as a resource
by another species.
 True
predators: the consumption of one species by
another
 Prey
is killed by predator
 Herbivores
 Parasitism:
relationship in which one species is
benefited and the other is harmed
 Parasites
rarely kill their hosts
Mutualism
A
relationship in which both organisms benefit
 Example:
Honey guide birds alert and direct badgers
to bee hives. The badgers then expose the hives and
feed on the honey first. Next the honey birds eat.
Both species benefit.
 Example:
Oxpeckers feed on the ticks found on a
rhinocerous. The oxpeckers get a meal and the
rhinocerous is helped by the removal of ticks.
Commensalism
A
relationship in which one organism benefits
and the other is not harmed nor helped.
 Example:
Barnacles attach to whales. Barnacles
get a free ride, but it doesn’t affect the whales.
 Example:
A sparrow will build its nest under the
next of the osprey. The smaller bird gets
protection. The osprey is neither helped nor
harmed.
Interactions between species
and their effects
Type of
Interaction
Competition
Predation
Mutualism
Commensalism
Species 1 Species 2
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