Download Chapter 6 Population and Community Ecology

Chapter 6
Population and Community Ecology
NATURE EXISTS AT SEVERAL LEVELS OF COMPLEXITY
FACTORS THAT REGULATE POPULATION
ABUNDANCE AND DISTRIBUTION
1.
2.
3.
4.
5.
Population size- the total number of
individuals within a defined area at a given
time.
Population density- the number of
individuals per unit area at a given time.
Population distribution- how individuals are
distributed with respect to one another.
Population sex ratio- the ratio of males to
females
Population age structure- how many
individuals fit into particular age categories.
1. POPULATION SIZE


Population size is the number of
individuals that a population
contains.
Determining the size can be tricky

Simply counting (if population is small

Estimating (if populations are too

& stationary)
abundant, too widespread or too mobile)
Mark-Capture-Recapture
(scientists use
traps to capture the animals alive and mark them in
some way. The animals are returned unharmed to
their environment. Over a long time period, the
animals from the population are continued to be
trapped and data is taken on how many are
captured with tags)
2. POPULATION DENSITY

Population density measures how
crowded a population is.
Usually expressed as:
 # individuals / unit of area
 30 people/km2

Macau (China) = 54,882 people/mi2

United States = 84 people/mi2 (182nd
in the world)

3. POPULATION DISPERSION

Population dispersion is the spatial distribution of individuals
within a population. Most populations live as clumped
individuals. Clumped helps protect against predators, provides
individuals to mate with and helps with gathering resources.
Random
Uniform
Clumped
4. POPULATION SEX RATIO
5. POPULATION AGE STRUCTURE




Sex ratio = the ratio of males to females
In monogamous species, 50:50 will increase population size.
Age Structure = the relative numbers of individuals of each age
within a population.
The more individuals in childbearing age, will increase population
size.
POPULATION CONTROL
 Density-Dependent
Population Control- the size
of the population will influence
an individual’s probability of
survival. Large populations are
at greater risk!
 Limiting Resource


Carrying Capacity (K)


A resource (water, food, etc) that a
community cannot live without and
those quantities are lower than what
is needed to sustain a growing
population.
How many individuals an
environment can sustain.
Disease
POPULATION CONTROL
 Density-Independent
Population Control- This is
•
•
•
•
•
•
when a population is controlled
by natural events other than
population density.
fires
floods
earthquakes
hurricanes
volcanoes
drought
GRAPHING POPULATION FACTORS

Density-Dependent
Factors - mortality
increases as population
density increases.
(food shortage)


Inverse-Dependent
Factors - mortality
decreases as a
population increases in
size. (mating)
Density-Independent
Factors - mortality
remains unchanged.
(natural disaster)
Last slide
GROWTH MODELS & POPULATION
CHANGES
Exponential & Logistic Growth Models
K-selected species, r-selected species
Survivorship Curves & Metapopulations
POPULATION SIZES CAN GROW
Growth rate- the number of
offspring an individual can
produce in a given time
period, minus the deaths of
the individual or offspring
during the same period.
Growth Rate = Birth - Deaths


Intrinsic growth rate (r)- is
the rate at which a
population would grow if it
had unlimited resources
POPULATION GROWTH MODELS
EXPONENTIAL GROWTH CURVE: J-SHAPED CURVE
Exponential
Growth Model
 J-shaped curve


When populations
are not limited by
resources, growth
can be very rapid.
POPULATION GROWTH MODELS
LOGISTIC GROWTH CURVE: S-SHAPED CURVE
Logistic Growth
Model
 S- shaped Curve

But as resources
become limited,
its growth rate
slows and levels
off.
 Environmental
resistance: water,
space, food,
predators or
disease

P
OPULATION
G
ROWTH
M
ODELS
VARIATIONS OF THE LOGISTIC MODEL
LOGISTIC GROWTH CURVE: S-SHAPED CURVE
 If
resources become scarce,
the population will
experience an overshoot
by becoming larger than
the carrying capacity and
will result in a die-off, or
population crash.
DIFFERENT REPRODUCTIVE STRATEGIES
CAN AFFECT POPULATION SIZES
species- the
population of a species that grows
slowly until it reaches the carrying
capacity.
 K-selected

Ex. elephants, whales, and
humans.
species- the population
of a species that grows quickly
and is often followed by
overshoots and die-offs.
 r-selected

Ex. mosquitoes and dandelions
DIFFERENT LIFESPANS
CAN AFFECT POPULATION SIZES
 Survivorship
Curves show
that
populations of
different
species vary
in how long
individual
members
typically live.
SURVIVORSHIP CURVES
 Type
I
long life span
 K-selected Species

 Type

II
medium life span
 Type
III
short life span
 r-selected Species

GEOGRAPHICAL MOVEMENT
CAN AFFECT POPULATION SIZES

Metapopulations- a group of spatially distinct
populations that are connected by occasional
movements of individuals between them.
SPECIES INTERACTIONS
Competition, predation, mutualism, commensalism
Keystone species
THERE ARE 4 MAJOR TYPES OF SPECIES INTERACTION
1.
2.
3.
4.
Competition
Predator-Prey
Mutualism
Commensalism
Symbiosis
COMPETITION
occurs when
two or more organisms
attempt to use the same
limited resource.
 Competition


A limited resource is any
resource that may run out.
Examples:
Interspecific: Different Species Hyenas fight with lions over the
same animals
 Intraspecific: Same Species Two cacti that are side by side
compete for water

COMPETITION – GAUSE’S LAW
The competitive exclusion
principle, sometimes
referred to as Gause’s Law of
Competitive Exclusion or
just Gause's law, is a
proposition that states that
two species competing for
the same resource cannot
coexist at constant
population values.
 One species must go.

RESOURCE PARTITIONING IN COMPETITION
PREDATION (PREDATOR – PREY)
Predation- the use of one
species as a resource by
another species.
 True predators- kill their
prey.
 Herbivores- consume
plants as prey.
 Parasites- live on or in
the organism they
consume.
 Parasitoids- lay eggs
inside other organisms.

With Predation,
Populations rise & fall
with each other.
SYMBIOSIS
is the close
association between two
or more organisms of
different species living
together
2 types:
 Mutualism
 Commensalism
Symbiosis
MUTUALISM
 Mutualism
is a cooperative partnership between
two species in which both species benefit.

An example is the bacteria in your intestines and you.
Billions of bacteria live in your intestines.
 They help break down food you would otherwise not be able to
digest.
 They also produce beneficial substances, such as Vitamin K for
you.
 In return, you give them a warm, dark, food-rich environment.

COMMENSALISM
is a relationship in which
one species benefits from another species
and the other is neither harmed nor helped
 This is the rarest and strangest type of
species interaction
 Commensalism

Example: Remoras and Sharks
 The remoras attach to the sharks and feed on scraps left
over from the sharks meals.
 The shark is neither harmed nor helped by this
relationship
Relationship
Commensalism
Mutualism
Parasitism
Predation
Competition
Organism #1
Organism #2
+
+
+
+
-
0
+
-
KEYSTONE SPECIES HAVE LARGE EFFECTS ON
COMMUNITIES

Keystone species- a species that plays a role in its
community that is far more important than its relative
abundance might suggest.

Example: Sea Otters in California Kelp Beds
Beavers build dams which
provides many species
with a home. Dams are
also vital in restoring
wetlands.
Gray wolf is an
effective predator in
Yellowstone National
Park.
Bees help to
pollinate flowers
providing food for
the ecosystem.
Pine trees are keystone
species in the Sierra
Nevada Mountains. They
regulate snowmelt runoff
and soil erosion.
Sharks prey on the
sick and weak
species in their
ecosystem.
COMMUNITIES CHANGE OVER
TIME
Ecological Disturbances
Primary succession
Secondary succession
Aquatic succession
ECOLOGICAL DISTURBANCE

A disturbance is an event that affects environmental
conditions rapidly and drastically, resulting in changes to
the community and ecosystem.
o A disturbance can be as
localized as a tree falling in a
forest, creating a gap in the
canopy that lets in light and
alters conditions for plants and
animals in the gap.
o A disturbance can be as large
and severe as a hurricane,
tornado or volcanic eruption.
ECOLOGICAL DISTURBANCE

A disturbance is an event that affects environmental
conditions rapidly and drastically, resulting in changes to
the community and ecosystem.
o Some disturbances are sudden,
such as landslides.
o Some disturbances are gradual,
such as climate change.
o Some disturbances regularly
reoccur and are considered
normal aspects of a system.
• Periodic fire
• Seasonal storms
• Cyclic insect outbreaks
ECOLOGICAL DISTURBANCE

Communities are dynamic systems and may respond to
disturbance in several ways.
Resistance – a
community that resists
change and remains
stable despite
disturbances is
resistance.
2) Resilience – a
community that changes
in response to
disturbance but later
returns to is original
state.
1)
ECOLOGICAL DISTURBANCE

Communities are dynamic systems and may respond to
disturbance in several ways.
Once a community is disturbed
and changes are set in motion,
there is no guarantee that the
community will ever return to
its original state.
3)Permanent change – the
characteristics of the
community fundamentally
change. This can occur when…
• If some crucial climatic threshold is
passed
• A keystone species is lost
• A invasive species enters
ECOLOGICAL SUCCESSION
What happens to an ecosystem after a disturbance?
The gradual, sequential growth of a community is called
ecological succession
This can occur in areas where no life has been before
(primary succession)
This can also occur when there is a disaster that
completely wipes out a community (secondary succession)
ECOLOGICAL SUCCESSION

In both primary and secondary
succession there is a specific
sequence of growth
 The first species to grow in
succession is called the
pioneer species
 Pioneer species are typically
small, grow quickly, good at
growing under harsh
conditions, and good at
dispersing offspring
PRIMARY SUCCESSION
 No
true soil is
present before
Primary Succession
 Because of this it is
slower than
secondary

Occurs on
 Newly exposed land
under melting ice
caps
 Newly created land
formed by volcanoes
PRIMARY SUCCESSION
are usually the
pioneer species
 They break down the rocks
into dirt and when they die,
they leave behind organic
material, which will be broken
down by decomposers
 This creates soil which is
basically dirt, minerals, and
organic material
 This process takes
thousands of years
 Lichens
PRIMARY SUCCESSION

Primary succession- occurs on surfaces that are initially
devoid of soil.
Lichens are the first to appear
SECONDARY SUCCESSION
 This
happens after an existing community is
disrupted by a disturbance like a fire
 Soil is already present
 Grasses and weeds tend to be the pioneer
species
 After many years bigger plants will begin to
grow
 This process takes about 100 years
SECONDARY SUCCESSION

Secondary succession- occurs in areas that have been
disturbed but have not lost their soil.
AQUATIC SUCCESSION
Aquatic
Succession turns
a freshwater lake
into a terrestrial
habitat.
CLIMAX COMMUNITY
When does succession end?
 When it reaches a climax
community



This is simply a stable end
point
At this point, the
community remains
relatively stable assuming
there are no more
disturbances
Some scientists don’t support
the concept of a climax
community because
environmental disturbances
are always occurring.
RESTORATION ECOLOGY
We can help!
Restoration ecology
emerged as a separate field in
ecology in the 1980s. It is the
scientific study supporting
the practice of ecological
restoration, which is the
practice of renewing and
restoring degraded,
damaged, or destroyed
ecosystems and habitats in
the environment by active
human intervention and
action.
COMMUNITY SUSTAINABILITY
Species Richness
Theory of Island Biography
COMMUNITY SUSTAINABILITY
Species-Rich Communities tend to be
more productive and sustainable.


The more diverse an ecosystem is, the
more productive it will be. That is, with
a greater variety of producer species, that
community will produce more plant
biomass, which in turn will support a
greater variety of consumer species.
The more diverse an ecosystem is, the
more sustainable it will be. That is,
with a greater variety the greater ability
to withstand environmental disturbances
such as drought or insect infestations.
FACTORS THAT DETERMINE SPECIES RICHNESS

Latitudinal Gradient



Time



As we move away from the
equator, species richness declines
Why?... milder climate at the
equator.
The older the communities have
more species richness
Why?… more time for speciation.
Habitat size


Larger habitats have more species
Why?… wider environmental
conditions and resources.
THEORY OF ISLAND BIOGEOGRAPHY

Theory of Island Biogeography- the theory that
explains that both habitat size and distance determine
species richness.