Download Community Structure and Biodiversity

Community Structure
and Biodiversity
Chapter 40
Ecosystems

Ecosystems vary in size. They can be
as small as a puddle or as large as the
Universe itself. Any group of living and
nonliving things interacting with each
other can be considered as an
ecosystem.

Within each ecosystem,
 there
are habitats which may also vary in
size.
 A habitat
is the place where a population lives.
 A population is a group of living organisms of
the same kind living in the same place at the
same time.
 All
of the populations interact and form a
community.
The community of living things interacts
with the non-living world around it to
form the ecosystem. The habitat must
supply the needs of organisms, such as
food, water, temperature, oxygen, and
minerals. If the population's needs are
not met, it will move to a better habitat.
 Two different populations can not
occupy the same niche at the same
time, however.

Community

All the populations that live together in
a habitat

Habitat is the type of place where
individuals of a species typically live

Type of habitat shapes a community’s
structure
Factors Shaping
Community Structure

Climate and topography

Available foods and resources

Adaptations of species in community

Species interactions

Arrival and disappearance of species

Physical disturbances
Niche
Sum of activities and relationships in
which a species engages to secure and
use resources necessary for survival
and reproduction
Realized &
Fundamental Niches

Fundamental niche
 Theoretical
niche occupied in the absence
of any competing species

Realized niche
 Niche

a species actually occupies
Realized niche is some fraction of the
fundamental niche
Species Interactions

Most interactions are neutral; have no effect
on either species

Commensalism helps one species and has
no effect on the other

Mutualism helps both species
Species Interactions

Interspecific competition has a negative
effect on both species

Predation and parasitism both benefit
one species at a cost to another
Symbiosis

Living together for at least some part of
the life cycle

Commensalism, mutualism, and
parasitism are forms of symbiosis
Mutualism

Both species benefit

Many examples in nature

Some mutualisms are obligatory;
partners depend upon each other
Mycorrhizae

Obligatory mutualism between fungus
and plant root

Fungus supplies mineral ions to root

Root supplies sugars to fungus
Yucca and Yucca Moth

Example of an obligatory mutualism

Each species of yucca is pollinated only
by one species of moth

Moth larvae can grow only in that one
species of yucca
Competition

Interspecific - between species

Intraspecific - between members of the
same species

Intraspecific competition is most intense
Forms of Competition

Competitors may have equal access to
a resource; compete to exploit resource
more effectively

One competitor may be able to control
access to a resource, to exclude others
Competitive Exclusion Principle
When two species compete for identical
resources, one will be more successful
and will eventually eliminate the other
Competitive Exclusion Expt
Paramecium caudatum
Paramecium aurelia
Resource Partitioning

Apparent competitors
may actually have
slightly different niches

Species may use
resources in a different
way or time

Minimizes competition
and allows coexistence
Predation

Predators are animals that feed on other
living organisms

Predators are free-living; they do not
take up residence on their prey
Coevolution

Natural selection promotes traits that
help prey escape predation

It also promotes traits that make
predators more successful at capturing
prey
Predators and Prey Density
• I represents abundant prey; II represents a leveling
off as predators consume their fill; III shows prey
levels dropping off and predator response slowing
Variation in Cycles

An association in predator and prey
abundance does not always indicate a
cause and effect relationship

Variations in food supply and additional
predators may also influence changes
in prey abundance
Species Interactions
• Canadian lynx
(dashed lines)
versus
snowshoe hares
(solid line)
Prey Defenses

Camouflage

Warning coloration

Mimicry
Predator Responses

Any adaptation that protects prey may
select for predators that can overcome
that adaptation

Prey adaptations include stealth,
camouflage, and ways to avoid
chemical repellents
Parasitism

Parasites drain nutrients from their
hosts and live on or in their bodies

Natural selection favors parasites
that do not kill their host too quickly
Types of Parasites

Microparasites

Macroparasites

Social parasites

Parasitoids
Biological Controls
Parasites and parasitoids are
commercially raised and release in
target areas as biological controls
 An alternative to pesticides
 Must be carefully managed to not upset
natural balances

Study: Cowbirds

Brown-headed cowbirds evolved in the Great
Plains region of the U.S., living as
commensalists with bison
 As they migrated along with the bison, the
cowbirds learned to lay eggs in the nests of
other birds
 Cowbird hatchlings push the “host” bird eggs
out of the nest
 Today, cowbirds parasitize some 15 species
of birds
End for now!
The exam will cover material to this point!
80% on material since the last exam
20% will include material from the
beginning of the class - general principles
which you should know
Succession
Change in the composition of different
species over time
Pioneer Species

Species that colonize barren habitats - first come,
first served!

Lichens, small plants with brief life cycles

Improve conditions for other species who then
replace them

lose out to better adapted species

who themselves lost out to even better adapted species.
Types of Succession

Primary succession new environments

Secondary succession communities were destroyed or
displaced

Following hurricanes

After the tsunami
Climax Community
Eventually we arrive at the climax community

Stable array of species that persists relatively
unchanged over time

Succession does not always move
predictably toward a specific climax
community; other stable communities may
persist

Multiple outcomes.
Cyclic Changes
DYNAMIC and STABLE
 Cyclic, nondirectional changes also
shape community structure
 Tree
falls cause local patchiness in tropical
forests
 Fires
periodically destroy underbrush in
sequoia forests
Community Instability
REMEMBER

Disturbances can cause a
community to change in ways that
persist even if the change is
reversed
Keystone Species

A species that can dictate community
structure

Removal of a keystone species can
cause drastic changes in a community;
can increase or decrease diversity…
Lubchenco Experiment
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Periwinkles promote or limit diversity in different habitats
Tidepools
Rocks exposed at high tide
Species Introductions

Introduction of a nonindigenous species can
decimate a community

Just recently Hawaii has introduces Sea Stars to
control sea weed

WHY? There are;

No natural enemies or controls

Can outcompete native species
Exotic Species

Species that has left its home natural
range and become established elsewhere

Becomes part of its new community

Can have beneficial, neutral, or harmful
effects on a community…
Kudzu in Georgia
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Kudzu in Georgia
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
Imported for erosion control

No natural herbivores, pathogens, or
competitors

Grows over landscapes and cannot be
dug up or burned out
 May
turn out to have some commercial use
for humans
Caulerpa taxifolia
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Caulerpa taxifolia
Researchers developed a hybrid, sterile
strain of this green alga - for fish tanks.
 Somehow it was released into the wild,
where it reproduces asexually - down
the drain!!!
 Thrives in varying conditions and emits
a toxin that poisons invertebrates and
fishes
 Illegal to import the “aquarium” strain
into the U.S.

Rabbits in Australia
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
Rabbits were introduced for food and
hunting

Without predators, their numbers
soared

Attempts at control using fences or
viruses have thus far been unsuccessful
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Biodiversity

The sum of all species occupying a
specified area during a specified
interval, past and present
Diversity by Latitude

Diversity of most groups is greatest
in tropics; declines toward poles
 a)
ant species
 b) breeding birds
Why Are Tropical Species Rich?

Resources are plentiful and reliable

Species diversity is self-reinforcing

Rates of speciation are highest in the
tropics
Surtsey: Volcanic Island
Primary succession
Distance Effect

The farther an island is from a
mainland, the fewer species. WHY?

Closer islands receive more immigrants

Species that reach islands far from
mainland are adapted for long-distance
dispersal and can move on
Distance Effect
Endangered Species

A species that is extremely vulnerable to
extinction
 Habitat loss is putting major pressure on
more than 90 percent of the endemic species
that now face extinction
Conservation Biology
Is a systematic survey of the full range
of biological diversity
 Entails efforts to decipher biodiversity’s
evolutionary and ecological origins
 Attempt to identify methods of
maintaining and using biodiversity in
ways that can benefit the human
population

 Only
real way to save the local ecosystem
Sampling of Current Species Diversity
Areas at Risk
Preserving Biodiversity
Requires identifying and protecting
regions that support the highest levels
of biodiversity
 It is possible to protect a habitat and still
withdraw resources in a sustainable
fashion - but based on human nature it
is a risky practice!
