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Chapter 53
Community Ecology
A Community
A community is all of the species within
a given area that have the ability to
interact with one another and their
environment.
Community structure is chiefly governed
by the interactions of the organisms and
their environments.
Interspecific Interactions
Interspecific interactions are the
relationships in the life cycles of the
organisms and their interactions with
other species in the community.
Possible Linking Interactions:
1.
2.
3.
4.
Competition
Predation
Herbivory
Symbiosis
• Parasitism,
• Mutualism,
• Commensalism
Interspecific Competition
Occurs when species compete for a
particular resource that is limited in
some way.
When both organisms compete for it, it
may be detrimental to one or both
organisms and may lead to competitive
exclusion.
Competitive Exclusion
Occurs when one organism has a
means to use a resource better than
another.
Thus, it is better able to reproduce and
ultimately leads to the elimination of the
the other organism.
G.F. Gause
Arrived at the Principle of Competitive
Exclusion while studying 2 species of
paramecium.
Each would grow well on their own-reaching a carrying capacity.
When grown together, one would drive
the other to extinction.
Niche
This is a species role in the
environment--where and how it fits into
an ecosystem.
A species ecological niche is the sum
total of all biotic and abiotic resources
available to an organism within an
environment.
Niche
In terms of the Competitive Exclusion
Principle, two species cannot coexist in
an ecosystem if their niches are
identical.
Niche
Similar species can coexist if they are in
a community where there are one or
more significant differences in their
niches
Niche
As a result of competition, a species
may occupy a realized niche rather than
a fundamental niche.
– Fundamental niche is the entire geographic
range suitable to a particular organism.
– Realized niche is the part of the
fundamental niche actually occupied.
QuickTime™ and a
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Resource Partitioning
As a direct result of competition, 2
organisms may evolve the capacity to
use a different set of resources.
This enables 2 competing species to
coexist.
Character Displacement
A comparison of 2 closely related
species whose populations overlap.
They may be allopatric or sympatric
species.
Character Displacement
In some cases, allopatric populations
have similar morphology and use similar
resources.
Character Displacement
In contrast, sympatric populations
compete for resources and show
differences in body structure and the
resources they use.
Character Displacement
Thus, character
displacement is the
tendency for
characteristics to be
more divergent in
sympatric populations
and convergent in
allopatric populations
as a result of
competition.
Predation
Predators kill things.
They have acute senses and many
adaptations.
– Claws, fangs, teeth, etc.
They have to have these adaptations
because they are chasing prey that are
often fast and agile, or bigger and
stronger.
Prey
They have evolved many adaptations to
avoid being caught.
Hiding, fleeing, self-defense, alarm
calls.
They have morphological and
physiological adaptations.
Cryptic coloration, mechanical and
chemical defenses.
Aposematic Coloration
Many times animals
with effective chemical
defenses have bright
warning coloration-aposematic coloration.
It is likely adaptive.
Evidence supports the
adaptive idea.
Predators avoid prey
with bright coloration.
Mimicry
This occurs when one species mimics
another for some benefit.
There are two types:
1. Batesian
2. Müllerian
1. Batesian Mimicry
This is where a nonpoisonous species
tricks (baits) a
potential predator
into thinking that it is
poisonous.
They mimic the
appearance of a
poisonous species.
2. Müllerian Mimicry
Two or more poisonous species
resemble one another.
When the prey mimic one another, it is
beneficial to both species because
predators will quickly learn to avoid
certain coloration patterns.
Convergent Evolution
Müllerian mimicry is a good example of
convergent evolution because many
different species have similar patterns
of coloration.
Example: bees
Predation
Predation can take on many different forms.
Herbivory--eating of plants.
Parasitism--deriving nutrients from a host with
no benefit to the host.
– Endoparasites, ectoparasites, parasitoidism
Mutualism--symbiotic type of relationship.
Commensalism--two species interact, one
benefits and the other is neither harmed not
benefits.
Predation
The interspecific interactions of the
species result in selective forces such
as those seen in coevolution and
convergent evolution.
Interspecific Interactions
Interspecific interactions and
adaptations that result in coevolution
must result in a genetic change
between two interacting species.
One species changes which results in a
change in another species, which
results in a change in the first species,
etc.
Convergent Evolution
In contrast, when more than two
species are involved, convergent
evolution occurs.
We see this with aposematic coloration.
Changes occur in multiple species as a
result of a selective force of a predator.
Community Structure
Community structure is governed by a
few species.
They control composition, relative
abundance and diversity among
species.
2 Fundamental Features of
Community Structure
1. Species diversity
2. Feeding Relationships
1. Species Diversity
The variety of different kinds of
organisms that comprise a community.
There are 2 components:
– A. Species richness
– B. Relative abundance
A. Species Richness
The number of different types of species in
a community.
Correlates to rates of evapotranspiration-the measure of evaporation of water from
soil plus the transpiration of water from
plants.
B. Relative Abundance
The proportion of the total each species
represents.
Consider 2 Communities:
Community #1: 25A,
25B, 25C, 25D
Community #2: 80A,
5B, 5C, 10D
Each community has
4 species: richness
is the same.
Relative abundance
is different.
2. Feeding Relationships
The structure and dynamics of a
community depend on the feeding
relationships between organisms for the
most part.
This makes up the trophic structure of
the community.
Food Webs
They are very
complex and
many species
weave in and out
at different levels.
They are linked to
food chains.
Food Chains
They are relatively short.
1. The energetic hypothesis:
– The length is limited by the inefficiency of
energy transfer.
2. The dynamic stability hypothesis:
– Long food chains are less stable than short
ones.
1. The Energetic Hypothesis
Most data supports this. Only about
10% of the energy stored in each
trophic level is converted into organic
matter of the next level.
2. The Dynamic Stability
Hypothesis
Wild fluctuations in smaller populations
are magnified at higher levels.
In variable environments, top predators
can have a difficult time adjusting with
shocks to the food chain.
Species Impact
Certain species have a large impact on the
structure of a community.
– They are highly abundant.
– They play a key role in community dynamics.
They can be classified as:
– Dominant species
– Keystone species
– Foundation Species
Dominant Species
Most abundant--greatest biomass.
Control the distribution of other species.
There is no single explanation for why a
species becomes dominant.
– They outcompete other species for
resources.
– They are successful at avoiding predation.
Keystone Species
Not the most abundant species.
Do exert a strong control--stems from
niche.
Sea-Star--Mussel Example:
The mussel Mytilus californianus is a
dominant species in the rocky intertidal
community of western N. America.
They compete for space.
The sea star Pisaster ocharaceous
preys on the mussel removing it and
allows for other animals to move in.
Sea-Star--Mussel Example:
When the sea star is experimentally
removed, the mussels dominate the
area and diversity declines.
QuickTime™ and a
TIFF (U ncompressed) decompressor
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Sea-Star--Mussel Example:
Thus, the sea star acts as a keystone
species and exerts an influence over
the entire community.
Models Describing Trophic
Levels
Useful for describing biological
communities.
– Bottom-Up model
– Top-Down model
– Numerous intermediate models.
– Nonequilibrium model
Bottom-Up Model
Hypothesis that there is a unidirectional
influence from lower to higher trophic
levels.
Vegetation→Herbivore linkage.
Top-Down Model
The hypothesis is that predators control
organization because they reduce the
herbivore population.
Nutrients←Vegetation←Herbivore←Predator
Intermediate Models
Many models between bottom-up and
top-down are proposed.
The direction of flow in these models is
also hypothesized to fluctuate from
bottom-up and top-down over time.
Nonequilibrium Model
Originally, scientists used to think that
communities were stable.
Now, it is obvious that communities
change much more than they are
stable.
This gave rise to the nonequilibrium
model.
Nonequilibrium Model
Communities are in a constant state of
change as a result of this continued
disturbances.
Disturbances: things that change a
community by altering its resources
and/or organisms.
– Example: fires, floods, droughts
Intermediate Disturbance
Hypothesis
Suggests that moderate levels of
disturbance can create conditions that
foster species diversity.
It is supported by a broad range of
studies from terrestrial and aquatic
communities.
Ecological Succession
The process by which a disturbed area
gets colonized by a variety of species.
These are gradually replaced by still
other species.
Primary Succession
Occurs when the process begins in a
“lifeless” area where soil has not yet
formed.
– Example: moraine, volcanic island.
• Prokaryotes are initially present
• Mosses and lichens are the 1st large enough to
see.
Primary Succession
As time passes, soil forms from
weathering and the chemical
breakdown and plants eventually
become the main form of vegetation.
Secondary Succession
Occurs when existing communities
become cleared by some disturbance
and get repopulated with plants over
time.
Succession
Mount St. Helens
Secondary Succession
There are three processes that link
early and late arriving species:
1. Early arrivals make the environment
more hospitable.
– They facilitate the appearance of later
species by making the environment
hospitable.
Secondary Succession
2. Early arrivals may inhibit the arrival
of later species.
– Colonization by later plants occurs in spite
of the plants rather than because of them.
Secondary Succession
3. Early and late arrivals are
independent of one another.
– Early arrivals tolerate later species but
neither help nor hinder them.
Biodiversity
Is controlled by biogeographical
features.
The location and size of the island are
correlated to species biodiversity.
As Darwin and Wallace pointed out, life
is more varied and abundant than in
other parts of the world.
Equatorial-Polar Gradients
There are two key factors observed in
equatorial-polar gradients:
– Evolutionary history and climate.
Tropical regions are “older” than polar
regions because their growing season is
longer.
Equatorial regions have tended to avoid
major disturbances such as glaciation
compared to temperate regions.
The Island Equilibrium Model
Island biogeography provides a great
way to study species.
The Island Equilibrium Model helps us
study this.
Islands--terrestrial islands and islands in
the water.
The Island Equilibrium Model
Consider a newly formed
island:
Species come from a
mainland.
2 factors determine the
number of species on the
island:
The rate of immigration
and the rate of extinction.
The Island Equilibrium Model
2 physical features of the island affect
immigration and extinction rates:
1. Size.
2. Distance from mainland.
The Island Equilibrium Model
1. Size
Small islands
generally have low
immigration rates.
The Island Equilibrium Model
2. Distance from the
mainland:
With 2 islands of the
same size, the one
closer to the
mainland will have a
higher immigration
rate and a lower
extinction rate.
The Island Equilibrium Model
It is called the island equilibrium model
because eventually extinction rates will
equal the immigration rates.
It is somewhat of an oversimplification.
It can only be applied over short time
periods and on small islands.
Large islands are subject to a number of
changes.