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What is a community?
• A biological community is an
assemblage of populations of various
species living close enough for
potential interaction
What are the types of interactions?
• relationships between species in a
community interspecific interactions
• Examples are competition, predation,
herbivory, and symbiosis (parasitism,
mutualism, and commensalism)
• Interspecific interactions can affect the
survival and reproduction of each
species, and the effects can be
summarized as positive (+), negative (–),
or no effect (0)
Competition
• Interspecific competition (–/– interaction)
occurs when species compete for a resource
in short supply
• Strong competition can lead to competitive
exclusion, local elimination of a competing
species
• The Gause competitive exclusion principle
states that two species competing for the
same limiting resources cannot coexist in
the same place
Ecological Niches
• The total of a species’ use of biotic and
abiotic resources is called the species’
ecological niche
• An ecological niche can also be thought
of as an organism’s ecological role
• It is the functional position of an
organism in its environment, comprising
its habitat and the resources it obtains,
periods of time it is active, etc.
Physical conditions
Substrate
Humidity
Sunlight
Temperature
Salinity
pH
Exposure
Attitude
depth
Other
organisms
Adaptations for
Locomotion
Biorhythms
Tolerance
Predator avoidance
Reproduction
feeding
Resources offered by the habitat
Food
Shelter
Mating sites
Nesting sites
Predator avoidance
• Ecologically similar species can coexist in
a community if there are one or more
significant differences in their niches
• Resource partitioning is differentiation of
ecological niches, enabling similar
species to coexist in a community
Fig. 54-2
A. distichus perches on fence
posts and other sunny surfaces.
A. insolitus usually perches
on shady branches.
A. ricordii
A. insolitus
A. aliniger
A. distichus
A. christophei
A. cybotes
A. etheridgei
• The full range of environmental conditions
under which an organism can exist is its
fundamental niche.
• Due to interactions and evironmental
pressures, organisms are usually forced to
occupy a niche that is narrower than
this…their realized niche.
Fig. 54-3
EXPERIMENT
Chthamalus
Balanus
High tide
Chthamalus
realized niche
Balanus
realized niche
Ocean
Low tide
RESULTS
High tide
Chthamalus
fundamental niche
Ocean
Low tide
• Question: Two species of Anolis lizards are often found perched and
feeding in the same trees, with species I in the upper and outer
branches, and species II occupying the shady inner branches. After
removing one or the other species in test trees, an ecologist observes
the following results: Species I is found throughout the branches of
trees in which it is now the sole occupant. Species II is still found only
in the shady interior when it is the sole occupant. What do these
results indicate about the niches of these two species?
The realized niche of Species I
is smaller than its fundamental
Species I niche when it is in competition
with SpeciesII.
Species II
Species II’s fundamental and
Realized niche are the same.
Predation
• Predation (+/– interaction) refers to
interaction where one species, the
predator, kills and eats the other, the prey
• Some feeding adaptations of predators
are claws, teeth, fangs, stingers, and
poison
• Prey display various defensive
adaptations - hiding, fleeing, forming
herds or schools, self-defense, coloration
patterns, mimicry, and alarm calls
Coloration Patterns and
Mimicry
Herbivory
• Herbivory (+/– interaction) refers to an
interaction in which an herbivore eats
parts of a plant or alga
• It has led to evolution of plant
mechanical and chemical defenses and
adaptations by herbivores
Fig. 54-6
A manatee is feeding on water hyacinth, an introduced species,
in Florida.
Symbiosis
• Symbiosis is a relationship where two or more
species live in direct and intimate contact with
one another
• parasitism (+/– interaction)
• mutualism (+/+ interaction), is an interspecific
interaction that benefits both species
A mutualism can be
– Obligate, where one species cannot survive
without the other
– Facultative, where both species can survive
alone
• commensalism (+/0 interaction)
Fig. 54-7
The tree and the
ant are locked
into relationship
where the
survival of both
partners
depends on the
other. The ants
provide the
Acacia with
protection from
herbivores and
from competing
plants, while the
tree provides the
ants with food
and shelter.
Facultative
mutualism
(a) Acacia tree and ants (genus Pseudomyrmex)
(b) Area cleared by ants at the base of an acacia tree
Clownfish and Sea Anemones
Facultative
Mutualism
Fig. 54-8
Facultative
Mutualism
Parasitism
Commensalism – epiphytes
protists in termite guts
Obligate
Mutualism
• In general, a few species in a community
exert strong control on that community’s
structure
• Two fundamental features of community
structure are species diversity and
feeding relationships
Species Diversity
• Species diversity of a community is the
variety of organisms that make up the
community
• It has two components: species richness
and relative abundance
• Species richness is the total number of
different species in the community
• Relative abundance is the proportion
each species represents of the total
individuals in the community
Fig. 54-9
A
B C D
Community 1
A: 25% B: 25% C: 25% D: 25%
Community 2
A: 80% B: 5% C: 5% D: 10%
Two communities can have the same species richness but
a different relative abundance
Trophic Structure
• Trophic structure is the feeding
relationships between organisms in a
community
• It is a key factor in community dynamics
• Food chains link trophic levels from
producers to top carnivores
Fig. 54-11
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Herbivore
Zooplankton
Primary
producers
Plant
Phytoplankton
A terrestrial food chain
A marine food chain
Fig. 54-12
Humans
A food web
is a
branching
food chain
with
complex
trophic
interactions
Smaller
toothed
whales
Baleen
whales
Crab-eater
seals
Birds
Leopard
seals
Fishes
Sperm
whales
Elephant
seals
Squids
Carnivorous
plankton
Euphausids
(krill)
Copepods
Phytoplankton
Limits on Food Chain Length
• Two hypotheses attempt to explain food chain
length:
• The energetic hypothesis suggests that
length is limited by inefficient energy
transfer
• The dynamic stability hypothesis proposes
that long food chains are less stable than short
ones
• Most data support the energetic hypothesis
Experimental data from the tree hole
communities showed that food chains were
longest when food supply (leaf litter) was
greatest. Which hypothesis about what ali
its food chain length do these results
suggest?
energetic
Number of trophic links
Fig. 54-14
5
4
3
2
1
0
High (control):
natural rate of
litter fall
Medium: 1/10
natural rate
Productivity
Low: 1/100
natural rate
Species with a Large Impact
• Certain species have a very large impact
on community structure
• Such species are highly abundant or play
a pivotal role in community dynamics
• Dominant species are those that are most
abundant or have the highest biomass
(the total mass of all individuals in a
population)
Why are they dominant?
• One hypothesis suggests that
dominant species are most
competitive in exploiting resources
• Another hypothesis is that they are
most successful at avoiding
predators
Invasive Species
• Species typically introduced to a new
environment by humans, often lack
predators or disease
Kudzu
• Kudzu is a vine which was brought to North America from
Asia in 1876 to help prevent soil erosion, which has since
become an utter nuisance in some areas of the country. It can
grow up to 6.5 feet a week and its roots are nearly impossible
to eradicate entirely.
Other examples
• Dutch Elm Disease – caused by a fungus
and accidentally spread into the United
States.
• Potato Blight – caused by a fungus that
caused the Great Potato Famine in
Ireland in the 1840’s. Spores have been
carried all over the world.
• Small Pox – spread of virus from Asia to
all over the world.
Dutch Elm Disease
• Dutch elm disease (DED) is
caused by a member of the sac
fungi (Ascomycota) affecting
elm trees, and is spread by the
elm bark beetle. Although
believed to be originally native
to Asia, the disease has been
accidentally introduced into
America and Europe, where it
has devastated native
populations of elms which had
not had the opportunity to
evolve resistance to the
disease. The name "Dutch elm
disease" refers to its
identification in 1921 in the
Netherlands by Dutch
phytopathologists.
Potato Blight caused by a fungus.
Smallpox caused by a virus.
Keystone Species
• Keystone species exert strong control on
a community by their ecological roles, or
niches
• In contrast to dominant species, they are
not necessarily abundant in a community
Fig. 54-15
EXPERIMENT
Field studies
of sea stars
exhibit their
role as a
keystone
species in
intertidal
communities
Number of species
present
RESULTS
20
15
With Pisaster (control)
10
5
Without Pisaster (experimental)
0
1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73
Year
They keep the
number of
mussels
controlled
that
outcompete
other species.
Fig. 54-16
80
60
40
20
0
(a) Sea otter abundance
Keystone species
Grams per
0.25 m2
400
After orcas
entered the
food chain and
preyed on the
otters, notice the
change in the
sea urchins and
kelp.
300
200
100
0
(b) Sea urchin biomass
Number per
0.25 m2
Observation
of sea otter
populations
and their
predation
shows how
otters affect
ocean
communities
Otter number
(% max. count)
100
10
8
6
4
2
0
1972
1985
(c) Total kelp density
1989
Year
1993 1997
Food chain
This resulted in a loss of kelp
forests.
Ecological Succession
• Ecological succession is the sequence of
community and ecosystem changes after
a disturbance
• Primary succession occurs where no soil
exists when succession begins
• Secondary succession begins in an area
where soil remains after a disturbance
Successive species can
• Inhibit growth of new organisms
sphagnum moss making boggy areas
in poorly drained sites
• Promote growth of new organisms
Dryas and Alder trees raising N
content
• Tolerate conditions that resulted from
former species
Fig. 54-22-4
Succession on the moraines in Glacier Bay, Alaska, follows a
predictable pattern of change in vegetation and soil characteristics
1941
1907
2
1
Pioneer stage, with
fireweed dominant
0
1860
Dryas stage
5 10 15
Kilometers
Glacier
Bay
Alaska
1760
4
Spruce stage
3
Alder stage
Succession at Mt. St. Helen’s in
1980
• Pioneer stage – first species
• Climax or dominant species – stable,
typically most biomass species
Mosses - pioneers
Hardwood Forests - dominant
Fig. 54-23
60
Soil nitrogen (g/m2)
50
40
Succession is the result of changes
induced by the vegetation itself.
On the glacial moraines, vegetation
lowers the soil pH and increases
soil nitrogen content.
30
20
10
0
Pioneer
Dryas
Alder
Successional stage
Spruce
Dune Succession
Primary Succession
Pond Succession
Secondary succession
Human Disturbance
• Humans have the greatest impact on
biological communities worldwide!
• Human disturbance to communities
usually reduces species diversity
• Humans also prevent some naturally
occurring disturbances, which can be
important to community structure
Fig. 54-24
Results
from trawling.
Biogeographic factors affect
community biodiversity
• Latitude and area are two key factors that
affect a community’s species diversity
- generally declines along an equatorialpolar gradient and is especially great in the
tropics
- two key factors are evolutionary history
and climate
• The greater age of tropical environments
may account for the greater species
richness – more growing time so more
chance for evolutionary changes
Area Effects
• The species-area curve quantifies the idea
that, all other factors being equal, a larger
geographic area has more species
• A species-area curve of North American
breeding birds supports this idea
Fig. 54-26
Number of species
1,000
100
10
1
0.1
1
10 100 103 104 105 106 107 108 109 1010
Area (hectares)
Island Equilibrium Model
• Species richness on islands depends on
island size, distance from the mainland,
immigration, and extinction
The number of species
found on an island can be
determined by a balance
between the immigration
rate (or the movement of
species onto the island
from other islands) and
the extinction rate (or the
rate at which species
already on the island
become nonexistent).
Effect of Island Size
Immigration and
extinction rates are
affected by the size of the
island and its distance
from a non-island source
of immigrant species
A larger island has higher species diversity for two reasons: it is a
larger target, giving it a greater probability of becoming the home to
immigrants, and it has a larger supply of resources necessary to
prevent extinctions.
Effect of Island Distance
• An island's distance from a
mainland source of new
immigrants, despite its
size, is an important factor
in species diversity. Even
if two islands are the exact
same size and all other
factors are constant,
the island closest to the
mainland is more likely to
attract a larger number of
immigrant species due to
its proximity and
convenience
Number of plant species (log scale)
Fig. 54-28
400
200
Studies of species richness on
the Galápagos Islands support
the prediction that species
richness increases with island
size
100
50
25
10
5
10
100
103
104
Area of island (hectares)
(log scale)
105
106
Community ecology is useful for
understanding pathogen life cycles and
controlling human disease
• Ecological communities are
universally affected by
pathogens, which include
disease-causing
microorganisms, viruses,
viroids (viral DNA), and
prions (proteins)
Pathogens can alter community
structure quickly and extensively
For example, coral reef communities are being
decimated by white-band disease
• Human activities are transporting
pathogens around the world at
unprecedented rates
• Community ecology is needed to help
study and combat them
• Zoonotic pathogens have been transferred
from other animals to humans
• The transfer of pathogens can be direct or
through an intermediate species called a
vector
• Many of today’s emerging human diseases
are zoonotic SWINE FLU!
Fig. 54-30
Avian
flu is a highly
contagious virus of birds
Ecologists are studying the
potential spread of the virus
from Asia to North America
through migrating birds