Community
Ecology
Concerned with community structure
and population interactions
Interactions
Five categories
1. Competition
2. Predation
3. Mutualism
4. Commensalism
5. Parasitism
Competition
Competition
• Interspecific competition
– The competition between two or more species
for some limiting resource
– food or nutrients, space, mates, nesting sites
• Resolutions
– Competitive exclusion principle (Gause’s principle)
– Resource partitioning
– Character displacement
– Realized niche
Competitive interactions between organisms can have a
great deal of influence on species evolution
The Competitive Exclusion Principle
• G.F. Gause-worked with 2 strains of Paramecium
-States that two species competing for the
same limiting resources cannot coexist in
the same place
• When two species compete, one is
likely to be more successful.
– Cannot coexist if they share the
same niche.
• One species could become
extinct or evolve to exploit
different resources
resource partitioning)
• Divergence in adaptation
is called character displacement.
Animation
Ecological Niches
• The ecological niche
– Is the total of an organism’s use of the biotic
and abiotic resources in its environment
• The niche concept allows restatement of the
competitive exclusion principle
– Two species cannot coexist in a community if
their niches are identical
Clip
Resource Partitioning
• Resource partitioning is the differentiation
of niches
–That enables
similar species
to coexist in a
community
A. distichus
perches on fence
posts and other
sunny surfaces.
A. insolitus usually
perches on shady
branches.
A. ricordii
A. insolitus
A. alinigar
A. distichus
A. christophei
A. cybotes
A. etheridgei
Minimizes competition & maximizes success
Resource Partitioning
Resource partitioning is a way in which different species can use the same
resource, such as food, without occupying the same physical location at the same
point in time. In this example, the different warblers eat the same caterpillar, but
they occupy different positions in the tree. Two primarily occupy the area near the
trunk, with the others share the edges of the branches, but at different heights.
The result is the warblers do not overtly compete for food in the same space.
Character Displacement (niche shift)
There is a tendency
for characteristics
to be more divergent
in sympatric
populations of two
species than in
allopatric
populations of the
same two species
Speciation:
Allopatric-geographic barriers
Sympatric- no geo barrier
Percentages of individuals in each size class
–As a result of resource partitioning.
G. fuliginosa
–-Divergence in adaptation
G. fortis
Beak
depth
Santa María, San Cristóbal
40
20
0
Sympatric
populations
Los Hermanos
40
G. fuliginosa,
allopatric
20
0
Daphne
40
20
0
G. fortis,
allopatric
8
10
12
Beak depth (mm)
14
16
Realized Niche
Niche theory distinguishes between fundamental & realized niches:
– fundamental = all possible conditions under which population reproduces
itself
– realized niche = the actual niche exhibited in particular time & place
•
•
•
•
Predation
True predator
Parasite
Parasitoid
Herbivore
Snowshoe Hare and Canadian Lynx
Camouflage
-passive defense
Cryptic coloration
– Fur of snowshoe hare is
white in winter and brown
in summer
– The larvae of some moths
are colored like bird
droppings
– Marking on animals to
blend in to background.
Lizard
• Aposematic coloration (warning coloration)
– Coloration that warns predators
– Predators learn to associate color with bad
taste or harmful chemical
• Mimicry
– Two types
• Mϋllerian
• Batesian
Müllerian mimicry
– Two or more unpalatable species resemble each
other
(a) Cuckoo bee
(b) Yellow jacket
Batesian mimicry
– A palatable or harmless species mimics an
unpalatable or harmful model
(b) Green parrot snake
(a) Hawkmoth larva
passive defense
• Spins, thorns,
poisons
• Hiding, fleeing
Community Interactions
•Symbiosis
•Predatory-Prey
•Competition
Symbiosis
• Mutualism (+/+)
• Commensalisms (+/o)
• Parasitism (+/-)
(+/+)
Mutualism
(+/o)
Commensalism
One species benefits and the other is not affected
It is difficult to determine true commensalism because it is difficult to
ensure host is not harmed.
Ex: Barnacles that attach themselves to the backs of whales
Parasitism (+/-)
• Relationship in which one species benefits;
while harming the other
• Tomato hornworm covered with cocoons of
pupating wasps
Review of
Community
Interactions
Ecological
Succession
Ecological Succession
Ecological Succession
• Primary succession
– Occurs where no soil exists when succession
begins
– Starts with pioneer species (lichens & mosses)
Ecological Succession
• Secondary succession
– Begins in an area where soil remains after a
disturbance
– takes place where a community has been
removed, e.g., in a plowed field or a clearcut
forest
(a) Soon after fire. As this photo taken soon after
the fire shows, the burn left a patchy landscape.
Note the unburned trees in the distance.
(b) One year after fire. This photo of the same
general area taken the following year indicates how
rapidly the community began to recover. A variety
of herbaceous plants, different from those in the
former forest, cover the ground.