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Community Ecology II:
Species Interactions
Ecological communities: Assemblages of two
or more species living and interacting in the same area.
Species interactions: the ways different organisms
affect each other and how they have adapted to each
other’s presence.
FOUR main kinds of species interactions; all play
a role in the formation of community structure...
Competition (-,-)
Predation, parasitism (+,-)
Mutualism (+,+)
Commensalism (+,0)
Interspecific Competition:
Use or defense of a resource by one species
that reduces availability of that resource
to a different species.
exploitative competition
interference competition
Competition in Paramecium
Competition in Paramecium
Competitive Exclusion:
A result of competition between species
for a limiting resource, in which one
species completely eliminates the other.
Ecological Niche:
• a species’ ecological ‘role’
• all the environmental factors that influence
the growth, survival, and reproduction of a species
• an n-dimensional hypervolume within which
a species’ population growth is positive
3-dimensional niche
Fundamental Niche vs Realized Niche
Fundamental Niche: the niche that a species potentially
could occupy, in the absence of competitors.
Realized Niche: the niche to which a species is restricted
in the presence of competitors.
Competition in Barnacles
Competitive Release:
Expansion of a species’ ecological
niche when a competitor is removed
(usually in a removal experiment).
Q: How do similar species coexist?
A1: Resource Partitioning
• Species using similar resources can coexist when
the habitat is spatially and temporally complex,
thus allowing for resource partitioning.
Anolis spp.
Number of different Anole species on different islands
Q: How do similar species coexist?
A2: Character Displacement
Character Displacement
The tendency for physical characteristics to be
more different in sympatric populations of two
closely related species than in allopatric
populations of the same two species.
FOUR main kinds of species interactions; all play
a role in the formation of community structure...
Competition (-,-)
Predation, parasitism (+,-)
Mutualism (+,+)
Commensalism (+,0)
True Predators
- prey is killed immediately upon successful attack
Great White Shark, Siberian Tiger,
Fox Squirrel
Partial Predators
- prey usually not killed but many prey organisms
affected during predators lifetime - grazers, browsers
Giraffe, Bison, Monarch Caterpillar
- lay eggs on or in prey and then eggs hatch and
developing offspring consume the prey
Parasitoid wasp attacking moth caterpillar
Predators can influence species within
communities by selecting for anti-predator
adaptations in prey…
Behavioral Defenses
Morphological Defenses
Crypsis - matching background
Cryptic coloration
Flexible crypsis
Morphological & Physiological Defenses
Aposematic or Warning Coloration
Monarch Butterfly and Caterpillar
Blue jay eats monarch, but vomits
due to cardiac glycoside from milkweed
• Batesian mimicry in which a palatable
species mimics a harmful “model” species
• Müllerian mimicry in which two or more
harmful species come to resemble each
Batesian mimicry – several fly species
mimicking bees and wasps
Batesian mimicry – coral and king snakes
Arizona Coral Snake
Central American coral snakes and
king snake mimic – coral snake on
left or in middle
Müllerian Mimicry
Complex of Batesian and Müllerian Mimics
Predators can also influence the numbers
and distribution of their prey.
e.g. Wolves and Moose on Isle Royale
Parasites can affect host populations by reducing
their survival, reproduction and density.
Parasitic effects on host species
can be indirect or direct.
• Ticks (ectoparasites) suck blood and cause irritation
• Lead to weakened moose, which are more susceptible
to predation by wolves and death from starvation.
Parasites can also affect the behavior of the
hosts, making them more likely to pass the
parasite from one host to another.
e.g. endoparasites in sticklebacks cause them to approach
water surface more often and increase predation risk.