Interactions in
Communities
1. Predator/Prey: one organism
kills another for food
Predator populations are affected by the
size of the prey population, and vice
versa.
Why do animals compete?
2. Competition occurs when two organisms
fight for the
same limited resource.
– Intraspecific
competition
– Interspecific
competition
Interspecific or intraspecific?
• Two red squirrels race up a tree to find a hidden
pile of nuts.
• A hyena chases off a vulture to feast on an
antelope carcass.
• Shrubs and grasses on the forest floor compete
for sunlight.
• Brown bears hunting for fish on a river’s edge
fight over space.
• Male big horn sheep butt heads violently in
competition for mates.
3. Symbiosis
• 2 species living closely connected to
one another
1. Mutualism
2. Commensalism
3. Parasitism
7
Mutualism
Both species
benefit.
Ex:
CLOWNFISH
AND SEA
ANEMONE
8
Commensalism
One member
benefits and the
other is not helped
or harmed.
• Ex: Whale
and
Barnacle
9
30
Parasitism
one organism
benefits and the
other is harmed
Types of Symbiosis
Interaction
Species A
Species B
Mutualism
Benefit (+)
Benefit (+)
Commensalism Benefit (+)
No Effect (0)
Parasitism
Harm (-)
Benefit (+)
More about Parasitism
– One organism feeds on
another organism called
the host
– Host is almost always
larger than the parasite
– Parasites often live on or
in host
– Host usually harmed but
not killed
Riverblindness: Parasitic worm
transmitted by blackfly
Isopod parasitizing a fish tongue
 Herbivores
don’t often kills
plants
Plant defenses
 thorns/spines
 chemical compounds
that taste bad, kill,
or make herbivores sick
5. Coevolution: back and forth
evolutionary adjustment between
two species that interact
What is the difference between a
habitat and a niche?
• A habitat is the area
in which an organism
lives. (where)
• A niche includes the
role that the organism
plays in the
environment. (how)
Are the following part of the habitat
or niche?
a) Lions eat zebras
b) Lions hunt in groups
c) Lions live in the
savanna
d) Lions live near a
watering hole
e) Lions are most active
just after dusk
f) Lions reproduce
starting at age 4
Fundamental niche : entire range of conditions where an
organism could survive.
Realized niche: the actual niche that a species occupies.
Competition between two species of barnacles.
The realized niche of each barnacle
growth
rate
Balanus
realized
niche
low
Balanus
and
Chthamalus
Chthamalus
realized
niche
middle
high
Location in intertidal zone
The fundamental niche of each barnacle.
Balanus alone
Balanus
fundamental
niche
growth
rate
Chthamalus alone
Chthamalus fundamental niche
low
middle
high
Location in intertidal zone
Streams with only
Planaria species A
Streams with only
Planaria species B
Streams with both
Planaria species
What are the fundamental
and realized niches for
each species?
Competitive exclusion occurs when one species
eliminates another through competition..
Two species of Paramecia
predict the outcome of
interspecific competition
P. aurelia
P. caudatum
Gause (1934)
A black rhinoceros grazes on the African
plains. As it forages, the rhino disturbs
multitudes of insects in the grass, crushing
some and inadvertently eating others. Birds
known as egrets forage on the ground near
the rhino, eating insects flushed out by the
hooves. Other birds known as oxpeckers
eat ticks from the
skin of the rhino.