Ecology 3
Community Ecology
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A biological community is a group of
populations of different species living close
enough to interact.
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Community interactions
are classified by
whether they help,
harm, or have no effect
on the species involved.
Some key relationships
in the life of an
organism are its
interactions with
individuals of other
species in the
community.
Lesser Long-nose Bat
feeds on nectar from
Saguaro Cacti. In turn, the
Saguaro is pollinated.
Lynx

Fox
Lynx and fox both
compete for the
Snowshoe Hare
Competition between
two species is a -/interaction that occurs
when individuals of
different species
compete for a
resource that limits
their growth and
survival.
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What happens in a
community over time
when two species
directly compete for
limited resources?
Two species cannot
coexist in the same
place.
One will use the
resource more
efficiently and thus,
reproduce more
successfully. The other
will be eliminated.
Paramecium
aurelia;
Paramecium
caudatum
E.F. Gause, 1934, studied two
closely related species of ciliated
protists. When cultured together,
P. caudatum was driven to
extinction in the culture. This is
called competitive exclusion.
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The sum of a species’
use of the biotic and
abiotic resources in its
environment.
If an organisms’ habitat
is its “address”, then an
organisms’ niche is its
“profession”.
Its niche is its ecological
role in the
community—how it
“fits into” an ecosystem.
This tropical lizard’s niche consists
of, among other things: the
temperature range it tolerates, the
size of branches on which it
perches, the time of day when it is
active, and the sizes and kinds of
insects it eats.


Two species cannot
coexist permanently in a
community if their
niches are the same.
Resource partitioning
allows two similar
species to coexist, if
their niches are
modified so they aren’t
identical.
Resource partitioning
among Dominican Republic
lizards (Anoles)
A. Distichus perches on
fence posts and sunny
surfaces.
A. Insolitus perches on
shady branches

As a result of
competition, a species
fundamental niche
(its’ optimum niche—
where it could be
living) may be
different than its’
realized niche (where
and how it actually
lives).
Green Tree Python

Ecologist J. Connell studied two barnacle species that
have a stratified distribution along the coast of
Scotland. Chthamalus is usually found higher on the
rocks than Balanus. He removed Balanus from certain
areas and found that Chthamalus moved to lower rocks.
Connell
found that
Chthamalus
has a much
smaller
realized niche
than its
fundamental
niche.

Predation is a +/- interaction between species
in which one species, the predator, kills and
eats the other, they prey. (Qu. Is an animal
killing and eating a plant a predator?)

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Just as predators
possess adaptations
for capturing prey,
prey animals have
adaptations that
help them avoid
being eaten.
Cryptic coloration,
or camouflage,
makes prey
difficult to spot.
Canyon
Tree
Frogs
Pygmy
Seahorse

Mutualism is an
interaction between
two species in a
community that
benefits both
organisms.
How is this relationship mutually
beneficial for both species?
Certain species of acacia trees in Central and
South America have hollow thorns that house
stinging ants of the species Pseudomyrmex.
The ants feed on nectar and proteins produced
by the trees. The acacia benefits because the
ants will attack anything that touches the tree,
remove fungal spores and clip vegetation that
grows close to the acacia.

The remora is a “hitchhiker” species that
hitches a ride on the shark. It benefits by
eating the scraps from the sharks’ meal.
The shark appears unaffected by the
presence of the remora……Or is it?
In what way could the shark be harmed?
In commensalism,
two species interact in
a community and one
species benefits while
the other is neither
helped nor harmed.
Cattle egrets
feed on
insects
flushed out of
the grass by
grazing
cattle.
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In parasitism, two
Brood parasitism is
seen here—a
species interact and
Common Cuckoo
one organism benefits is being raised by
a Reed Warbler.
by feeding on or
The mother
inside another
Cuckoo laid her
organism.
egg in the
Warbler’s nest!
The other organism is
harmed by this
This tomato
interaction, but is
hornworm is
covered with
rarely killed.
cocoons of
pupating
braconid
wasps