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COMMUNITY ECOLOGY
Chapter 54
What is a community?

Many species living closely together, so
that they interact with each-other.
Population Interactions:
Intraspecific competition - within a
species
 Interspecific interactions - Individuals of
different species compete for a resource
that limits their growth and survival

Interspecific competition
-/- interaction
 Competition must occur:

◦ interference competition - actual fighting
occurs
◦ exploitation competition - consumption or
use of similar resource

Ex: weeds growing in a garden compete
with garden plants, lynx and foxes
compete for snowshoe hares
Competitive Exclusion Principle
Two species cannot coexist in a
community if their niches are identical
 Ecologically similar species can coexist in
a community if there are one or more
differences in their niches

Competitive Exclusion Principle
Ecological Niche
Sum total of a species’ use of the biotic
and abiotic resources in its environment
 Habitat=address
 Niche=habitat + occupation

Resource partitioning of lizards
Fundamental / Realized Niches
Character
displacement
Upon What Do These Animals Prey?
Upon What Do These Animals Prey?
Animals
Upon What Do These Animals Prey?
Plants
Animals
Animal Defenses
Running
 Hiding
 Physical or Chemical Defenses

Chemical
Defense and
Aposematic
coloration
are seen
here
Aposematic coloration
Cryptic
coloration
is seen
here
Cryptic Coloration
Mimicry
Batesian mimicry - harmless species
resembles harmful or distasteful species
 Mullerian mimicry - two or more
unpalatable species resemble one another.

Batesian Mimicry
Batesian Mimicry
Mullerian Mimicry
Plant Defenses Against Predators:
What defense is seen here?
Other defenses:
• Chemicals that cause plant to taste bad
• Poisonous chemicals
In this relationship, the ant hollows out the thorns of the
plant for nests, feed on sweet secretions from the four
nectaries at the base of each petiole, and protect these trees
from invertebrate herbivores.
Heliconius butterflies (Heliconius sp.)
lay their eggs on the leaves of the
passionflower vine (Passiflora sp.).
When the eggs hatch into hungry, leafeating Heliconius caterpillars, the
passionflower vines protect themselves
with an array of defenses.
Fake eggs . . . some passionflower vines
grow parts that look like Heliconius
caterpillar eggs. This keeps the female
Heliconius butterfly from laying eggs on
leaves that appear to be occupied by
other cannibalistic caterpillars.
Dumpers . . . Some
Heliconius butterflies like to
lay their eggs on upwardcurling tendrils of
passionflower vines. Some
vines produce fake tendrils
that are weak and fall off
under the weight of the
developing butterfly eggs.
Symbiosis
Close association between two species
 Parasitism +/ Commensalism +/0 interaction
 Mutualism +/+ interaction

Predation
+/- interaction
 Predator eats prey
◦ Ex. Lion eats an antelope, animal eats a
plant

Predators
Can reduce relative abundance
 Can reduce diversity by eliminating
competitors - keystone predators

◦ sea star feeds on mussels and other organisms,
remove sea star and mussels take over the
region and eliminate species diversity.
Parasites are also +/Endoparasites-live within the body of their host
◦ Ex. tapeworms
 Ectoparasites-parasites that feed on the
external surface of a host
◦ Ex. Ticks, lice
 Parasitoid insects-lay eggs on the body of the
host, the larva then feed on the body of the
host, eventually killing it

One parasite example:
Mutualism

+/+ interaction
◦ Ex. Nitrogen fixation by bacteria in root
nodules of legumes
◦ Ex. Termites and microorganisms in their gut
Interspecific Interactions:
What is co-evolution?
 Reciprocal evolutionary changes between
two or more species.
 Results in a close association between
certain species

Commensalism

+/0 interaction
◦ Ex. Cowbirds and cattle egrets feed on
insects flushed out of the grass
Which symbiosis is seen here?
Which symbiosis is seen here?
parasitic
Which symbiosis is seen here?
Which symbiosis is seen here?
Mutualism
Community Structure
Feeding relationships
 Richness - number of different species
 Relative abundance - number in individuals in
each species
 Diversity - combination of abundance and
richness
 Communities with even distribution of
species are more diverse than communities
with greater richness but skewed
distributions

Trophic Structure
Feeding relationships-transfer of food
energy
 Plants to herbivores to carnivores to
decomposers = food chain
 Food web - interconnected food chains

Control of community structure

Dominant species-species in a community that
have highest abundance or biomass
◦ Sugar maples

Keystone species-not abundant but exert strong
control on community structure
◦ Remove the species to determine if it is a keystone
species
◦ Sea otters

Invasive species-no natural predators so their
population is not held in check
◦ Buckthorn, garlic mustard, purple loosestrife
Nonequilibrium
Most communities are not stable
 Disturbances such as storms, floods
overgrazing etc. can effect stability
 Humans bring disturbance

Succession
Example of disturbance leading to change
 primary succession - soil never formed
before
 secondary succession - existing
community was disturbed and returned
to original state.

Secondary Succession
Biogeography
Study of past and present distribution of
individual species and communities
 Limitation of species to one region:

◦ species never dispersed
◦ pioneers did not survive
◦ species retracts from once larger range to
smaller boundaries
Island biogeography
Review the following slides and
identify which community interaction
you are observing
Where is the Moth?
Can you see it now?
What coloration is
seen in the next
slides?
What does this
tell you about the
way these animals
live in their
communities?
This animal is not a bee, why does it look
like one?
Species Richness and Island Size
Species diversity
Which forest is more diverse?