Community Ecology
Chapter 53
 An
assemblage of
populations in an
area or habitat
 Communities
differ in
# they contain
Relative abundance
of diff. species
Different Views of Communities
An individualistic hypothesis depicts a
community as a chance assemblage of species
found in the same area because they happen
to have similar abiotic
An interactive hypothesis depicts a community
as an assemblage of closely linked species locked
in by mandatory biotic interactions. (Clements
In most actual cases, the composition of
communities does seem to change
Rivet Model & Redundancy Model
 The
rivet model of communities is a
reincarnation of the interactive
 The redundancy model states that
most species in a community are not
closely associated with one another.
 No matter which model is correct, it
is important to study species
relationships in communities.
Interspecific Interactions
 Populations
may be linked by
Competition, Predation,
Mutualism and Commensalism
Interspecific Interactions
A. Interspecific Competition
 Resources
are in short supply
 Potential for competition between
any two species that need the same
limited resource.
Competitive Exclusion Principle
G.F. Gause P. aurelia & P.
caudatum, grown separately-did well
but tog. P. aurelia drove P. caudatum
to extinction
 Two species with similar needs for
same limiting resources cannot
coexist in the same place
 The better adapted of the two will
replace the other
 1934,
Ecological Niche
All the biotic and abiotic factors needed to
maintain the species.
An organism’s niche is its role in the
Hawks and Owls have the same niche but can
coexist b/c hawks are diurnal, owls are
The competitive exclusion principle can
be restated to say that two species
cannot coexist in a community if their
niches are identical.
Resource Partitioning
Differentiation of niches that enables
two similar species to coexist in a
Character Displacement
The tendency for
characteristics to be more
divergent in sympatric
populations of two species
than in allopatric
populations of the same two
Hereditary changes
evolve that bring
about resource
B. Predation (-/+)
 A predator eats prey.
Herbivory, in which animals eat plants.
 In parasitism, predators live on/in a host
and depend on the host for nutrition.
 Predator adaptations: many important
feeding adaptations of predators are
both obvious and familiar.
 Claws,
teeth, fangs, poison, heat-sensing
organs, speed, and agility
Plant Defenses
 Chemical
compounds that are toxic
 Thorns
 Secondary
compounds not part of
normal met, ex poisonous berries
Animal Defenses
Behavioral defenses include fleeing, hiding,
self-defense, noises, and mobbing.
a. Camouflage includes cryptic coloration,
deceptive markings.
b. Mechanical defenses include spines.
c. Chemical defenses include odors and
Aposematic coloration is indicated by
warning colors, and is sometimes
associated with other defenses (toxins).
Fig. 53.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Mimicry- organisms resemble other
Batesian mimicry is where a harmless
species mimics a harmful one
Hawkmoth larva
2. Müllerian Mimicry
or more unpalatable species resemble
each other
Both produce a toxin& predators learn
quickly to avoid them
Cuckoo Bee
Yellow jacket
Batesian mimicry
Convergent evolution
Monarch male
Viceroy male
Parasites and pathogens as
A parasite derives nourishment from a
host, which is harmed in the process.
 Endoparasites live inside the host and
ectoparasites live on the surface of the
 Parasitoidism is a special type of
parasitism where the parasite eventually
kills the host.
 Pathogens are disease-causing organisms
that can be considered predators.
C. Mutualism (+)
 Two
benefit from their
 Acacia & ants
Ants sting any
D.Commensalism (+/o)
One species benefits from the interaction,
but other is not affected.
 An example would be barnacles that attach
to a whale
 Bird’s nests in trees
E. Coevolution
 Two
species with intertwined life
history- the evolution of one
affects the other
one species evolves, it exerts
selective pressure on the other to
evolve to continue the interaction
Ex humming bird & flower
F.Trophic structure
Feeding relationships
 Food chains
 food
chain usually 4 or
5 links = trophic levels
 length of food chain
limited by inefficiency
of energy transfer
Food webs
Food chains are hooked
together into food webs
Who eats whom?
species may weave into
web at more than 1 trophic
“there’s always a bigger fish”
What limits the length
of a food chain?
G. Dominant & Keystone Species
Dominant species most abundant
species or highest biomass (total weight)
 If we remove a dominant species from a
community, it can change the entire
community structure.
 Keystone species exert important
regulating effect on other species in
Keystone species
 Influential
ecological role
dominant or
most abundant
species increases
of habitat
Keystone species
Sea otter is
predator in
North Pacific
What is the
impact of the
II. Disturbances
Most communities are in a state of
non-equilibrium due to disturbances
 fire,
weather, human activities, etc.
 not all are negative
Disturbances are often necessary for
community development & survival
Ecological cycle
fire as part of a natural community cycle
IIB. Ecological succession
The sequence of community changes
after a disturbance
 transition
in species composition over
ecological time
years or decades
Mt. St. Helens
Primary succession
Begins with
virtually lifeless
area without soil,
 bacteria
make  lichens
& mosses
 grasses
 shrubs
 trees
from mosses & lichens
= pioneer species
to shrubs & trees
Secondary succession
Existing community cleared, but soil is
Climax forest
Plant community dominated by trees
representing final stage of natural
succession for specific location
 stable
plant community developed through stages
 remains essentially unchanged in species composition
for as long as site remains undisturbed
 birch, beech, maple,
 oak, hickory, pine
Climax Forest
The species mix of
climax forest is
dependent on the
abiotic factors of
the region
 solar energy levels
 temperature
 rainfall
 fertility & depth of soil
birch, beech, maple, hemlock
III. Biodiversity of Communities
Two components
1. Species
2. Relative
Greater diversity =
greater stability
Which is more
Species richness is related to a
community’s geographic size
The species-area curve : the larger the
geographic area, the greater
the number
of species
Patterns of Biodiversity
Equatorial/Polar gradient:
greatest at equator, less &less
as move to poles, because
Climate: poles frozen at
times,long growing season @eq,
more solar energy & precip.@ eq
2. Age: eq comm. Are older, more
Patterns of Biodiversity
Island Biogeography: diff islands have
diff # of species b/c of immigration &
Immigration & extinction determined by
the size of the island & distance from the
Factors that determine the
number of species that eventually
inhabit the island
 The
rate at which new species
immigrate to the island.
 The rate at which species become
New Island
Size of Island
Distance from Mainland

Community Ecology