Community Ecology
Chapter 53
Community
 An
assemblage of
populations in an
area or habitat
 Communities
differ in
Species
richness
# 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
requirements.(Gleason.1900s)


An interactive hypothesis depicts a community
as an assemblage of closely linked species locked
in by mandatory biotic interactions. (Clements
1900s)
In most actual cases, the composition of
communities does seem to change
continuously
Rivet Model & Redundancy Model
 The
rivet model of communities is a
reincarnation of the interactive
model.
 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
environment.
Hawks and Owls have the same niche but can
coexist b/c hawks are diurnal, owls are
nocturnal
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
community
Character Displacement


The tendency for
characteristics to be more
divergent in sympatric
populations of two species
than in allopatric
populations of the same two
species.
Hereditary changes
evolve that bring
about resource
partitioning
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
toxins
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
species
1.
Batesian mimicry is where a harmless
species mimics a harmful one
Hawkmoth larva
Snake
2. Müllerian Mimicry
Two
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
poisonous
Viceroy male
edible
Parasites and pathogens as
predators
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
host.
 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
species
benefit from their
interaction
 Acacia & ants
Acaciafood
&
cover
Ants sting any
intruder
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
When
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?
a
species may weave into
web at more than 1 trophic
level


bears
“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)
community
 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
community

Keystone species
 Influential
ecological role
not
necessarily
dominant or
most abundant
keystone
species increases
diversity
of habitat
Keystone species
Sea otter is
keystone
predator in
North Pacific
What is the
impact of the
whale?
II. Disturbances

Most communities are in a state of
non-equilibrium due to disturbances
 fire,
weather, human activities, etc.
 not all are negative
Disturbances

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,
then…
 bacteria
{
make  lichens
soil
& mosses
 grasses
 shrubs
 trees
Succession
from mosses & lichens
= pioneer species
to shrubs & trees
Secondary succession

Existing community cleared, but soil is
intact
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,
hemlock
 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
Richness
2. Relative
abundance
Greater diversity =
greater stability
Which is more
diverse?
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
1.
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
established
1.
Patterns of Biodiversity
2.
Island Biogeography: diff islands have
diff # of species b/c of immigration &
extinction.
Immigration & extinction determined by
the size of the island & distance from the
mainland
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
extinct.
New Island
Size of Island
Distance from Mainland
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Community Ecology