Chapter 53:
Community Ecology
Trophic Structure, Predation,
and Competition
Assemblage of populations of various species
living close enough for potential interaction
Symbiosis
Competition
Disease
Herbivory
Predation
Competitive
Exclusion Principle
Ecological
Niche
Character
Displacement
• 2 species competing for same
resource can’t coexist in same place
at same time
• Sum of species’ use of biotic &
abiotic resources
• Resource partitioning:
differentiation of niches, so similar
species coexist in same community
• More divergent characteristics in
sympatric populations of 2 species than
in allopatric populations of same
species
• sympatric: geographically overlapping,
need diff. niches to survive; allopatric:
separate, can have same niches
+/- interaction; predator kills
& eats prey
Prey
Predator
+/- interaction; herbivore
eats plant, kills it
Herbivore
Plant
Self-defense
Have senses
Have special
Chemical
mechanisms:
that enable
sensors to
weapons/
camouflage,
them to locate
recognize
thorns protect
bright coloration, prey
appropriate food them
mimicry
+/- interaction; one organism (parasite) gets nourishment
from other organism (host), which is harmed
endoparasite
Live within host’s body
ectoparasite
parasitoidism
Insect lays eggs in/on living
Feed on external host, leaving the larvae to feed
surface of host
on body, and kill it
Inflict lethal harm, can
limit populations
+/0 interaction; 1 species
benefits, & the other is not
+/+ interaction; both
affected; hard to find true
species help each other example in nature
Pathogen: disease-causing agent; unlike
parasite: microscopic, lethal; +/- interaction
Variety of diff. kinds of organisms that make
up the community
Total # of different species
Proportion each species represents
of total individuals in the community
Eventually to
Feeding relationships between organisms decomposers
Food Chain:
Energetic Hypothesis: length
of food chain is limited by
inefficiency of energy transfer
along chain
Dynamic Stability Hypothesis:
long food chains are less stable
than short food chains;
population fluctuations at lower
levels are more profound at
higher levels, causing potential
extinction of high level
predators
Food Web: food chains are linked
together; 1 species, such as
plant, is eaten by several species
Biomass: total mass of all individuals in a population
Dominant species
Keystone species
Species that: collectively have
highest biomass OR most abundant
Ecosystem engineers
Not necessarily abundant;
exert strong control on
community by their ecological
roles
“foundation species”; facilitators; cause
physical changes that benefit
community, by increasing survival and
reproduction
Model of community organization
Predation controls community
Mineral nutrients control
community organization; nutrients organization; predators control
herbivores, which control plants,
control plant #’s, which control
which control nutrient levels
herbivore #’s, which control
predator #’s
Polluted State Restored State
Biomanipulation
Fish
Abundant
Rare
Zooplankton
Rare
Abundant
Algae
Abundant
Rare
An event that changes a community, removes
organisms from it, and alters resource availability
Not always negative
Burned trees release nutrients; no trees to block sunlight for
new plants
Primary Succession: occurs in virtually lifeless area, with no
organisms or soil; only autotrophic prokaryotes are present
Secondary Succession: occurs where an existing community was
cleared by disturbance, that leaves soil intact; first plants to
recolonize: grow from wind-blown/animal-borne seeds
Geographic Location
Tropical habitats have the most species;
tropical habitats are older (long growing
season), and greatest evapotranspiration
Size
Greater area=more diverse
habitats, more species
New Species
Immigration Rate
Island Size
Influenced By:
Species Extinction Rate
Island’s Distance
from Mainland
new colonizers: unlikely to
reach small island = low
immigration rate, high
extinction rate (less resources)
# of Species Already
Present
# Species
immigration rate (those reaching
island are less likely to represent
species not already there)
Islands near mainland: high immigration
rate, low extinction rate (new colonizers
sustain presence of species)
Extinction rate (competitive
exclusion)
Integrated Hypothesis
Individualistic Hypothesis
(Clements)
(Gleason)
Describes a community as an
States that species are found in
assemblage of closely linked
the same area simply because
species, locked into association by they happen to have similar
mandatory biotic interactions
abiotic requirements
These interactions cause the
community to function as an
integrated unit
Rivet Model
Redundancy Model
Suggests: most of the species in Exact opposite of the rivet model
a community are associated
Suggests: species in a community
tightly with particular other
are redundant; species operate
species in a web of life
independently, & aren’t affected if
one species increases/decreases
**the Ehrlich’s agree with
Ex. – if one pollinator disappears,
Clements
then another species will do the
job
**Walker agrees with Gleason
DDT and other toxins, when in tissues and fats of an
organism, can accumulate and be passed onto the next level
of the food chain, affecting many organisms. Rachel Carson
states in Silent Spring that the concentration of DDT in a low
level organism’s fat is a lot lower than that of a higher level
organism on the food chain
If phytoplankton is infected
Zooplankton & other primary
consumers, secondary
consumers, etc. are affected
Everything occurring at low food chain levels is magnified
at high food chain levels, causing possible extinction of top
level predators
Similar effect of DDT; If organism is affected by pollution, it
can spread through food chain, greatly affecting top-level
predators
If water is polluted and filled with garbage, the fish living
there drink bad water and aren’t getting the right nutrients
Make fish sick, and all those who eat them: fish’s predators,
fish’s predators’ predators, and even human who eat fish for
dinner
**If one organism is infected by either DDT, pollution, or by severa
other possibilities, the harms done to that animal certainly don‘t
stop there**
DDT Impacts Predation
•The producers absorb the
DDT and are eaten by the
consumers.
•The predators will continue
imbalance of predators to prey will
to consume the DDT laden
prey, accumulating the DDT
cause the ecosystem to go out of
in their body fats.
balance
The spraying will cover
the soil and the
nutrients it contains.
The reduction of 4th
level predators, for
example, will cause:
overabundance of the 3rd level
prey, which consume so much 2nd
level prey
Decrease 2nd level prey, so there’s not as
many organisms to eat primary producers
Effects of Global Warming
on Predation
As the environment becomes
increasingly warmer each year,
organisms might not be able to
adapt or migrate to other areas
The introduction of organisms
disrupts and changes the
ecosystem of the environment.
If organisms die, their predators This migration will bring new
predators and new prey, which
would starve, and there would
would alter the trophic structure.
be an increase in the dying
organisms’ prey
Altering the trophic structure would change the predatorprey relationships; the top level carnivores may not be
the top level carnivores anymore, having an effect like the
top-down model
DDT Affects Competition
• DDT can increase competition
DDT may kill many organisms of a
species that two other species
compete for.
Because the resource is in short
supply, interspecific competition
between the two competing
species will increase, hurting both
species
eventually eliminating one of the two
competing species (competitive exclusion).
DDT Affects Competition
•DDT can also lessen competition
Reproduction Rates
Biological Magnification
DDT influences reproduction rates
DDT’s concentration with each
link in the food chain. By eating Ex.: Eagles in Silent Spring by
DDT-contaminated food, DDT
Rachel Carson
accumulates in carnivores,
-DDT stored in eagles’ fatty cells
which may be enough to kill the
cause the deposition of calcium in
carnivores.
their eggshells to be hindered.
Less carnivores; some organisms’
populations from lower trophic
levels (top-down model),
providing more of those organisms
for the surviving carnivores (more
food for them).
less competition among remaining
carnivores for resources.
Weak eggs, so many parent eagles
accidentally crushed the egg during
incubation; some eggs also didn’t
hatch
eagle population; the population of
eagles’ prey increased, so
competition for those prey isn’t as
intense.
Global Warming’s Impact on
Competition
Some organisms may not be able to Species may become extinct; its
adapt fast enough to the rapidly
predators may starve/be eliminated
changing conditions of the world
& its prey is more abundant
Some organisms may adapt
well to the new conditions
caused by global warming
Have edge over other species to get
resources, reproduce rapidly; lead to
local elimination of interior competitor
(not able to obtain resources)
If species is extinct or
adapts, a competitor is still Survivors reproduce, pass genes onto
eliminated, leading to less offspring; over evolutionary time, may
evolve to new species
competition for resources
A.P. Biology
Pd. A
9/15/08