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Chap. 53 Community Ecology
AP Biology
Mr. Orndorff
May 2005
What is community structure?
• Species composition: species present
• Species diversity: variety of species present
– Species richness: number of different species
– Relative abundance: number of individuals of
each species
How does species diversity vary?
• Latitude gradient: species diversity gets
less going from the equator to the poles.
• Depth gradient: species diversity increases
with depth to 3000 meters and then
decreases; very high on deep sea bottom.
• Pollution gradient: species diversity gets
less as pollution increases.
Species Interactions
• Predation and Parasitism (+/-)
• Interspecific Competition (-/-)
– Competitive exclusion principle
– Resource partitioning
– Character displacement
Interspecific Competition (53.10)
Resource partitioning in Anolis lizards at La
Palma in the Dominican Republic (Fig. 53.11)
Character
displacement:
circumstantial
evidence for
competition in
nature
(Fig. 53.12)
Experimental evidence for
competition in nature (Fig. 53.13)
Keystone species
• Species believed to play a more important
role in maintaining ecosystem processes
than their abundance or biomass suggests.
• Example: In intertidal pools, Pisaster
(starfish) acts as a keystone species by
feeding on Mytilus (mussel). This allows
other species to compete with Mytilus for
attachment space on rocks.
Symbiosis
• Parasitism (+/-)
– Tapeworms, pathogenic microorganisms
• Commensalism (+/0)
– Epiphytes, cowbirds on grazing herbivores
• Mutualism (+/+)
– Lichens, termites and hindgut microbes
Ecosystem disturbances
• Storms, fire, drought, flood, overgrazing
and human activities that damage
communities.
• Most ecosystems are kept in a state of
nonequilibrium by some amount of
disturbance.
Ecological succession
• Transition of species composition over time in
response to ecological disturbances.
• Primary succession = process in which lifeless
area without soil (e.g. new volcanic island, glacial
morraine) develops a living community.
• Secondary succession = process in which a
disturbance to an existing community leaves the
soil intact so that the early stages of primary
succession are not necessary.
Stages of succession
• Pioneer species = hardy species such as lichens
and mosses able to colonize bare rock and begin
the soil making process.
• Early successional species = often r-selected
species able to tolerate extremes of abiotic
conditions.
• Midsuccessional species and late successional
species = often K-selected species which can
inhibit, facilitate, or tolerate other species
producing unique combinations of species in the
community.
Diversity of life and periods of mass extinction
Continental drift and natural
barriers (deserts, mountains)
create biogeographical realms
Factors influencing range of
species
• Species may never have dispersed beyond
current boundaries.
• Pioneers which did spread beyond observed
range died.
• Species may have retracted from a once
larger range to present boundaries.
Hypothesis of island biogeography (Fig. 53.21)
Species richness and island size (Fig. 53.22)