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Chapter 53
Community Ecology
I. Community
Assemblage of populations in an
area.
A. Communities Differ In …
1. Species Richness – number of species
2. Relative Abundance – amount of
individuals of each species
B. Why are Species Found
Together?
1. Individualistic Hypothesis – species have
similar abiotic requirements /
communities lack distinct boundries
2. Interactive Hypothesis – species locked
into association by biotic interactions /
noticeable boundries
B. Why are Species Found
Together?
3. Rivet Model – remove multiple species
and community falls apart / similar to
interactive hypothesis
4. Redundancy Model – species are not
closely related / remove one species and
it will be replaced
II. Interspecific Interactions
A. Competition -/1. Interspecific Competition – different species
fight for resources
2. Competitive Exclusion Principle – slight
reproductive advantage will lead to the
elimination of inferior species
3. Ecological Niche – sum total of a species use
of its biotic and abiotic resources
4. Resource Partitioning – differentiation of
niches
5. Character Displacement – tendency for
characteristics to be more divergent in
sympatric populations of 2 species then in
allopatric populations of the same 2 species
C-E Principle and Resource
Partitioning
Character Displacement
B. Predation +/1. Predator Adaptations – claws, teeth,
fangs, stingers, poison, speed,
camouflage
2. Plant Defenses – toxins, thorns
3. Animal Defenses – hiding, escape, alarm
calls, cryptic coloration, aposematic
coloration, mimicry (Batesian, Mullerian)
4. Parasites as Predators – live on host but
don’t kill / pathogens kill
Animal Defenses
C. Mutualism +/+
- both species benefit
D. Commensalism +/0
- one benefits one is not hurt or
helped
E. Coevolution
- reciprocal evolutionary
adaptations of 2 interacting species
III. Community Structure
Trophic structure is the feeding
relationships in a community
A. Food Chain
1. Light – primary producers – primary
consumers – secondary consumers –
detritovours
2. Limited to 4-5 links
3. Energetic Hypothesis – inefficiency of
energy exchange (10%)
4. Dynamic Stability Hypothesis – long
chains are less stable
B. Food Web
1. Group of food chains
C. Species Types
1. Dominant Species – highest abundance
2. Keystone Species – not the most
abundant necessarily but have a strong
hold because of their ecological niche
D. Community Control
1. Bottom-up Model – nutrients control
community
2. Top-down Model – predation controls
community / trophic cascade model
IV. Disturbance
Nonequalibrium Model – communities are
constantly changing because of
disturbance (storms, fire, flood, drought,
humans)
Small scale disturbance is good! Why?
A. Primary Succession
1. Life develops from lifeless area with no
soil
2. Bacteria – lichens, moss – grasses,
shrubs – climax community
B. Secondary Succession
1. Develops after an existing community
has been cleared.
2. Facilitate, inhibit, tolerate
V. Biogeographic Factors
Affecting Biodiversity
Biodiversity depends on species
richness and relative abundance.
A. Equatorial – Polar Gradient
1. More species richness if you are closer
to the equator
2. Caused by…
a. Evolutionary history / tropical areas
are older and have longer “growing”
seasons
b. Climate / more energy and water
input at equator
B. Size
1. Bigger areas equate to more species
richness.
2. More diverse habitats and microhabitats.
C. Islands
1. Depends on size and distance from the
mainland.
2. More immigration with large/close islands
3. More extinction with small/far islands
4. WHY?