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Chapter 6: Population and
Community Ecology
Key Ideas
There are clear patterns in the distribution and
abundance of species across the globe.
Understanding the factors that generate these
patterns can help us preserve global
biodiversity. These factors include the ways in
which populations increase and decrease in
size and the ways in which species interact
with one another in their communities
Population Characteristics
•
•
•
•
•
population size (N)
population density
population distribution
population sex ratio
population age structure
Population Dispersal Patterns
• Random
• Clumped
• Uniform
Factors that Influence Population Size
• Density-dependent factors- the size of
•
•
the population will influence an
individual’s probability of survival.
Density-independent factors- the size
of the population has no effect on the
individual’s probability of survival.
Carrying capacity (K)
population growth
• birth
• immigration
population decline
• death
• emigration
population growth
environmental
resistance
carrying capacity
population
logistic growth
biotic
potential
exponential growth
time
Logistic Growth of Sheep Population
Number of sheep (millions)
2.0
Overshoot
1.5
Carrying Capacity
dieback
1.0
.5
1800
1825
1850
1875
1900
1925
Year
Fig. 6-12, p. 121
Variations of the Logistic Model
• If food becomes scarce, the population
will experience an overshoot by
becoming larger than the spring carrying
capacity and will result in a die-off, or
population crash.
Estimating Population in the Wild
• sampling-
• mark and recapture-
Assumptions for validity of mark
and recapture techniques
• tagging does not change the behavior of
the animal
• tagged animals must have time to freely
mix with the rest of the population
• animals must be captured randomly
Species Interactions
Keystone Species
• play critical ecological roles
• top predators
• pollinators
• engineers
Competition
• interspecific
• intraspecific
• resource partitioning reduces
competition
Mutualism
• Mutualism- A type of interspecific
interaction where both species benefit.
Ecological Succession
Change Over Time
primary succession
secondary succession
Aquatic Succession
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