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Population Equilibrium
A dynamic balance between births and deaths.
Births = Deaths
Population Growth Curves
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Reproductive strategies:
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Many offspring with low parental care
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Example  fish
J-shaped growth curve
Few offspring with high parental care
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Example  mammals
S-shaped growth curve
Population Growth Curves
Population Dynamics
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Environmental resistance: combination of
biotic and abiotic factors that may limit
population increase
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Predators, competitors, disease
Adverse weather, limited food/nutrients
Biotic Potential and Environmental
Resistance
Density Dependence and Critical Numbers
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Factors of environmental resistance are
either:
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density-independent: effect does not vary with
population density; e.g., adverse weather
density-dependent: effect varies with
population density; e.g., infectious disease
Critical number: the lowest population
level for survival and recovery
Mechanisms of Population Equilibrium
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Predator–prey dynamics
Competition
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Interspecific
Intraspecific
Introduced species
Predator–Prey Balance: Wolves and
Moose
Predator–Prey Balance
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Absence of natural enemies allows a
herbivore population to exceed carrying
capacity, which results in overgrazing of
the habitat.
The herbivore population subsequently
crashes.
The size of the herbivore population is
maintained so that overgrazing or other
overuse does not occur.
Keystone Species
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A single species that
maintains biotic structure of
the ecosystem
Example:
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Pisaster ochraceus: a starfish
that feeds on mussels
(Mytilus californianus) ,
keeping them from blanketing
the rocks
Pacific coast intertidal zone
Competition: Intraspecific
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Territoriality: defense of a resource against
individuals of the same species
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Results in priority access and use of resources
How do wolves and songbirds establish
territory?
Competition: Interspecific
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Grasslands contain
plants with both
fibrous roots and
taproots
Coexist by accessing
resources from
different soil levels
Introduced Species
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Examples (Long Island):
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Mute swans
Starling
House Sparrows
Gypsy Moths
Japanese Beetles
Pheasants
Phragmites
Eurasian fresh water clam
And LOTS MORE!!!
European Green Crab – Native
Species
Asian Shore Crab – Introduced
Competitor
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Introduced to
New Jersey in
1988
Spread up to
Maine
Native Species
are diminishing
European
Green Crab
Asian Shore
Crab
Mechanisms of Species Adaptation

Change through natural selection

Selective pressure determines which organisms
survive and reproduce and which are eliminated.
Adaptations
to the
Environment
The Limits of Change

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Adapt
Move (migrate)
Die (extinction)
Vulnerability of different organisms to
environmental changes
Prerequisites for Speciation
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Original population must separate into
smaller populations that do not interbreed
with one another.
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List some ways this might happen.
Separated populations must be exposed
to different selective pressures.
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Example: arctic and gray fox
Speciation: Foxes
Speciation: Galápagos Finches
Ecosystem Responses to Disturbance
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Ecological succession
Disturbance and resilience
Evolving ecosystems