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ECOLOGY
Populations and Limiting Factors
Characteristics of Populations
What is a Population?
– A group of individuals of a species that
occupy the same area
• Two parts:
– 1) need to be same species (can interbreed)
– 2) spatial delineation
(Where located? How are they dispersed?)
Population Dispersions
1.
Aggregated (clumped) dispersionindividuals concentrated in specific
portions of the habitat; most
common scenario, resulting from
patchy distribution of resources in
habitat.
Population Dispersions
1.
Aggregated (clumped) dispersionindividuals concentrated in specific
portions of the habitat; most
common scenario, resulting from
patchy distribution of resources in
habitat.
2.
Uniform dispersion - all individuals
are more evenly spaced than one
might expect by chance.
Population Dispersions
1.
Aggregated (clumped) dispersionindividuals concentrated in specific
portions of the habitat; most
common scenario, resulting from
patchy distribution of resources in
habitat.
2.
Uniform dispersion - all individuals
are more evenly spaced than one
might expect by chance.
3.
Random dispersion - individuals in a
population are spaced in an
unpredictable and random fashion
that is unrelated to the presence of
others.
Aggregated?
Uniform?
Random?
Aggregated?
Uniform?
Random?
Aggregated?
Uniform?
Random?
What makes a Population grow?
• Birth rate
• Longevity of individuals’ lives
• Immigration
• If this kept on, what would a population
growth curve look like on a graph?
Exponential
Growth
Rate
Human population…
Sustainability
• Populations cannot keep on getting that large,
that fast. What works against that?
• LIMITING FACTORS
– Two types
• Density-dependent
• Density-independent
Density-Dependent
• The growing size of the population
eventually affects:
– birthrate, death rate
– emigration (leaving), competition
– abundance/scarcity of food,
– disease, parasitism, predation
– physical space
Consider the predator/prey relationship:
Eventually…
• In any given ecosystem,
a Carrying Capacity (K) is reached:
– The maximum average number of organisms
of a given species that can survive in good
condition in a particular ecosystem on a longterm basis.
Logistic
(restricted)
growth
(K)
In reality, the population density line may look like this:
So, what is the carrying capacity in this environment for
the hare? The lynx?
Density-Independent
• Some things occur that affect populations,
no matter how dense or sparse they may
be:
– unusual weather,
– natural disasters,
– seasonal cycles,
– human activities
• damming,
• clear-cutting
Tsunami…
Tsunami…
Eruptions…
Mount St. Helens on May 17, 1980, the day before…
September 10, 1980
For more than
nine hours a
vigorous plume
of ash erupted,
eventually
reaching 12 to
15 miles (20-25
km) above sea
level. The plume
moved eastward
at an average
speed of 60
mph (95 km/hr),
with ash
reaching Idaho
by noon.
The eruption of May 18, 1980 sent volcanic ash, steam, water, and debris to a height of
60,000 feet. The mountain lost 1,300 feet of altitude and about 2/3 of a cubic mile of
material stream downward from the center of the plume and the formation and movement of
pyroclastic flows down the left flank of the volcano.
Forest Fire…
Another way to classify factors:
• Biotic
– Biological influences on organisms within an
ecosystem
• how living things affect other living things
• “density dependent factors”
• Abiotic
– Physical, non-living factors that influence
organisms within an ecosystem
• “density independent factors”