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Ecosystems
Part 2
Edge effects
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How the local environment changes
along the boundary or edge in a habitat
Caused by:
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natural events (tree fall)
humans (deforestation)
Results:
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Changes in species composition and
diversity
Vocabulary
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Interior species: can only live in the
undisturbed core of a habitat
Edge species: can live in edge habitats
Open community: edge is gradual or
has indistinct boundaries which many
species cross over
Closed community: community is
sharply devided from its neighbors
U.S. Roads

3.9 million miles of public roads in the
United States.
Wildlife and highways
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Road kill – animals
that are killed by
being run over by
cars each year
Millions of animals
die each year
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Have a significant
impact on
populations
Costs $8 billion per
year
Wildlife crossings:
bridges and tunnels
Ecosystem diversity

Involves:
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Biodiversity
Natural selection
Evolution
Ecosystem services
Biodiversity
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Attempts to describe diversity of life
at three levels:
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Genetic – range of all genetic traits in a
population
Species – number of different species
that inhabit a different area
Estimated between 10 and 30 million
species on Earth
 Named around 1.5 million
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
Ecosystem – the range of habitats that
can be found in a defined area
Biodiversity
Increases
 Diverse habitats
 Moderate disturbance
in the habitat
 Environmental
conditions with low
variation
 Trophic levels with
high diversity
 Middle states of
succession
 Evolution
Decreases
 Extreme stress
 Extreme environments
 Extreme limitations in
the supply of a
fundamental resource
 Extreme amounts of
disturbance
 Introduction of
species from other
areas
 Geographic isolation
Natural Selection

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The mechanism of how organisms
evolve
Works on the individual level by
determining which organisms
survive and reproduce
The range of genetic variation in a
population determines if the species
Natural Selection

“Survival of the fittest”

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Fittest = how many babies you have
who go on to have babies
New genes enter the population
through mutation
Natural selection
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Types of selection
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Stabilizing
Directional
Disruptive
Stabilizing Selection
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Selects against the
extremes of a
population
Most common
form of natural
selection
Results:
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Decreased diversity
Maintenance of a
stable gene pool
No evolution
Directional Selection
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Affects one
extreme of a
population
Gives an
advantage to other
extreme
Results

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Change in
population
characteristics
Can lead to
evolution of a new
species
Disruptive Selection

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Acts against the
individuals in the
middle
Favors both
extremes
Results:
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Splitting of
population into two
smaller populations
Results in evolution
of two new species
Evolution
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Change in the genetic composition
of a population over successive
generations
Supported by evidence from:
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Fossil record
Genetics
Homologous traits
Embryological similarities
Computer models
Speciation
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Results when segments of a
population becomes isolated so
gene flow stops
The isolated populations eventually
become new species
Maintaining gene flow
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Some species of spiders balloon to
disperse through the environment
They let out a long strand of silk which is
caught by air currents and transports
them over long distances
Patterns in Evolution
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(A) Divergent (adaptive radiation) – similar species
become less similar
(B) Convergent – Unlike species evolve similar
(analogous) traits while evolving in separate
ecosystems
(C) Parallel – Two independent species evolve at the
same time, in the same ecosystem, and acquire similar
traits
Convergent Evolution
Adaptive Radiation
Parallel Evolution
Gradualism vs. Punctuated Equilibrium
Gradualism
 Views evolution as
slow, stepwise
development of
species over a
long period of time
Punctuated Equilibrium
 Proposes some
species arose
suddenly in a short
period of time after
long periods of
stability
Ecosystem services
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Things the environment does for us
for free.
Includes but not limited to:
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Moderate weather extremes and their
impacts
Disperse seeds
Mitigate droughts and floods
Cycle and move nutrients
Detoxify and decompose waste
Purify air and water