Environmental Science: Toward a Sustainable Future Richard T. Wright

1
Environmental Science Toward a Sustainable
Future Richard T. Wright
Chapter 4

• Ecosystems How They Change
• PPT by Clark E. Adams

2
Factors That Contribute to Ecosystem Change

• Dynamics of natural populations
• Mechanisms of population equilibrium
• Mechanisms of species adaptation
• Ecosystem response to disturbance
• Lessons to learn

3
Dynamics of Natural Populations

• Population growth curves
• Biotic potential versus environmental resistance
• Density dependence and critical number

4
Biotic Potential and Environmental Resistance
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Population Equilibrium
A dynamic balance between births and deaths.

• Births

Deaths
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Population Growth Curves
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Population Growth Curves

• Reproductive strategies

Many offspring with low parental care
Few offspring with high parental care
J-shaped growth curve
S-shaped growth curve
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Population Dynamics

• Environmental resistance combination of biotic
  and abiotic factors that may limit population
  increase
• Predators, competitors, disease
• Adverse weather, limited food/nutrients

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Biotic Potential and Environmental Resistance
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Density Dependence and Critical Numbers

• Factors of environmental resistance are either
• 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

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Mechanisms of Population Equilibrium

• Predatorprey dynamics
• Competition
• Interspecific
• Intraspecific
• Introduced species

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PredatorPrey Balance Wolves and Moose
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Lessons to Be Learned about PredatorPrey Balance

• 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.

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PlantHerbivore Dynamics
Reindeer on St. Matthew Island

• No regulatory control (predation) on herbivores
• Went into exponential growth pattern
• Overgrazed habitat
• Massive die-off of herbivores

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Mechanisms of Population Equilibrium
PlantHerbivore

• Compare the predatorprey with plantherbivore
  methods of controlling the size of the herbivore
  population.
• How would the herbivore population growth curve
  look if diseases or predators were used as the
  control mechanism?

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Keystone Species

• A single species that maintains biotic structure
  of the ecosystem
• Pisaster ochraceus a starfish that feeds on
  mussels, keeping them from blanketing the rocks

http//www.marine.gov/
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Competition Intraspecific

• Territoriality defense of a resource against
  individuals of the same species
• Examples of wolves and songbirds
• Results in priority access and use of resources
• How do wolves and songbirds establish territory?

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Competition Interspecific

• Grasslands contain plants with both fibrous roots
  and taproots
• Coexist by accessing resources from different
  soil levels

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Introduced Species

• Rabbits in Australia (next slide)
• Chestnut blight in United States
• Japanese beetles, fire ants, gypsy moths in
  United States
• Water hyacinth, kudzu, spotted knapweed, purple
  loosestrife (see Fig. 4-13 in text) in United
  States

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Rabbits Overgrazing in Australia
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Introduced Species

• Why have introductions of nonnative and exotic
  species resulted in a degradation of ecosystems?
  (Think in terms of environmental resistance and
  biotic potential.)
• An example of the answer to this question is
  given in the next slide.

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Introduced Species Rabbits in Australia

• Introduced into Australia from England in 1859
• No natural enemies rabbit population exploded
• Overabundant herbivore population devastated
  natural vegetation (see Fig. 4-11 in text).
• Using disease as control measure why will this
  procedure fail in the long term?

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Mechanisms of Species Adaptation

• Change through natural selection
• Selective pressure determines which organisms
  survive and reproduce and which are eliminated.

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Recipe for Change
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Adaptations to the Environment
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The Limits of Change

• Adapt
• Move (migrate)
• Die (extinction)

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Vulnerability of different organisms to
environmental changes
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Prerequisites for Speciation

• Original population must separate into smaller
  populations that do not interbreed with one
  another.
• List some ways this might happen.
• Separated populations must be exposed to
  different selective pressures.
• Example arctic and gray fox (next slide)

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Speciation Foxes
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Speciation Galápagos Finches
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Ecosystem Responses to Disturbance

• Ecological succession
• Disturbance and resilience
• Evolving ecosystems

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Equilibrium Theory

• Ecosystems are stable environments in which the
  biotic interactions among species determine the
  structure of the communities present.

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Succession and Disturbance

• Ecological succession transition between biotic
  communities
• Primary no previous biotic community
• Secondary previously occupied by a community
• Aquatic transition from pond or lake to
  terrestrial community

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Primary Succession
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Primary Succession

• Mosses invade an area and provide a place for
  soil to accumulate.
• Larger plants germinate in the new soil layer,
  resulting in additional soil formation.
• Eventually shrubs and trees will invade the area.

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Secondary Succession
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Aquatic Succession
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Disturbance and Resilience

• Removes organisms
• Reduces populations
• Creates opportunities for other species to
  colonize

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Fire and Succession
http//www.fs.fed.us/photovideo/
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Ground Fire
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Fire and Succession

• Fire climax ecosystems dependent upon fire for
  maintenance of existing balance e.g.,
  grasslands, pine and redwood forests
• What significance does this have for humans and
  where they live?

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Resilience in Ecosystems
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Resilience Mechanisms after a Forest Fire

• Nutrient release to soil
• Regrowth by remnant roots and seeds
• Invasions from neighboring ecosystems
• Rapid restoration of energy flow and nutrient
  cycling

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Lessons to Learn

• Managing ecosystems
• The pressure of population

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Managing Ecosystems

• Protecting and managing the natural environment
  to maintain the goods and services vital to human
  economy and survival.

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The Pressures of Population

• What is the carrying capacity for the human
  population on Earth?
• How will the human ecological footprint impact on
  natures goods and services?

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Carrying Capacity and Overshoot
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End of Chapter 4