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 (modified)

2
Factors That Contribute to Ecosystem Change

• Dynamics of natural populations
• Mechanisms of population equilibrium
• Ecosystem response to disturbance

3
Dynamics of Natural Populations

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

4
Population Equilibrium
A dynamic balance between births and deaths.

• Births

Deaths
5
Population Growth Curves
6
Population Growth Curves

• Reproductive strategies

Not really- think insects, fish, frogs
Many offspring with low parental care
Maybe, maybe not
Few offspring with high parental care
J-shaped growth curve
S-shaped growth curve
R strategists
K strategists
7
Population Dynamics

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

8
Biotic Potential and Environmental Resistance
9
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

10
Mechanisms of Population Equilibrium

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

11
PredatorPrey Balance Wolves and Moose
12
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.
• Predators reduce the herbivore population it is
  maintained so that overgrazing or other overuse
  does not occur.

13
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

14
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?

15
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/
16
Competition- an overview

• Interspecific competition- different species
  compete for the same resource- my tomatoes and
  sheep sorel in my garden!
• It is often less of a problem because or specific
  habitats and niches, until an invader appears-
  zebra mussel, ruffe, etc.
• Intraspecific competition- competition within a
  species- songbirds sing mainly to warn other
  songbirds to stay out!

17
Ecosystem Responses to Disturbance

• Ecological succession
• Disturbance and resilience
• Evolving ecosystems

18
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

19
Primary Succession
20
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.

21
Secondary Succession
22
Aquatic Succession
23
Disturbance and Resilience

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

24
Disturbance and Resilience

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

25
Fire and Succession
http//www.fs.fed.us/photovideo/
26
Ground Fire
27
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?

28
Resilience in Ecosystems
29
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

30
What you should know from Ch. 4Population growth
curves- J vs SBiotic potentialEnvironmental
ResistanceDensity dependent and independent
resistance factorsK vs R strategistsKeystone
speciesInter intraspecific competitionSuccessi
on- primary secondaryDisturbance and resilience