Transcript wfsc420 lesson04

Environmental Science: Toward a
Sustainable Future
Richard T. Wright
Chapter 4
Ecosystems: Populations and
Succession
PPT by Clark E. Adams
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
Dynamics of Natural Populations



Population growth curves
Biotic potential versus environmental
resistance
Density dependence and critical number
Biotic Potential and Environmental
Resistance
Population Equilibrium
A dynamic balance between births and deaths.
Births
Deaths
Population Growth Curves
Population Growth Curves

Reproductive strategies:
Many offspring with
low parental care
J-shaped growth curve
Few offspring with
high parental care
S-shaped growth curve
Population Dynamics

Environmental resistance: combination of
biotic and abiotic factors that may limit
population increase


Predators, competitors, disease
Adverse weather, limited food/nutrients
Biotic Potential and Environmental
Resistance
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
Mechanisms of Population Equilibrium


Predator–prey dynamics
Competition



Interspecific
Intraspecific
Introduced species
Predator–Prey Balance: Wolves and
Moose
Lessons to Be Learned about Predator–Prey
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.
Plant–Herbivore Dynamics
Reindeer on St. Matthew Island




No regulatory control
(predation) on
herbivores
Went into exponential
growth pattern
Overgrazed habitat
Massive die-off of
herbivores
Mechanisms of Population Equilibrium:
Plant–Herbivore


Compare the predator–prey with plant–herbivore
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?
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/
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?
Competition: Interspecific


Grasslands contain
plants with both
fibrous roots and
taproots
Coexist by accessing
resources from
different soil levels
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
Rabbits Overgrazing in Australia
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.
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. 411 in text).
Using disease as control measure – why
will this procedure fail in the long term?
Mechanisms of Species Adaptation

Change through natural selection

Selective pressure determines which organisms
survive and reproduce and which are eliminated.
Recipe for Change
+
ADAPTATIONS
NATURAL SELECTION: For? or Against?
Adaptations to the Environment
The Limits of Change



Adapt
Move (migrate)
Die (extinction)
Vulnerability of different organisms to
environmental changes
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)
Speciation: Foxes
Speciation: Galápagos Finches
Ecosystem Responses to Disturbance



Ecological succession
Disturbance and resilience
Evolving ecosystems
Equilibrium Theory

Ecosystems are stable environments in
which the biotic interactions among
species determine the structure of the
communities present.
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
Primary Succession
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.
Secondary Succession
Aquatic Succession
Disturbance and Resilience



Removes organisms
Reduces populations
Creates opportunities for other species to
colonize
Fire and Succession
http://www.fs.fed.us/photovideo/
Ground Fire
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?
Resilience in Ecosystems
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
Lessons to Learn


Managing ecosystems
The pressure of population
Managing Ecosystems

Protecting and managing the natural
environment to maintain the goods and
services vital to human economy and
survival.
The Pressures of Population


What is the carrying
capacity for the
human population on
Earth?
How will the human
ecological footprint
impact on nature’s
goods and services?
Carrying Capacity and Overshoot
End of Chapter 4