Download Environmental Science

Richard T. Wright
Environmental
Science
Tenth Edition
Chapter 4
Ecosystems: How They Change
Copyright © 2008 Pearson Prentice Hall, Inc.
Fire and Succession
http://www.fs.fed.us/photovideo/
One Year Later
Thirteen Years Later
Moving on
• Chapters 2 and 3:Structure
– Structure and Function of Ecosystems as Sustainable
Units in Natural Landscapea
• Chapter 4 : Ecosystem Dynamics
– What does it mean for an ecosystem to be in
balance?
– What happens after a major natural disturbance such
as a fire?
– Changes are the only constant in ecosystems
– The notion of balance must be carefully defined
KEY TOPICS
•
•
•
•
Dynamics of natural populations
Mechanisms of population equilibrium
Evolution as a force for change
Ecosystem response to disturbance
4.1 Dynamics of Natural Populations
• Population growth curves
• Biotic potential versus environmental
resistance
• Density dependence and critical number
Population Equilibrium
A dynamic balance between births and deaths.
Births
Deaths
Population Growth Curves
Exponential Growth
• Growth by a factor (by
multiplication, by a
percentage)
• Unrestricted Growth under
ideal conditions
• Doubling time is constant
• Population grows faster and
faster in numbers
• Leads to population
explosion
Genera Number of Rabitts
-tion
0
2 = 2 x 100
1
2
20 = 2 x 101
200 = 2 x 102
4
2000 = 2 x 103
5
20000 = 2 x104
Exponential Growth
Rabbits Population Growth Curve
Polulation
250000
200000
y = 2e
150000
2.30x
Series 1
100000
Ex pon. (Series1)
50000
0
0
1
2
3
4
5
6
Months
Biotic Potential
Doubling Time
y = 2 × e 2.30 x
e = 2.72
P (t ) = P (0)e rt
DT ~
70
%r
r : growth rate
Fate of Exponential Growth (J-curve)
• Population crash due to starvation and
disease
• Natural control mechanisms return the
population into the S-curve balance
Dynamic Equilibrium
• In natural ecosystems, undisturbed by
human activities, populations are in a
dynamic equilibrium represented by the Scurve
• J-curves occur in cases of disturbances or
sudden alteration of a habitat
• Environmental resistance limits population
growth
Population Dynamics
• Environmental resistance: combination of biotic
and abiotic factors that may limit population
increase.
• Biotic potential is the number of offspring that a
species may produce under ideal conditions
• Recruitment is survival to reproducing age
• Replacement level is a recruitment level that just
replaces the parents
Biotic Potential and Reproductive Strategies
• Two Reproductive Strategies:
Many offspring (high biotic potential)
withlow parental care
(low recruitment)
Few offspring (low
biotic potential)
With high parental
care
J-shaped growth curve
S-shaped growth curve
Factors of Environmental Resistance
• Abiotic factors
–
–
–
–
Temperature, Moisture, Light
Salinity, pH
Lack of Nutrients
Fire
• Biotic factors
– Predation
– Competition
– Parasites
Carrying Capacity
• The carrying capacity is maximum population of
a species that a given habitat can support
sustainably
Population dynamics are regulated by the interplay
between Biotic Potential and Environmental
Resistance
• Biotic Potential is a
constant for any given
species
• Therefore, it is ER
that determines
growth or decline of
population
• Density dependence
of ER factors limit the
population size
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, parasites
• Density dependent environmental factor keep
the population within a limited range
– As population density increases, environmental
resistance increases too leading to increase in
mortality
• Critical number: the lowest population level for
survival and recovery
Critical Number and Extinction
• Endangered species: population size near
critical number
• Threatened species: rapidly declining
population
4.2 Close Look at Specific Mechanisms of
Population Equilibrium
• Top-down regulation is control of population by
predation
• Bottom-up regulation is control of population by
scarcity of resources
-----------------------------------------------------------------• Predator-prey dynamics
• Competition
– Interspecific
– Intraspecific
• Introduced species
4.2.1 Predator-Prey Dynamics
Herbivore Population is Regulated by their Predators
Simple ecosystem
Isle Royale
Moose-Wolves : Top-down or Bottom-up
Population Regulation?
• Both
• Top-down for the moose
• Bottom-up for the wolves
• Parasites, like predators, can also regulate
the population of their host in a density
dependent fashion
4.2.2 Plant-Herbivore Dynamics
Reindeer on St. Matthew Island
• No regulatory control
(predation) on
herbivores
• Population grew,
exploded, overgrazed
and crashed
• Overgrazing: eating
plants faster than the
plants grow.
4.2.3 Keystone Species
• A single species that
plays a crucial role in
maintaining the biotic
structure of the
ecosystem
• Pisaster ochraeceus a starfish that feeds
on mussels, keeping
them from blanketing
the rocks.
http://www.marine.gov/
4.2.5 Competition: Intraspecific
• Territoriality: defense of a resource
against individuals of the same species
– Results in priority access and use of
resources
– Breeding is restricted to those who can claim
and defend their territory
– Survival of the fittest!
• How do wolves and songbirds establish
territory?
4.2.5.Competition: Interspecific
• Grassland contains
plants with both
fibrous and tap roots
• Coexist by accessing
resources from
different soil levels
4.2.6 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.)
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).
Rabbits Overgrazing in Australia
Phillip Island 1978
Phillip Island 1988 After Rabbit Removal
The Third Principle of Ecosystem
Sustainability
• The size of the consumer population is
maintained so that overgrazing or other
overuse does not occur.
• First?
• Second?
4.3 Evolution as a Force for Change
• Adaptations through natural selection
• Drifting continents
Mechanisms of Species Adaptation
• Change through natural selection.
– Selective pressure determines which organisms
survive and reproduce and which are eliminated
Recipe for Change
GENES
+
EN
V
IRO
NM
EN
T
ADAPTATIONS
NATURAL SELECTION
Adaptations for Survival and Reproduction
The Limits of Change
• Adapt
• Move (migrate)
• Die (extinction)
Vulnerability of different organisms to
environmental changes.
Which Species Will Survive in a
Rapidly Changing Environment?
• Panda
Narrow distribution
Small population
Low genetic variation
Large size
Small # of offspring
Limited migration
• Mouse
Wide distribution
Large population
High genetic variation
Small size
Large # of offspring
High migration
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
Drifting Continents: Pangaea 225 Million
Years Ago
Earth’s Crust: 14 Tectonic Plates
4.4 Ecosystem Responses to Disturbance
• Ecological succession
• Disturbance and resilience
• Evolving ecosystems?
Equilibrium Theory
• Ecosystems are stable environments in
which species interact constantly in well
balanced predator-prey and competitive
relationships.
• Balance of nature idea derived from this
theory.
Succession and Disturbance
• Ecological succession: the orderly and
progressive replacement of one
community type by another until a climax
stage is reached.
– Primary - no previous biotic community
– Secondary - previously occupied by a
community
– Aquatic - transition from pond or lake to
terrestrial community
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
This used to be a lake!
Disturbance and Resilience
• Removes organisms
• Reduces populations
• Creates opportunities for other species to
colonize
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
Living Beyond Our Means (MA)
• Managing ecosystems
• The pressure of population
• We have already exceeded the earth’s
carrying capacity
Managing Ecosystems
• Protecting and managing the natural
environment to maintain the goods and
services vital to human economy and
survival.
Pressures on Ecosystems
• What is the carrying
capacity for the
human population on
Earth?
• How will the human
ecological footprint
impact on nature’s
goods and services?