A.P. Environmental Science
Objective: The Learner will be able to:
1) Explain the dynamics of natural populations.
2) Discuss the mechanisms of populations equilibrium
and mechanisms of species adaptation.
3) Explain ecosystem responses to disturbance.
4) Identify several invasive species, explain how they
arrived and the impact they have had on ecosystems.
Yellowstone – June 1988
 Lightning started fire
 Burned thousands of acres of
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

forest
Park Service Manages
Yellowstone
Smokey Bear Policy – put out
fires before they get started!
Park Service policy -burn itself
out unless near human
habitation
Largest fire-fighting effort in
U.S. History
Finally put out by snowfall
The Yellowstone fire was a “crown fire”.
Yellowstone
 Fires marked in red
with infrared satellite
image.
 10% of park area was
burned
Yellowstone - Recovery
Here is a
picture of
Yellowstone
July 1986
before the
fire
Yellowstone 1989
Same
Site
August
1989
 Same site in July
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
1999
Recovery was
immediate. Within
two weeks grasses
began sprouting
Herbivores fed on
the new growth
25 years later the
diversity of plants
and animals in the
burned area have
completely recovered
Predators – prey
reacted positively to
disturbance
Dynamics of Natural Populations
 Each species exists in
a population
 Over time most
species remain more
or less constant in size
and geographic
distribution
 Deaths = Births
– This balance is called
Population equilibrium
Growth Curves – Two Types J or S
 Exponential growth results
in population explosion
 Rule of 70
 to find the doubling time of a
quantity growing at a given
annual percentage rate,
divide the percentage number
into 70 to obtain the
approximate number of years
required to double.
 For example, at a 10% annual
growth rate, doubling time is
70 / 10 = 7 years.
 This results in a J curve
graph.
Dynamics of Natural Populations
 Population growth curves
 Biotic potential (the ability to increase
population numbers) versus environmental
resistance (the combination of all the biotic
and abiotic factors that limit a population’s
increase.)
 If recruitment is at replacement level, then
the population will remain constant.
 Carrying capacity – the upper limit to the
population of any particular organism that
an ecosystem can support
Density Dependence And Critical Number
 Environmental resistance factors can be
density dependent.
– If population density increases, environmental
resistance becomes more intense and causes in
increase in mortality.
– If population density decreases, environmental
resistance lessens, allowing the population to
recover.
 Environmental factors that cause mortality
can be density independent
– A sudden deep freeze in spring
– A fire that may kill all small mammals
Environmental Resistance
 Environmental resistance
 Biotic Potential
– Reproductive rate
– Ability to migrate
(animals) or disperse
(seeds)
– Ability to invade new
habitats
– Defense mechanisms
– Ability to cope with
adverse conditions
–
–
–
–
–
–
–
–
Lack of food or nutrients
Lack of water
Lack of suitable habitat
Adverse weather
Predators
Disease
Parasites
Competitors
Critical Number
 The survival and recovery of a population
depends on a certain minimum population
base – this is known as the population’s
critical number.
Reproductive Strategies
 Reproduce in massive




quantities
Leave nature to raise the
young
“low recruitment’ = high
mortality rate
Favors quickly changing
environments
Organisms are usually
–
–
–
–
–
Small
Rapid reproductive rate
Short life spans
J – curves
Instinct important
 Few offspring
 Spend a lot of time
nurturing their young
 Organisms
–
–
–
–
Large
Long life spans
S – curves
Learned behavior
Environmental Resistance
 Population explosions seldom
occur in nature
 Biotic and abiotic factors cause
mortality in populations
 Biotic factors
– Predators
– Parasites
– Competitors
 Abiotic factors
–
–
–
–
Changes in temperatures
Moisture
Salinity
pH
Carrying Capacity
 Upper Limit to
population number
 Maximum population
that a given habitat
can support without
the habitat being
degraded over time
 (sustainable system)
Density Dependence & Critical Number
 The size of a population remains within a certain range when
environmental resistance factors are density dependent
 As population density (the number of individuals per unit
area) increases, Environmental resistance becomes more
intense and causes an increase in mortality, resulting in the
population decreasing.
Critical number
 The survival and
recovery of a
population depends on
a certain minimum
population base
(critical number)
 If population is
depleted below this
number surviving
members become
more vulnerable,
breeding fails, and
extinction is almost
inevitable
Human impact activities
 Altering habitat
 Pollution
 Over-hunting
 Intro alien species
 Pet exploitation
 Collectors
Alien Plants
Mechanisms of Population Equilibrium
 Predator – Prey
– A rise in the moose
population
– Rise in the wolf
population
– Higher predation
– Lower moose
population
– Lag time from year to
year
Parasites
 Worms, bacteria, viruses,




protists, fungi
Similar to predator – prey
relationship
Most parasites do not
cause the host to die
Population density is high
– parasites and vectors
find hosts easily
Population density is low
– parasites have trouble
finding hosts
Plant – Herbivore Dynamics
 Overgrazing
 Herbivore population
is high, feed on
grasses to the roots
 Causes loss of
secondary consumers
 May take long time to
recover food chain
 Lead to erosion
Keystone species
 An organism that
plays a crucial role in
maintaining ecosystem
biostructure
Competition
 Interspecific
– No individual lives in
isolation from other
members
– competition between
two or more species for
some limiting resource.
– have identical
requirements
– Niches (roles) over-lap
– Competition from
members of the same
species
Competition
 Intraspecific -the struggle between members of a
population for scarce resources. As in intraspecific
cooperation, there are two basic types of competition:
1. Adapted or programmed - results from aggressive social behavior
such as dominance hierarchies and territoriality.
2. Only certain individuals high in the peck-order, or holding territories,
succeed in breeding. This is sometimes called contest competition
because it involves aggressive contests between competing individuals.
3. Unadapted or incidental - results from the accidental interaction
between individual organisms utilizing the same resources, for
resources used by one are unavailable to others. This is sometimes
called scramble competition because everybody is involved in a mad
scramble for the scarce resources.
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
The
herbivore population subsequently crashes.
size of the herbivore population is maintained so
that overgrazing or other overuse does not occur.
Reindeer on St. Matthew Island
Plant–Herbivore
Dynamics
No
regulatory
control (predation)
on herbivores
Went
into
exponential growth
pattern
Overgrazed
Massive
habitat
die-off of
herbivores
Rabbits Overgrazing in Australia
Mechanisms of Species Adaptation (selective pressure)
Selective Pressure: How it works
 All Individuals of a
species have differences
 If one difference is able to
be inherited and it allows
for the individual to
survive, and it lives to
reproduce, its young will
have the gene to survive
and over time adapt to
changes in the
environment
 Red & black in nature
means “leave me alone”
Limits of Change
 Organisms facing extreme
selective pressure can
 Adapt – Population of
survivors may adapt to the
new conditions through
natural selection
 Migrate – Surviving
populations may migrate
and find an area where
conditions are suitable to
them
 Extinction – Failing the
first two choices, the third
is inevitable
Four Keys to Survival
 Geographical
distribution
 Specialization to a
given habitat or food
supply
 Genetic variation
within the gene pool
of the species
 The reproductive rate
relative to the rate of
environmental change
Vulnerability of different organisms to
environmental changes
Prerequisites for
Speciation
 Original population must
separate into smaller
populations that do not
interbreed with one another
 Reproductive isolation
 Unable to interbreed and
produce fertile offspring
 Separated populations must be
exposed to different selective
pressures
Speciation
Darwin’s Finches
Ecosystem Response to Disturbance
 Equilibrium theory
– ecosystems are stable
environments in which
species interact constantly
in well-balanced predatorprey and competitive
relationships
 Biotic interactions
– Determine the structure of
living communities within
ecosystems
Ecological Succession
 Ecological succession:
transition between biotic
communities
– Primary: no previous biotic
community
• Ex: Newly formed island
– Secondary: previously
occupied by a community
• Ex: Mount St. Helens blast
area
– 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
Disturbance and Resilience
 Removes organisms
 Reduces populations
 Creates opportunities
for other species to
colonize
Fire and Succession
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
 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!