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Chapter 3
Ecosystems: What Are
They and How Do They
Work?
Core Case Study:
Have You Thanked the Insects
Today?
 Many
plant species depend on insects for
pollination.
 Insect can control other pest insects by
eating them
Figure 3-1
Core Case Study:
Have You Thanked the Insects
Today?
 …if
all insects disappeared, humanity
probably could not last more than a few
months [E.O. Wilson, Biodiversity expert].

Insect’s role in nature is part of the larger
biological community in which they live.
THE NATURE OF ECOLOGY
 Ecology
is a study
of connections in
nature.

How organisms
interact with one
another and with
their nonliving
environment.
Figure 3-2
Ecology
‣
Ecology: the study of the relationships between
organisms & the abiotic (nonliving) and biotic (living)
environment.

The physical conditions influence the habitat in which an
organism lives. These include:
substrate
CO
2
O
2
humidity
sunlight
temperature
salinity
pH (acidity)
exposure
altitude
depth

Nutrients
Each abiotic (or physical) factor may be well suited to the
organism or it may present it with problems to overcome.
Universe
Galaxies
Solar systems
Biosphere
Planets
Earth
Biosphere
Ecosystems
Ecosystems
Communities
Populations
Realm of ecology
Organisms
Organ systems
Communities
Organs
Tissues
Cells
Populations
Protoplasm
Molecules
Atoms
Organisms
Subatomic Particles
Fig. 3-2, p. 51
Biological Complexity
‣ Living organisms can
be studied at different
levels of complexity.
‣ From least to most
complex, these levels
are (in an ecological
context):
Individual
Population
Community
Ecosystem
Biome
Biosphere
Biosphere
Biome
Ecosystem
Community
Population
Individual
Organisms and Species
 Organisms,
the different forms of life on
earth, can be classified into different species
based on certain characteristics.
Figure 3-3
Other animals
281,000
Known species
1,412,000
Insects
751,000
Fungi
69,000
Prokaryotes
4,800
Plants
248,400
Protists
57,700
Fig. 3-3, p. 52
Case Study:
Which Species Run the World?
 Multitudes
of tiny microbes such as bacteria,
protozoa, fungi, and yeast help keep us alive.






Harmful microbes are the minority.
Soil bacteria convert nitrogen gas to a usable
form for plants.
They help produce foods (bread, cheese, yogurt,
beer, wine).
90% of all living mass.
Helps purify water, provide oxygen, breakdown
waste.
Lives beneficially in your body (intestines, nose).
Populations, Communities, and
Ecosystems
 Members
of a species interact in groups
called populations.
 Populations of different species living and
interacting in an area form a community.
 A community interacting with its physical
environment of matter and energy is an
ecosystem.
Populations
A
population is a
group of interacting
individuals of the
same species
occupying a specific
area.

The space an
individual or
population normally
occupies is its habitat.
Figure 3-4
Populations
 Genetic

diversity
In most natural
populations
individuals vary
slightly in their
genetic makeup.
Figure 3-5
Community
 The
population of all species living &
interacting in an area.
Ecosystems
 Ecosystems
consist of nonliving (abiotic) and
living (biotic) components.
Figure 3-10
The Biosphere

The biosphere is the region within which all living things
are found on Earth, extending from the bottom of the
oceans to the upper atmosphere.
 The biosphere is but one of the four separate
components of the geochemical model along with the
lithosphere, hydrosphere, and atmosphere.
The Gaia Hypothesis maintains that the Earth is a single
self-regulating complex evolving system. An example being
the exchange of elements between oceans and land.
THE EARTH’S LIFE SUPPORT
SYSTEMS
 The
biosphere
consists of several
physical layers that
contain:





Air
Water
Soil
Minerals
Life
Figure 3-6
Biosphere
 Atmosphere

Membrane of air around the planet.
 Stratosphere

Lower portion contains ozone to filter out most of
the sun’s harmful UV radiation.
 Hydrosphere

All the earth’s water: liquid, ice, water vapor
 Lithosphere

The earth’s crust and upper mantle.
Oceanic
Crust
Atmosphere
Vegetation
Biosphere
and animals
Soil
Crust
Rock
Continental
Crust
Lithosphere
Upper mantle
Asthenosphere
Lower mantle
Core
Mantle
Crust (soil
and rock)
Biosphere
Hydrosphere (living and dead
(water)
organisms)
Lithosphere
Atmosphere
(crust, top of upper mantle)
(air)
Fig. 3-6, p. 54
Habitat
An organism’s habitat is the physical place or
environment in which it lives.
 Organisms show a preference for a particular
habitat type, but some are more specific in their
requirements than others.

Lichens, fungi & algae or bacteria, are
found on rocks, trees, and bare ground.
Most frogs, like this leopard frog, live in or
near fresh water, but a few can survive in
arid habitats.
Habitat Needs
 Cover – shelter; trees, shrubs,
etc.
 Water
 Nutrients
Macronutrients
 Chemicals
organisms need in
large numbers to live, grow, and
reproduce.
 Ex. carbon, oxygen, hydrogen,
nitrogen, calcium, and iron.
Micronutrients
 These
are needed in small or even
trace amounts.
 Ex. sodium, zinc copper, chlorine,
and iodine.
Ecological Niche

The ecological niche
describes the
functional position of
an organism
in its environment.
 A niche comprises:
the habitat in which
the organism lives.
the organism’s
activity pattern: the
periods of time
during which it is
active.
the resources it
obtains
from the habitat.
Adaptations
Habitat
Activity
patterns
Presence of
other organisms
Physical
conditions
The Fundamental Niche

The fundamental niche of
an organism is described
by the full range of
environmental conditions
(biological and physical)
under which the organism
can exist.
 The realized niche of the
organism is the niche that
is actually occupied. It is
narrower than the
fundamental niche.
This contraction of the
realized niche is a result
of pressure from, and
interactions with, other
organisms.
Factors That Limit Population Growth
 Availability
of matter and energy resources
can limit the number of organisms in a
population.
Figure 3-11
Law of Tolerance

The law of tolerance states that “For each abiotic factor, an
organism has a range of tolerances within which it can
survive.”
Tolerance range
Number of organisms
Optimum range
Unavailable Marginal
niche
niche
Examples of abiotic
factors that influence
size of the realized niche
Too
acidic
Too cold
Preferred
niche
Marginal Unavailable
niche
niche
pH
Temperature
Too
alkaline
Too hot
Factors That Limit Population Growth
 The
physical
conditions of the
environment can
limit the
distribution of a
species.
Figure 3-12
Population Growth Cycle
Limited Resources
 A population can grow until competition
for limited resources increases & the
carrying capacity (C.C.) is reached.
Typical Phases
 1.
The population overshoots the C.C.
 2. This is because of a reproductive
time lag (the period required for the
birth rate to fall & the death rate to
rise).
 3. The population has a dieback or
crashes.
 4. The carrying capacity is reached.
What Happens to Solar Energy
Reaching the Earth?
 Solar
energy
flowing through
the biosphere
warms the
atmosphere,
evaporates and
recycles water,
generates winds
and supports
plant growth.
Figure 3-8
Producers: Basic Source of All Food
 Most
producers capture sunlight to produce
carbohydrates by photosynthesis:
Producers: Basic Source of All Food
 Chemosynthesis:

Some organisms such as deep ocean bacteria
draw energy from hydrothermal vents and
produce carbohydrates from hydrogen sulfide
(H2S) gas .
Photosynthesis:
A Closer Look
 Chlorophyll
molecules in the
chloroplasts of plant cells
absorb solar energy.
 This initiates a complex
series of chemical reactions
in which carbon dioxide and
water are converted to
sugars and oxygen.
Figure 3-A
Sun
Chlorophyll
H2O
Light-dependent
Reaction
Chloroplast
in leaf cell
O2
Energy storage
and release
(ATP/ADP)
CO2
6CO2 + 6 H2O
Lightindependent
reaction
Sunlight
Glucose
C6H12O6 + 6 O2
Fig. 3-A, p. 59
Consumers: Eating and Recycling to
Survive
 Consumers
(heterotrophs) get their food by
eating or breaking down all or parts of other
organisms or their remains.

Herbivores
• Primary consumers that eat producers

Carnivores
• Primary consumers eat primary consumers
• Third and higher level consumers: carnivores that eat
carnivores.

Omnivores
• Feed on both plant and animals.
Decomposers and Detrivores


Decomposers: Recycle nutrients in ecosystems.
Detrivores: Insects or other scavengers that feed
on wastes or dead bodies.
Figure 3-13
Scavengers
Longhorned
beetle
holes
Decomposers
Termite
and
Bark beetle Carpenter
carpenter
ant
engraving
galleries ant work Dry rot
fungus
Time
progression
Wood
reduced
to
Mushroom
powder
Powder broken down by decomposers
into plant nutrients in soil
Fig. 3-13, p. 61
Aerobic and Anaerobic Respiration:
Getting Energy for Survival
 Organisms
break down carbohydrates and
other organic compounds in their cells to
obtain the energy they need.
 This is usually done through aerobic
respiration.

The opposite of photosynthesis
Aerobic and Anaerobic Respiration:
Getting Energy for Survival
 Anaerobic


respiration or fermentation:
Some decomposers get energy by breaking
down glucose (or other organic compounds) in
the absence of oxygen.
The end products vary based on the chemical
reaction:
•
•
•
•
Methane gas
Ethyl alcohol
Acetic acid
Hydrogen sulfide
Two Secrets of Survival: Energy Flow
and Matter Recycle
 An
ecosystem
survives by a
combination of
energy flow
and matter
recycling.
Figure 3-14
Decomposition
 As
plant or animal matter dies it will break
down and return the chemicals back to the
soil.
 This happens very quickly in tropical
rainforest which results in low-nutrient soils.
 Grasslands have the deepest and most
nutrient rich of all soils
BIODIVERSITY
Figure 3-15
Biodiversity Loss and Species
Extinction: Remember HIPPO
H
for habitat destruction and degradation
 I for invasive species
 P for pollution
 P for human population growth
 O for overexploitation
Biodiversity Loss and Species
Extinction: Remember HIPPCO
H
for habitat destruction and degradation
 I for invasive species
 P for pollution
 P for human population growth
 C for Climate Change
 O for overexploitation
Why Should We Care About
Biodiversity?
 Biodiversity



provides us with:
Natural Resources (food water, wood, energy,
and medicines)
Natural Services (air and water purification, soil
fertility, waste disposal, pest control)
Aesthetic pleasure
Solutions
 Goals,
strategies
and tactics for
protecting
biodiversity.
Figure 3-16
ENERGY FLOW IN ECOSYSTEMS
 Food
chains and webs show how eaters, the
eaten, and the decomposed are connected to
one another in an ecosystem.
Figure 3-17
Food Webs
 Trophic
levels are
interconnected
within a more
complicated food
web.
Figure 3-18
Energy Flow in an Ecosystem: Losing
Energy in Food Chains and Webs
accordance with the 2nd law of
thermodynamics, there is a decrease in the
amount of energy available to each
succeeding organism in a food chain or web.
 In
Energy Flow in an Ecosystem: Losing
Energy in Food Chains and Webs
 Ecological
efficiency:
percentage of
useable energy
transferred as
biomass from
one trophic level
to the next.
Figure 3-19
10% Rule
 We
assume that 90% of the energy at
each energy level is lost because the
organism uses the energy. (heat)
 It is more efficient to eat lower on the
energy pyramid. You get more out of it!
 This is why top predators are few in
number & vulnerable to extinction.
Productivity of Producers:
The Rate Is Crucial
 Gross
primary
production
(GPP)

Rate at which an
ecosystem’s
producers
convert solar
energy into
chemical energy
as biomass.
Figure 3-20
Net Primary Production (NPP)
 NPP

= GPP – R
Rate at which
producers use
photosynthesis to
store energy minus
the rate at which they
use some of this
energy through
respiration (R).
Figure 3-21
 What
are nature’s three most productive and
three least productive systems?
Figure 3-22
What Sustains Life on Earth?
 Solar
energy, the cycling of matter, and gravity
sustain the earth’s life.
Figure 3-7
MATTER CYCLING IN
ECOSYSTEMS
 Nutrient



Cycles: Global Recycling
Global Cycles recycle nutrients through the
earth’s air, land, water, and living organisms.
Nutrients are the elements and compounds that
organisms need to live, grow, and reproduce.
Biogeochemical cycles move these substances
through air, water, soil, rock and living
organisms.
The Water Cycle
Figure 3-26
Water’ Unique Properties
 There
are strong forces of attraction between
molecules of water.
 Water exists as a liquid over a wide
temperature range.
 Liquid water changes temperature slowly.
 It takes a large amount of energy for water to
evaporate.
 Liquid water can dissolve a variety of
compounds.
 Water expands when it freezes.
Effects of Human Activities
on Water Cycle
 We




alter the water cycle by:
Withdrawing large amounts of freshwater.
Clearing vegetation and eroding soils.
Polluting surface and underground water.
Contributing to climate change.
The Carbon Cycle:
Part of Nature’s Thermostat
Figure 3-27
The Carbon
Cycle
Effects of Human Activities
on Carbon Cycle
 We
alter the
carbon cycle by
adding excess CO2
to the atmosphere
through:


Burning fossil fuels.
Clearing vegetation
faster than it is
replaced.
Figure 3-28
Carbon Cycling
‣
‣
Carbon cycles as gaseous carbon is fixed in
the process of photosynthesis and returned
to the atmosphere in respiration.
Burning fossil fuels
Carbon may remain locked up in sinks or
reservoirs that are biotic or abiotic for long
periods of time, e.g. in the wood of trees,
oceans or in fossil fuels such as coal or oil.
‣
‣
Indirectly carbon forms carbonate deposits
as carbon is removed from the atmosphere.
(The carbonate is stored mostly in the
marine ecosystem.)
Humans have disturbed the balance of
the carbon cycle through activities such
as combustion and deforestation.
Petroleum & Coal
The Nitrogen Cycle: Bacteria in Action
Nitrogen Cycle
Most of the work done by bacteria & decomposers
Process
Product
Formulas
Nitrogen Fixation
Ammonia
N2 → NH4
Nitrification
Nitrite then nitrate
NH4 → NO2 →NO3
Assimilation
Proteins: amino
acids, DNA
NO3 (some NH4) →
uptake by plant
Ammonification
(Decay & waste
products)
Ammonia
Plant & animal
decay & excretions
→ NH4
Denitrification
Nitrogen gas
NO3 → N2
Nitrogen in the Environment
‣
Nitrogen cycles between the biotic and
abiotic environments. The largest
reservoir for nitrogen is the atmosphere
and thus it is difficult to fix, bacteria play
an important role in this transfer.
Nitrogen-fixing bacteria are able to fix
atmospheric nitrogen.
Lightning can fix
atmospheric nitrogen
Nitrifying bacteria convert ammonia to
nitrite, and nitrite to nitrate.
Denitrifying bacteria return fixed
nitrogen to the atmosphere.

Atmospheric fixation also occurs as a
result of lightning discharges.
‣
Humans intervene in the nitrogen cycle by
producing and applying nitrogen (nitrates)
fertilizers which is the main cause of
eutrophication.
Bacteria on the roots of
legumes can fix nitrogen
Effects of Human Activities
on the Nitrogen Cycle
 We




alter the nitrogen cycle by:
Adding gases that contribute to acid rain.
Adding nitrous oxide to the atmosphere through
farming practices which can warm the
atmosphere and deplete ozone.
Contaminating ground water from nitrate ions in
inorganic fertilizers.
Releasing nitrogen into the troposphere through
deforestation.
Effects of Human Activities
on the Nitrogen Cycle
 Human
activities
such as
production of
fertilizers now fix
more nitrogen
than all natural
sources
combined.
Figure 3-30
The Phosphorous Cycle
mining
excretion
Fertilizer
Guano
agriculture
uptake by
uptake by weathering
autotrophs
autotrophs
leaching, runoff
Dissolved
Land
Marine
Dissolved
in Soil Water,
Food
Food
in Ocean
Lakes, Rivers
Webs
Webs
Water
death,
death,
decomposition
decomposition
weathering
sedimentation
settling out
uplifting over
geologic time
Rocks
Marine Sediments
Fig. 3-31, p. 77
Phosphorus Cycling

Phosphorus cycling is very slow and
tends to be local and stable; in aquatic
and terrestrial ecosystems, phosphorous
is a sedimentary cycle.
Phosphorous is lost from ecosystems
through run-off, precipitation, and
sedimentation.
A very small amount of phosphorus
returns to the land as guano (manure
of fish-eating birds). Weathering and
phosphatizing bacteria return
phosphorus to the soil.
Deposition as guano…
Loss via sedimentation…
Often the Limiting factor in the
ecosystem
The phosphorous cycle has no real
significant gas phase and under
many conditions will form stable
insoluble compounds
Fertilizer production
Effects of Human Activities
on the Phosphorous Cycle
 We
remove large amounts of phosphate from
the earth to make fertilizer.
 We reduce phosphorous in tropical soils by
clearing forests.
 We add excess phosphates to aquatic
systems from runoff of animal wastes and
fertilizers contributing to eutrophication
The Sulfur Cycle
Figure 3-32
Sulfur
trioxide
Water
Acidic fog and
precipitation
Sulfuric acid
Ammonia
Oxygen
Sulfur dioxide
Ammonium
sulfate
Hydrogen sulfide
Plants
Dimethyl
sulfide
Volcano
Industries
Animals
Ocean
Sulfate salts
Metallic
sulfide
deposits
Decaying matter
Sulfur
Hydrogen sulfide
Fig. 3-32, p. 78
Sulfur Cycling

Sulfur is naturally occurring in rock or
mineral forms and is a sedimentary
cycle.

Sulfur is an essential component of
proteins and is important in determining
the acidity of precipitation, surface
water, and soil.

Sulfur circulates through the biosphere
as:
hydrogen sulfide (H2S), sulfur
dioxide (SO2), sulfate (SO42-), and
elemental sulfur (S)
Sulfur in petrol
Molecular bridges in proteins
Elemental sulfur
Effects of Human Activities
on the Sulfur Cycle
 We



add sulfur dioxide to the atmosphere by:
Burning coal and oil
Refining sulfur containing petroleum.
Convert sulfur-containing metallic ores into free
metals such as copper, lead, and zinc releasing
sulfur dioxide into the environment.
HOW DO ECOLOGISTS LEARN ABOUT
ECOSYSTEMS?
 Ecologist
go into ecosystems to observe, but
also use remote sensors on aircraft and
satellites to collect data and analyze
geographic data in large databases.


Geographic Information Systems
Remote Sensing
 Ecologists
also use controlled indoor and
outdoor chambers to study ecosystems
Geographic Information Systems (GIS)
A
GIS organizes,
stores, and analyzes
complex data
collected over broad
geographic areas.
 Allows the
simultaneous
overlay of many
layers of data.
Figure 3-33
Critical nesting site
locations
USDA Forest Service
USDA
Private Forest Service
owner 1
Private owner 2
Topography
Forest
Habitat type
Wetland Lake
Grassland
Real world
Fig. 3-33, p. 79
Systems Analysis
 Ecologists
develop
mathematical and
other models to
simulate the
behavior of
ecosystems.
Figure 3-34
Systems
Measurement
Define objectives
Identify and inventory variables
Obtain baseline data on variables
Data
Analysis
Make statistical analysis of
relationships among variables
Determine significant interactions
System
Modeling
Objectives Construct mathematical model
describing interactions among
variables
System
Simulation
System
Optimization
Run the model on a computer,
with values entered for different
Variables
Evaluate best ways to achieve
objectives
Fig. 3-34, p. 80
Importance of Baseline
Ecological Data
 We
need baseline data on the world’s
ecosystems so we can see how they are
changing and develop effective strategies for
preventing or slowing their degradation.

Scientists have less than half of the basic
ecological data needed to evaluate the status of
ecosystems in the United Sates (Heinz
Foundation 2002; Millennium Assessment 2005).