Chapter 4
Ecosystems: What Are
They and How Do They
Work?
Chapter Overview Questions
• What is ecology?
• What basic processes keep us and other
organisms alive?
• What are the major components of an
ecosystem?
• What happens to energy in an ecosystem?
• What are soils and how are they formed?
• What happens to matter in an ecosystem?
• How do scientists study ecosystems?
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
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
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
THE EARTH’S LIFE SUPPORT
SYSTEMS
• The biosphere
consists of several
physical layers that
contain:
– Air
– Water
– Soil
– Minerals
– Life
Figure 3-6
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
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.
What Sustains Life on Earth?
• Solar energy,
the cycling of
matter, and
gravity sustain
the earth’s life.
Figure 3-7
Biosphere
Carbon
cycle
Phosphorus
cycle
Nitrogen
cycle
Water
cycle
Oxygen
cycle
Heat in the environment
Heat
Heat
Heat
Fig. 3-7, p. 55
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
Solar
radiation
Energy in = Energy out
Reflected by
atmosphere (34% )
UV radiation
Absorbed
by ozone
Visible
Light
Absorbed
by the
earth
Radiated by
atmosphere
as heat (66%)
Lower Stratosphere
(ozone layer)
Troposphere Greenhouse
effect
Heat
Heat radiated
by the earth
Fig. 3-8, p. 55
ECOSYSTEM COMPONENTS
• Life exists on land systems called biomes
and in freshwater and ocean aquatic life
zones.
Figure 3-9
Average annual precipitation
100–125 cm (40–50 in.)
75–100 cm (30–40 in.)
50–75 cm (20–30 in.)
25–50 cm (10–20 in.)
below 25 cm (0–10 in.)
4,600 m (15,000 ft.)
3,000 m (10,000 ft.)
1,500 m (5,000 ft.)
Coastal
mountain
ranges
Sierra
Nevada
Mountains
Great
American
Desert
Coastal chaparral Coniferous
and scrub
forest
Rocky
Mountains
Desert
Great
Plains
Coniferous
forest
Mississippi
River Valley
Prairie
grassland
Appalachian
Mountains
Deciduous
forest
Fig. 3-9, p. 56
Nonliving and Living
Components of Ecosystems
• Ecosystems consist of nonliving (abiotic) and
living (biotic) components.
Figure 3-10
Oxygen
(O2)
Sun
Producer
Carbon dioxide (CO2)
Secondary consumer
Primary
(fox)
consumer
(rabbit)
Precipitation
Falling leaves
and twigs
Producers
Soil decomposers
Water
Fig. 3-10, p. 57
Factors That Limit Population
Growth
• Availability of matter and energy resources
can limit the number of organisms in a
population.
Figure 3-11
Abundance of organisms
Upper limit of
tolerance
Few
No
organisms organisms
Population size
Lower limit of
tolerance
No
Few
organisms
organisms
Zone of
intolerance
Low
Zone of
physiological
stress
Optimum range
Temperature
Zone of
physiological
stress
Zone of
intolerance
High
Fig. 3-11, p. 58
Factors That Limit Population
Growth
• The physical
conditions of the
environment can
limit the
distribution of a
species.
Figure 3-12
Sugar Maple
Fig. 3-12, p. 58
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
Fig. 3-A, p. 59
Animation: Linked Processes
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
Heat
Abiotic chemicals
(carbon dioxide,
oxygen, nitrogen,
minerals)
Heat
Solar
energy
Heat
Producers
(plants)
Decomposers
(bacteria, fungi)
Heat
Consumers
(herbivores,
carnivores)
Heat
Fig. 3-14, p. 61
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
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
The Ecosystem Approach The Species Approach
Goal
Goal
Protect populations
of species in their
natural habitats
Protect species
from premature
extinction
Strategy
Preserve sufficient
areas of habitats in
different biomes and
aquatic systems
Strategies
Tactics
•Protect habitat areas
through private
purchase or
government action
•Eliminate or reduce
populations of
nonnative species
from protected areas
•Manage protected
areas to sustain
native species
•Restore degraded
ecosystems
•Identify endangered
species
•Protect their critical
habitats
Tactics
•Legally protect
endangered species
•Manage habitat
•Propagate
endangered
species in captivity
•Reintroduce
species into
suitable habitats
Fig. 3-16, p. 63
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
First Trophic
Level
Second Trophic
Level
Third Trophic
Level
Producers
(plants)
Primary
consumers
(herbivores)
Secondary
consumers
(carnivores)
Heat
Heat
Fourth Trophic
Level
Tertiary
consumers
(top carnivores)
Heat
Solar
energy
Heat Heat
Heat
Heat
Heat
Detritivores
(decomposers and detritus feeders)
Fig. 3-17, p. 64
Animation: Prairie Trophic
Levels
ANIMATION
Food Webs
• Trophic levels are
interconnected
within a more
complicated food
web.
Figure 3-18
Humans
Blue whale
Sperm whale
Crabeater
seal
Elephant
seal
Killer whale
Leopard
seal
Adelie
penguins
Emperor
penguin
Petrel
Fish
Squid
Carnivorous plankton
Krill
Herbivorous
plankton
Phytoplankton
Fig. 3-18, p. 65
Animation: Categories of Food
Webs
Animation: Prairie Food Web
PLAY
ANIMATION
Energy Flow in an Ecosystem:
Losing Energy in Food Chains
and Webs
• In 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.
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
Heat
Tertiary
consumers
(human)
Heat
Decomposers
Heat
10
Secondary
consumers
(perch)
Heat
100
1,000
Primary
consumers
(zooplankton)
Heat
10,000
Producers
Usable energy (phytoplankton)
Available at
Each tropic level
(in kilocalories)
Fig. 3-19, p. 66
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
Gross primary productivity
(grams of carbon per square meter)
Fig. 3-20, p. 66
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
Sun
Respiration
Gross primary
production
Growth and reproduction
Energy lost
and unavailable
to consumers
Net primary
production
(energy
available to
consumers)
Fig. 3-21, p. 66
• What are nature’s three most productive and
three least productive systems?
Figure 3-22
Terrestrial Ecosystems
Swamps and marshes
Tropical rain forest
Temperate forest
North. coniferous forest
Savanna
Agricultural land
Woodland and shrubland
Temperate grassland
Tundra (arctic and alpine)
Desert scrub
Extreme desert
Aquatic Ecosystems
Estuaries
Lakes and streams
Continental shelf
Open ocean
Average net primary productivity (kcal/m2 /yr)
Fig. 3-22, p. 67
SOIL: A RENEWABLE
RESOURCE
• Soil is a slowly renewed resource that
provides most of the nutrients needed for
plant growth and also helps purify water.
– Soil formation begins when bedrock is broken
down by physical, chemical and biological
processes called weathering.
• Mature soils, or soils that have developed
over a long time are arranged in a series of
horizontal layers called soil horizons.
SOIL: A RENEWABLE
RESOURCE
Figure 3-23
Oak tree
Wood
sorrel
Lords and
ladies
Fern
O horizon
Leaf litter
Dog violet
Grasses and
small shrubs
Earthworm
Millipede
Honey
fungus
Mole
Organic debris
builds up
Rock
fragments
Moss and
lichen
A horizon
Topsoil
B horizon
Subsoil
Bedrock
Immature soil
Regolith
Young soil
Pseudoscorpion
C horizon
Mite
Parent
material
Nematode
Root system
Mature soil
Red Earth
Mite
Springtail
Actinomycetes
Fungus
Bacteria
Fig. 3-23, p. 68
Animation: Soil Profile
PLAY
ANIMATION
Layers in Mature Soils
• Infiltration: the downward movement of
water through soil.
• Leaching: dissolving of minerals and
organic matter in upper layers carrying
them to lower layers.
• The soil type determines the degree of
infiltration and leaching.
Soil Profiles of the
Principal
Terrestrial Soil
Types
Figure 3-24
Mosaic of
closely
packed
pebbles,
boulders
Weak humusmineral mixture
Desert Soil
(hot, dry climate)
Dry, brown to
reddish-brown
with variable
accumulations
of clay, calcium
and carbonate,
and soluble
salts
Alkaline,
dark,
and rich
in humus
Clay,
calcium
compounds
Grassland Soil
semiarid climate)
Fig. 3-24a, p. 69
Acidic
light-colored
humus
Iron and
aluminum
compounds
mixed with
clay
Tropical Rain Forest Soil
(humid, tropical climate)
Fig. 3-24b, p. 69
Forest litter leaf
mold
Humus-mineral
mixture
Light, grayishbrown, silt loam
Dark brown
firm clay
Deciduous Forest Soil
(humid, mild climate)
Fig. 3-24b, p. 69
Acid litter
and humus
Light-colored
and acidic
Humus and
iron and
aluminum
compounds
Coniferous Forest Soil
(humid, cold climate)
Fig. 3-24b, p. 69
Some Soil Properties
• Soils vary in the size
of the particles they
contain, the amount
of space between
these particles, and
how rapidly water
flows through them.
Figure 3-25
Sand
0.05–2 mm
diameter
Silt
0.002–0.05 mm
diameter
Water
High permeability
Clay
less than 0.002 mm
Diameter
Water
Low permeability
Fig. 3-25, p. 70
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
Condensation
Rain clouds
Transpiration Evaporation
Transpiration
Precipitation
to land
from plants
Precipitation
Runoff
Surface runoff
(rapid)
Precipitation
Evaporation
from land Evaporation
from ocean
Precipitation
to ocean
Surface
runoff
(rapid)
Infiltration and
Percolation
Groundwater movement (slow)
Ocean storage
Fig. 3-26, p. 72
Animation: Hydrologic Cycle
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
Fig. 3-27, pp. 72-73
Animation: 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
CO2 emissions from fossil fuels
(billion metric tons of carbon equivalent)
High
projection
Low
projection
Year
Fig. 3-28, p. 74
The Nitrogen Cycle:
Bacteria in Action
Figure 3-29
Gaseous nitrogen (N2)
in atmosphere
Food webs on land
Nitrogen fixation
Fertilizers
Uptake by autotrophs Excretion, death,
decomposition
Ammonia, ammonium in soil
Nitrogen-rich wastes,
remains in soil
Ammonification
Loss by
leaching
Nitrification
Uptake by
Loss by
autotrophs denitrification
Nitrate in soil
Nitrification
Nitrite in soil
Loss by
leaching
Fig. 3-29, p. 75
Animation: Nitrogen Cycle
PLAY
ANIMATION
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
Global nitrogen (N) fixation
(trillion grams)
Nitrogen fixation by natural processes
Year
Fig. 3-30, p. 76
The Phosphorous Cycle
Figure 3-31
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
Animation: Phosphorous Cycle
PLAY
ANIMATION
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.
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
Animation: Sulfur Cycle
PLAY
ANIMATION
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.
The Gaia Hypothesis:
Is the Earth Alive?
• Some have proposed that the earth’s various
forms of life control or at least influence its
chemical cycles and other earth-sustaining
processes.
– The strong Gaia hypothesis: life controls the
earth’s life-sustaining processes.
– The weak Gaia hypothesis: life influences the
earth’s life-sustaining processes.
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).