Ecosystems: What Are They and
How Do They Work?
Chapter 3
3-1 What Is Ecology?
 Concept 3-1 Ecology is the study of how
organisms interact with one another and with
their physical environment of matter and energy.
3-1 What Is Ecology?
 Ecology is a study of connections in nature.
• How organisms interact with one another and with
their nonliving environment.
Species Make Up the Encyclopedia of Life
 Organisms – individual forms of life
• Classified into species based on certain characteristics
 Species – a set of individuals that can mate and
produce viable, fertile offspring
• 1.75 Million species
identified
• Up to 10–14 million
species not yet
identified??
• Insects make up most
of the known species
Science Focus:
Have You Thanked the Insects Today?
 Many times we classify insects as pests:
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Compete with us for food
Spread human diseases
Bite or sting us
Can be a nuisance
 However, they are VERY beneficial:
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Pollinators
Eat other insects
Loosen and renew soil
Estimated $57 billion/yr in ecological services
Species Make Up the Encyclopedia of Life
 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.
Species Make Up the Encyclopedia of Life
 Genetic diversity
• In most natural populations
individuals vary slightly in their
genetic makeup.
• These genetic diversity is crucial
to the survival of the population.
Levels of Organization of
Matter in Nature
 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.
 The biosphere is where all
of the Earth’s life is found
• A “global ecosystem”
3-2 What Keeps Us and
Other Organisms Alive?
 Concept 3-2 Life is sustained by the flow of energy
from the sun through the biosphere, the cycling of
nutrients within the biosphere, and gravity.
Natural Capital:
General Structure of the Earth
 The biosphere consists of
several physical layers
that contain:
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Air
Water
Soil
Minerals
Life
The Earth’s Life-Support System Has Four
Major Components
 Biosphere – All of the life on the planet
 Atmosphere – Membrane of air around the planet.
• Troposphere – lowest layer - most gas of any layer
• where weather occurs
• Stratosphere – contains ozone to filter out most of
the sun’s harmful UV radiation.
 Hydrosphere – All the earth’s water: liquid, ice,
water vapor
 Geosphere – The earth’s crust and upper mantle
• Lithosphere – hard outer crust
• Asthenosphere – molten interior
Three Factors Sustain Life on Earth
1. One-way flow of
energy beginning
with the sun
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Follows the laws of
thermodynamics
Most is lost as heat
2. Cycling of matter
or nutrients
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Nutrients exist in a
fixed amount
Follows the law of
conservation of
matter
3. Gravity
Flow of Energy to and from the Earth
 Solar energy
flowing through
the biosphere
warms the
atmosphere,
evaporates and
recycles water,
generates winds
and supports
plant growth.
 Natural greenhouse effect – natural process that
keeps the Earth warmer than it would otherwise be
Life Exists on Land and in Water
 Life exists on land systems called biomes and in
freshwater and ocean aquatic life zones.
3-3 What Are the Major Components
of an Ecosystem?
 Concept 3-3A Ecosystems contain living
(biotic) and nonliving (abiotic) components.
 Concept 3-3B Some organisms produce the
nutrients they need, others get their nutrients by
consuming other organisms, and some recycle
nutrients back to producers by decomposing the
wastes and remains of organisms.
Ecosystems Have Living and
Nonliving Components
 Ecosystems consist
of living (biotic) and
nonliving (abiotic)
components.
Biotic or Abiotic?
• Plants
• Water
• Animals
• Temperature
• Rocks/Minerals
• Bacteria
• Solar Energy
• Landforms
• Soil
Several Abiotic Factors Can
Limit Population Growth
 Populations have a range of tolerance to variations
in their physical and chemical environments.
Several Abiotic Factors Can
Limit Population Growth
 Availability of matter and energy resources can
limit the number of organisms in a population.
 Limiting factor principle
• Too much or too little of any abiotic factor can
limit or prevent growth of a population, even if
all other factors are at or near the optimal
range of tolerance
Several Abiotic Factors Can
Limit Population Growth
 The physical conditions of the environment can limit
the distribution (range) of a species.
Producers and Consumers Are the Living
Components of Ecosystems
 Producers, or autotrophs, make their own food
from nutrients/energy obtained from the environment
 Most producers capture sunlight to produce
carbohydrates by photosynthesis:
** You must memorize this equation if you haven’t already. It will come up
throughout the year.
Photosynthesis
 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.
Producers and Consumers Are the Living
Components of Ecosystems
 Some organisms can obtain energy from inorganic
compounds in their environment WITHOUT sunlight
through a process called chemosynthesis.
• Deep ocean bacteria draw energy from hydrothermal
vents and produce carbohydrates from hydrogen
sulfide (H2S) gas.
Producers and Consumers Are the Living
Components of Ecosystems
 Consumers, or heterotroph, cannot produce their
own food and, therefore, must obtain their nutrients
by feeding on other organisms.
• Herbivores
• Primary consumers that eat producers
• Carnivores
• Secondary consumers eat primary consumers
• Third- and higher-level consumers: carnivores that eat
carnivores
• Omnivores
• Feed on both plant and animals
Producers and Consumers Are the Living
Components of Ecosystems
• Detritivores: Insects or other scavengers that feed on
wastes or dead bodies (detritus) – physically break down
• Decomposers: Bacteria and/or fungi that use enzymes to
recycle nutrients in ecosystems – chemically break down
Producers and Consumers Are the Living
Components of Ecosystems
 Organisms break down carbohydrates and other
organic compounds in their cells to obtain the
energy they need.
 This is usually done through
aerobic respiration.
• Aerobic means “with oxygen”
** You must memorize this equation if you haven’t already. It will come up
throughout the year.
Producers and Consumers Are the Living
Components of Ecosystems
 Some decomposers get energy by breaking down
glucose (or other organic compounds) WITHOUT
oxygen through a process called anaerobic
respiration, or fermentation
• Anaerobic means “without oxygen”
 The end products vary based
on the chemical reaction:
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Methane gas
Ethyl alcohol
Acetic acid
Hydrogen sulfide
Energy Flow and Nutrient Cycling Sustain
Ecosystems and the Biosphere
 An ecosystem is
sustained by a
combination of a
one-way flow of
energy and the
cycling of key
nutrients.
Science Focus: Many of the World’s Most
Important Species Are Invisible to Us
 Multitudes of tiny microbes such as bacteria,
protozoa, fungi, and yeast help keep us alive.
• Harmful microbes are the minority
• Soil bacteria (nitrogen fixing bacteria) convert nitrogen
gas to a usable form for plants
• They help produce foods: bread, cheese, yogurt, beer, wine
• Make up 90% of all living mass on Earth
• Help purify water, provide oxygen, breakdown waste
• Live beneficially in your body (intestines, nose)
3-4 What Happens to Energy in an Ecosystem?
 Concept 3-4A Energy flows through ecosystems
in food chains and webs.
 Concept 3-4B As energy flows through
ecosystems in food chains and webs, the amount
of chemical energy available to organisms at each
succeeding feeding level decreases.
Energy Flows Through Ecosystems in Food
Chains and Food Webs
 Food chains and webs show how eaters, the eaten, and the
decomposed are connected to one another in an ecosystem.
 The arrows point in the direction of the energy flow.
Energy Flows Through Ecosystems in Food
Chains and Food Webs
 A food web shows the
connections of all
organisms within an
ecosystem.
Usable Energy Decreases with Each Link in a
Food Chain or Web
 Biomass – the dry
weight of all organic
matter contained in an
organism(s).
 Chemical energy in
biomass is transferred
from one trophic level
to another.
Pyramid of Energy Flow
 Ecological efficiency: percentage of useable energy
transferred as biomass from one trophic level to the next.
• Typically, only 10% efficient (90% lost as heat)
Usable Energy Decreases with Each Link in a
Food Chain or Web
 Food chains rarely have
more than 4 steps or 3
trophic levels. Why?
Usable Energy Decreases with Each Link in a
Food Chain or Web
 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.
• Cellular respiration - the
breakdown of glucose to release
energy – is only 38% efficient
• Energy is lost as heat
• Energy is also used for growth,
reproduction, movement
• Ultimately, only about 10% of the original energy is
available for the next trophic level (90% lost as heat)
Some Ecosystems Produce Plant Matter
Faster Than Others Do
 Gross primary production (GPP)
• Rate at which an ecosystem’s producers convert solar
energy into chemical energy (photosynthesis) as biomass.
Some Ecosystems Produce Plant Matter
Faster Than Others Do
 Net Primary Production (NPP)
• Rate at which producers use
photosynthesis to store energy minus
the rate at which they use some of this
energy through respiration (R).
 NPP = GPP – R
Estimated Annual Average NPP in Major Life
Zones and Ecosystems
 What are nature’s most productive/least productive systems?
Info from notes handout
 Sample food chain
Info from notes handout
1. Pyramid of Numbers
2. Pyramid of Biomass
3. Pyramid of Energy
3-5 What Happens to Matter in
an Ecosystem?
 Concept 3-5 Matter, in the form of nutrients,
cycles within and among ecosystems and the
biosphere, and human activities are altering
these chemical cycles.
Nutrients Cycle in the Biosphere
 Biogeochemical 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.
 Connect past, present , and future forms of life
The Water Cycle
The Water Cycle
Effects of Human Activities
on Water Cycle
 Humans alter the water cycle by:
• Withdrawal of large amounts of freshwater at rates
faster than nature can replace it
• Clearing vegetation and eroding soils.
• Increased flooding when wetlands are drained
• Polluting surface and underground water.
• Contributing to climate change.
• Increases melting, evaporation, precipitation, etc.
The Carbon Cycle
The Carbon Cycle
Effects of Human Activities
on Carbon Cycle
 Humans alter the carbon cycle by adding excess
CO2 to the atmosphere through:
• Burning fossil fuels
• Clearing/burning vegetation faster than it is replaced
The Nitrogen Cycle
The Nitrogen Cycle
Nitrogen Cycles through the Biosphere:
Bacteria in Action
 Nitrogen fixation – converting atmospheric N2 into
ammonia (NH3) or ammonium (NH4) that can be used
by plants
• Lightning – ammonia falls to the ground with the rain
• Nitrogen-fixing bacteria
 Nitrification – Nitrifying bacteria convert ammonium
(NH4) to nitrates (NO3) and/or nitrites (NO2)
 Denitrification – Denitrifying bacteria convert
ammonia, nitrates, or nitrites back into atmospheric N2.
 Ammonification – Decomposers convert ditritus into
ammonia or ammonium.
Nitrogen Fixation:
A Closer Look
Effects of Human Activities
on the Nitrogen Cycle
 Humans alter the nitrogen cycle by:
• Adding gases that contribute to acid rain
• NO and N2O
• Contaminating bodies of
water with excess nitrates
from inorganic fertilizers
• Humans fix more nitrogen than all
natural sources combined
• Adding nitrogen to the
troposphere through
deforestation
• Remove nitrogen from topsoil
The Phosphorous Cycle
The Phosphorous Cycle
Effects of Human Activities
on the Phosphorous Cycle
 Cycles through water, the earth’s crust, and living
organisms (NOT the atmosphere like the others)
 May be limiting factor for plant growth
 Humans alter the phosphorus cycle by:
• 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
The Sulfur Cycle
Effects of Human Activities
on the Sulfur Cycle
 Sulfur found in organisms, ocean sediments, soil,
rocks, and fossil fuels.
• Sulfur dioxide (SO2) in the atmosphere
 Humans alter the sulfur cycle by adding sulfur dioxide
to the atmosphere through:
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Burning sulfur-containing coal and oil
Refining sulfur-containing petroleum
Smelting sulfur-containing metallic mineral ores
These can all lead to acid rain
• Sulfuric acid (H2SO4) when SO4 mixes with H2O