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Chapter 3
Ecosystems: What Are
They and How Do They
Work?
LT5
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
Organisms and Species
 Organisms,
the different forms of life on
earth, can be classified into different species
based on certain characteristics.
Figure 3-3
Classification of Life
 Domain
 Kingdom
 Phylum
 Class
 Order
 Family
 Genus
 Species
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
What Sustains Life on Earth?
 Solar
energy,
the cycling of
matter, and
gravity sustain
the earth’s life.
Figure 3-7
ECOSYSTEM COMPONENTS
 Life
exists on land systems called biomes
and in freshwater and ocean aquatic life
zones.
Figure 3-9
Nonliving and Living Components of
Ecosystems
 Ecosystems
consist of nonliving (abiotic) and
living (biotic) components.
Figure 3-10
Factors That Limit Population Growth
 Availability
of matter and energy resources
can limit the number of organisms in a
population.
Figure 3-11
Factors That Limit Population Growth
 The
physical
conditions of the
environment can
limit the
distribution of a
species.
Figure 3-12
LT 6
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
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
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
LT 2
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
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
LT9
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
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
The Nitrogen Cycle:
Bacteria in Action
Figure 3-29
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
Figure 3-31
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
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.