Chapter 28
Energy Flow and Nutrient Cycling in
Ecosystems
How Do Nutrients and Energy Move
Through Ecosystems?
• All ecosystems consist of two components
– Biotic - living organisms in a given area—bacteria,
fungi, protists, plants, and animals
– Abiotic - all nonliving physical or chemical aspects
of the environment, such as the climate, light,
temperature, availability of water, and minerals in
the soil
How Do Nutrients and Energy Move
Through Ecosystems?
• Nutrients are atoms and molecules that
organisms obtain from their environment
– The same nutrients have been sustaining life on
Earth for about 3.5 billion years
– Your body includes oxygen, carbon, hydrogen, and
nitrogen atoms that were once part of a dinosaur
or a wooly mammoth
– Nutrients are transported around the Earth, but
they never leave Earth
How Do Nutrients and Energy Move
Through Ecosystems?
• Energy, in contrast, takes a one-way journey
through ecosystems
– Solar energy is captured by photosynthetic
bacteria, algae, and plants, and then flows from
organism to organism
– Eventually, all of life’s energy is converted to heat
that is given off to the environment and cannot be
used to drive the chemical reactions of living
organisms
– Life requires a continuous input of energy
Figure 28-1 Energy flow, nutrient cycling, and feeding relationships in ecosystems
energy from
sunlight
producers
primary
consumers
nutrients
detritivores and
decomposers
higher-level
consumers
solar energy
heat energy
energy stored in
chemical bonds
nutrients
How Does Energy Flow Through
Ecosystems?
• Much of the energy reaching Earth from the sun is
reflected back into space by the atmosphere,
clouds, and the Earth’s surface
• Some is absorbed by the Earth to warm the planet
• Less than 0.03% of the energy reaching Earth from
the sun is captured by photosynthetic organisms,
and supports life on Earth
How Does Energy Flow Through
Ecosystems?
• Energy enters ecosystems through
photosynthesis
– Plants, algae, and photosynthetic bacteria acquire
nutrients such as carbon, nitrogen, oxygen, and
phosphorus from the abiotic portions of
ecosystems
– Photosynthesizers bring energy and nutrients into
ecosystems
How Does Energy Flow Through
Ecosystems?
• Energy is passed from one trophic level to the
next
– Each category of organisms is called a trophic level
• Producers (or autotrophs) photosynthesizing organisms
• Consumers (or heterotrophs) cannot photosynthesize
– They acquire energy and nutrients from molecules in the bodies
of other organisms
How Does Energy Flow Through
Ecosystems?
• Energy is passed from one trophic level to the
next (continued)
– There are several levels of consumers
• Primary consumers (herbivores) feed directly and
exclusively on producers
• Carnivores act as secondary consumers when they prey
on herbivores
• Some carnivores eat other carnivores and are called
tertiary consumers
How Does Energy Flow Through
Ecosystems?
• Net primary production is a measure of the energy
stored in producers
– The amount of life that a particular ecosystem can support is
determined by the energy captured by the producers in that
ecosystem
– Biomass, or dry biological material, is usually a good measure
of the energy stored in organisms’ bodies
open
ocean
(125)
tundra
(140)
continental
shelf (140)
tropical rain
forest (2,200)
estuary
(1,500)
coniferous
forest (800)
temperate
deciduous
forest (1,200)
grassland (600)
desert (90)
How Does Energy Flow Through
Ecosystems?
• The net primary production of an ecosystem is
influenced by many factors
• The amount of sunlight
• The availability of water and nutrients
• The temperature
• An ecosystem’s contribution to Earth’s total production is
determined by the ecosystem’s productivity and by the
portion of Earth that the ecosystem covers
How Does Energy Flow Through
Ecosystems?
• Food chains and food webs describe the
feeding relationships within communities
– A food chain is a linear feeding relationship with
just one representative at each trophic level
tertiary consumer
(fourth trophic level)
secondary consumer
(third trophic level)
primary consumer
(second trophic level)
producer
(first trophic level)
Figure 28-3b A simple marine food chain
phytoplankton
zooplankton
secondary consumer
(third trophic level)
producer
(first trophic level)
primary consumer
(second trophic level)
tertiary consumer
(fourth trophic level)
quaternary consumer
(fifth trophic level)
A simple marine food chain
How Does Energy Flow Through
Ecosystems?
• Food chains and food webs describe the feeding
relationships within communities (continued)
– Animals in natural communities often do not fit neatly
into the categories of primary, secondary, and tertiary
consumers depicted in simple food chains
– A food web shows many interconnected food chains,
and actual feeding relationships in a community
– Some animals, such as raccoons, bears, rats, and
humans, are omnivores (“everything eaters”) and act
as primary, secondary, and tertiary consumers
Figure 28-4 A simplified grassland food web
How Does Energy Flow Through
Ecosystems?
• Detritivores and decomposers release nutrients
for reuse
– Among the most important strands in a food web are
the detritivores and decomposers
• Detritivores (“debris eaters”) are an army of mostly small
and often unnoticed organisms
–
–
–
–
–
Nematode worms and Earthworms
Vultures
Millipedes
Dung beetles
Slugs
How Does Energy Flow Through
Ecosystems?
• Detritivores and decomposers release nutrients
for reuse (continued)
– Decomposers are primarily fungi and bacteria
• They feed on the same material as detritivores
• They do not ingest chunks of organic matter
• They secrete digestive enzymes outside their bodies, where
the enzymes break down nearby organic material
• Decomposers absorb some of the resulting nutrient
molecules but leave the rest
How Does Energy Flow Through
Ecosystems?
• Detritivores and decomposers are absolutely
essential to life on Earth
– Without detritivores and decomposers, ecosystems
would gradually be buried by accumulated wastes and
dead bodies, whose nutrients would be unavailable to
enrich the soil and water
How Does Energy Flow Through
Ecosystems?
• Energy transfer through trophic levels is inefficient
– Second Law of Thermodynamics
– Inefficiency is a rule in living systems
• Waste is heat produced by all biochemical reactions that
keep cells alive
• Only a fraction of the energy captured by producers of the
first trophic level can be used by organisms in the second
trophic level
How Does Energy Flow Through
Ecosystems?
• Energy transfer through trophic levels is
inefficient (continued)
– The average net energy transfer between trophic
levels is roughly 10% efficient and is known as the
“10% law”
• An energy pyramid illustrates the energy relationships
between trophic levels—widest at the base, and
progressively narrowing in higher trophic levels
• A biomass pyramid for a community has the same
general shape
Figure 28-5 An energy pyramid for a grassland ecosystem
tertiary
consumer
(1 calorie)
secondary
consumer
(10 calories)
primary
consumer
(100 calories)
producers
(1,000 calories)
How Does Energy Flow Through
Ecosystems?
• Energy transfer through trophic levels is
inefficient (continued)
– The most abundant organisms are plants
– The most abundant animals are herbivores
– Carnivores are relatively scarce because there is
far less energy available to support them
– Energy losses within and between trophic levels
mean that long-lived animals at higher trophic
levels eat many times their body weight in food
How Does Energy Flow Through
Ecosystems?
• Energy transfer through trophic levels is
inefficient (continued)
– If the food contains certain types of toxic
substances, they may be stored and become more
concentrated
– This biological magnification can lead to harmful
and even fatal effects
• Mercury in fish
How Do Nutrients Cycle Within and
Among Ecosystems?
• Nutrient cycles, also called biogeochemical
cycles, describe the pathways that nutrients
follow as they move from their major sources in
the abiotic parts of ecosystems, called reservoirs,
through living communities and back again
How Do Nutrients Cycle Within and
Among Ecosystems?
• The hydrologic cycle has its major reservoir in
the oceans
– The water cycle, or hydrologic cycle, is the
pathway that water takes as it travels from its
major reservoir—the oceans—through the
atmosphere, to reservoirs in freshwater lakes,
rivers, and groundwater, and then back again to
the oceans
How Do Nutrients Cycle Within and
Among Ecosystems?
• The hydrologic cycle
– The hydrologic cycle would continue even if life on
Earth disappeared because the biotic portion of
ecosystems plays a small role in the hydrologic cycle
– The hydrologic cycle is crucial for terrestrial
communities because it continually restores the fresh
water needed for land-based life
– The oceans cover 70% of the Earth’s surface and
contain more than 97% of Earth’s water
– Solar energy evaporates water, and it comes back to
Earth as precipitation
Figure 28-6 The hydrologic cycle
reservoirs
processes
water vapor in
the atmosphere
precipitation
over the ocean
precipitation
over land
evaporation
from the land
and from the
leaves of plants
evaporation
from the
ocean
evaporation from
lakes and rivers
lakes and rivers
seepage through soil
into groundwater
groundwater,
including
aquifers
runoff
from rivers
and land
extraction for
agriculture
water in
the ocean
How Do Nutrients Cycle Within and
Among Ecosystems?
• The carbon cycle has major reservoirs in the
atmosphere and oceans
– Carbon atoms form the framework of all organic
molecules
– The carbon cycle is the pathway that carbon takes
from its major short-term reservoirs in the
atmosphere and oceans, through producers and
into the bodies of consumers, detritivores, and
decomposers, and then back again to its
reservoirs
Figure 28-7 The carbon cycle
reservoirs
processes
trophic levels
CO2 in the
atmosphere
burning
fossil fuels
CO2 dissolved
in the ocean
respiration
fire
photosynthesis
consumers
producers
detritivores
and decomposers
decomposition
fossil fuels
(coal, oil, natural gas)
How Do Nutrients Cycle Within and
Among Ecosystems?
• The carbon cycle
– The complementary processes of uptake by
photosynthesis and release by cellular respiration
continually recycle carbon from the abiotic to the
biotic portions of an ecosystem and back again
How Do Nutrients Cycle Within and
Among Ecosystems?
• The carbon cycle
– Much of Earth’s carbon is bound up in limestone
rock, formed from calcium carbonate (CaCO3)
deposited on the ocean floor in the shells of
prehistoric phytoplankton
• This cycling requires millions of years
– Fossil fuels, which include coal, oil, and natural
gas, are additional long-term reservoirs for carbon
How Do Nutrients Cycle Within and
Among Ecosystems?
• The nitrogen cycle has its major reservoir in the
atmosphere
– Nitrogen is a crucial component of proteins, many
vitamins, nucleotides (such as ATP), and nucleic acids
(such as DNA)
– The nitrogen cycle is the pathway taken by nitrogen
from its primary reservoir—nitrogen gas (N2) in the
atmosphere—to much smaller reservoirs of ammonia
and nitrate in soil and water, through producers,
consumers, detritivores and decomposers, and back
to its reservoirs
How Do Nutrients Cycle Within and
Among Ecosystems?
• The nitrogen cycle
– While nitrogen gas (N2) makes up 78% of the
atmosphere, this form of nitrogen cannot be
utilized by plants
• Plants utilize nitrate (NO3) or ammonia (NH3) as their
nitrogen source
– N2 is converted to ammonia by specific bacteria
during a process called nitrogen fixation
Figure 28-8 The nitrogen cycle
reservoirs
processes
trophic levels
N2 in the
atmosphere
burning
fossil fuels
lightning
application of
manufactured fertilizer
consumers
ammonia and
nitrates in water
detritivores
and decomposers
producers
uptake by
producers
nitrogen-fixing
bacteria in soil
and legume roots
decomposition
denitrifying
bacteria
ammonia
and nitrates
in soil
How Do Nutrients Cycle Within and
Among Ecosystems?
• The nitrogen cycle
– People significantly manipulate the nitrogen cycle,
both deliberately and unintentionally
• Plant legumes to fertilize fields
• About 150 million tons of nitrogen-based fertilizer are
applied to farms each year
• The heat produced by burning fossil fuels combines
atmospheric N2 and O2, generating nitrogen oxides that
form nitrates
– Human activities now dominate the nitrogen cycle
How Do Nutrients Cycle Within and
Among Ecosystems?
• The phosphorus cycle has its major reservoir in
rock, bound to oxygen as phosphate
– Phosphorus is found in biological molecules such as
nucleic acids and the phospholipids of cell membranes
– It also forms a major component of vertebrate teeth
and bones
– The phosphorus cycle is the pathway taken by
phosphorus from its primary reservoir in rocks to much
smaller reservoirs in soil and water, producers,
consumers, detritivores and decomposers and back
Figure 28-9 The phosphorus cycle
reservoirs
processes
trophic levels
phosphate
in rock
geological
uplift
application of
manufactured
fertilizer
runoff
from rivers
consumers
producers
detritivores and
decomposers
decomposition
runoff from
fertilized
fields
uptake by
producers
phosphate
in water
phosphate
in soil
phosphate
in sediment
formation of
phosphate-containing
rock
Review
1. Why is energy flow not cyclical like nutrients?
2. Name the trophic levels in a community.
3. Why are detritivores and decomposers
essential to ecosystem function?
4. What are the four major nutrient cycles?
What Happens When Humans Disrupt
Nutrient Cycles?
• As the human population grew and
technology increased, people began to act
more independently of natural ecosystem
processes
– The Industrial Revolution resulted in a
tremendous increase in our reliance on energy
stored in fossil fuels for heat, light, transportation,
industry, and agriculture
What Happens When Humans Disrupt
Nutrient Cycles?
• Overloading the nitrogen and phosphorus cycles
damages aquatic ecosystems
– Fertilizers are applied to farm fields
– Water dissolves and carries away some of the
phosphate and nitrogen-based fertilizer
– Overstimulating the growth of phytoplankton in the
ocean “bloom”
What Happens When Humans Disrupt
Nutrient Cycles?
• Overloading the nitrogen and phosphorus cycles
damages aquatic ecosystems (continued)
– The phytoplankton die, and their bodies sink into
deeper water and provide food for decomposer
bacteria
– The decomposers use up most of the available
oxygen, and other aquatic organisms, such as
invertebrates and fish, die, creating “dead zones” in
many waters (Gulf of Mexico)
What Happens When Humans Disrupt
Nutrient Cycles?
• Overloading the sulfur and nitrogen cycles
causes acid deposition
– Burning of sulfur-containing fossil fuels, primarily
coal, accounts for about 75% of all sulfur dioxide
emissions worldwide
– Days later and often hundreds of miles from the
source, these acids fall to Earth in rain or snow
What Happens When Humans Disrupt
Nutrient Cycles?
• Interfering with the carbon cycle is warming
Earth’s climate
– Natural process called the greenhouse effect,
which keeps our atmosphere relatively warm and
allows life on Earth as we know it
– For Earth’s temperature to remain constant, the
total amount of energy entering and leaving
Earth’s atmosphere must be equal
Figure 28-13 The greenhouse effect
Most heat is radiated
into space
Sun
Sunlight energy
enters the atmosphere
Some atmospheric heat is
retained by greenhouse gases
Some energy
is reflected back
into space
Most sunlight strikes
Earth’s surface and is
converted into heat
vehicle
emissions
Heat is
radiated back into
the atmosphere
agricultural
activities
volcanoes
power plants
and factories
homes and other
buildings
forest
fires
What Happens When Humans Disrupt
Nutrient Cycles?
• Interfering with the carbon cycle is warming
Earth’s climate (continued)
– If atmospheric concentrations of greenhouse
gases increase, more heat is retained than is
radiated into space, causing Earth to warm
– Greenhouse gases are increasing because people
burn fossil fuels, releasing CO2
– Other important greenhouse gases include
methane (CH4), released by agricultural activities
and burning fossil fuels
What Happens When Humans Disrupt
Nutrient Cycles?
• Burning fossil fuels is causing climate change
– Climate scientists predict that the warming
atmosphere will cause more severe storms,
including stronger hurricanes
– Greater amounts of rain or snow will fall in single
storms
– More frequent and more prolonged droughts will
occur
– Increased CO2 makes the oceans more acidic