1. Energy Flow and Nutrient Cycling in Ecosystems
Chapter 28
Energy Flow and Nutrient Cycling in Ecosystems 1

2. 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

3. 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

4. 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

5. Figure 28-1 Energy flow, nutrient cycling, and feeding relationships in ecosystems
energy from sunlight
producers
primary consumers
nutrients
detritivores and decomposers
Second law of thermodynamics – some energy is always lost when converting
higher-level consumers
solar energy
heat energy
energy stored in chemical bonds
nutrients 5

6. 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

7. 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
Photosynthetic organisms capture sunlight’s energy
Nutrients and energy contained in biological molecules move from photosynthetic organisms to nonphotosynthetic organisms

8. 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
Energy flow through ecosystems begins with photosynthetic organisms and passes through several levels of nonphotosynthetic organisms that feed on the photosynthesizers or each other
autotrophs - make their own food using inorganic nutrients and solar energy from the environment

9. 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
These herbivores (plant eaters) include animals such as grasshoppers, mice, and zebras, and form the second trophic level
Carnivores (meat eaters) such as spiders, hawks, and salmon, make up the higher-level consumers
May be higher trophic levels (like in the oceans)

10. 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)
continental shelf (140)
tundra (140)
coniferous forest (800)
temperate deciduous forest (1,200)
desert (90)
grassland (600)
estuary (1,500)
tropical rain forest (2,200)
The energy that photosynthetic organisms store and make available to other members of the community over a given period is called net primary production
Biomass is presented as grams per square meter per year.
Picture is net primary production of various ecosystems.

11. 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
In the desert, lack of water limits production
In the open ocean, light is a limiting factor in deep waters
The oceans have low net primary production, but they cover about 70% of Earth’s surface, so they contribute about 25% of Earth’s total production
The rainforests are about the same because they have high productivity but cover less than 5% of Earth’s surface.

12. 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)
A simple terrestrial food chain
Plants are the dominant producers in land-based (terrestrial) ecosystems
Plants support plant-eating insects, reptiles, birds, and mammals, each of which can be preyed on by the other animals

13. Figure 28-3b A simple marine food chain
phytoplankton
zooplankton
secondary consumer (third trophic level)
producer (first trophic level)
primary consumer (second trophic level)
In contrast, microscopic photosynthetic protists and bacteria collectively called phytoplankton are the dominant producers in most aquatic food chains, such as those found in lakes and oceans
Phytoplankton are consumed by zooplankton, which consist of protists and small, shrimp-like crustaceans
tertiary consumer (fourth trophic level)
quaternary consumer (fifth trophic level)
A simple marine food chain 13

14. 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

15. Figure 28-4 A simplified grassland food web
A hawk is a secondary consumer when it eats a mouse (herbivore) and a tertiary consumer when it eats a meadowlark that feeds on insects. 15

16. 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
Eat fallen leaves, wastes, and dead bodies

17. 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

18. 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

19. 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
When energy is converted from one form to another, the amount of useful energy decreases.
Burning ATP releases heat.

20. 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

21. Figure 28-5 An energy pyramid for a grassland ecosystem
tertiary consumer (1 calorie)
secondary consumer (10 calories)
primary consumer (100 calories)
When a grasshopper (a primary consumer) eats grass (a producer), only some of the solar energy trapped by the grass is available to the insect
Some was converted into the chemical bonds of cellulose, which a grasshopper cannot digest
If the grasshopper is eaten by a robin (the third trophic level), the bird will not obtain all the energy that the insect acquired from the plants
Some of the energy will have been used up to power hopping, flying, and eating
Some energy will be found in the grasshopper’s indigestible exoskeleton
Much of the energy will have been lost as heat
producers (1,000 calories) 21

22. 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
1. They have the most energy available to them from sunlight.

23. 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

24. 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

25. 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

26. 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
Soil nutrients and CO2 must have water to be taken up.
2% of water is trapped as ice
1% is liquid fresh water

27. Figure 28-6 The hydrologic cycle
reservoirs
reservoirs
water vapor in the atmosphere
processes
processes
precipitation over land
precipitation over the ocean
evaporation from the land and from the leaves of plants
evaporation from the ocean
evaporation from lakes and rivers
lakes and rivers
Of the water that falls on land as rain or snow:
Flows downhill
Most evaporates from the ocean, soil, lakes, and streams
Some is absorbed by the roots of plants
A portion runs back to the oceans in rivers
An extremely miniscule fraction is stored in the bodies of living organisms
Some enters natural underground reservoirs called aquifers
aquifers – water permeable rock (sandstone, sand, gravel)
runoff from rivers and land
water in the ocean
seepage through soil into groundwater
extraction for agriculture
groundwater, including aquifers 27

28. 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

29. Figure 28-7 The carbon cycle
reservoirs
CO2 in the atmosphere
processes
trophic levels
burning fossil fuels
CO2 dissolved in the ocean
respiration
fire
photosynthesis
producers
consumers
Carbon enters communities through capture of carbon dioxide (CO2) during photosynthesis
Producers on land get CO2 from the atmosphere which is then “fixed” in biological molecules
Primary consumers eat producers and acquire carbon stored in their tissues
These herbivores release some of the carbon through respiration as CO2, excrete carbon compounds in their feces, and store the rest in their bodies, which may be consumed by higher trophic levels
All living things eventually die, and their bodies are broken down by detritivores and decomposers, whose cellular respiration returns CO2 to the atmosphere and oceans
detritivores and decomposers
fossil fuels (coal, oil, natural gas)
decomposition 29

30. 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

31. 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
These substances were produced from the remains of prehistoric organisms buried deep underground and subjected to high temperature and pressure
In addition to carbon, the energy of prehistoric sunlight is trapped in these deposits
When human beings burn fossil fuels to tap this stored energy, CO2 is released into the atmosphere, with potentially serious consequences

32. 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

33. 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

34. Figure 28-8 The nitrogen cycle
reservoirs
N2 in the atmosphere
processes
trophic levels
burning fossil fuels
lightning
application of manufactured fertilizer
consumers
producers
nitrogen-fixing bacteria in soil and legume roots
Nitrogen fixers:
Some of these bacteria live in water and soil and convert the ammonia into nitrate that plants can directly use
Others live in symbiotic associations with plants called legumes, which include alfalfa, soybeans, clover, and peas
Some nitrogen is released in wastes and dead bodies
Decomposer bacteria convert this back to nitrate and ammonia in the soil or water, which is then available to plants
Denitrifying bacteria break down nitrate, releasing N2 back to the atmosphere completing the cycle.
ammonia and nitrates in water
uptake by producers
detritivores and decomposers
decomposition
denitrifying bacteria
ammonia and nitrates in soil 34

35. 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

36. 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

37. Figure 28-9 The phosphorus cycle
reservoirs
processes
trophic levels
phosphate in rock
geological uplift
application of manufactured fertilizer
runoff from rivers
consumers
producers
runoff from fertilized fields
Throughout its cycle, almost all phosphorus is bound to oxygen, forming phosphate (PO43)
There are no gaseous forms of phosphate, so there is no atmospheric reservoir in the phosphorus cycle
As phosphate-rich rocks are exposed by geological processes, some of the phosphate is dissolved by rain and flowing water
carries it to soil, lakes, and the ocean
Dissolved phosphate is absorbed by producers, which incorporate it into biological molecules
From producers, phosphate is passed through food webs
At each level, excess phosphate is excreted
Detritivores and decomposers return the phosphate to the soil and water
uptake by producers
phosphate in water
detritivores and decomposers
formation of phosphate-containing rock
phosphate in soil
phosphate in sediment
decomposition 37

38. Review Why is energy flow not cyclical like nutrients?
Name the trophic levels in a community.
Why are detritivores and decomposers essential to ecosystem function?
What are the four major nutrient cycles?

39. 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
Fertilizer use on commercial farms grew exponentially
Human use of fossil fuels and chemical fertilizers has significantly disrupted the global nutrient cycles of nitrogen, phosphorus, sulfur, and carbon

40. 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”
to help satisfy the agricultural demands of a growing human population
3. As water drains into lakes, rivers, and ultimately the oceans, these fertilizers can disrupt the delicate balance of food webs by …

41. 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)
Gulf of Mexico dead zone covers 6000 to 8500 square miles each summer. Hurricanes break it up each fall.

42. 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
Nitrogen oxides and sulfur dioxides combine with atmospheric water and form nitric and sulfuric acids
This “acid rain”—more accurately called acid deposition—was first recognized in New Hampshire, where a sample of rain collected in 1963 had a pH of 3.7, about the same as that of orange juice
Acid deposition damages forests, can render lakes lifeless, and even eats away at buildings and statues
Many lakes and ponds in the Adirondack Mountains are too acidic to support fish
Acid rain also increases the extent to which organisms are exposed to toxic metals, such as aluminum, mercury, lead, and cadmium, which are far more soluble in acidified water
Since 1990, government regulations have resulted in substantial reductions in emissions of both sulfur dioxide and nitrogen oxides from U.S. power plants
Sulfur dioxide emissions are down about 40% and nitrogen oxide levels have been reduced by more than 50%
Air quality has improved, and rain has become less acidic

43. 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

44. 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
volcanoes
Most sunlight strikes Earth’s surface and is converted into heat
Heat is radiated back into the atmosphere
forest fires
power plants and factories
Some of the energy from sunlight is reflected back into space by the atmosphere, and by Earth’s surface, especially by areas covered with snow or ice
Most sunlight strikes relatively dark areas of the surface and is converted into heat that is radiated into the atmosphere
Water vapor, CO2, and several greenhouse gases trap some of the heat in the atmosphere
vehicle emissions
agricultural activities
homes and other buildings 44

45. 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

46. 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