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Energy Flow and Nutrient Cycling in Ecosystems

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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


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