13.4 nutrient cycling in ecosystems · 13.4 nutrient cycling in ecosystems i n addition to energy,...

451Chapter 13 Ecological Principles • MHR

Nutrient Cycling in Ecosystems13.4

In addition to energy, organisms must also obtain17 chemical elements (termed essential nutrients)for proper growth and repair of body tissue. Severalof these elements, including carbon, hydrogen,oxygen, and nitrogen, are required in largeamounts, since they make up 95 percent of themass of all living organisms. Calcium, iron, andother elements are needed in smaller amounts,while only trace (very small) amounts are requiredof the remaining nutrients.

Energy and nutrients are vital for the survival oforganisms. Although both are carried as part of thesame complex molecules from one trophic level toanother, their overall paths through ecosystems aredifferent. Energy is continuously supplied to Earthby the Sun, and while some is captured and flowsthrough the ecosystem, much is lost to theenvironment along the way.

In contrast, the supply of nutrients is notconstantly replenished. The existence of life in thebiosphere depends on the recycling of chemicalelements. Recycling begins when organisms die,parts are lost (such as dead skin cells or brokenbranches), or wastes are eliminated.

Decomposers then go to work, releasing thenutrients that were contained in these bodies intothe atmosphere or soil. From these “pools,”nutrients are picked up by various types oforganisms and re-used to build new bodies —which can later be eaten by other organisms.

Since the routes these chemicals travel involveboth biotic and abiotic components of theenvironment (including rocks and soil that aregeological features), they are referred to asbiogeochemical cycles. Read on to learn about thegeneral features of biogeochemical cycles.

Biogeochemical CyclesThe route a specific element takes in itsbiogeochemical cycle depends on the element and on the trophic structure of the ecosystem inwhich it is travelling. However, there are twogeneral types of cycles. The first type includes thecycles of carbon, oxygen, nitrogen, and sulfur —elements that can occur as gases in the atmosphere.These nutrients have global cycles becauseindividual nutrient atoms may travel long distances.For example, a plant living in one location maytake up carbon (in a carbon dioxide molecule) thatwas released by an animal living far away.

The second type of cycle involves nutrients that are more static and tend not to move around(including phosphorus, potassium, calcium, andtrace elements). These elements are typically foundin the soil rather than the atmosphere. They areabsorbed by plant roots and return to the same soil“pool” when the plant dies.

In general, all biogeochemical cycles involve themovement of elements between four reservoirs —which can be thought of as nutrient “banks” (seeFigure 13.25 on the following page). Nutrients canbe easily withdrawn by organisms from two ofthese banks. The first of these consists of livingorganisms and the bodies of recently deceasedorganisms; therefore it is a biotic bank. The second storehouse consists of abiotic parts of theenvironment from which nutrients can be easilyaccessed — atmosphere, soil, and water.

In the other two reservoirs, nutrients are heldmore tightly and cannot be accessed by livingorganisms. Again, one is biotic and the other isabiotic. The biotic reservoir is formed from thecompressed (fossilized) remains of organisms thatdied long ago. Over time, the nutrients containedin the bodies of these organisms became incorporated

EXPECTAT IONS

Distinguish between how energy and nutrients move in ecosystems.

Describe the general features shared by all biogeochemical cycles.

Differentiate between biogeochemical cycles of materials that exist in theenvironment in gaseous form and those that do not.

Illustrate some of the processes that move nutrients through ecosystems,using one nutrient cycle as an example.

Recognize the impact that humans and plants have on biogeochemical cycles.

452 MHR • Unit 5 Population Dynamics

solarenergy transport over the land

precipitationover the land(95)

percolationin soil

run-off and ground water (36)

net movementof water vapourby wind (36)

evaporationfrom the sea(319)

evapotranspiration(59)

precipitationover the sea(283)

Figure 13.26 The water or hydrologic cycle. The width of the arrows indicates therelative amounts of water being carried in each route. The numbers in parenthesesrepresent estimates of global water flow in billion billion (1018) grams per year.

into oil, peat, or coal deposits, where they areunavailable to living organisms. When we burnthese fossilized remains as fuel (or when they areeroded), the elements move from this biotic “vault”into the accessible abiotic bank formed by soil,water, and atmosphere.

Figure 13.25 The four nutrient reservoirs are categorizedwith respect to (a) whether they involve biotic or abioticcomponents of the ecosystem and (b) whether the nutrientsthey contain are directly available to living things or not.

The abiotic vault in which nutrients are lockedconsists of rock. When soil is converted to rock, thechemicals contained in the rock are held very tightlyand become inaccessible to living things. However,these chemicals can be released back into the soil,water, or atmosphere by weathering or erosion.

Having looked at some of the features shared byall biogeochemical cycles, it is useful to examine a few specific cycles in more detail. Becauseecosystems are complex and can exchange materialswith each other, it is difficult to trace the routetaken by any of the cycling elements. For example,an ecosystem with finite boundaries (such as a pond) exchanges nutrients with many otherecosystems. Water running off adjacent landintroduces chemicals to the pond from distantsources, and visiting birds carry away nutrients inthe bodies of the fish or insects they have eaten.Following the inputs and outputs of ecosystems ischallenging, but researchers have worked out thebasic routes followed by materials in severalbiogeochemical cycles. One of the most familiar isthe water (hydrologic) cycle, in which oxygen andhydrogen cycle together, as shown in Figure 13.26.

fossilization

photosynthesis

respiration,decomposition,excretion

erosion, burningof fossil fuels

weathering,erosion

formation ofsedimentary

rock

Biotic componentavailable as nutrients

living or recentlyliving organisms

Biotic componentunavailable as nutrients

coal, oil,peat

Abiotic componentavailable as nutrients

atmosphere,soil, water

Abiotic componentunavailable as nutrients

minerals in rocks


The Carbon CycleAnother important biogeochemical cycle is thecarbon cycle (shown in Figure 13.27). Other thanoxygen and hydrogen, which are mostly locked upin water molecules, carbon is the most abundantelement in living things. Plants and otherautotrophs take in carbon dioxide as a raw materialfor photosynthesis, and almost all organisms(including autotrophs) give it off as a waste productof cellular respiration. The carbon that accumulatesin long-lived, durable plant material (such as thewood of trees) does not cycle as rapidly, unless thetrees are burned.

In some environments, living things die fasterthan decomposers can break them down. This wasparticularly evident in the warm, moist conditionsof the Mesozoic era, roughly 150 to 250 millionyears ago. During that time, giant ferns and amultitude of other plants grew thickly in vastforests. Under these conditions, so much detritusbuilt up that it became compressed, resulting in theformation of coal or oil deposits. Today, we refer tothese substances as fossil fuels. Since thesedeposits were formed from what were once livingthings, they contain substantial amounts of carbon.

As described above, these carbon supplies are notaccessible to living things unless the carbon isreleased by burning — either naturally or in a coalstove or oil furnace.

In one sense, the burning of fossil fuels simplyreturns to the atmosphere the carbon that wasremoved by photosynthetic activity in Mesozoicforests. However, during the time these carbonsupplies were locked up underground, a newbalance developed in the global carbon cycle. Withthe addition of substantial amounts of so-callednew carbon to the cycle, the environment ischanging. What effects do you think this mighthave on the biosphere? This is an issue you willconsider further in Chapter 15.

The Nitrogen CycleNitrogen is another element that is relativelyabundant in ecosystems, making up almost 80 percent of Earth’s atmosphere. But sincenitrogen gas (N2) cannot be taken up byphotosynthetic organisms, it is not available toheterotrophs. In fact, plants can only use nitrogenwhen it is a part of ammonium (NH4

+) or nitrate(NO3

−) molecules. Nitrogen gas is occasionally

decomposers(soil microbes and others)

cellular respirationphotosynthesis

plants, algae,cyanobacteria

primary consumers

higher-levelconsumers

detritus

burning wood and fossil fuels

carbon in atmosphere ( )CO2

Figure 13.27 On a global scale, the amount of carbonentering the atmosphere as a result of decomposition andrespiration balances the carbon removed by photosynthesis.

The burning of wood and fossil fuels unbalances theequation. What might result from this practice?

nitrogen-fixingbacteria in rootnodules of legumes

nitrogen-fixing soil bacteria

ammonification

nitrification

assimilation

nitrifying bacteria

nitrifyingbacteria

denitrifyingbacteria

plants

decomposers(aerobic and anaerobic

bacteria and fungi)

nitrogen in atmosphere ( )N2

nitrates ( )NO −3

ammonium ( )NH +4 nitrites ( )NO −

2


converted to these materials in the atmosphere if it becomes dissolved in rain or attached to dustparticles. It can then enter the soil and be taken up by plants (see Figure 13.28).

More commonly, the conversion of nitrogen to aform useful to plants occurs as a result of theactivity of bacteria. Certain bacterial species arecapable of a process known as nitrogen fixation,which converts nitrogen gas to ammonium. Othertypes of bacteria can convert ammonium to nitrite(NO2

−), in processes referred to as nitrification.Bacteria and cyanobacteria are the only organismsthat carry out these processes. Without them, verylittle nitrogen would be available for living thingsin ecosystems.

Nitrogen is a vital component of amino acids,which are the building blocks of which proteins aremade. Since proteins make up the majority of thestructural material of living organisms, act asenzymes, and perform many other functions, it isclear that without the bacteria involved in thenitrogen cycle the biosphere as we know it todaywould be devoid of life.

The Phosphorus CycleWater, carbon, and nitrogen exist as gases in theatmosphere and therefore cycle over large areas. In contrast, phosphorus is an example of a nutrientthat cycles on a more localized scale (see Figure13.29). Most phosphorus is cycled through foodwebs in both aquatic and terrestrial ecosystems.Some phosphorus escapes from the land and entersEarth’s supply of ground water (the reserve ofunderground water), drains into rivers, andeventually flows into the sea. However, an almostequal amount re-enters the soil from theweathering of rocks.

Over long periods of time, phosphates that reachthe ocean accumulate in sediments and eventuallybecome incorporated into rock. These rocks mayre-enter terrestrial environments as a result ofgeological processes that either raise the sea flooror lower seawater levels at a particular location.Most phosphorus cycles locally through soil,plants, and consumers, but some will cycle over alonger time and larger scale. This is true for mostelements that do not exist in the atmosphere as a gas.

Figure 13.28 The width of the arrows in this diagramrepresents the relative amount of nitrogen being moved byeach process. Why do you think some gardeners inoculate

some types of plants with a specially prepared mix ofbacteria before planting them?


The Impact of Humans onBiogeochemical Cycles: A First LookCould the destruction of the Easter Islandecosystem have been due to the upset of one ormore biogeochemical cycles? Can humans actuallyhave a large enough impact to disrupt naturalcycles? These questions will be examined in moredetail in Chapter 15; for now, you need only look atsome general aspects.

All over the world, researchers have realizedthat because of weather variations, the rates ofbiogeochemical cycling and the routes taken bynutrient molecules differ from time to time, evenwithin the same ecosystem. As a result, it isnecessary to study ecosystems for long periods oftime to fully understand the movement of nutrientsand the time required to complete certain cycles orparts of cycles. Many scientists are thereforeconducting what is often referred to as long-termecological research (LTER), either by monitoringchange in certain variables over a broad area or bystudying specific ecosystems in detail.

One very productive LTER project has been inexistence since 1963 at the Hubbard BrookExperimental Forest in New Hampshire (see Figure 13.30A on the following page). At thislocation, scientists began by monitoring the amountof important nutrients that entered the ecosystem(contained in rain and snow) and the amount thatleft it (flowing out in streams or being given off tothe atmosphere by plants). The results indicatedthat the amounts were almost balanced — cyclesoperating within the forest ecosystem conservedthe nutrients the ecosystem contained.

www.mcgrawhill.ca/links/biology12In some cases, LTER involves members of the public assistingin the collection of data that are later analyzed by scientists. InCanada (as well as elsewhere in North America), programs likeFrogwatch, Feeder Watch, Tree Watch, The Breeding BirdSurvey, and The Christmas Bird Count encourage theparticipation of individuals and groups. Various web sites showyou how to get involved, obtain data, analyze populations, orlearn more about various LTER projects. To get started, go tothe web site above, and click on Web Links.

WEB L INK

decomposers

animals

plants

rain

geological uplifting

weathering of phosphate from rocks

run-off

phosphate in solution

chemical precipitation

detritus settling to bottom

sedimentation = new rocksleaching

phosphate in soil

Figure 13.29 Manufacturers have responded to thedemands of various environmental groups and removedmost of the phosphates from laundry and dishwashing

detergents. Why do you think these groups campaigned forthe removal of phosphates?


In 1966, one of the valleys in the forest area waslogged (as in Figure 13.30B). All the trees were cutdown and left to decay, and the area was sprayedwith herbicides to prevent new plant growth. The inflow and outflow of nutrients in this valleywas then compared with that of an untouchedexperimental forest. The amount of water that ranthrough and out of the logged valley increased by35 percent, presumably because there were noplants to absorb the water from the soil. Substantialamounts of nutrients were lost from the ecosystem,with most flowing into the stream that ran throughthe valley. The amounts of calcium and potassiumin the stream increased by 4 and 15 timesrespectively, and the nitrite concentration was 60 times higher after logging. Not only were thesenutrients being lost from the forest ecosystem, butin some cases they were making the water from thestream unsafe to drink.

These results, as well as other data collectedover the last 35 years, have demonstrated that plantscontrol the amount of nutrients leaving a forestecosystem. When plants are removed, nutrients arelost. The loss begins immediately and continues foras long as the plants are absent. After successionstarts, nutrient loss is reduced. It may take a longtime to achieve the balance of loss and gain seen inan intact ecosystem — if it ever does return.

Environmental factors can have a significantinfluence on the variety, distribution, andproductivity of autotrophs in an ecosystem. Toenable you to understand the potential effect thesefactors can have, you will design and test your ownmodel ecosystem in the investigation on page 458.Using controlled testing, you will observe andmake comparisons between a control ecosystemset-up and one in which you have altered theenvironmental factors.

In this chapter, you recognized the challenge oflearning about the complexity of the interactionsamong organisms so they can be better understood.You also learned about trophic structure, includingvarious ecological principles involving abiotic andbiotic components; the complex relationships thatexist among individuals, populations, commu-nities, and ecosystems; ecological succession; andthe differentiation between the habitat, range, andniche of a population. You also learned abouttrophic structure, including the ecosystem role ofproducers, consumers, and decomposers, and sawhow energy flows and nutrients cycle through anecosystem. In Chapter 14, you will learn aboutpopulation ecology, and examine features ofpopulations, including their size and density. Youwill also investigate the factors that influence thegrowth and decline of populations.

Figure 13.30 Hubbard Brook Experimental Forest (A)Dams built along streams running through the area allowedresearchers to collect data on the usual outflow of nutrients.Rain and snow samples were collected and analyzed to

measure nutrient inflow. (B) After this area had been logged,the loss of nutrients increased dramatically. Do you think theloss would have stopped if secondary succession had beenallowed to occur naturally? Defend your answer.

A B


At WorkBiology

Ecological Policy Maker“Aboriginal wisdom teaches that all species should beprotected, and all species interrelate,” says Dr. DeborahMcGregor, Head of Aboriginal Policy and IntergovernmentalCo-ordination for Environment Canada, Ontario Region.Similarly, Dr. McGregor stresses that society shouldsafeguard all natural habitats, not merely those deemedenvironmentally sensitive. “You have to protect the wholesystem,” she says. “It’s not enough to just create a park.”

“Certain responsibilities come with being in Creation,” sheadds. “One key principle is that everything has life, whileanother is the ethics of non-interference. For example, weshouldn’t interfere with water and prevent it from carryingout its responsibilities by polluting it. If we do, we createimbalances. The water can no longer quench our thirst.”A member of the Whitefish First Nation on ManitoulinIsland, Dr. McGregor believes that Aboriginal wisdom isspiritually derived — it comes from the Creator.

According to Dr. McGregor, all Aboriginal people arescientists in the sense that they have ecological wisdompassed down to them by their elders. “Aboriginal scienceis different from traditional western (non-Aboriginal)science,” she says. “It’s important for western-trainedscientists to work with Aboriginal people — to considermore bases of knowledge.” Dr. McGregor provides adviceto staff at Environment Canada on how to interact withAboriginal communities. “Visitors shouldn’t just come with

a predetermined agenda, a clipboard, and tape recorder,”she says. “The way to really learn is to build relationships.”

Among traditional western professions, Aboriginal peoplehave tended to enter teaching, law, social work, andhealth sciences. Dr. McGregor and her EnvironmentCanada colleagues try to encourage more Aboriginalpeople to go into ecology and other natural resourcefields. Dr. McGregor has a master’s degree inenvironmental studies from York University, and adoctorate in forestry from the University of Toronto. She stresses, however, that academic achievement goes only so far in preparing a person for a career.“Community service is important too,” she says. “Youhave to prove that you can deliver at the community level — that you’re not just book learning and talk.”

Career Tips1. What community services could you perform to help

prepare yourself for a career that interests you? Makea list.

2. “Traditional western science doesn’t have amonopoly on truth,” says Dr. McGregor. In 1997, she was among Aboriginal scientists featured in anOntario Science Centre exhibit called Question ofTruth. Use the Internet and/or other resources tolearn more about Aboriginal scientists. Choose onescientist and write a report about his or her work.

Manitoulin Island

S K I L L F O C U S

Initiating and planning

Hypothesizing

Identifying variables

Performing and recording

Analyzing and interpreting

I n v e s t i g a t i o nDESIGN YOUR OWN

1 3 • B


Problem

How are model ecosystems used to investigate theimpact of various environmental factors on theproduction of biomass at various trophic levels of anenergy pyramid?

Hypothesis

Devise a hypothesis about how one or more specificenvironmental factors may affect the productivity andbiomass of each trophic level within a specific type ofecosystem.

CAUTION: Treat all of the organisms you haveselected with care and respect. Wash your handsafter handling the organisms.

Materials

From your procedure, develop a list of requiredmaterials. Be sure to include an appropriate number ofequal-sized containers for your ecosystem simulations.

The ecosystem in this aquarium shows three trophiclevels (primary producer, consumer, and detrivore). Thereis also an invisible fourth trophic level. Can you identify it?

Experimental Plan

1. For this investigation, organize yourselves into smallwork groups. Each group will be responsible forinvestigating how a specific variable (such as lightintensity) affects some aspect of its modelecosystem. Groups may wish to co-ordinate theirefforts by testing different ranges or values of thesame variable. This approach will generate a widerange of data for analysis regarding the impact of asingle variable. Alternatively, work groups maychoose to investigate different variables —generating a variety of data about the impact ofdifferent variables. Whichever organizational strategyyou select, each group will prepare one control set-up and a minimum of two experimental set-upsfor the investigation.

2. Use containers to set up terrariums or aquariumsthat will serve as simulated ecosystems for yourinvestigation (each set-up should be identical).Select organisms (such as phytoplankton,zooplankton, snails, aquarium plants, and small fishspecies) that will inhabit the ecosystems. A typicalecosystem consists of three to four trophic levels.

3. Identify the organism(s) in each trophic level anddraw a food web of your ecosystems. Include lessobvious organisms (such as bacteria and fungi) thatform important links in the food web.

4. Identify a specific limiting factor or variable, such as the intensity of light, temperature, fertilizer, orspecific nutrient that will impact the productivity of your model ecosystems. Make this yourindependent (manipulated) variable.

5. Identify the other factors or variables to becontrolled; that is, those that must remain the same in each of your ecosystem set-ups.

6. Design a procedure to test the effect of the selectedexperimental variable on productivity and biomass.This procedure should allow for quantitativemeasurement of changes in biomass production ineach ecosystem. Select an appropriate parameterto be measured, such as transparency of water, pH,CO2 concentration in aquarium water, populationsamples of micro-organisms, and so on.

Ecosystem ProductivityVarious vital environmental factors (such as sunlight, water quality, nutrientavailability, and temperature) influence the variety, distribution, and productivity of autotrophs within an ecosystem. In this investigation, you will devise modelecosystems and use them to study the impact of specific environmental factors on the productivity of various trophic levels of a food web. You will also investigatehow energy flows through the various trophic levels in these ecosystems.


7. Devise a sampling procedure (be sure to includesample size, procedure, timetable and frequency ofsampling, and sampling instruments) to measurechanges in productivity and biomass of the targetorganism(s) at each trophic level in each modelecosystem. Ensure that your sampling techniqueswill provide an accurate estimation without undulydisturbing the integrity of the ecosystems.

8. You may wish to set up and run the ecosystems fora period of time to ensure they are stable, and togive you an opportunity to sample and identify thevarious species inhabiting your systems. You mayalso wish to monitor biotic and abiotic conditionssuch as temperature, water transparency, nutrientcontent, population size of various species, and thepH of the water. The data derived from this initialsampling could provide you with a baseline orstandard for your subsequent experimentalobservations.

9. Design a procedure for evaluating the effects ofchanging the selected variable and measuringecosystem response.

10. Your procedure should provide quantitative as wellas qualitative data. Determine how your sampledata could be translated into specific biomassvalues at each trophic level for the food webs inyour model ecosystems.

11. Determine your method of data analysis. Select theformat for graphs and charts that will illustrate mosteffectively how key parameters of your ecosystemsresponded to the variables manipulated in yourexperimental procedures.

12. Establish the amount of time you will require. Askyour teacher to approve your experimental designand your safety plans and arrange for the equipmentand materials you require.

Checking the Plan

1. Describe the dependent and independent variablesin this investigation.

2. What will you measure and how?

3. Design a table to record your data and observations.

4. Select the factors to be graphed, such as sunlightversus biomass.

Data and Observations

Conduct your investigation and record the observationsand measurements. Enter your data on a summary tableand graph your results.

Analyze

1. Describe any changes in biomass production andother factors in your ecosystem models over theduration of the experimental procedure.

2. What did the results indicate about how the variabletested for in this procedure affected biomass ateach trophic level?

3. Compare the biomass of heterotrophs with thebiomass of autotrophs.

4. How did your results relate to your originalhypothesis?

5. Discuss possible sources of error in your procedure(relating to such factors as the sampling proceduresused and the ability to control extraneous variables)that may have influenced your results.

6. How do the results of your group compare with thedata provided by the other groups working onsimilar variables? Did dividing the class into groupsprovide you with a more complete understanding ofthe effects of certain variables on the quality offunctioning ecosystems?

Conclude and Apply

7. What can you conclude about the accuracy of youroriginal hypothesis?

8. How could your original procedure be improved?

9. Discuss the benefits and drawbacks of studyingmodel ecosystems rather than studying actualecosystems in the field.

If you have access to probeware, do the activity“Temperature and Dissolved Oxygen Use by Goldfish.”

Probeware


S E C T I O N R E V I E W

1. Describe the similarities and differences youwould expect to see between biogeochemical cyclesthat occur in aquatic ecosystems (such as a pond)and those that occur in terrestrial ecosystems (suchas a grassland).

2. What are the essential differences between the biogeochemical cycles for elements like nitrogenor oxygen and for elements like phosphorous orcalcium?

3. Draw a flowchart that illustrates the carbonbiogeochemical cycle.

4. Explain why oil and coal are often referred to asfossil fuels.

5. Describe the role of nitrification in supplyingessential nutrients to the species found at higherlevels in a food web.

6. Explain the similarities and differences betweenterrestrial and aquatic ecosystems with respect tocarbon cycling.

7. Describe the role of bacteria in terrestrialbiogeochemical cycles.

8. Make a diagram that illustrates each phase of the hydrologic cycle. Identify the biotic and abioticelements of the cycle depicted in your diagram.

9. Identify the types of green plants that play acritical role in the perpetuation of the nitrogen cycle.

10. Speculate about the possible impact of globalwarming on the carbon cycle in the northern borealforest, which is one of Canada’s more predominantbiomes.

11. Some concerns now exist about the purity offruits and vegetables that are labelled organic. Someacademics suggest that it is not possible to growfood that is completely free of pesticides, fertilizers,and other commercial additives that are commonlyused by today’s highly industrialized agriculturaloperations. Discuss these concerns within thecontext of your knowledge of how biotic and abioticfactors interact in natural ecosystems.

12. In Earth’s northern hemisphere, the amount ofcarbon dioxide in the atmosphere is less during thesummer than during the rest of the year. Examine thecarbon cycle and suggest why this is so. (Note: thereis more land area in the northern than the southernhemisphere, and therefore more vegetation).

13. Design an investigation to compare the ecologicalefficiency of a small terrestrial ecosystem (such as asmall terrarium) with an aquatic ecosystem (such as asmall aquarium). Specifically, you may want to addressthe following generalization: aquatic ecosystems aremore efficient than terrestrial ecosystems because thebodies of aquatic producers are made up of a smallerpercentage of indigestible material than the bodies ofterrestrial producers.

LTER projects are used to study ecosystems for longperiods of time to fully understand the movement andtime required for nutrients to complete certain cycles. Are LTER projects being undertaken in your country? What impact might changed biogeochemical cycles haveon humans?

UNIT ISSUE PREP

I

K/U

MC

MC

K/U

C

K/U

K/U

K/U

K/U

C

K/U

K/U

As you continue to prepare for the Biology CourseChallenge, consider the ecological principles presented inthis chapter. These principles offer many opportunities forselecting an issue to analyze from various perspectives. For example, look again at the carbon cycle presented inFigure 13.27 on page 453. This figure shows the effects ofcarbon on the following (keep in mind that there are alsoeffects on living things you cannot see, such as insects and bacteria):

• plants • animals

• atmosphere • land surfaces

• industries

COURSE CHALLENGE

13.4 nutrient cycling in ecosystems · 13.4 nutrient cycling in ecosystems i n addition to energy,...

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