Chapt 36

8/13/2019 Chapt 36

1/20

Ecosystems andHuman Interferences

Chapter Concepts

36.1 The Nature of Ecosystems An ecosystem is a community of organisms

along with its physical and chemicalenvironment. 744

Autotrophs make their own food;photoautotrophs carry on photosynthesis.Heterotrophs, on the other hand, take inpreformed food. 744

Solar energy enters biotic communities via

photosynthesis, and as organic molecules passfrom one organism to another, heat is returnedto the atmosphere. 745

36.2 Global Biogeochemical Cycles Chemicals cycle within and between ecosystems

in global biogeochemical cycles. 748 Biogeochemical cycles are gaseous (carbon cycle,

nitrogen cycle) or sedimentary (phosphoruscycle). 748

The addition of carbon dioxide (and other gases)to the atmosphere is associated with globalwarming. 750

The production of fertilizers from nitrogen gas isassociated with acid deposition, photochemicalsmog, and temperature inversions. 752

Fertilizer also contains mined phosphate;fertilizer runoff is associated with waterpollution. 754

36.3 Human Impact on Biodiversity Global warming, acid deposition, photochemical

smog, water pollution, ozone depletion, andtropical rain forest destruction are all involved inreducing biodiversity. 756

Conservation biology is the scientific study ofbiodiversity and the management of ecosystemsfor the preservation of all species, including

Homo sapiens. 757

Humans usually live in developed areas with a limited variety ofspecies, and these areas are sources of pollution that is harmful toall forms of life.

743

Back Forward Main Menu TOC Study Guide TOC Textbook Website Student OLC MHHE Website
http://toc.pdf/http://toc.pdf/http://toc.pdf/

8/13/2019 Chapt 36

2/20

8/13/2019 Chapt 36

3/20

Energy Flow and Chemical CyclingWhen we diagram all the biotic components of an ecosys-tem, as in Figure 36.2 it is possible to illustrate that every

ecosystem is characterized by two fundamental phenom-ena: energy flow and chemical cycling. Energy flowbeginswhen producers absorb solar energy, and chemical cycling

begins when producers take in inorganic nutrients fromthe physical environment. Thereafter, producers makefood for themselves and indirectly for the other popula-tions of the ecosystem. Energy flow occurs because all theenergy content of organic nutrients is eventually convertedto heat, which dissipates in the environment. Therefore

most ecosystems cannot exist without a continual supplyof solar energy. Chemicals cycle when inorganic nutrientsare returned to the producers from the atmosphere or soil,as appropriate.

Only a portion of the food made by autotrophs is passedon to heterotrophs because plants use organic molecules tofuel their own cellular respiration. Only about 55% of thefood made by producers is available to heterotrophs. Simi-larly, only a small percentage of food taken in by het-erotrophs is available to higher level consumers. Figure 36.3

shows why. A certain amount of the food eaten by a herbi-vore is never digested and is eliminated as feces. Metabolicwastes are excreted as urine. Of the assimilated energy, alarge portion is utilized during cellular respiration andthereafter becomes heat. Only the remaining food which is

converted into increased body weight (or additional off-spring) becomes available to carnivores.

The elimination of feces and urine by a heterotroph,and indeed the death of all organisms, does not mean thatsubstances are lost to an ecosystem. They represent thefood made available to decomposers. Since decomposerscan be food for other heterotrophs of an ecosystem, the sit-uation can get a bit complicated. Still, we can conceive thatall the solar energy that enters an ecosystem eventually be-comes heat. And this is consistent with the observation thatecosystems are dependent on a continual supply of solarenergy.

The laws of thermodynamics support the concept that

energy flows through an ecosystem. The first law statesthat energy cannot be created (nor destroyed). This ex-plains why ecosystems are dependent on a continual out-side source of energy, usually solar energy, which is used

by photosynthesizers to produce food. The second lawstates that with every transformation some energy is de-graded into a less available form such as heat. Becauseplants carry on cellular respiration, for example, onlyabout 55% of the original energy absorbed by plants isavailable to an ecosystem.

Energy flows through an ecosystem, while

chemicals cycle within and between ecosystems.

Chapter 36 Ecosystems and Human Interferences 74536-3

solarenergy

inorganic

nutrientpool

heat heat

heatnutrients

energy

producers consumers

decomposers

growthand

repro-duction

Food eaten by herbivore

cellularrespirationde

fec

ation

death

excre

tion

Energy to detritus feeders

Energy to carnivores

Heat toenvironment

Figure 36.2 Nature of an ecosystem.Chemicals cycle but energy flows through an ecosystem. All the

energy derived from the sun eventually dissipates as heat as energy

transformations repeatedly occur.

Figure 36.3 Energy balances.Only about 10% of the food energy taken in by a herbivore is passed

on to carnivores. A large portion goes to detritus feeders in the ways

indicated, and another large portion is used for cellular respiration.

http://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0598l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0599l.pdfhttp://vrl/0598l.pdf

8/13/2019 Chapt 36

4/20

Figure 36.5 Food chain.Trace this grazing food chain in the grazing food web depicted in Figure 36.4.

746 Part 7 Behavior and Ecology 36-4

Grazing food web

Detritus food web

fruits and nuts

leaves

old leaves,dead twigs

deer

rabbits

leaf-eatinginsects

mice

chipmunks

birds

bacteria and fungidetritusinvertebrates

carnivorousinvertebrates

shrews

salamanders

foxes

fishers

skunks

owls

snakes

hawks

producers primary consumers secondary consumers tertiary consumers

photosynthesizers herbivorescarnivores

Figure 36.4 Forest food webs.Two linked food webs are shown for a forest ecosystem: a grazing food web and a detrital food web.

http://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0601l.pdfhttp://vrl/0602l.pdfhttp://vrl/0602l.pdfhttp://vrl/0602l.pdfhttp://vrl/0602l.pdfhttp://vrl/0601l.pdf

8/13/2019 Chapt 36

5/20

Food Webs and Trophic LevelsThe principles we have been discussing can now be ap-plied to an actual examplea forest in New Hampshire. In

this forest, the producers include sugar maple, beech, andyellow birch trees. The complicated feeding relationshipsthat exist in natural ecosystems are called food webs. Afood web shows how organisms acquire their food. For ex-ample, Figure 36.4 shows that insects in the form of cater-pillars feed on leaves, while mice, rabbits, and deer feed onleaf tissue at or near the ground. Birds, chipmunks, andmice feed on fruits and nuts, but they are in fact omnivores

because they also feed on caterpillars. These herbivores

and omnivores all provide nutrients for a number of differ-ent carnivores. This portion of the diagram is called a graz-ing food web because it begins with aboveground plantmaterial.

The lower half of Figure 36.4 is devoted to the detritalfood web. Detritus, along with the bacteria and fungi of de-cay, can be food for larger decomposers. Because some ofthese, like shrews and salamanders, become food for above-ground carnivores, the detrital and the grazing food websare connected.

We naturally tend to think that aboveground vegetationlike trees are the largest storage form of organic matter andenergy, but this is not necessarily the case. In this particularforest, the organic matter lying on the forest floor and mixedinto the soil contains much more energy than does the leafmatter of living trees. The soil contains over twice as muchenergy as the forest floor. Therefore, more energy in a forestmay be funneling through the detrital food web thanthrough the grazing food web.

Trophic LevelsYou can see that Figure 36.4 would allow us to link organ-isms one to another in a straight line manner, according towho eats whom. Such diagrams are called food chains (Fig.36.5). For example, in the grazing food web we can find thisgrazing food chain:

leaves caterpillars tree birds hawks

And in the detrital food web we could find thisdetrital foodchain:

dead organic matter soil microbes earthworms etc.

A trophic level is all the organisms that feed at a particu-lar link in a food chain. In the grazing food web, goingfrom left to right, the trees are primary producers (firsttrophic level), the first series of animals are primary con-sumers (second trophic level), and the next group of ani-mals are secondary consumers (third trophic level) and so

forth.

Ecological PyramidsEcologists portray the energy relationships betweentrophic levels in the form of ecological pyramids, dia-

grams whose building blocks designate the varioustrophic levels (Fig. 36.6). (We need to keep in mind thatsometimes organisms dont fit into one trophic level. Forexample, chipmunks feed on fruits and nuts, but they alsofeed on leaf-eating insects.)

A pyramid of numbers simply tells how many organ-isms there are at each trophic level. Its easy to see that apyramid of numbers could be completely misleading. Forexample, in Figure 36.4 you would expect each tree to con-tain numerous caterpillars; therefore there would be moreherbivores than autotrophs! The problem, of course, has todo with size. Autotrophs can be tiny, like microscopic al-gae, or they can be big like beech trees; similarly, herbi-

vores can be small like caterpillars, or they can be largelike elephants.

Pyramids of biomass eliminate size as a factor since bio-mass is the number of organisms multiplied by theirweight. You would certainly expect the biomass of pro-ducers to be greater than the biomass of the herbivores,and that of the herbivores to be greater than the carni-vores. In some aquatic ecosystems such as lakes and openseas, where algae are the only producers, the herbivoresmay have a greater biomass than the producers when youtake their measurements. Why? The reason is that overtime, the algae reproduce rapidly, but they are also consumed


producers

herbivores

carnivores

top carnivores

Figure 36.6 Ecological pyramid.An ecological pyramid shows the relationship between either the

number of organisms, the biomass, or the amount of energy

theoretically available at each trophic level.

http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://vrl/0605l.pdfhttp://vrl/0605l.pdfhttp://vrl/0605l.pdfhttp://vrl/0605l.pdfhttp://vrl/0605l.pdfhttp://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/

8/13/2019 Chapt 36

6/20

at a high rate. Pyramids, like this one, that have moreherbivores than producers are called inverted pyramids:

There are ecological pyramids of energy also, and theygenerally have the appearance of Figure 36.6. Ecologists arenow beginning to rethink the usefulness of utilizing pyra-mids to describe energy relationships. One problem is whatto do with the decomposers, which are rarely included in

pyramids, and yet a large portion of energy becomes detri-tus in many ecosystems.There is a rule of 10% with regard to biomass (or energy)

pyramids. It says that, in general, the amount of biomass (orenergy) from one level to the next is reduced by a magnitudeof 10. Thus, if an average of 1,000 kg of plant material is con-sumed by herbivores, about 100 kg is converted to herbivoretissue, 10 kg to first-level carnivores, and 1 kg to second-level carnivores. The rule of 10% suggests that few carni-vores can be supported in a food web. This is consistent with

the observation that each food chain has from three to fourlinks, rarely five.

36.2 Global Biogeochemical Cycles MAll organisms require a variety of organic and inorganic nu-trients. Carbon dioxide and water are necessary for photo-synthesis. Nitrogen is a component of all the structural and

functional proteins and nucleic acids that sustain living tis-sues. Phosphorus is essential for ATP and nucleotide

zooplankton

relativedry weight

phytoplankton

production. In contrast to energy, inorganic nutrients areused over and over again by autotrophs.

Since the pathways by which chemicals circulatethrough ecosystems involve both living (biosphere) and

nonliving (geological) components, they are known asbio-geochemical cycles. For each element, chemical cycling mayinvolve (1) a reservoira source normally unavailable toproducers, such as fossilized remains, rocks, and deep-seasediments; (2) an exchange poola source from which or-ganisms do generally take chemicals, such as the atmo-sphere or soil; and (3) the biotic communitythrough whichchemicals move along food chains, perhaps never entering apool (Fig. 36.7).

There are two general categories of biogeochemical cy-cles. In agaseous cycle, exemplified by the carbon and nitro-gen cycles, the element returns to and is withdrawn from theatmosphere as a gas. In the sedimentary cycle, exemplified bythe phosphorus cycle, the element is absorbed from the sed-iment by plant roots, passed to heterotrophs, and is eventu-ally returned to the soil by decomposers, usually in the samegeneral area.

The diagrams on the next few pages make it clear thatnutrients can flow between terrestrial and aquatic ecosys-tems. In the nitrogen and phosphorus cycles, these nutrientsrun off from a terrestrial to an aquatic ecosystem and in thatway enrich aquatic ecosystems. Decaying organic materialin aquatic ecosystems can be a source of nutrients for inter-tidal inhabitants like fiddler crabs. Sea birds feed on fish butdeposit guano (droppings) on land, and in that way phos-phorus from the water is deposited on land. It would seemthat anything put into the environment in one ecosystemcould find its way to another ecosystem. Scientists find the

soot from urban areas and pesticides from agricultural fieldsin the snow and animals of the Arctic.


reservoir exchangepool

bioticcommunity

atmospheresoilwater

fossil fuelsmineral in rockssediment in oceans

human activities

rp

odu

cers

decom

posers

consumer

s

Figure 36.7 Model for chemical cycling.Nutrients cycle between these components of ecosystems: Reservoirs such as fossil fuels, minerals in rocks, and sediments in oceans are

normally relatively unavailable sources, but pools such as those in the atmosphere, soil, and water are available sources of chemicals for the

biotic community. Human activities remove chemicals from reservoirs and pools and make them available to the biotic community, and the result

can be pollution.

http://sgch36.pdf/http://sgch36.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://vrl/0606l.pdfhttp://sgch36.pdf/http://glossary.pdf/http://glossary.pdf/

8/13/2019 Chapt 36

7/20

The Water CycleThe water (hydrologic) cycle is described in Figure 36.8.Fresh water is distilled from salt water. The suns rays cause

fresh water to evaporate from seawater, and the salts are leftbehind. Vaporized fresh water rises into the atmosphere,cools, and falls as rain over the oceans and the land.

Water evaporates from land and from plants (evapora-tion from plants is called transpiration). It also evaporatesfrom bodies of fresh water, but since land lies above sealevel, gravity eventually returns all fresh water to the sea. Inthe meantime, water is contained within standing waters(lakes and ponds), flowing water (streams and rivers), and

groundwater.When rain falls, some of the water sinks or percolatesinto the ground and saturates the earth to a certain level. Thetop of the saturation zone is called the groundwater table, orsimply, the water table. Sometimes groundwater is also lo-cated in aquifers, rock layers that contain water and will re-lease it in appreciable quantities to wells or springs.

Aquifers are recharged when rainfall and melted snow per-colate into the soil. In some parts of the country, especiallyarid areas and southern Florida, withdrawals from aquifersexceed any possibility of recharge. This is called ground-

water mining. In these locations the groundwater is drop-ping, and residents may run out of groundwater, at least forirrigation purposes, within a few short years. Fresh water,which makes up only about 3% of the worlds supply of wa-ter, is called a renewable resource because a new supply isalways being produced. But it is possible to run out of freshwater when the available supply is not adequate and/or ispolluted so that it is not usable.

In the water cycle, fresh water evaporates from the

bodies of water. Water that falls on land enters the

ground, surface waters, or aquifers. Water

ultimately returns to the oceaneven the quantity

that remains in aquifers for some time.


rainfallover ocean

rainfall

over land

evaporationfrom ocean

runoff

net transport ofwater vapor by wind

OCEAN

ICE

GROUNDWATERS

H2O IN ATMOSPHERE

transpiration from plantsandevaporation from soil

lake

aquifer

Figure 36.8 The water (hydrologic) cycle.

http://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdfhttp://vrl/0607l.pdf

8/13/2019 Chapt 36

8/20

The Carbon CycleIn the carbon cycle, both terrestrial and aquatic organismsexchange carbon dioxide with the atmosphere (Fig. 36.9).

On land, plants take up carbon dioxide from the air, andthrough photosynthesis they incorporate carbon into foodthat is used for other living things. When organisms (e.g.,plants, animals, and decomposers) respire, a portion of thiscarbon is returned to the atmosphere as carbon dioxide. Inaquatic ecosystems, the exchange of carbon dioxide with theatmosphere is indirect. Carbon dioxide from the air com-

bines with water to produce bicarbonate ion (HCO3), a

source of carbon for algae, which also produce food through

photosynthesis. And when aquatic organisms respire, thecarbon dioxide they give off becomes bicarbonate ion.Living and dead organisms are reservoirs for carbon. If

decomposition of dead remains fails to occur, they are subjectto physical processes that transform them into coal, oil, andnatural gas. We call these reservoirs for carbon the fossil fuels.Most of the fossil fuels were formed during the Carboniferousperiod, 286 to 360 million years ago, when an exceptionallylarge amount of organic matter was buried before decompos-ing. Another reservoir for carbon is calcium carbonate shells,

which accumulate in ocean bottom sediments.

Carbon Dioxide and Global WarmingA transfer rate is defined as the amount of a nutrient thatmoves from one component of the environment to anotherwithin a specified period of time. The width of the arrows inFigure 36.9 indicates the transfer rate of carbon dioxide. Thetransfer rates due to photosynthesis and respiration, which in-cludes decay, are just about even. However, there is now morecarbon dioxide being deposited in the atmosphere than beingremoved. In 1850, atmospheric carbon dioxide was about 280parts per million (ppm) and today it is about 350 ppm. This in-crease is largely due to the burning of fossil fuels and the de-struction of forests to make way for farmland and pasture.

The emission of other gases due to human activities is

also taking place. Altogether the following gases are ex-pected to contribute significantly to global warming:

Gas From

Carbon dioxide (CO2) Fossil fuel and wood burning

Nitrous oxide (N2O) Fertilizer use and animal wastes

Methane (CH4) Biogas (bacterial decomposi-

tion, particularly in the guts of

animals, in sediments, and in

flooded rice paddies)


SOILSOCEAN

CO2IN ATMOSPHERE

combustion

destructionof vegetation

photosynthesis

respiration

FOSSIL FUELS

dead organismsand animal waste

decay

runoff

diffusion

bicarbonate ( HCO3 )

sedimentation

LAND PLANTS

Figure 36.9 The carbon cycle.

http://glossary.pdf/http://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://vrl/0608l.pdfhttp://glossary.pdf/

8/13/2019 Chapt 36

9/20

as the oceans warm, temperatures in the polar regionswill rise to a greater degree than other regions. If so, glac-iers would melt, and sea levels will rise, not only due tothis melting but also because water expands as it warms.

Water evaporation will increase, and most likely therewill be increased rainfall along the coasts and dryer con-ditions inland. The occurrence of droughts will reduceagricultural yields and also cause trees to die off. Expan-sion of forests into Arctic areas might not offset the loss offorests in the temperate zones. Coastal agricultural landssuch as the deltas of Bangladesh, India, and China would

be inundated, and bill ions of dollars will have to be spentto keep coastal cities, like New York, Boston, Miami, and

Galveston in the United States from disappearing intothe sea.

The atmosphere is an exchange pool for carbon

dioxide. Fossil fuel combustion in particular has

increased the amount of carbon dioxide in the

atmosphere. Global warming is predicted because

carbon dioxide and other gases impede the

escape of infrared radiation from the surface of the

earth.


These gases are called greenhouse gases because just like thepanes of a greenhouse they allow solar radiation to passthrough but hinder the escape of infrared rays (heat) backinto space. Figure 36.10 shows the earths radiation balances.

One thing to be learned from this diagram is that water va-por is a greenhouse gas: clouds also reradiate heat back toearth. If the earths temperature rises due to thegreenhouseeffect, more water will evaporate, forming more clouds, set-ting up a positive feedback effect that could increase globalwarming.

Today, data collected around the world show a steadyrise in the concentration of the various greenhouse gases.Methane, another significant greenhouse gas, given off by

oil and gas wells, rice paddies, and organisms, is increasingby about 1% a year. Such data are used to generate computermodels that predict the earth may warm to temperaturesnever before experienced by living things. The global cli-mate has already warmed about 0.6C since the industrialrevolution. Computer models are unable to consider all pos-sible variables, but the earths temperature may rise 1.5-4.5C by 2100 if greenhouse emissions continue at the cur-rent rates.

Global warming will bring about other effects, which

computer models attempt to forecast. It is predicted that

absorbedreflected

greenhouse gases

GreenhouseEffect

Solar radiationthat passes throughthe atmospherewarms the earth'ssurface.

Clouds, which contain water vapor,absorb infrared rays (heat) from theearth's surface and warm the surfaceby downward reradiation of a portionof these rays.

Greenhouse gases, including carbon dioxide and methane,also cause the atmosphere to absorb and reradiate infraredrays toward the surface of the earth. As the concentrationof greenhouse gases increases, the temperature of theearth is expected to increase also.

Troposphere

Stratosphere

reflectedreleased by

clouds

infrared raysfrom

surface

reradiated

infraredrays

solarradiation

absorbed by clouds

Figure 36.10 Earths radiation balances.The contribution of greenhouse gases (far right) to the earths surface is called global warming.

http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://vrl/0609l.pdfhttp://glossary.pdf/http://glossary.pdf/http://glossary.pdf/

8/13/2019 Chapt 36

10/20

The Nitrogen Cycle

Nitrogen is an abundant element in the atmosphere. Nitrogen(N2) makes up about 78% of the atmosphere by volume, yet

nitrogen deficiency sometimes limits plant growth. Plantscannot incorporate nitrogen gas into organic compounds andtherefore depend on various types of bacteria to make nitro-gen available to them in the nitrogen cycle (Fig. 36.11).

Nitrogen fixationoccurs when nitrogen (N2) is convertedto a form that plants can use. Some nitrogen-fixing bacterialive in nodules on the roots of legumes. They make nitrogen-containing organic compounds available to a host plant.Cyanobacteria in aquatic ecosystems and free-living bacteria

in soil are able to fix nitrogen gas as ammonium (NH4

).Plants can use NH4 and nitrate (NO3

) from the soil. AfterNO3

is taken up, it is enzymatically reduced to NH4, which

is used to produce amino acids and nucleic acids.Nitrification is the production of nitrates. Nitrogen gas

(N2) is converted to nitrate (NO3) in the atmosphere when

cosmic radiation, meteor trails, and lightning provide thehigh energy needed for nitrogen to react with oxygen. Am-

monium (NH4) in the soil is converted to nitrate by

chemoautotrophic soil bacteria in a two-step process. First,nitrite-producing bacteria convert ammonium to nitrite(NO2

), and then nitrate-producing bacteria convert nitrite

to nitrate. Notice the subcycle in the nitrogen cycle that in-volves dead organisms and animal wastes, ammonium, ni-trites, nitrates, and plants. This subcycle does notnecessarily depend on nitrogen gas at all (Fig. 36.11).

Denitrification is the conversion of nitrate to nitrous ox-ide and nitrogen gas. There are denitrifying bacteria in bothaquatic and terrestrial ecosystems. Denitrification balancesnitrogen fixation, but not completely.

Nitrogen and Air PollutionHuman activities significantly alter transfer rates in the ni-trogen cycle. Because we produce fertilizers, thereby con-verting N2 to NO3

, and burn fossil fuels, the atmospherecontains three times the nitrogen oxides (NOx) than it wouldotherwise. Fossil fuel combustion also pumps much sulfurdioxide (SO2) into the atmosphere. Both nitrogen oxides andsulfur dioxide are converted to acids when they combine


plants

dead organisms

and animal waste

BIOTICCOMMUNITY BIOTIC

COMMUNITY

phyto-plankton

decomposers

NO3

NO3

NO2

NH4+

NH4+

sedimentation

cyanobacteria

denitrifying bacteria

nitrifyingbacteria

denitrifyingbacteria

N2fixation

N2fixation

nitrification

denitrificationdenitrification

runoff

humanactivities

decomposers

nitrogen-fixingbacteria in

nodules and soil

N2 IN ATMOSPHERE

Figure 36.11 The nitrogen cycle.

http://glossary.pdf/http://glossary.pdf/http://glossary.pdf/http://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://vrl/0610l.pdfhttp://glossary.pdf/http://vrl/0610l.pdfhttp://glossary.pdf/http://glossary.pdf/

8/13/2019 Chapt 36

11/20

plants, resulting in leaf mottling and reduced growth.Normally, warm air near the ground is able to escape

into the atmosphere. Sometimes, however, air pollutants,such as those in smog and soot, trap warm air near the earth.

During a thermal inversion there is cold air at ground levelbeneath a layer of warm stagnant air above. Some areas sur-rounded by hills are particularly susceptible to the effects ofa temperature inversion because the air tends to stagnate,and there is little turbulent mixing (Fig. 36.13).

Fertilizer use also results in the release of nitrous oxide(N2O), a greenhouse gas and a contributor to ozone shielddepletion in the stratosphere, a topic to be discussed later.

Atmospheric N2, a reservoir and exchange pool fornitrogen, must be fixed by bacteria in order to

make nitrogen available to plants. Environmental

problems are associated with the release of nitrous

oxide (N2O) and nitrogen oxides (NOx) due to the

action of bacteria on fertilizers and fossil fuel

combustion, respectively.


with water vapor in the atmosphere. These acids return toearth as either wet deposition (acid rain or snow) or dry de-position (sulfate and nitrate salts).

Increased deposition of acids has drastically affected

forests and lakes in northern Europe, Canada, and northeast-ern United States because their soils are naturally acidic andtheir surface waters are only mildly alkaline (basic) to beginwith. The forests in these areas are dying (Fig. 36.12), andtheir waters cannot support normal fish populations. Aciddeposition reduces agricultural yields and corrodes marble,metal, and stonework, an effect that is noticeable in cities.

Nitrogen oxides (NOx) and hydrocarbons (HC) reactwith one another in the presence of sunlight to produce

photochemical smog, which contains ozone (O3) and PAN(peroxyacetylnitrate).Hydrocarbons come from fossil fuelcombustion, but additional amounts come from variousother sources as well, including paint solvents and pesti-cides. Breathing ozone affects the respiratory and nervoussystems, resulting in respiratory distress, headache, andexhaustion. These symptoms are particularly apt to ap-pear in young people. Ozone is especially damaging to

cool air

warm inversion layer

cool air

Thermal inversion

cooler air

cool air

warm air

a. Normal pattern

b.

c.

Figure 36.12 Acid deposition.

a. Many forests in higher elevations of northeastern North Americaand Europe are dying due to acid deposition. b.Air pollution due to

emissions from factories and fossil fuel burning is the major cause of

acid deposition, which contains nitric acid (H2NO3) and sulfuric acid

(H2SO4).

Figure 36.13 Thermal inversion.a. Normally, pollutants escape into the atmosphere when warm air

rises. b. During a thermal inversion, a layer of warm air (warminversion layer) overlies and traps pollutants in cool air below. c. Los

Angeles is particularly susceptible to thermal inversions, and this

accounts for why this city is the air pollution capital of the United

States.

b.

a.

http://glossary.pdf/http://glossary.pdf/http://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://vrl/0611l.pdfhttp://glossary.pdf/http://glossary.pdf/http://vrl/0611l.pdf

8/13/2019 Chapt 36

12/20

8/13/2019 Chapt 36

13/20

pollutants to the oceans. Some 5 million metric tons of oil ayearor more than one gram per 100 square meters of theoceans surfacesend up in the oceans. Large oil spills killplankton, fish fry, and shellfishes, as well as birds and ma-

rine mammals. The largest tanker spill in U.S. territorial wa-ters occurred on March 24, 1989, when the tanker ExxonValdez struck a reef in Alaskas Prince William Sound andleaked 44 million liters of crude oil.

In the last 50 years, we have polluted the seas and ex-ploited their resources to the point that many species are atthe brink of extinction. Fisheries once rich and diverse, suchas Georges Bank off the coast of New England, are in se-vere decline. Haddock was once the most abundant species

in this fishery, but now it accounts for less than 2% of the to-tal catch. Cod and bluefin tuna have suffered a 90% reduc-tion in population size. In warm, tropical regions, manyareas of coral reefs are now overgrown with algae becausethe fish that normally keep the algae under control have

been killed off.

Sedimentary rock is a reservoir for phosphorus; for

the most part producers are dependent on

decomposers to make phosphate available to

them. Fertilizer production and other humanactivities add phosphate to aquatic ecosystems,

contributing to water pollution.


Runoff of phosphate and nitrogen due to fertilizer use, ani-mal wastes from livestock feedlots, as well as discharge fromsewage treatment plants results in eutrophication (overen-richment). Eutrophication can lead to an algal bloom, appar-

ent when green scum floats on the water. When the algae dieoff, decomposers use up all available oxygen during cellularrespiration. The result is a massive fish kill.

Figure 36.15 lists the various sources of water pollution.Point sources are sources of pollution that are specific, andnonpoint sources are those caused by runoff from the land.Industrial wastes can include heavy metals and organochlo-rides, such as those in some pesticides. These materials arenot degraded readily under natural conditions nor in con-

ventional sewage treatment plants. They enter bodies of wa-ter and are subject to biological magnificationbecause theyremain in the body and are not excreted. Therefore, they be-come more concentrated as they pass along a food chain. Bi-ological magnification occurs more readily in aquatic foodchains because aquatic food chains have more links than ter-restrial food chains. Humans are the final consumers in foodchains, and in some areas, human milk contains detectableamounts of DDT and PCBs, which are organochlorides.

Coastal regions are the immediate receptors for local

pollutants, and are the final receptors for pollutants carriedby rivers that empty at a coast. Waste dumping occurs at sea,but ocean currents sometimes transport both trash and pol-lutants back to shore. Offshore mining and shipping add

industrial wastes

Pointsources

Nonpoint

sources

Nonpoint

sources

Pointsources

Pointsources

nuclear reactor

oil pollution

barnyard wastes

fertilizerrunoff

cropdusting

sewagetreatment plant

suburbandevelopment

city

acidic waterfrom mines

sediments

Biodegradable organic compounds (e.g., sewage, wastes from food processing plants, paper mills, and tanneries)

Nitrates and phosphates from detergents, fertilizers, and sewage treatment plants

Enriched soil in water due to soil erosion

Heated water from power plants

disease-causing agents

synthetic organic compounds

inorganic chemicals and minerals

radioactive substances

Bacteria and viruses from sewage (e.g., food poisoning and hepatitis)

pesticides, industrial chemicals (e.g., PCBs)

Acids from mines and air pollution; dissolved salts; heavy metals (e.g., mercury) from industry

From nuclear power plants, medical and research facilities, and nuclear

weapons testing

oxygen-demanding wastes

plant nutrients

sediments

thermal discharges

Sources of Water Pollution

Leading to Cultural

Eutrophication

Health Hazards

Figure 36.15 Sources of surface water pollution.Many bodies of water are dying due to the introduction of pollutants from point sources, which are easily identifiable, and nonpoint sources,

which cannot be specifically identified.

http://glossary.pdf/http://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://vrl/0615l.pdfhttp://glossary.pdf/

8/13/2019 Chapt 36

14/20

36.3 Human Impact on BiodiversityAll of the human activities we have discussed thus far havea negative impact on biodiversity. Global warming maymean that coastal ecosystems, such as marshes, swamps,and bayous, will have to move inland to higher ground asthe sea level rises, but many of these are blocked in by artifi-cial structures and may be unable to move inland. Acid de-position is associated with dead or dying lakes and forests,particularly in North America and Europe. We have pol-luted the seas and exploited their resources to the point thatmany species are on the brink of extinction. Still there areother human activities that will adversely affect the number

of species on earth.

Stratospheric Ozone DepletionThe earths atmosphere is divided into layers. The tropo-sphere envelops us as we go about our day-to-day lives.Ozone in the troposphere is a pollutant, but in the strato-sphere, some 50 km above the earth, ozone (O3) forms alayer, called the ozone shield, that absorbs most of the

wavelengths of harmful ultraviolet (UV) radiation so thatthey do not strike the earth. Life on earth is threatened ifthe ozone shield is reduced. UV radiation impairs cropand tree growth and also kills offplankton (microscopic plant and ani-mal life) that sustain oceanic life. With-out an adequate ozone shield, livingthings, our food sources, and healthare threatened. UV radiation causes

mutations that can lead to skin cancerand can make the lens of the eyes de-velop cataracts. It also is believed toadversely affect the immune systemand our ability to resist infectiousdiseases.

Depletion of the ozone shieldwithin the stratosphere in recent yearsis, therefore, of serious concern. It be-came apparent in the 1980s that some

worldwide depletion of ozone had oc-curred, and that by the 1990s there wasa severe depletion of some 4050%above the Antarctic every spring (Fig.36.16). Severe depletions of the ozonelayer are commonly called ozoneholes. Nitrous oxide is one cause ofozone depletion, but, in large part, thecause of ozone depletion can be traced

to chlorine atoms (Cl) that are releasedin the troposphere but rise into thestratosphere. Chlorine atoms combinewith ozone and strip away the oxygenatoms one by one. One atom of chlorine

can destroy up to 100,000 molecules of ozone before set-tling to the earths surface as chloride many years later.These chlorine atoms come from the breakdown of chloro-fluorocarbons (CFCs), chemicals much in use by humans

from 1955 to 1990. The best-known CFC is Freon, a heattransfer agent still found in refrigerators and air condition-ers today. CFCs were used as cleaning agents and duringthe production of styrofoam found in coffee cups, egg car-tons, insulation, and paddings. Their use as a propellent inspray cans has been outlawed in the United States and sev-eral other countries but not in western Europe. Althoughmost countries of the world have agreed to stop usingCFCs by the year 2000, CFCs already in the atmosphere

will be there for over a hundred years before they stoptheir destructive activity.

Ozone depletion is one of our air pollution

problems. The others are global warming, acid

deposition and photochemical smog.


Figure 36.16 Ozone shield depletion.These satellite observations show that the amount of ozone over the South Pole between

October 1979 and October 1994 fell by more than 50%. Green represents an average amount

of ozone, blue less, and purple still less.

http://sgch36.pdf/http://sgch36.pdf/http://glossary.pdf/http://sgch36.pdf/http://glossary.pdf/

8/13/2019 Chapt 36

15/20

Tropical Rain Forest DestructionTropical rain forests are much more biologically diverse thantemperate forests (see Fig. 36.4). For example, temperate

forests across the entire United States contain about 400 treespecies. In the rain forest, a typical ten-hectare area holds asmany as 750 types of trees. Tropical rain forests are also notedfor their animal diversity. On the eastern slopes of the Andes,there are 80 or more species of frogs and toads, and inEcuador, there are more than 1,200 species of birdsroughlytwice as many as those inhabiting all of the United States andCanada. Therefore, a very serious side effect of deforestationin tropical countries is a loss of biological diversity.

A National Academy of Sciences study estimated that amillion species of plants and animals are in danger of disap-pearing within 20 years as a result of deforestation in tropicalcountries. Many of these life forms have never been studied,and yet they may be useful sources of food or medicines. Fig-ure 36.17 lists other deleterious effects of deforestation.

Logging of tropical forests occurs because industrializednations prefer furniture made from costly tropical woods and

because people want to farm the land. In Brazil, the govern-ment allows citizens to own any land they clear in the Amazon

forest (along the Amazon River). When they arrive, the peoplepractice slash-and-burn agriculture, in which trees are cutdown and burned to provide inorganic nutrients and space toraise crops. Unfortunately, the fertility of the land is sufficientto sustain agriculture for only a few years. Once the clearedland is incapable of sustaining crops, the farmer moves on toanother part of the rain forest to slash and burn again. In themeantime, cattle ranchers move in. Cattle ranchers are thegreatest beneficiaries of deforestation, and increased ranchingis therefore another reason for tropical rain forest destruction.A newly begun pig-iron industry in Brazil also indirectly re-sults in further exploitation of therain forest. The pig iron must beprocessed before it is exported, andsmelting the pig iron requires the useof charcoal (burnt wood).

Conservation BiologyConservation biologyis a relatively new scientific disciplinethat brings together people and knowledge from many dif-

ferent fields to attempt to solve the biodiversity crisis. Con-servation biology wants to understand the effects of humanactivities on species, communities, and ecosystems, and de-velop practical approaches to preventing the extinctions ofspecies and the destruction of ecosystems. In the past, ecolo-gists have preferred to study the workings of ecosystems nottainted by human activities, and wildlife managers have

been concerned with managing a small number of species forthe marketplace and for recreation. Therefore, neither en-deavor has addressed the possibility of preserving entire bi-ological communities, although humans are active in thearea. Conservation biologists want to draw from scientific re-search and experience in the field to develop a managementprogram that will preserve an ecosystem.

Many conservation biologists believe that each specieshas a value all its own, regardless of its direct material valueto humans. Other conservation biologists are willing to testthe hypothesis of sustainability; that it is possible to manageecosystems so that biodiversity is preserved while still meet-

ing the economic needs of humans.Much scientific research is being directed on how to pre-

serve ecosystems and, therefore, biodiversity. The readingon the next page describes work being done at the Univer-sity of Rhode Island.

Conservation biology is the scientific study of

biodiversity, leading to the preservation of species

and the management of ecosystems for

sustainable human welfare.


Figure 36.17 Tropical rain forest.Forest destruction leads to the

detrimental effects listed.

Deforestation:

forests take up carbon dioxide from theatmosphere

forests are home for plants and animalstropical rain forests contain unique plantstrees hold the soilsaw mills and paper mills pollutefirst step toward converting land to industrializedor urbanized areas

Loss of CO2sink:

Loss of biodiversity:Loss of possible medicinal plants:Soil erosion:Water pollution:Ecosystems destruction:

There is much concern

worldwide about the lossof biological diversity due

to the destruction of

tropical rain forests.

http://glossary.pdf/http://glossary.pdf/

8/13/2019 Chapt 36

16/20

The MERL (Marine Ecosystem Research Laboratory) enclo-sures shown in Figure 36A provide marine researchers with a

unique means to experiment with an entire marine ecosystem.

The tanks measure 1.8 meters in diameter and 5 meters deep

and are located outdoors, exposed to natural sunlight. To initi-

ate a typical experiment, a benthic (bottom of the ocean) com-

munity is collected, usually from a silt-clay area in

Narragansett Bay, off the coast of Rhode Island. A 37-cm-thick

bed of sediment weighing roughly a ton is placed into each

tank. Thirteen cubic meters of unfiltered seawater are trans-ferred from the adjacent bay with nondisruptive displacement

pumps. Mechanical mixing provides water movement in the

enclosures to simulate wind and wave movement in the field.

In the summer, cooling is provided, and in the winter, heat is

provided to keep the enclosure temperatures similar to those

in the bay.

When set up in this manner, unmanipulated enclosures

maintain healthy ecosystems for many months and with prop-

erties that are similar to those actually found in the bay. Their

large size and proximity to laboratory facilities allows repetitive

sampling of all biological populations at relatively short time in-

tervals. Because there are 14 enclosures, replication of experi-

ments with controls is possible.

Experiments are done to determine the effect of a contami-

nant and to study the fate of chemicals within an entire coastal

ecosystem. One set of MERL experiments addressed the prob-

lem of chronic additions of oil hydrocarbons to coastal waters.

Water runoff from land, especially in urban areas, carries a

continuous trickle of oil to coastal environments. The totalamounts of petroleum hydrocarbons introduced into coastal

waters through urban runoff and river

runoff are believed to be greater than in-

troductions through oil spills. Daily ad-

ditions of fuel oil, of the type regularly

used in home furnaces, were made into

three replicate enclosures. Two experi-

ments were conducted. The first 5.5-month

experiment added oil to achieve about

0.2 ppm total hydrocarbons in the water,

while the second 4-month experiment achieved about 0.1 ppm

total hydrocarbons. Thereafter, recovery from the oil additions

was studied for one year.

These levels of fuel hydrocarbons are below those that

cause most tested marine species to die. One objective of the

experiments was to develop an index system that could be

used to indicate the health of an ecosystem. It was hypothe-

sized that community diversity and primary productivity

would be lower in the treated enclosures compared to the con-

trols. The oil additions had a clear effect on the populations inthe enclosures. Zooplankton and benthic macroorganisms

were greatly reduced in abundance. Benthic populations re-

mained depressed for at least a year after the treatments were

stopped.

Even though the additions of oil clearly had a major impact

on the communities in the enclosures, neither of the original hy-

potheses turned out to be correct. Although the population lev-

els were quite different in treated and control tanks, measures of

the diversity of benthic organisms in treated and control enclo-

sures were indistinguishable. With oil treatments, there were in-

creases in phytoplankton abundance and primary productivity

instead of the expected decrease. In hindsight, the reason for the

increase in phytoplankton abundance was clear. The oil was

more toxic to the organisms that may graze the phytoplankton

than to the phytoplankton itself. With the population of grazers

reduced, the abundance and production of phytoplankton

increased.

This is a good example of the interactions within an ecosys-

tem, and the inherent difficulty of predicting how any compo-nent of an ecosystem will respond to stress.

758

Marine Enclosures for Whole-Ecosystem Studies

Figure 36A Marine EcosystemResearch Laboratory (MERL).

MERL enclosures at the University of

Rhode Islands Graduate School of

Oceanography. Students are shown

taking samples and measurements.


8/13/2019 Chapt 36

17/20

Summarizing the Concepts36.1 The Nature of EcosystemsEcosystems contain biotic (living) components and abiotic (physical)

components. The biotic components of ecosystems are either producers

or consumers. Producers are autotrophs that produce their own food.

Consumers are heterotrophs that take in preformed food. Consumers

may be herbivores, carnivores, omnivores, or decomposers.

Energy flows through an ecosystem. Producers transform solar

energy into food for themselves and all consumers. As herbivores feed

on plants (or algae), and carnivores feed on herbivores, some energy is

converted to heat. Feces, urine, and dead bodies become food for de-

composers. Eventually, all the solar energy that enters an ecosystem is

converted to heat, and thus ecosystems require a continual supply of

solar energy.

Chemicals are not lost from the biosphere as is energy. They recy-

cle within and between ecosystems. Decomposers return some propor-

tion of inorganic nutrients to autotrophs, and other portions are

imported or exported between ecosystems in global cycles.

Ecosystems contain food webs, and a diagram of a food web

shows how the various organisms are connected by eating relation-

ships. Grazing food chains begin with vegetation that is fed on by aherbivore, which becomes food for a carnivore, and so forth. In detrital

food chains, a decomposer acts on organic material in the soil, and

when it is fed on by a carnivore, the two food webs are joined. Atrophic

level is all the organisms that feed at a particular link in food chains.

Ecological pyramids show trophic levels stacked one on the other like

building blocks. Generally they show that biomass and energy content

decrease from one trophic level to the next. Most pyramids pertain to

grazing food webs and largely ignore the detrital food web portion of

an ecosystem.

36.2 Global Biogeochemical CyclesBiogeochemical cycles contain reservoirs, components of ecosystems

like fossil fuels, sediments, and rocks that contain elements available

on a limited basis to living things. Pools are components of ecosystems

like the atmosphere, soil, and waterwhich are ready sources of nutri-

ents for living things. Nutrients cycle among the members of the bioticcomponent of an ecosystem.

In the water cycle, evaporation over the ocean is not compensated

for by rainfall. Evaporation from terrestrial ecosystems includes tran-

spiration from plants. Rainfall over land results in bodies of fresh wa-

ter plus groundwater, including aquifers. Eventually all water returns

to the oceans.

In the carbon cycle, organisms add as much carbon dioxide to the

atmosphere as they remove. Shells in ocean sediments, organic com-

pounds in living and dead organisms, and fossil fuels are reservoirs for

carbon. Human activities such as the burning of fossil fuels and treesare adding carbon dioxide to the atmosphere. Like the panes of a

greenhouse, carbon dioxide and other gases allow the suns rays to

pass through but impede the release of infrared wavelengths. It is pre-

dicted that a buildup of these greenhouse gases will lead to a global

warming. The effects of global warming could be a rise in sea level and

a change in climate patterns with disastrous effects.

In the nitrogen cycle, the biotic community, which includes sev-

eral types of bacteria, keeps nitrogen recycling back to the producers. A

few organisms (cyanobacteria in aquatic habitats and bacteria in soil

and root nodules) can fix atmospheric nitrogen. Other bacteria return

nitrogen to the atmosphere. Human activities convert atmospheric ni-trogen to fertilizer which is broken down by soil bacteria and they burn

fossil fuels. In this way a large quantity of nitrogen oxide (NO x) and

sulfur dioxide (SO2) is added to atmosphere where it reacts with water

vapor to form acids that contribute to acid deposition. Acid deposition

is killing lakes and forests and also corrodes marble, metal, and

stonework. Nitrogen oxides and hydrocarbons (HC) react to form

smog, which contains ozone and PAN (peroxyacetylnitrate). These ox-

idants are harmful to animal and plant life.

In the phosphorus cycle, the biotic community recycles phospho-

rus back to the producers, and only limited quantities are made avail-able by the weathering of rocks. Phosphates are mined for fertilizer

production; when phosphates and nitrates enter lakes and ponds,

over-enrichment occurs. Many kinds of wastes enter rivers which flow

to the oceans now degraded from added pollutants.


Peter Jutro, a scientist working for theU.S. Environmental Protection Agency,wants to research native traditions forclues on how to preserve ecosystems. Hehas run into opposition from the indige-nous groups because they mistrust conser-vationists. Take, as an example, the factthat the Kuna people in Panama refused torenew the lease for a Smithsonian Institu-tion studying reef ecology for the past 21years. Much of the trouble seems to havecome from the failure of scientists to ex-plain their program to local communities.In a meeting of the Kuna congress, the sci-entists were accused by the Kuna of steal-ing their knowledge, stealing their reefs,stealing their sand. Local people find ithard to see a difference between a scien-

tific study and commercial ventures whichexploit their areas for minerals, timber,and other resources.

Laura Snook, a forester from DukeUniversity, researches the growing habitsof mahogany trees in Mexico. She came tothe conclusion that because the trees growslowly, logging shouldnt be done too fast.Most local foresters resented her findings,

but she was able to establish a good rela-tionship with two women foresters whowere open to her ideas. Now local peopleare changing the way they replant ma-hogany trees so that the resource will bethere for some time to come. The point isthat conservationists working with peoplefrom different cultural backgrounds prob-ably need to communicate their goals

more clearly and involve local people inproject planning. Perhaps they shouldlearn to accept slower time scales andstyles of decision-making different fromour own.

Questions1. Should U.S. scientists be studying

ecosystems in such far-flung places asPanama, Alaska, Mexico, and theAmazon? Why or why not?

2. Should we insert political correctness intonegotiations with local groups in orderto bring about conservation? Why orwhy not?

3. How far should a scientist go to establishcommunication with local groups in orderto preserve the environment in othercountries? Explain.


Go To Student OLC

8/13/2019 Chapt 36

18/20

36.3 Human Impact on BiodiversityGlobal warming, acid deposition, and water pollution all act to reduce

biodiversity. Ozone shield destruction, which is particularly associated

with CFCs, is expected to result in decreased productivity of the

oceans. The tropical rain forests are being cut to provide wood for ex-port. Slash-and-burn agriculture also reduces tropical rain forests. The

loss of biological diversity due to the destruction of tropical rain forests

will be immense. Many of these threatened organisms could possibly

be of benefit to humans if we had time to study and domesticate them.

Conservation biology is a new discipline that pulls together informa-

tion from a number of biological fields to determine how best to man-

age ecosystems for the benefit of all species, including humans.

Studying the Concepts1. Distinguish between autotrophs and heterotrophs, and

describe four different types of heterotrophs found in naturalecosystems. Explain the terms producer and consumer. 744

2. Tell why energy must flow but chemicals can cycle in anecosystem. 745

3. Describe two types of food webs and two types of foodchains typically found in terrestrial ecosystems. Which ofthese typically moves more energy through anecosystem? 74647

4. What is a trophic level? an ecological pyramid? 747

485. Give examples of reservoirs and pools in biogeochemical

cycles. Which is less accessible to biotic communities? 7486. Draw a diagram to illustrate the water cycle and the carbon

cycle. 749507. How and why is the global climate expected to change, and

what are the predicted consequences of this change? 750518. Draw a diagram of the nitrogen cycle. What types of bacteria

are involved in this cycle? 7529. What causes acid deposition, and what are its

effects? 752

5310. How does photochemical smog develop, and what is a ther-

mal inversion? 75311. Draw a diagram of the phosphorus cycle. 75412. What are several ways in which fresh water and marine

waters can be polluted? What is biologicalmagnification? 75455

13. Of what benefit is the ozone shield? What pollutant in partic-ular should be associated with ozone shield depletion, andwhat are the consequences of this depletion? 756

14. What are the primary ecological concerns associated with thedestruction of rain forests? 75715. Explain the primary causes of the biodiversity crisis and the

goals of conservation biology. 75657

Testing Yourself

Choose the best answer for each question.1. Of the total amount of energy that passes from one trophic

level to another, about 10% is

a. respired and becomes heat.b. passed out as feces or urine.c. stored as body tissue.d. All of these are correct.


2. Compare this food chain:algaewater fleas fish green herons

to this food chain:trees tent caterpillars red-eyed vireos hawks.

Both water fleas and tent caterpillars area. carnivores. c. detritus feeders.b. primary consumers. d. Both a and b are correct.

3. Which of the following contribute(s) to the carbon cycle?a. respiration c. fossil fuel combustion

b. photosynthesis d. All of these are correct.4. How do plants contribute to the carbon cycle?

a. When they respire, they release CO2 into the atmosphere.b. When they photosynthesize, they consume CO2 from the

atmosphere.c. They do not contribute to the carbon cycle.

d. Both a and b are correct.5. How do nitrogen-fixing bacteria contribute to the nitrogen

cycle?a. They return nitrogen (N2) to the atmosphere.

b. They change ammonium to nitrate.c. They change N2 to ammonium.d. They withdraw nitrate from the soil.

6. In what way are decomposers like producers?a. Either may be the first member of a grazing food chain.

b. Both produce oxygen for other forms of life.

c. Both require a source of nutrient molecules and energy.d. Both supply organic food for the biosphere.

7. Which statement is true concerning this food chain: grassrabbits snakes hawks?a. Each predator population has a greater biomass than its

prey population.b. Each prey population has a greater biomass than its preda-

tor population.c. Each population is omnivorous.d. Both a and c are correct.

For questions 8

11, match the terms with those in the key:Key:a. sulfur dioxide c. carbon dioxide

b. ozone d. chlorofluorocarbons(CFCs)

8. acid deposition9. ozone shield destruction

10. greenhouse effect11. photochemical smog12. Which of these is mismatched?

a. fossil fuel burning

carbon dioxide given offb. nuclear powerradioactive wastesc. solar energygreenhouse effectd. biomass burningcarbon dioxide given off

13. Acid deposition causesa. lakes and forests to die.

b. acid indigestion in humans.c. the greenhouse effect to lessen.d. All of these are correct.

14. Water is a renewable resource, anda. there will always be a plentiful supply.

b. the oceans can never become polluted.c. it is still subject to pollution.d. primary sewage treatment plants assure clean drinking

water.


8/13/2019 Chapt 36

19/20


15. Label this diagram.

Match the terms to these definitions:a. Partially decomposed remains of plants and ani-

mals found in soil and on the beds of bodies of water.

b. Formed from oxygen in the upper atmosphere, itprotects the earth from ultraviolet radiation.

c. Remains of once living organisms that are burnedto release energy, such as coal, oil, and natural gas.

d. Process by which atmospheric nitrogen gas ischanged to forms that plants can use.

e. Complex pattern of interlocking and crisscrossingfood chains.

Understanding the Terms

acid deposition 753aquifer 749

autotroph 744biogeochemical cycle 748biological magnification 755carbon cycle 750carnivore 744chlorofluoro-

carbons 756conservation biology 757consumer 744decomposer 744

deforestation 757denitrification 752detrital food chain 747detrital food web 747detritus 744ecological pyramid 747ecosystem 744food chain 747food web 747fossil fuel 750

global warming 751grazing food chain 747

grazing food web 747greenhouse effect 751herbivore 744heterotroph 744nitrification 752nitrogen cycle 752nitrogen fixation 752omnivore 744ozone hole 756ozone shield 756

PAN (peroxyacetylnitrate)753

phosphorus cycle 754photochemical smog 753producer 744thermal inversion 753transfer rate 750trophic level 747water (hydrologic) cycle 749

sun

heat

energy

nutrients

a.

heat

heat

b.

c.d.

d.

c.

b.

a.

16. Label the trophic levels.

Thinking Scientifically

1. Considering an ecological pyramid:

a. Why would you expect mice (herbivores) to be more com-mon than weasels, foxes, or hawks (carnivores) in theenvironment?

b. Why you would expect food chains to be short4 or 5links at most?

c. The population size of a top predator is not held in checkby another predator population. Why does a top predatorpopulation not increase constantly in size?

d. What would you expect to happen to an ecosystem if oneof the secondary consumer populations suffered a col-lapse?

2. You are an ecologist who has been hired by a less-developedcountry to help them increase their agricultural yield peracre. Why might you recommend that theya. retain labor intensive methods instead of adopting mecha-

nized means of growing food?b. limit their consumption of meat and use grains and

legumes as a source of protein?c. not keep cattle in feedlots and feed them grain?d. plant as many different varieties of crops as possible?e. grow crops that require as little irrigation as possible?f. begin a program of population control?

Using Technology

Your study of ecosystems and human interferences issupported by these available technologies.

Essential Study Partner CD-ROMEcology Ecosystems

Visit the Mader web site for related ESP activities.

Exploring the Internet

The Mader Home Page provides resources andtools as you study this chapter.

http://www.mhhe.com/biosci/genbio/mader

http://www.mhhe.com/biosci/genbio/maderinquiry9/student/olc/chap36espactivities_s.htmlhttp://www.mhhe.com/biosci/genbio/maderhttp://www.mhhe.com/biosci/genbio/maderinquiry9/student/olc/chap36espactivities_s.htmlhttp://www.mhhe.com/biosci/genbio/mader

8/13/2019 Chapt 36

20/20

Further Readings for Part 7

Bavendam, F. July 1998. Lure of the frogfish. National Geographic194(1):40. Use of camouflaging coloration to obtain prey.

Begon, M., et al. 1996. Ecology: Individuals, populations, andcommunities. London: Blackwell Science Ltd. Thedistribution and abundance of organisms and their physcialand chemical interactions within ecosystems is discussed.

Bennet-Clark, H. C. May 1998. How cicadas make their noise.Scientific American 278(5):58. This 2.3-inch-long insect canproduce mating calls at 100 decibels.

Boyd, C. E., and Clay, J. W. June 1998. Shrimp aquaculture and theenvironment. Scientific American 278(6):58. Building shrimpponds for shrimp farming can result in destructive flooding.

Cunningham, W. P., and Saigo, B. W. 1997. Environmental science: Aglobal concern. Dubuque, Iowa: Wm. C. Brown Publishers.Provides scientific principles plus insights into the social,political, and economic systems impacting the environment.

Dobson, A. P. 1996. Conservation and biodiversity. New York:Scientific American Library. Discusses the value of

biodiversity; describes endangered species management.

Dugatkin, L. A., and Godin, J. J. April 1998. How females choosetheir mates. Scientific American 278(4):56. Female choice is

studied in relation to a number of fish and bird species.Fox, G. 1997. Conservation ecology. 2d ed. Dubuque, Iowa: Wm. C.

Brown Publishers. Discusses the nature of the biosphere, thethreats to its integrity, and ecologically sound responses.

Gorman, J. January 1998. They saw it coming. Discover 18(1):82.Article discusses how El Nio was forecast.

Hedin, L. O., and Likens, G. E. December 1996. Atmospheric dustand acid rain. Scientific American 274(6):88. Despite pollutionreduction, acid rain continues to be a problem.

Kovacs, K. March 1997. Bearded seals. National Geographic191(3):124. This article describes a behavioral study of

bearded seals in their natural environment.

Long, M. E. April 1998. The vanishing prairie dog. NationalGeographic 193(4):116. Prairie dogs and their ecosystems aredisappearing from the American West.

McClintock, J. B., and Baker, B. J. May/June 1998. Chemicalecology in Antarctic seas.American Scientist 86(3):254.Sessile benthic dwellers of polar seas use chemical defensesto ward off predators.

Miller, G. T. 1996. Living in the environment. 9th ed. Belmont, Calif.:Wadsworth Publishers. This introductory environmentalscience text discusses how the environment is being abused,and what can be done to protect it.

Mitchell, J. G. February 1996. Our polluted runoff. NationalGeographic 189(2):106. 80% of U.S. water pollution is due toland runoff not resulting from industrial sources.

Morgan, M., et al. 1997. Environmental science: Managing biologicaland physical resources. Dubuque, Iowa: Wm. C. BrownPublishers. Written for the undergraduate, this book

explains how various environmental issues are linked.

National Geographic. October 1998. Millennium supplement:Population. Articles survey the needs of the worldwidepopulation, and address issues such as birthrate, globalfood production, and migration.

Natural History Magazine.July/August 1998. 107(6):3451. Articlesaddress the preservation of Amazon rain forest diversity.

Nemecek, S. August 1997. Frankly, my dear, I dont want a dam.Scientific American 277(2):20. Discusses how dams affect

biodiversity.Newman, E. 1997.Applied ecology. Oxford: Blackwell Scientific

Publications. Presents the role of biological science inenvironmental preservation.

Nicol, S., and Allison, I. September/October 1997. The frozen skinof the southern ocean.American Scientist 85(5):426. Sea-iceand the organisms that occupy it interact with the ocean-atmosphere system in ways that may influence climate.

Odum, E. 1997.A bridge between science and society. 3d ed.Sunderland, Mass.: Sinauer Associates. Introduces the

principles of modern ecology as they relate to threats to thebiosphere.

Ostfeld, R. S. July/August 1997. The ecology of Lyme-disease risk.American Scientist 85(4):338. Article discusses the history ofLyme disease, its symptoms and diagnosis, and the lifecycle of the deer tick.

Pitelka, L. F., et al. September/October 1997. Plant migration andclimate change.American Scientist 85(5):464. There may be arelationship between plant migration and climate change, asevidenced by the fossil record and computer models.

Rice, R. E., et al. April 1997. Can sustainable management savetropical forests? Scientific American 276(4):44. The strategy ofreplacing harvested trees in rain forests often fails.

Robinson, G. E. September/October 1998. From society to geneswith the honey bee.American Scientist 86(5):456. The lifestages of a honey bee are regulated by hormones,neurobiology, genes, and environment.

Rutowski, R. L. July 1998. Mating strategies in butterflies. ScientificAmerican 279(1):64. Visual attributes (colorful wing patterns)and chemical signals (pheromones) play important roles in

butterfly mating.

Schmidt, M. J. January 1996. Working elephants. ScientificAmerican 274(1):82. In Asia, teams of elephants serve as analternative to destructive logging equipment.

Schoech, S. J. January/February 1998. Physiology of helping inFlorida scrub jay.American Scientist 86(1):70. Birds, whichhelp rear the offspring of others, apparently experiencedelayed reproduction due to hormonal effects.

Scientific American Quarterly. Fall 1998. The oceans. Scientific

American 9(3). This issues articles discuss the origins ofearths water, polar ice cap melting, weather, pollution andlegal issues, aquaculture, mineral mining, and marinediversity.

Simmons, L. M. August 1998. Indonesias plague of fire. NationalGeographic 194(2):100. Slash-and-burn agriculturaltechniques result in air pollution and respiratory disease, aswell as deforestation.

Steiner, R. September/October 1998. Resurrection in the wind.International Wildlife 28(5):12. The short-tailed albatross is

recovering from near-extinction.Suplee, C. May 1998. Unlocking the climate puzzle. National

Geographic 193(5):38. Our use of fossil fuels may be alteringthe earths natural warming and cooling cycles.



Date post:	04-Jun-2018
Category:	Documents
Upload:	silax
View:	217 times
Download:	0 times

Chapt 36

Documents