Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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Acidiﬁcation, Including Acid Mine Dra inage Oxidation of soluble ferrous iron appears

to consume acidity:

2þ

þ H

! Fe

3þ

However, the subsequent precipitation of ferric hydroxide has the effect of producing

3 mol of acid, so that the overall reaction can be written as

2þ

þ 2:5H

O ! FeðOHÞ

þ 2H

ð13:21Þ

During mining of coal and other minerals, anaerobic deposits containing pyrites

(FeS

) are commonly exposed to the air. This begins a complex process involving both

abiotic and biochemical mechanisms that leads eventually to oxidation of both the iron

and sulfur. Acidiﬁcation initially results from the oxida tion of reduced sulfur to sulfate

(sulfuric acid), which then makes oxidation of the iron favorable. The ﬁrst step, or initia-

tor reaction, is typically abiotic at the starting neutral pH:

FeS

þ 3:5O

þ H

O ! Fe

2þ

þ 2SO

2

þ 2H

ð13:22Þ

The production of acid leads to a drop in pH and the development of favorable con-

ditions for bacteria such as Thiobacillus ferrooxidans. The overall reaction can be

written as

FeS

þ 3:75O

þ 3:5H

O ! FeðOHÞ

þ 2SO

2

þ 4H

ð13:23Þ

Drainage from such areas can thus be highly acidic, with a pH of 2 not uncommon. This

may have a drastic effect on the ecosystem of any nearby streams. An iron sulfate mineral

known as yellow boy may also precipitate and color a stream bed dramatically.

Pyrites are also found in some clay minerals. This has been found to be a problem with

the clay caps that are sometimes put over landﬁlls—acid formation inhibits establishment

of cover vegetation. Leachate from landﬁlls containing reduced iron and/or sulﬁdes may

also become acidiﬁed through similar processes.

Drinking Water With certain groundwater supplies, iron-oxidizing bacteria such as

Gallionella in wells can cause clogging of the screens, resulting in a decrease in pumping

capacity. Beyond the growth itself, the large amounts of precipitated iron they produce

may be responsible for much of the problem. A similar effect can occur with the oxidation

of reduced manganese (Section 13.5.3), but iron concentrations are typically much

(1000) higher. Clogging of sand ﬁlters is also possible if the feed water contain s sufﬁ-

cient reduced iron.

High iron concentrations can provoke consumer complaints about the aesthetic quality

of drinking water. Dissolved iron can create taste problems, particularly with hot bev-

erages such as coffee or tea. Also, the oxidation of reduced iron over time may produce

a noticeable brownish discolo ration (mainly from ferric hydroxide) on porcelain surfaces

such as kitchen sinks, toilet bowls, and bathtubs, and even in clothes during washing.

Additionally, there may be concern among customers over the safety of the ‘‘rusty’’

water itself. For these reasons, the secondary standard for iron in drinking water is

0.3 mg/L.

438 MICROBIAL TRANSFORMATIONS

13.5 MANGANESE

Manganese can have an oxidation state as high as þ7 in the form of permanganate

(KMnO

), which has some use in the environmental ﬁeld as an oxidizing agent. However,

within the biological realm, the natural states are þ2toþ4, called, respectively, the

‘‘mangan ous’’ and ‘‘manganic’’ forms. Soluble Mn

2þ

and insoluble MnO

represent

the principal forms that are found.

The dominant reservoir for manganese is the lithosphere; it forms  0.1% of Earth’s

crust. Waters, both fresh and marine, rarely contain manganese concentrations above

a few tenths of a part per million, and often far less. Like iron, manganese can also

be chemically oxidized by atmospheric oxygen, but in this case pH levels of 8 or higher

(as opposed to pH 4 to 5 for iron) are necess ary to promote this spontaneous reac-

tion. (This fact is used in the Winkler test, the standard ch emical method of measur-

ing dissolved oxygen; a solution of Mn

2þ

is added to the sample, followed by alkali,

leading to formation of a stoichiometric amount of MnO

that is then analyzed

iodometrically.)

13.5.1 Manganese Reduction

Manganese in the þ4 state (i.e., MnO

) can be reductively converted to Mn(II) by micro-

bial activity under anoxic conditions. At least in some cases it appears that it is being used

as an alternative electron acceptor for anaerobic respiration.

13.5.2 Manganese Oxidation

There is a relatively small group of bacteria capable of directly using manganese as an

oxidative energy source, including some strains of Arthrobacter, Bacillus manganicus,

Corynebacterium, Flavobacterium, Gallionella, Hyphomicrobium, Pseudomonas, and

Vibrio. Several strains of iron-oxidizing bacteria, such as Leptothrix and Sphaerotilus,

are also believed to be capable of cooxidizing manganese, precipitating it within their

sheaths and metallic coatings.

13.5.3 Manganese in Environmental Engineering and Science

The cycling of manganese is not often regarded as a major environmental engineering and

science concern, particularly since it is usually present only in the low parts per billion

range. However, as with iron, higher levels of manganese may cause aesthetic problems in

waters, or even difﬁculties with groundwater pumping. These issues tend to develop in

relation to site-speciﬁc conditions.

Like iron, manganese can impose objectionable tastes in waters at quite low concen-

trations, particularly with hot coffee and tea. Discoloration with manganese generates a

pinkish color. To address these concerns, the secondary standard for drinking water has

been set at 50 parts per billion.

Typically, the problem is seen in communities using a lake or reservoir as their pot-

able water source. Such water bodies tend to stratify (Section 15.2.1), with an aerobic

zone (epilimnion) above an anaerobic one (hypolimnion). Seasonally, as the depths of

these layers vary, the point of withdrawal for these waters (Figure 13.29) may extend

MANGANESE 439

into the anaerobic zone. This can draw reduced manganese into the system, where it may

later be oxidized.

Another potential problem occurs when groundwater is contaminated by landﬁll lea-

chate or other degradable organic compounds, leading to anaerobic conditions. This can

lead to mobilization of manganese and iron, potentially causing clogging of nearby wells

(Figure 13.30).

Nominal Mn

at water intake point

Fixed

elevation

intake pipe

Seasonal

upward shift

in hypolimnion

(typically ‘‘peaks’’

during Fall)

settling

MnO

Thermocline

Hypolimnion

MnO

reduced

Epilimnion

MnO

oxidation

Thermocline

Hypolimnion

Epilimnion

MnO

reduced

MnO

oxidation

Elevated Mn

at water intake point

Figure 13.29 Seasonal shifts in lake stratiﬁcation leading to increased presence of soluble reduced

manganese (Mn

2þ

) in potable waters.

Figure 13.30 Mobilization of reduced manganese and iron by groundwater contamination.

440

MICROBIAL TRANSFORMATIONS

PROBLEMS

13.1. What is the theoretical oxygen demand of a 20-mg/L solution of (a) pentane,

; (b) ribose, C

13.2. What are the total organic carbon concentrations of the solutions in Problem 13.1?

13.3. How much oxygen is utilized to nitrify 25 mg=LNH

-N to nitrate? How much

CaCO

alkalinity?

13.4. In a particular wastewater treat ment plant, 200 mg/L of C-BOD and 20 mg/L of

ammonium-N are oxidized using oxygen. Suppose that anoxic zones could be

introduced so that all the nitrate produced was consumed by denitriﬁcation while

oxidizing some of the wastewater C-BOD. By what percentage could the oxygen

requirement potentially be reduced in this way?

13.5. A water quality limit of 0.04 mg/L NH

is placed on a stream receiving the efﬂuent

from a wastewater treatment plant. (a) How much total ammonia-N can be present

in the water if the ﬁnal pH of the stream is 7.3? (b) If the stream above the discharge

point contains 0.2 mg/L total ammonia, and if the plant discharge is 20% of the total

ﬂow, how much total ammonia can the efﬂuent contain without resulting in a

violation of the limit?

13.6. In mg/L, how much methanol would be needed to denitrify 20 mg/L of nitrate-N

stoichiometrically in an anoxic wastewater?

13.7. Suppose that a ﬁnished drinking water contains 0.5 mg/L ammonium nitrogen that

becomes nitriﬁed by bioﬁlms in the water distribution system. How much chlorine

residual will this nitrite consume?

REFERENCES

Atlas, R. M., 1997. Principles of Microbiology, 2nd ed., Wm. C. Brown, Dubuque, IA.

Atlas, R. M., and Richard Bartha, 1993. Microbial Ecology: Fundamentals and Applications, 3rd ed.,

Benjamin/Cummings, Redwood City, CA.

Madigan, M. T., J. M. Martinko, and J. Parker, 2000. Brock Biology of Microorganisms, 9th ed.,

Prentice Hall, Upper Saddle River, NJ.

Rittmann, B. E., and P. L. McCarty 2001. Environmental Biotechnology: Principles and Applications,

McGraw-Hill, New York.

University of Minnesota Biocatalysis/Biodegradation Database, 2004. http://umbbd.ahc.umn.edu/.

Updated regularly.

U.S. EPA, 1993. Process Design Manual: Nitrogen Control, EPA/625/R-93/010, U.S. Environmental

Protection Agency, Ofﬁce of Research and Development, Washington, DC.

Wackett, L. P., and C. D. Hershberger 2001, Biocatalysis and Biodegradation: Microbial Transfor-

mation of Organic Compounds, ASM Press, Washington, DC.

Young, L. Y., and C. Cerniglia (Eds.), 1995. Microbial Transformation and Degradation of Toxic

Organic Chemicals, Wiley-Liss, New York.

REFERENCES 441

ECOLOGY: THE GLOBAL VIEW OF LIFE

‘‘Ecology is the scientiﬁc study of the interactions that determine the distribution and

abundance of organisms’’ (Krebs, 1994). The effects of pollutants can be detected at

all levels of the biological hierarchy. Our concern about these impacts falls mainly into

two groups of organisms: humans and everything else. We are concerned directly with all

types of impacts on humans. However, in nonhuman populations our primary conce rn is

about loss of desirable species and the proliferation of undesirable ones resulting from our

activities. Th erefore, ecology provides an appropriate framework for examining our

impact on the natural world.

The word ecology has as its root oikos, the Greek word for ‘‘house.’’ The word

economy is based on the same root. Thus, one can be thoug ht of as the study of where

we live; the other is its management. It is intere sting that these two disciplines are com-

monly considered to conﬂict with each other. However, economics is now being brought

to bear on environmental problems. Some think that if the true value of unspoiled ecosys-

tems were taken into account, much destructive human activity would actually be seen not

to be econom ically beneﬁcial. In this view, the apparent economic advantages of polluting

or destroying ecosystems comes from the costs being borne by others than those who

receive the proﬁts (the ‘‘tragedy of the commons’’).

Recall the following deﬁnitions from Section 2.2:

Population A group of individuals of the same species living in the same environment

and are actively interbreeding.

Community A group of interacting populations occupying the same environment.

Environment The physical and chemical surroundings in which individuals live.

Ecosystem The combination of a community and its environment.

Environmental Biology for Engineers and Scientists, by David A. Vaccari, Peter F. Strom, and James E. Alleman

442

These are the entities with which we are concerned directly in the study of ecology. A

biome is a major type of ecosystem. Table 14.1 lists the most important biomes. The indi-

vidual biomes are discussed in more detail in Chapter 15.

The range of environments in which a species lives is called its habitat. Each type of

organism is thought to occupy a unique functional position, called its niche, in an ecosystem.

The idea of a niche can be deﬁned as a subset of the environmental and ecological variables

that are favorable to a species. For example, two aquatic insects, the backswimmer (Noto-

necta) and the water-boatman (Corixa), occupy the same habitat among aquatic plants at

the edge of a pond. But they have different niches because the former is a predator and the

latter feeds on dead plant matter. Niche may be characterized by many variables, such as

food source or tolerated physical conditions. The niches of two species may overlap but

not coincide. It is said that two organisms in a particular ecosystem cannot occupy the

same niche. However, they can coexist if there are even slight differences in their behavior.

Let us now systematize and categorize the ecological roles taken by organisms. Much

of this is in terms that engineers will ﬁnd familiar. For example, one important way to

look at ecosystems is to examine their energy and material balances. Changes in sizes

of populations are modeled in a way not very different from chemic al or microbial

kinetics. Most unique, perhaps, are the types of interactions between species such as com-

petition or symbiosis. However, even these are studied in part using the mathematics of

population dynamics. As you learn the ways in which we describe ecological roles and

phenomena, think about how pollution could affect each.

14.1 FLOW OF ENERGY IN THE ECOSYSTEM

With minor exceptions, the biosphere is powered by the sun. The average amount of solar

energy reaching Earth’s surface (the insolation) in the United States ranges from 1250 to

TABLE 14.1 Biomes of the Biosphere

Terrestrial biomes

Tundra: arctic and alpine

Boreal coniferous forests

Temperate deciduous forests

Temperate grassland

Tropical grassland and savanna

Chaparral: winter rain / summer drought

Desert: herbaceous and shrub

Semievergreen tropical forest: pronounced wet and dry seasons

Evergreen tropical rain forest

Freshwater ecosystems

Lentic (standing water): lakes, ponds, etc.

Lotic (running water): rivers and streams

Wetlands: marshes and swamp forests

Marine ecosystems

Pelagic (open ocean)

Continental shelf (inshore waters)

Upwelling regions

Estuaries (coastal bays, sounds, river mouths, salt marshes, etc.)

Source: Odum (1987).

FLOW OF ENERGY IN THE ECOSYSTEM

443

2750 kcal/m

 day in January to 5250 to 7000 kcal/m

 day in July. For the entire year,

averaged over all regions of the country, the input is about 3940 kcal/m

 day. Much smal-

ler quantities originate from nonthermal sources. Geothermal energy derives from radio-

active decay in the Earth and contributes about 0.5% of the solar input. Tidal friction

extracts energy from the kinetic energy of the Earth–sun–moon system and is about

0.0017% of solar. A portion of the wind’s energy comes from the kinetic energy of Earth’s

rotation. Oxidation of reduced inorganic minerals transported to the biosphere from deep

in Earth’s crust by hot springs and deep ocean vents provide energy for unique ecosystems

adjacent to them. (However, this source is not completely independent of solar input, as it

relies on oxygen produced mostly in the photosphere by solar energy.) These are the ulti-

mate sources of energy for the biosphere. However, sensible heat and mechanical forms of

energy cannot be used to form carbohydrates from CO

. Organisms require high-quality

energy in the form of photons or chemical bond energy in order to drive the reactions that

produce biomass from inorganic precursors.

14.1.1 Primary Productivity

Most of the solar energy arriving at Earth’s surface is converted to sensible heat or drives

the processes of evaporation, wind, and waves. About 0.8%, however, is captured by

photosynthetic conversion to organics. This autotrophic activity supplies practically all

of the energy for the biosphere. The autotrophs responsible for the initial generation of

ﬁxed carbon from CO

are called producers in ecological terminology. The rate at

which carbon is ﬁxed in an environment is called its primary productivity. It can be mea-

sured either in energy terms (kcal/m

 day), or equivalently, in terms of biomass produc-

tion rate (e.g., g/m

yr). The basis area (m

) refers to the area of the land. Biomass is

usually measured as the total dry weight of the organisms under consideration. Carbohy-

drates have about 4.1 kcal/g (17.2 kJ/g).

Note that much of the activity of chemoautotrophs is not ‘‘primary’’ in the sense that

the reduced minerals they are oxidizing were produced by other organisms. For example,

hydrogen sulﬁde and ammonia are produced by anaerobi c bacteria in marsh sediments.

These can then be used by chemoautotrophic bacteria to ﬁx CO

. The energy for the anae-

robes comes from organics that probably originated from plants. Other sources of these

minerals, such as deep ocean vents or volcanoes, are abiotic in origin.

Primary productivity has two main com ponents. Gross primary productivity (GPP) is

the total amount of carbon ﬁxed by the autotrophs. GPP may also be called the rate of

assimilation. Only part of the gross productivity is available to other components of the

ecosystem because the autotrophs themselves respire, consuming some of the fruits of

their own labor. Net primary productivity (NPP) is the rate at which biomass or energy

is accumulated by the autotrophs. It is equal to the difference between gross productivity

and respiration. In practice, the net productivity is measured by harvesting and respiration

measured by CO

production.

Gross primary productivity can be computed from measurements of net primary pro-

ductivity and respiration. In some systems, such as the ocean , GPP is strongly correlated

to the chlorophyll content. Chlorophyll can be measured by solvent extraction followed

by spectrophotometry. Then one can apply a factor known as the assimilation ratio,

which is the ratio of carbon ﬁxation rate to chlorophyll concentration. For the ocean

the assimilation ratio is fairly constant at 3.7 g of carbon ﬁxed per hour per gram of chlor-

ophyll. However, this ratio can vary widely for other ecosystems. Plants that are adapted

444 ECOLOGY: THE GLOBAL VIEW OF LIFE

to grow in shade usually have much higher chlorophyll concentrations in order to capture

more effectively the energy available to them. This results in a lower assimilation ratio.

Forests typically have about 0.4 to 3.0 g of chlorophyll per square meter, and an assim-

ilation ratio that varies from about 0.4 to 4.0. Thinly vegetated areas such as new crops in

the ﬁeld, on the other hand, may have only 0.01 to 0.60 g of chlorophyll per square meter

but ﬁx from 8 to 40 g of carbon per gram of chlorophyll.

It is important to understand the relationship between the gross and net productivity,

which are rates, and the biomass, or standing crop. Keep in mind that the amount of bio-

mass present is not a measure of the gross or net productivity. As an analogy, consider a

tank with water being pumped in, and with some ﬂowing out through a hole in the bottom.

The inﬂow is like the gross primary productivity, and the outﬂow is like respiration. If

more is entering the tank than is draining out the hole, the difference accumulates. The

rate of accumulation is like the net primary productivity. If the amount entering and

leaving are equal, the amount in the tank stays the same and the system is at steady

state. This would be equivalent to having zero net productivity and could occur with

the tank full or almost empty. Consider a mature tropical rain forest (Table 14.2). The

gross primary productivity is of such a forest is about 45,000 kcal/m

yr. However,

most of that is taken up by respiration to maintain the large biomass. Thus, the net primary

productivity is essentially zero.

On the other hand, an alfalfa ﬁeld is adding considerably to its biomass during the

growing season. Its gross primary productivity is 24,400 kcal/m

yr. Respiration accounts

for 9200 kcal/m

yr, leaving a net primary productivity of 15,200 kcal/m

yr. However,

the actual yield that farmers can obtain from the ﬁeld will be lower because other hetero-

trophs (such as ﬁeld mice) consume part of the crop. This leads to a third type of produc-

tivity, net community productivity (NCP), which is the gross primary productivity minus

plant (autotrophic) respiration and minus the amount consumed by other organisms in the

ecosystem (heterotrop hic respiration). In the case of the alfalfa ﬁeld, 800 kcal/m

yr is

consumed by heterotrophic respiration. This yields a net community productivity of

14,400 kcal/m

yr, for an efﬁciency of 59%.

Primary productivity (GPP) can be limited by numerous factors, such as the amount of

sunlight, availability of moisture and nutrients, soil properties, and so on. On land, moist-

ure is the most serious limiting factor. In the open ocean, productivity is limited by

nutrient availability and the ability of sunlight to penetrate the water. Ideal conditions

can result in productivities that are as much as double the ‘‘typical’’ values (e.g., see

Table 15.1); 50,000 kcal/m

yr is thought to be the practical upper limit for any natural

ecosystem. Changes in primary productivity, or comparisons to similar ecosystems, can be

TABLE 14.2 Forms of Annual Primary Productivity for Several Ecosystems (kcal/m

)

Medium Age Mature

Alfalfa Field Oak/Pine Forest Rain Forest

Gross primary productivity 24,400 11,500 45,000

Net primary productivity 15,200 5,000 13,000

Net community productivity 14,400 2,000 Negligible

NPP/GPP 62% 43% 29%

NCP/GPP 59% 17% 0%

Source: Odum (1987).

FLOW OF ENERGY IN THE ECOSYSTEM

445

used as indicators of the health of an ecosystem. Worldwide, the total GPP is estimated to

be about 10

kcal/yr.

The primary productivity of the world’s agriculture has only been keeping pace with

increases in human population. Humans greatly increas e the productivity of cultivated

land by energy subsidies. This energy originates mostly from fossil fuels and takes the

form of synthetic nitrogen fertilizer, pesticides, and mechanized agriculture (tilling, har-

vesting, etc.). Productivity of crops has also been increased by breeding new varieties. The

greatly increased food production has resulted in an increase in the carrying capacity of

the Earth for humans, and has been called the green revolution. However, it must be noted

that the new crops are more dependent on energy subsidies than are traditional cultivars.

Although the improved productivity has been important, underdeveloped countries have

increased their food production as much by increasing the amount of land under cultiva-

tion. This results in destruction of forests and resulting loss in habitat for other species.

Humans have other uses for primary productivity besides as food. Cotton and ﬂax

ﬁbers are used for clothing and wood for structures, paper, and fuel. Overharvesting of

wood in poor regions of the world has led to deforestation and erosion.

14.1.2 Trophic Levels, and Food Chains and Webs

One of the most important relationships among organisms in an ecosystem is, of course,

who eats whom. Ecologists discover these relationships by direct observation as well as

by examining the stomach contents of animals captured from the wild. One species of bird

may eat seeds, another might only eat certain types of insects, whereas a third may prey

mostly on small mammals.

The energy and ﬁxed carbon from the autotrophic producers are available to hetero-

trophs in the ecosystem, and these are called the consumers. Consumers that feed directly

on the producers are called primary consumers, also called herbivores (plant eat ers).

Those that feed on herbivores are called secondary consumers,or carnivores (meat

eaters). Next may come tertiary consumers,ortop carnivores, which feed on carni-

vores.

This structure describes the food chain, the seque ntial feeding rel ationship from pri-

mary producers to higher consumers. The feeding levels in the food chain are collectively

called trophic levels. The organisms in a particular trophic level have the same number of

steps in their food chain between themselves and the primary producers.

The portion of each trophic level that is not used as food by a higher level will even-

tually die. A parallel food chain is based on this dead material. The organisms that rely on

this are called detritivores, decomposers, saprobes,orsaprotrophs, and a food chain

based on them is called a detritus food chain . (In contrast, a food chain based on living

biomass is called a grazing food chain.) The detritus food chain starts with organisms

that feed directly on dead material, including bacteria, fungus, earthworms and other

soil invertebrates, and some marine crustaceans, such as lobsters and crabs. These may,

themselves, be fed upon. In fact, the grazing and detritus food chains are linked at all

levels. For example, an owl may eat an insectivorous (insect-eating) shrew, which in

turn has fed on saprobic soil invertebrates, and the same owl may also eat an herbivorous

mouse (a grazer).

Ultimately, all the dead organic material is mineralized, that is, converted back to inor-

ganic minerals such as CO

, water , ammonia or nitrates, and salts. Along the way, the partially

biodegraded biomass forms an important part of the soil and aquatic environments. The

446 ECOLOGY: THE GLOBAL VIEW OF LIFE

decomposers are the ultimate recyclers, making the material available for reuse by the

primary producers. By releasing the minerals close to the roots of plants where they

can be rapidly taken up again, they reduce the rate at which scarce minerals leach out

of the ecosystem.

The energy for each trophic level is obtained from the level below. Some of the energy

in a trophic level will be unavailable because it might be indigestible material, such as

cellulose, lignin, chitin, or bone, or may be protected by defensive measures. Biomass

may be stored, as in the case of peat formation in marshes, o r it may be exported from

the ecosystem, as in the case of migratory animals or tidal estuaries.

The energy input to a trophic level is the gross energy intake. Part of this is unused

and is eliminated in the feces where it enters the detritus food chain. The remainder is the

assimilated energy. Part of the assimilated energy is excreted with urine. A large part is

used in respiration for maintenance and activity. The remainder is available for growth

and reproduction, forming the net productivity of that trophic level. The production efﬁ-

ciency (also called transfer efﬁciency) of a species or of a trophic level as a whole can be

deﬁned as:

Production efficiency ¼

net productivity

assimilation

ð14:1Þ

Typical values for production efﬁciency are:

Birds 1.3%

Small mammals 1.5%

Other mammals 3.1%

Fish and social insects 9.8%

Other invertebrates 21–56%

Warm-blooded animals seem to pay a penalty in efﬁciency. The rate of respiration in

animals can be related to body mass empirically by a log-log relationship. For example,

for herbivorous mammals:

log R ¼ 0:774 þ 0:727 log M ð14:2Þ

where R is the respiration rate in kJ/day per individual and M is the body mass in grams

(1 kJ ¼ 0.239 kcal). Cold-blooded animals may use more than an order of magnitude less

energy, but the value will be strongl y dependent on ambient temperature.

Although they vary widely, a rule of thumb is that the production efﬁciency of trophic

levels is typically about 10%. Thus, if the gross prima ry productivity is 60 kcal/m

day,

the net primary productivity would be estimated at 6 kcal/m

day, and the herbivores

would have a net productivity of about 0.6 kcal/m

day. Carnivores typically have a

20% efﬁciency, so net tertiary productivity would be estimated at 0.12 kcal/m

day.

As just described, the trophic structure of an ecosystem is commonly represented as an

energy pyramid, which shows the relative proportions of energy or production in the var-

ious trophic levels. As Figure 14.1 shows, the pyramid can also be used to compare gra-

phically quantities other than energy, such as population or standing crop. Note that the

food chain is rarely longer than four trophic levels. Although this was originally thought

to be due to low production efﬁciency, it now seems that the reason is due to the greater

sensitivity of top carnivores to ﬂuctuations in environmental conditions.

FLOW OF ENERGY IN THE ECOSYSTEM 447