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but controlled energy input (organics in the wastewater) takes its place. This system is

called heterotrophic succession. Respiration (per organism) peaks, then is reduced as sub-

strate concentrations drop and respiration achieves balance with energy input.

As succession proceeds, nutrien ts are stored in biomass, and in soil in the case of pri-

mary succe ssion. As these reservoirs build up, internal recycling increases, improving the

efﬁciency of nutrient use. Species diversity may increase throughout the succession,

although climax communities sometimes have less diversity than a peak. For example,

relatively moist forests in the Great Lakes region showed a peak diversity in middl e

ages, whereas drier forests increase in diversity continuously.

The succession just described has a single direction of change. Some ecosystems exhibit

cyclical changes even in the absence of external disturbance. The Calluna heath in Scotland

has a cycle of 20 to 30 years. Heather establishes itself on bare ground and matures in

about 7 to 15 years. After 14 to 25 years, this perennia l loses its vigor, and other lichens

and mosses invade. They eventually die out, leaving bare ground, and the cycle repe ats.

14.4.4 Distribution vs. Abundance: Spatial Structure of Communities

Earlier sections treated the population of each species within an ecosystem as if it were a

variable that depended only on time. However, their spatial distributions are interesting as

well. Factors that control where an organism will be found include:

TABLE 14.10 Trends Expected During Undisturbed (Autogenic) Succession

Energetics

Biomass (B) and organic detritus increase.

Gross productivity (P) increases in primary succession, little change in secondary.

Respiration (R) increases.

P/R ratio moves toward unity (balance).

B/P ratio increases.

Nutrient cycling

Element cycles increasingly closed.

Turnover time and storage of essential elements increase.

Cycling ratio (recycle/throughput) increases.

Nutrient retention and conservation increase.

Population and community structure

Species composition changes.

Species diversity peaks in middle or end of sere.

Species evenness peaks in middle or end of sere.

r-Selected organisms replaced by K-selected.

Life cycles increase in length and complexity.

Size of organism and/or propagule (seed, offspring, etc.) increases.

Mutualistic symbiosis increases.

Stability

Resistance increases.

Resilience decreases.

Overall strategy

Increasing efﬁciency of energy and nutrient utilization.

Trend based on theoretical considerations, not yet validated in the ﬁeld.

Source: Based on Odum (1987).

488 ECOLOGY: THE GLOBAL VIEW OF LIFE

 Method of dispersal

 Physical factors

 Habitat selection

 Interactions with other species (predation, competition, disease)

Organisms disperse to colonize new areas in many ways. Plant seeds may be spread by

wind, water, or animals, or they may be conﬁned to the margins of the parent. Animals

may disperse by their own motility or may be carried passively, like seeds, as in the case

of insects. If the environment in the new area ﬁts within their niche, they may thrive.

Some of the most general physical factors include climate, light availability, and the

chemical environment. Geo graphically, these translate to such factors as latitude, altitude

or depth (in soil or water), and position on a moun tain slope. Soil or water chemistry may

vary locally or regionally. Fire can alter the balance between grassland and forest, con-

trolling the position of their border.

These factors vary in gradients throughout the world. For example, temperature, rain-

fall, depth of soil and light all vary with elevation along the slope of a moun tain. Along

the gradient various species rise and fall in abundance. In what are known as open com-

munities, the population distributions overlap considerably, resulting in continuous var-

iation in species along the gradient (Figure 14.19a). In the other extreme, known as closed

communities, population abundances are strongly correlated (Figure 14.19b ).

Closed communities

Ecotone

Abundance

Environmental gradient

Open communities

Abundance

Environmental

radient

Figure 14.19 Population distributions along a hypothetical environmental gradient for both closed

and open communities.

POPULATIONS AND COMMUNITIES 489

Closed communities are more easily deﬁned as communities, since their boundaries

are marked more clearly. Sharp boundaries between closed communities are called

ecotones. They are especially prominent when the environmental gradient is fairly

rapid, such as the transition between a wetland and adjacent woodlands. Nevertheless,

they can also be present along gradual boundaries, such as the transition between grass-

lands and forest along long-ra nge moisture gradient s. In this case the boundaries are main-

tained by mutual inhibition, as when grasses prevent tree seeds from germinating and

trees prevent grasses from growing by shading them.

Ecotones have greater species diversity not only because they may have representatives

of two communities, but because some species beneﬁt directly from both communities.

This is called the edge effect, a concept that is attributed to Aldo Leopold. Many

birds, such as the American robin, for example, nest in the forest but feed in

ﬁelds. Human alterat ions to the landscape, such as by clearing ﬁelds or putting roads

through wilderness areas, result in increasing the edges. This also has the effect of

decreasing the area of each community, which can reduce the number of species each

can support [equation (14.34)], and particular species with large area requirements may

be eliminated.

We saw previously how the number of species tends to increase with the area of the

ecosystem. However, each species may not necessarily range over the entire area, but

rather, be limited by environmental conditions to a subset. It turns out, that most species

have sma ll ranges, and fewer have large ranges. Species with larger ranges also tend to

have greater population density. These tendencies may be due to the differing dispersion

capabilities of species or (as some think) to the artifacts of sampling, which are b iased

against rarer species.

Individuals within a popul ation tend to distribute themselves more or less evenly due to

intraspecies competition. Individuals or groups of vertebrates tend to restrict themselves

to an area known as the home range. If the home range is defended aggressively and does

not overlap with those of neighbors, it is called a territory. Species with complicated

reproductive behavior, such as nest building and extended care of young, are more likely

to be territorial. Birds form territories more often than other groups. Territoriality is a way

to avoid the need for aggressiveness, since once a territory is formed and marked, others

of the same species tend to respect it.

14.5 HUMANS IN THE BALANCE

As of the year 2005, the human population exceeds 6.4 billion people, versus 3.5 billion

in 1950 and 1.6 billion at the start of the twentieth century. The growth rate in 2005 is

74 million per year (almost 1.2% per year). However, growth in food production is not

keeping pace.

The amount of ﬁsh caught has increased 4.6 times from the 1950s to the late 1980s.

Over the same period, world grain produc tion has tripled. But since then, ﬁshing has

reached the sustainable-yield limit and has stopped increasing, and grain production

has slowed its increase to 0.5% per year, far less than world population growth. Thus,

the world’s annual per capita grain production has dropped almost 10% from its highest

value of 346 kg in 1984 to 313 kg in 1996.

Agricultural food production can be increased by either increasing the area

under cultivation or by increasing the yield. The oldest means of increasing yield is to

490 ECOLOGY: THE GLOBAL VIEW OF LIFE

initiate irrigation. In the twentieth century, fertilizer use provided the most dramatic

improvement. But fertilizer use is linked to water use. The more rain or irrigation that

is applied, the more fertilizer the crops can utilize. World fertilizer use has actually

dropped recently, as farmers have learned to apply it more precisely. A ﬁnal way to

improve yields was to develop better varieties of crops, but the high-yielding varieties

require higher fertilizer doses.

Some of these practices cannot be extended or even be sustained. The per capita area

under cultivation has been falling sinc e the 1950s, and the total area has been dropping

since 1981. Increasing the area now means bringing marginal lands under cultivation and/

or destroying sensitive ecosystems such as tropical rainforests or wetlands. In the former

Soviet Union the marginal land was subject to soil erosion. Over the 18 year s leading to

1995, 26% of the land used for grain production had to be abandoned for this reason.

Agricultural practices tend to increase erosion and do not contribute much organic matter

by which natural systems rejuvenate the soil. Since 1950 some 20% of the world’s topsoil

has been lost. Industrial and urban development is also taking much prime farmland out of

production, even in China.

The per capita amount of land under irrigation has been dropping since 1979. A

principal reason is the depletion of underground aquifers. Twen ty-one percent of U.S.

cropland under irrigation gets its water from aquifers that are being drawn down.

Many of the world’s great rivers are pumped dry before they reach the sea. In the United

States, these include the Colorado River and the Arkansas River. The river feeding

the Aral Sea in Uzbekistan has run dry, and the sea itself is disappearing. China’s

Yellow River ﬁrst ran dry in 1972, and does so now each year for longer and longer

periods.

These dismal statistics call for an evaluation of the carrying capacity of Earth. The total

world grain production is currently about 1.82 billion tons per year. This might be

increased to 2 billion tons. Am ericans use about 800 kg per person per year, much of

it indirectly in the form of meat and dairy products . Indians on average use only 200 kg

per person per year, in a diet consisting mostly of grain only. Some think that the health-

iest diet is what is known as the Mediterranean diet, which is lower in fat and protein than

the American diet. It requires about 400 kg per person per year. Thus, the world’s grain

supply could support 2.5 billion people on the American diet, 5 billion on the Mediterra-

nean diet, or 10 billion on the Indian diet.

The world population growth rate is 1.2% per year. The good news is that this is a

slowing, down from a peak of 2.5% per year in the 1960s. Fertility has fallen signiﬁcantly

in most of the large developing countries. Modern contraceptives are becoming more

accepted by these countries. For example, they are used by 66% of married women in

Brazil. Indonesia is a model for population control in developing countries because it

has linked it to health care and educa tion for women and children, reducing infant mor-

tality from 133 per 1000 births in the 1960s to 57 in the 1990s. Some think that feelings of

economic security are an even more potent factor in controlling population than birth con-

trol education.

However, we are already beyond the number that could be supported on the Mediter-

ranean diet. Even if population control is pursued aggressively by developing countries,

the population will continue to grow because of the high proportion that is yet to reach

childbearing age. Under this scenario the population is expected to level off at about 10 to

16 billion during the twenty-ﬁrst century.

HUMANS IN THE BALANCE 491

14.6 CONCLUSION

In the next chapter we study particular types of ecosystems. Based on what we have

learned from this chapter, we can formulate a few questions to think about when examin-

ing any ecosyste m:

 What is its primary productivity? Which organisms are responsible for most of its

productivity?

 How much energy is stored and transferred at each trophic level? How much

biomass?

 What do the biogeochemical cycles look like for this ecosystem? What are the

amounts of nutrient storage and the ﬂux rates between compartments?

 What are the important organisms and groups of organisms in this ecosystem? What

adaptations make them abundant in this ecosystem?

 What is the niche of each of these organisms (habitat; food source; is it food for

another organism; in what type s of speci es interactions does it participate)?

 What limits the population of each species?

 What stage of development is the ecosystem in?

 How do human activities affect all of these factors?

In the next several chapters we travel from the land to the sea, examining several types

of ecosystems with these questions in mind.

PROBLEMS

14.1. Decay of radi oactive elements deep within the Earth produces a ﬂux of heat

energy toward the surface in the form of geothermal energy. Visible results include

hot springs, geysers, and volcanoes. Humans can extract this energy to run

machines. Could plants or microorganisms use this energy directly for primary

productivity in nature? Explain.

14.2. The net primary productivity of a tall form of cord grass (Spartina alterniﬂora) has

been measured to be 5800 g/m

yr in a Georgia salt marsh. What is the efﬁciency

of energy capture assuming an average solar energy input for the southeastern

United States of 3888 kcal/m

 day? Comment on the result.

14.3. The United States as a whole receives an energy subsidy in the form of fuels in the

amount of about 1:8  10

kcal/m

yr. The world receives about 100 kcal/m

yr.

Compare these ﬁgures to the gross and net primary productivity ﬁgures given in

the text for the United States and for particular ecosystems.

14.4. Tommy’s Pond av erages 2.0 f deep and has an area of 0.40 acres (1 acre ¼43,560 ft

The stormwater entering the pond was measured to have an average phosphorus

concentration of 0.300 mg/L. The average ﬂow rate of the stormwater may be

assumed to be 5000 ft

/day (1 ft

¼28.3 L). Thirty ducks live on the pond. Each

duck is estimated to contribute phosphorus at the rate of 0.50 kg/y.

492 ECOLOGY: THE GLOBAL VIEW OF LIFE

(a) What will be the steady state phosphorous concentration in the pond?

(b) What is the turnover rate for phosphorus in Tommy’s Pond at steady state?

14.5. It can be difﬁcult to visualize the huge quantities in global biogeochemical cycles.

Earth has a land area of 148,429,000 km

and a water area 361,637,000 km

Normalize the ﬂuxes and reservoirs in the carbon cycle as shown in Figure 14.5 on

a per square meter basis. Separate the atmosphere into the portions over land

(29.1%) and over water (70.9%). How does productivity on land compare to that

on water?

14.6. Look at the magnitude of abiotic ﬁxation of nitrogen. How long would it take to

convert all the atmospheric nitrogen to nitrate if life suddenly disappeared from

Earth? Use information from Figures 14.7 and 14.8, plus Earth’s area from

Problem 14.4.

14.7. Write a the balanced chemical equation for the reaction of water, oxygen, and

nitrogen to nitric acid. How many moles of oxygen are needed for each mole of

nitrogen? Would there be an excess left over of either, based on the current

atmosphere of 79 mol% N

and 21 mol% O

? Are there any other sources of

oxygen?

14.8. Write a spreadsheet program to solve matrix (14.8). To do this in Microsoft Excel,

ﬁrst place the coefﬁcients in a 4  4 array of cells, then the right-hand-side (RHS)

vector in an adjacent column of cells. Say that the matrix is in cells A1 to D4 and

the RHS vector is in cells E1 to E4. Use the cursor to highlight another empty

4  4 array: say, cells A6 to D9. Then type the following: ‘‘¼minverse(A1:D4).’’

Now, hit ‘‘Cntrl-Alt-Enter.’’ Cells E1 to E4 will display the inverse of the matrix.

To get the solution, we must multiply the inverse by the RHS vector. To do this,

highlight four empty cells in a vertical column. Then type

‘‘¼mmult(A6:D9, E1:E4)’’ and hit ‘‘C ntrl-Alt-Enter.’’ The cells will now hold

the solution of the matrix system in order.

Notice that you can now change entries in the original matrix without reentering

the functions. The solution changes automatically. The coefﬁcients in Table 14.4

represent summer conditions. In winter the situation can be represented by the

following values: k

¼ 1:0; k

¼ 8:0; k

¼ 4:4 ; k

¼ 2:5; k

¼ 1:7. Use your pro-

gram to compute the resulting distribution of nitrogen forms in this ecosystem.

Compare and explain the differences between summer and winter.

14.9. What type of species interaction describes the human–mosquito pair?

14.10. To see how doubling can rapidly cause populations to outstrip their resources, try

the following math experiment. Suppose that you were to take a piece of paper

0.1 mm thick and fold it in half twice. This would quadruple the thickness to

0.4 mm. Suppose, instead, that you had folded it 50 times, if you could. Before

computing the thickness, take a guess (a big guess). Then do the calculation. To

what physical distance can you compare this?

14.11. A population of insects lives for two years. The ﬁrst year they are a juvenile form

that does not reproduce; the second year is an adult form that reproduces and then

dies. Sampling ﬁnds 1000 juveniles and 10 adults per square meter. Create a life

table for this insect. Compute the natality and mortality for each age group, and

PROBLEMS 493

use the results to compute the net reproductive rate, the mean length of a

generation, and the growth rate. Is the growth rate what you would expect?

14.12. For a carrying capacity K, what population, N, would result in a speciﬁc growth

rate that is one-half of the max imum?

14.13. Both the logistic equation and the Monod model produce sigmoidal growth curves.

(a) Why do we not use the logistic model for microorganism growth in wastewater

treatment plants? (b) Conversely, why not use the Monod model for ecological

population modeling?

14.14. (a) How many possible pairwise interactions are there among nine species? (b)If

each species has, on average, only two interactions, how many interactions will

there be in this case?

14.15. As of the year 2005, the human population exceeds 6 billion people vs. 3.5 billion

in 1950 and 1.6 billion at the start of the twentieth century. Find the values of r and

K that make the logistic equation ﬁt these data. According to this, what is the

carrying capacity of Earth? How does this compare with the estimates given in the

text? What are possible explanations for the discrepancy?

14.16. In the Lo tka–Volterra predator–prey model, the period of the oscillations depends

on the initial conditions. But in the vicinity of the steady-state solution given in

Section 14.4.1, it approaches 2pðadÞ

1=2

. Thus, a decrease in the host growth rate

or the predator death rate would increase the period. Can you explain in words

why this is so?

14.17. Suppose that someone were to enter a large temperate-zone hardwood forest and

harvest all of the trees in a 100-ft circle. Describe the changes that might occur at

the site if it were left undisturbed for the next 100 years.

14.18. Compute the Simpson and Shannon–Weaver diversity indices and Pielou’s

evenness index for a community consisting of four species that each comprises

one-eighth of the organisms and a ﬁfth species that comprises the other 50%.

Which species distribution in Table 14.9 does this community mos t resemble?

REFERENCES

Berner, E. K., and R. A. Berner, 1987. The Global Water Cycle: Geochemistry and Environment,

Prentice Hall, Englewood Cliffs, NJ.

Brown, L., C. Flavin, H. French, et al,, 1997. State of the World 1997, W.W. Norton, New York.

Krebs, C. J., 1994. Ecology, HarperCollins, New York.

Mattson, C. P., and N. C. Vallario, 1975. Hackensack Meadowlands Development Commission report.

Murray, J. D., 1993, Mathematical Biology, Springer-Verlag, New York.

Odum, E. P., 1987. Basic Ecology, Saunders College Publishing.

Odum, E. P., 1993. Ecology and Our Endangered Life-Support Systems, Sinauer Associates,

Sunderland, MA.

Prigogine, Ilya, 1972. Introduction to Nonequilibrium Thermodynamics Wiley, New York.

Prigogine, Ilya, 1978. Time structure and ﬂuctuations, Science, Vol. 201, pp. 777–785.

494

ECOLOGY: THE GLOBAL VIEW OF LIFE

Prigogine, Ilya, G. Nicolis, and A. Babloyantz, 1972. Thermodynamics and evolution, Physics Today,

Vol. 25, No. 11, pp. 23–38 and Vol. 12, pp. 138–144.

Raven, P. H., R. F. Evert, and S. E. Eichhorn, 1992. Biology of Plants, Worth Publishers, New York.

Ricklefs, R. E., 1993. The Economy of Nature: A Textbook in Basic Ecology, 3rd ed., W. H. Freeman,

New York.

Smil, Vaclav, 1997. Global population and the nitrogen cycle, Scientiﬁc American, Vol. 277, No. 1,

pp. 76–81.

U.S. Census Bureau, 2004. International data base, September, http://www.census.gov/ipc/www/

idbsum.html.

Utida, Syunro, 1957. Population ﬂuctuation, and experimental and theoretical approach, Cold Spring

Harbor Symposia on Quantitative Biology, Vol. 22.

Whittaker, R. H., 1975. Communities and Ecosystems, Macmillan, New York.

REFERENCES 495

ECOSYSTEMS AND APPLICATIONS

The major ecosystem types, or biomes, were listed in Table 14 .1. Some of these are

described in more detail here in a sequence from terrestrial to marine ecosystems, with

stops along the way for aquatic and wetland systems. The gross primary productivity of

the major ecosystems is listed in Table 15.1. The most productive ecosystems are estu-

aries, reefs, and tropical forests. Some of Earth’s environments are practically sterile,

such as the ice sheets of the Arctic and Antarctica and new land formed from volcanic

ash or lava ﬂows. The volcanic landscapes, will, however, soon be colonized if environ-

mental conditions are favorable.

15.1 TERRESTR IAL ECOSYSTEMS

Terrestrial biomes can be distinguished on the basis of the dominant climax vegetation.

The basic factors determining terrestrial ecosystems are temperature and moisture. Some

of these determinations can be made based on annual temperature and precipitation (see

Figure 15.1). Some biomes do not ﬁt into this scheme, such as the chaparral, which

depends on wet and dry seasons. Also note the overlap in regimes, since other factors

may affect the ecosystem type that results. Fire affects whether a region will have forest

or grassland, and the north slope of a mountain may have a different biome than the south-

facing side.

The tundra is essentially a wet grassland that exists in cold climates at high latitudes

(arctic tundra) or high altitudes (alpine tundra). The harsh climate excludes trees but

supports shrubs and lichens. Altho ugh the growing season is short, arctic tundra has long

photoperiods in the summer. This can result in primary productivity as high as 5 g of

biomass per square meter per day. This can suppor t a substantial animal population,

the larger of which tend to be migratory. Only the upper few inches thaws during the

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

496

summer. Below this is the deep, permanently frozen soil called permafrost. Tundra is

notoriously sensitive to disturbance and slow to recover. A footprint is said to remain

visible for years.

The taiga, also called boreal (northern) forest, occupies vast area s of Canada and

Russia and other areas of similar latitude. The dominant plants are conifers, including

TABLE 15.1 Gross Annual Primary Productivity of Some

Major Ecosystems

Ecosystem GPP (kcal/m

)

Marine

Open ocean 1,000

Coastal ocean 2,000

Upwelling zones 6,000

Estuaries and reefs 20,000

Salt marsh 36,000

Terrestrial

Deserts and tundras 200

Grasslands and pastures 2,500

Dry forests 2,500

Boreal coniferous forests 3,000

Cultivated lands without energy subsidy 3,000

Moist temperate forests 8,000

Cultivated lands with energy subsidy

(mechanized agriculture) 12,000

Wet tropical and subtropical forests 20,000

Average for the biosphere 2,000

Recall that 1000 g corresponds to about 4100 kcal.

Source: Odum (1987).

Arctic-Alpine

Cold temperate

Warm temperate

Tropical

Rainforest

Tropical

seasonal

forest

Temperate

forest

Temperate

rainforest

Woodland

Savanna

Thorn scrub

Woodland

Grassland

Shrubland

Desert

Taiga

Tundra etc.

Precipitation

Temperature

Figure 15.1 Temperature–moisture regimes of terrestrial biomes.

TERRESTRIAL ECOSYSTEMS 497