Townsend C.R., Begon M., Harper J.L. Essentials of Ecology

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acceptable standard of living. The higher estimates come closer to the concept

of a carrying capacity we normally apply to other organisms (see Chapter 5) – a

number ‘imposed’ by the limiting resources of the environment. But it is unlikely

that many of us would choose to live crushed up against an environmental ceiling

or wish it on our descendants.

In any case, it is a big step to assume that the human population is limited

‘from below’ by its resources rather than ‘from above’ by its natural enemies.

Infectious disease in particular, which not long ago was considered to be an

enemy largely vanquished, is now once again perceived, for example by the

World Health Organization, as a major threat to human welfare. Just consider

the growing epidemics in tuberculosis and HIV and AIDS and the deaths caused

by malaria. We saw in Chapter 7 that many infectious diseases thrive best in the

densest populations.

Any suggestions we make about a global carrying capacity clearly depend on

choices we make both for ourselves and for others. Most of us would choose to

live at least as well as we do at present, but can the global population afford to

choose for the whole world to live at least as well as those in developed countries

do now? The answer to any question depends on what is meant by the question –

deﬁning what we mean by ‘the global carrying capacity’ is far from straightforward.

12.3 Harvesting living resources from

the wild

A major limit to the number of people the Earth can support is the food that

can be obtained. Populations of many species living freely in the wild are exploited

for food by humans, who ‘cull’ or ‘harvest’ a proportion of the population,

leaving some individuals behind to grow and reproduce for future harvests.

Primitive human societies obtained all their resources like this, by hunting and

gathering from nature, and humans continue to garner some resources in this

way. The resources may be ﬁsh from the sea, deer from a moorland or timber from

a forest. There is an important difference between resources obtained in this

way and those that are farmed (Sections 12.4 and 12.6). Farmed resources are

obtained by taking chosen species of plant or animal, domesticating them (often

changing them genetically) and growing or rearing them in more or less con-

trolled monocultures. These resources tend to be owned and managed by a farmer

or organization. In contrast, most of the oceans and forests that are ﬁshed and

hunted have at one time been common property, open to free-for-all unsustain-

able looting by all-comers. Recently, though, ﬁshing and hunting have also come

under increasing national and international regulation and national claims to

‘ownership’. Many of our examples in this section are of ﬁsh or ﬁsheries, but the

principles apply to the harvesting of any natural resource.

12.3.1 Fisheries: maximum sustainable yields

Whenever a natural population is exploited there is a risk of overexploitation:

too many individuals are removed and the population is driven into biological

jeopardy or economic insigniﬁcance – perhaps even to extinction. Global catches

of marine ﬁsh rose ﬁve-fold between 1950 and 1989 and many of the world’s ﬁsh

Chapter 12 Sustainability

399

aiming for the narrow path

between over- and

underexploitation

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Part IV Applied Issues in Ecology

400

Underdeveloped

Developing

Fully exploited

Overexploited

Crashed

1950 1960 1970 1980 1990 2000

100

Stock (%)

Year

Figure 12.5

Changes in the contribution to global marine ﬁsh production made by

ﬁsheries in different phases of their exploitation. In the 1950s most

of the catches were from undeveloped ﬁsheries, but by 2000 most

ﬁsheries were fully exploited (near their maximum sustainable yield),

or overexploited or had already collapsed.

AFTER KHAN ET AL., 2006

Net recruitment

Population size

population dynamics in the

absence of exploitation –

humped net recruitment curves

stocks are now beyond the point of overexploitation (Figure 12.5). But harvesters

also want to avoid underexploitation: if fewer individuals are removed than the

population can bear, the harvested crop is smaller than necessary, potential con-

sumers are deprived and those who do the harvesting are underemployed. It is not

easy to tread the narrow path between under- and overexploitation. It is asking

a great deal of a management policy to combine the well-being of the exploited

species, the proﬁtability of the harvesting enterprise, continuing employment for

the workforce and the maintenance of traditional lifestyles, social customs and

natural biodiversity.

The most fundamental aspects of ecology that we need to understand here

were introduced in Chapter 5 when the effects of intraspeciﬁc competition on

populations were discussed. To determine the best way to exploit a population,

it is necessary to know what the consequences will be of different exploitation

‘strategies’. But in order to know these consequences, we ﬁrst need some under-

standing of the dynamics of the population in the absence of, or prior to, exploita-

tion. It is usual to assume that, before it is exploited, a harvestable population is

crowded and intraspeciﬁc competition is intense. Summarizing from Chapter 5,

and remembering that these are broad generalizations:

populations in the absence of exploitation can be expected to settle around

their carrying capacity, but exploitation will reduce numbers to less than this;

exploitation, by reducing the intensity of competition, moves the population

‘leftwards’ along the humped net recruitment curve, increasing the net

number of recruits to the population per unit time (Figure 12.6).

Figure 12.6

The humped relationship between the net

recruitment into a population (births minus

deaths) and the size of that population, resulting

from the effects of intraspeciﬁc competition

(see Chapter 5). Population size increases from

left to right, but increasing rates of exploitation

take the population from right to left.

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In fact, we can go further with Figure 12.6, since it is clear from the shape of

the curve that there must be an ‘intermediate’ population size at which the rate of

net recruitment is highest. Consider a time scale of years. The peak of the curve

might be ‘10 million new ﬁsh recruited each year’. This is then also the highest

number of new ﬁsh that could be removed from the population each year that the

population itself could replenish. It is known as the maximum sustainable yield

(MSY): the largest harvest that can be removed from the population regularly

and indeﬁnitely. It looks as though a ﬁshery could tread the narrow path between

under- and overexploitation if the ﬁshers could ﬁnd a way to achieve this MSY.

The MSY concept has been the guiding principle in resource management for

many years in ﬁsheries, forestry and wildlife exploitation, but it is very far from

being the perfect answer for a variety of reasons.

1 By treating the population as a number of similar individuals, it ignores

all aspects of population structure such as size or age classes and their

differential rates of growth, survival and reproduction.

2 By being based on a single recruitment curve, it treats the environment

as unvarying.

3 In practice, it may be impossible to obtain a reliable estimate of the MSY.

4 Achieving an MSY is by no means the only, nor necessarily the best,

criterion by which success in the management of a harvesting operation

should be judged. (It may, for example, be more important to provide

stable, long-term employment for the workforce.)

12.3.2 Obtaining MSYs through ﬁxed quotas

There are two simple ways of obtaining an MSY on a regular basis: through

a ‘ﬁxed quota’ and through a ‘ﬁxed effort’. With ﬁxed quota MSY harvesting

(Figure 12.7), the same amount, the MSY, is removed from the population every

year. If (and it is a big if) the population stayed exactly at the peak of its net

recruitment curve, then this could work: each year the members of the popula-

tion, through their own growth and reproduction, would add exactly what the

harvesting removed. But if by chance numbers fell even slightly below those

at which the curve peaked, then the numbers harvested would exceed those

recruited. Population size would then decline to below the peak of the curve,

Chapter 12 Sustainability

401

MSY – the narrow path?

the MSY concept has

shortcomings

Population density

Recruitment or

harvesting rate

Recruitment rate

Harvesting rate

the fragility of ﬁxed quota

harvesting...

Figure 12.7

Fixed quota harvesting. The ﬁgure shows a single recruitment curve

(solid line; recruitment in relation to density, N) and two ﬁxed quota

harvesting curves (dashed lines): high quota (h

), and MSY quota

). The arrows in the ﬁgure refer to changes to be expected in

abundance under the inﬂuence of the harvesting rate to which the

arrows are closest. The black dots indicate equilibria. At h

the only

‘equilibrium’ is when the population is driven to extinction. The MSY

is obtained at h

because it just touches the peak of the recruitment

curve (at a density N

): populations greater than N

are reduced to

, but populations smaller than N

are driven to extinction. K is the

carrying capacity, the density where the population is expected to

settle in the absence of exploitation.

9781405156585_4_012.qxd 11/5/07 15:01 Page 401

and if a ﬁxed quota at the MSY level were maintained the population would

carry on declining until it was extinct (Figure 12.7). Furthermore, if the MSY

was even slightly overestimated (and reliable estimates are hard to come by) then

harvesting rate would always exceed the recruitment rate and extinction would

again follow. In short, a ﬁxed quota at the MSY level might be desirable and

reasonable in a wholly predictable world about which we had perfect knowledge.

But in the real world of ﬂuctuating environments and imperfect data sets, these

ﬁxed quotas are open invitations to disaster.

Nevertheless, a ﬁxed quota strategy has frequently been used – a management

agency formulates an estimate of the MSY, which is then adopted as the annual

quota. On a speciﬁed day in the year, the ﬁshery is opened and the accumulated

catch is logged. A fairly typical example is provided by the Peruvian anchovy

(Engraulis ringens) ﬁshery (Figure 12.8). From 1960 to 1972 this was the world’s

largest single ﬁshery, and it constituted a major sector of the Peruvian economy.

Fisheries experts advised that the MSY was around 10 million tonnes annually,

and catches were limited accordingly. But the ﬁshing capacity of the ﬂeet expanded,

and in 1972 the catch crashed. Overﬁshing seems, at the least, to have been a major

cause of the collapse, although its effects were compounded with the inﬂuences

of profound environmental ﬂuctuations, discussed below. A moratorium on

ﬁshing would have been an ecologically sensible step, but this was not politically

feasible: 20,000 people were dependent on the anchovy industry for employment.

The Peruvian government therefore allowed ﬁshing to continue. The stock took

more than 20 years to recover.

12.3.3 Obtaining MSYs through ﬁxed effort

An alternative to trying to maintain a constant harvest is to maintain a constant

‘harvesting effort’ (e.g. the number of ‘trawler-days’ in a ﬁshery or the number of

‘gun-days’ with a hunted population). With such a regime the amount harvested

should increase with the size of the population being harvested (Figure 12.9).

Now, in contrast to Figure 12.7 if density drops below the peak, new recruit-

ment exceeds the amount harvested and the population recovers. The risk of

extinction is much reduced. The disadvantages, however, are ﬁrst, because there

is a ﬁxed effort, the yield varies with population size (there are good, but, more

to the point, bad years), and second, steps need to be taken to ensure that nobody

makes a greater effort than they are supposed to. Nonetheless, there are many

Part IV Applied Issues in Ecology

402

Landings (million tonnes)

Year

20001995199019851980197519701965196019551950

Figure 12.8

Landings of the Peruvian anchovy since 1950. Note the dramatic

crash that resulted mainly as a result of overﬁshing. The stock has

taken 20 years to rebuild.

AFTER JENNINGS ET AL., 2001

. . . borne out in practice

relative robustness of ﬁxed

effort harvesting

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examples of harvests being managed by legislative regulation of effort. Harvesting

of the important Paciﬁc halibut (Hippoglossus stenolepis), for example, is limited

by seasonal closures and sanctuary zones, though heavy investment in ﬁsheries

protection vessels is needed to control law breakers.

12.3.4 Beyond MSYs

There is no doubt that ﬁshing pressure often exerts a great strain on populations.

But the collapse of ﬁsh stocks in one year rather than any other is often the result

of an occurrence of unusually unfavorable environmental conditions, rather than

simply overﬁshing.

Harvests of the Peruvian anchovy (see Figure 12.8) collapsed from 1972 to

1973, but a previous steady rise in catches had already dipped in the mid-1960s

as a result of an El Niño event: this happens when warm tropical water from the

north reduces the upwelling, and hence the productivity, of the nutrient-rich cold

Peruvian current coming from the south. By 1973, however, commercial ﬁshing

had so greatly increased that the subsequent El Niño event had even more severe

consequences. There were some signs of recovery from 1973 to 1982, but a

further collapse occurred in 1983 associated with yet another El Niño event. It is

unlikely that the El Niño events would have had such severe effects if the anchovy

had only been lightly ﬁshed. It is equally clear, though, that the history of the

Peruvian anchovy ﬁshery cannot be explained simply in terms of overﬁshing.

So far, this account has ignored population structure of the exploited species.

This is a bad fault for two reasons. First, most harvesting practices are primarily

interested in only a portion of the harvested population (mature trees, ﬁsh that

are large enough to be saleable, etc.). Second, ‘recruitment’ is, in practice, a

complex process incorporating adult survival, adult fecundity, juvenile survival,

juvenile growth and so on, each of which may respond in its own way to changes

in density and harvesting strategy. An example of a model that takes some of

these variables into account was that developed for the Arcto-Norwegian cod

ﬁshery, the most northerly ﬁsh stock in the Atlantic Ocean. The numbers of ﬁsh

Chapter 12 Sustainability

403

Population density

Recruitment rate or harvesting rate

Recruitment rate

Harvesting rate

Figure 12.9

Fixed effort harvesting. Curves, arrows and dots as in Figure 12.7.

The MSY is obtained with an effort of E

, leading to a stable

equilibrium at a density of N

with a yield of h

. At a somewhat

higher effort (E

), the equilibrium density and the yield are both

lower than with E

, but the equilibrium is still stable. Only at a

much higher effort (E

) is the population driven to extinction.

environmental ﬂuctuations – the

anchovy and El Niño

population structure and the

Arctic cod (Gadus morhua)

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in different age classes were known for the late 1960s and this information was

used to predict the tonnage of ﬁsh likely to be caught with different intensities of

harvesting and with different net mesh sizes. The model predicted that the long-

term prospects for the ﬁshery were best ensured with a low intensity of ﬁshing (less

than 30%) and a large mesh size. These gave the ﬁsh more opportunity to grow

and reproduce before they were caught (Figure 12.10). The recommendations from

the model were ignored and, as predicted, the stocks of cod fell disastrously.

Indigenous harvesters have long had their own ‘regulations’ to reduce the chance

of overexploitation. In their harvesting of moi (Polydactylus sexﬁlis), Hawaiian

ﬁshermen, using traditional methods along the shore, take only intermediate-sized

ﬁshes, leaving both juveniles and large females. Thus they go a stage further than

simply increasing net mesh size, which, while reducing the numbers of smaller

individuals taken, nevertheless captures the largest individuals in the population.

The good sense of the Hawaiian strategy has been reinforced by the discovery

that large females of some ﬁsh not only produce exponentially more offspring

but also that each of their offspring grows faster (Figure 12.11) and is more likely

to reach adulthood. Protecting the largest individuals may give a great boost to

sustainability.

Managing most marine ﬁsheries to achieve perfect, optimum yields is an unattain-

able dream. There are generally too few researchers to do the work and, in many

parts of the world, no researchers at all. In these situations, a precautionary

approach to ﬁsheries management might involve locking away a proportion of a

coastal or coral community in marine-protected areas (Hall, 1998). The term

Part IV Applied Issues in Ecology

404

800

600

400

200

Catch (thousand tons)

600

400

200

600

400

200

5010152025

Mesh sizes

Years of this regime

160 mm

145 mm

130 mm

160 mm

145 mm

130 mm

160 mm

145 mm

130 mm

Fishing intensity

26%

Fishing intensity

33%

Fishing intensity

45%

Figure 12.10

Predictions for the stock of Arctic cod under three intensities of ﬁshing

and three different sizes of mesh in the nets. Larger meshes allow more

and larger ﬁsh to escape capture. The largest effort (45%, bottom panel)

is clearly unsustainable, regardless of the mesh size used. The largest

sustainable catches are achieved with a low ﬁshing effort (26%, upper

panel) and a large mesh size.

AFTER PITCHER & HART, 1982

a strategy of taking only

intermediate-sized ﬁshes

precautionary management,

closed areas and ‘data-less’

management

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data-less management has been applied to situations where local villagers follow

simple prescriptions to make sustainability more likely – for example locals on the

Paciﬁc island of Vanuatu were provided with some simple principles of manage-

ment for their trochus (Tectus niloticus) shellﬁshery: stocks should be harvested

every 3 years and left unﬁshed in between. The outcome has apparently been

successful: continued economic viability (Johannes, 1998).

12.4 The farming of monocultures

Globally there is abundant food. Between 1961 and 1994 the per capita food

supply in developing countries increased by 32% and the proportion of the

world’s population that was undernourished fell from 35% to 21%, though this

is very unevenly distributed. Yet 800 million people remain hungry worldwide,

and the rate of increase in per capita food production is falling.

Fishing and hunting (Section 12.3) have been human activities since our early

history as hunter-gatherers. But the harvest that can be taken from nature was

totally inadequate to support the main phases of growth of human populations.

Increasingly, both animals and plants were domesticated and managed in ways

that allowed much greater rates of production. The great bulk of the human food

resource is now farmed – usually produced as dense populations of single species

(monocultures). This allows them to be managed in specialized ways that can

maximize their productivity, whether as immense monocultures of rice, corn or

wheat (Figure 12.12), or as livestock factories producing beef, pork or poultry.

Fish, indeed, are increasingly managed in the same way (aquaculture) – reared in

enclosures, fed with controlled diets and harvested in mass production. Nearly a

quarter of the ﬁsh supply in Asia is already produced in this way.

Only monoculture can maximize the rate of food production. This is because

it allows the farmer to control and optimize with high precision the density of the

populations (livestock or crop plants), the quantity and quality of their resources

(food supplied to livestock; fertilizer and water to the crops) and often even the

physical conditions of temperature and humidity. With many animals, the mono-

cultures extend to segregating livestock or poultry into narrow age bands or age

classes. If the only important criteria are economic ones, then there need be

none of the uneconomic mixing of calves with cows or chickens with hens; ﬁsh

eggs and fry can be segregated from potentially cannibalistic adults; the grossly

Chapter 12 Sustainability

405

AFTER BOBKO & BERKELEY, 2004

0.08

0.06

0.04

0.02

4 6 8 10 12 14 16 18

Maternal a

e (years)

Growth in length (mm day

–1

)

Figure 12.11

The black rockﬁsh (Sebastes melanops), off the coast of Oregon,

USA, is a long-lived ﬁsh that produces live young. Not only do

bigger ﬁsh produce more eggs to be fertilized, but the proportion

of these that are in fact fertilized is itself greater in larger females.

Futhermore, as shown in the graph, larvae produced by older

(larger) females grow more than three times as fast as do larvae

produced by their younger (smaller) counterparts.

monoculture – and beyond

9781405156585_4_012.qxd 11/5/07 15:01 Page 405

uneconomic equality of the sex ratio that is common in nature can be distorted

by culling to give efﬁcient all-female dairy herds of cattle, or all-hen populations

in batteries for egg production. This is a far cry from the ecology of the primitive

human hunter-gatherers, who subsisted on their gleanings from the tangled web

of wild nature!

To what extent, though, are modern farming methods sustainable? There

is abundant evidence that a high price has to be paid to sustain the high rates of

food production achieved by farmed monocultures. For example, they offer ideal

conditions for the epidemic spread of diseases such as mastitis, brucellosis and

swine fever among livestock and coccidiosis among poultry. Farmed animals

are normally kept at densities far higher than their species would meet in nature

with the result that disease transmission rates are magniﬁed (see Chapter 7). In

addition, high rates of transmission between herds occur as animals are sold

from one farming enterprise to another, and it is easy for the farmers themselves,

with mud on their boots and their vehicles, to act as vectors of pests and disease.

The dramatic spread of foot and mouth disease in 2001 among British livestock

provides a graphic example.

Crop plants, too, provide illustrations of the fragility of human dependence on

monocultures. The potato, for example, was not introduced across the Atlantic to

Europe until the second half of the 16th century, but three centuries later other

foods had given way to it, and it had become the almost exclusive food crop of the

poorer half of the population of Ireland. Dense monoculture, though, provided

ideal conditions for the devastating spread of late blight (the fungal pathogen

Phytophthora infestans) when it also crossed the Atlantic in the 1840s. The disease

spread rapidly, dramatically reducing potato yields and also decomposing the tubers

in storage. Out of the Irish population of about 8 million, 1.1 million died in the

resulting famine and another 1.5 million emigrated to the UK and the USA.

In more modern history, an outbreak of southern corn leaf blight (caused

again by a fungus, Helminthosporium maydis) developed in southeastern USA

in the late 1960s and spread rapidly after 1970. Most of the corn grown in the

area had been derived from the same stock and was genetically almost uniform.

This extreme monoculture allowed one specialized race of the pathogen to have

devastating consequences. The damage was estimated as at least $1 billion in the

USA and had repercussions on grain prices worldwide. One of our favorite fruits

is also at great risk of economic disaster (Box 12.3).

Part IV Applied Issues in Ecology

406

Figure 12.12

Agricultural monoculture:

wheat as far as the eye

can see.

but disease spreads in

monocultures

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12.4.1 Degradation and erosion of soil

A United Nations report (1998) stated:

Agricultural intensiﬁcation in recent decades has taken a heavy toll on the

environment. Poor cultivation and irrigation techniques and excessive use of

pesticides and herbicides have led to widespread soil degradation and water

contamination.

Around 300 million hectares are now severely degraded around the world and

a further 1.2 billion hectares – 10% of the Earth’s vegetated surface – can be

described as moderately degraded. Clearly much of agricultural practice has not

been sustainable.

Land without soil can support only very small primitive plants such as lichens

and mosses that can cling onto a rock surface. The rest of the world’s terrestrial

Chapter 12 Sustainability

407

12.3 TOPICAL ECONCERNS

12.3 Topical ECOncerns

In June 2005, Dan Koeppel ﬁled the story below.

For nearly everyone in the US, Canada and

Europe, a banana is a banana: yellow and sweet,

uniformly sized, ﬁrmly textured, always seedless.

The Cavendish banana – as the slogan of

Chiquita, the globe’s largest banana producer,

declares – is ‘quite possibly the world’s perfect

food’. . . . It also turns out that the 100 billion

Cavendish bananas consumed annually worldwide

are perfect from a genetic standpoint, every

single one a duplicate of every other. It doesn’t

matter if it comes from Honduras or Thailand,

Jamaica or the Canary Islands – each Cavendish

is an identical twin to one ﬁrst found in Southeast

Asia, brought to a Caribbean botanic garden in

the early part of the 20th century, and put into

commercial production about 50 years ago.

That sameness is the banana’s paradox. After

15,000 years of human cultivation, the banana is

too perfect, lacking the genetic diversity that is

key to species health. What can ail one banana

can ail all. A fungus or bacterial disease that

infects one plantation could march around the

globe and destroy millions of bunches, leaving

supermarket shelves empty.

A wild scenario? Not when you consider that

there’s already been one banana apocalypse.

Until the early 1960s, American cereal bowls

and ice cream dishes were ﬁlled with the Gros

Michel, a banana that was larger and, by all

accounts, tastier than the fruit we now eat. Like

the Cavendish, the Gros Michel, or ‘Big Mike’,

accounted for nearly all the sales of sweet

bananas in the Americas and Europe. But

starting in the early part of the last century, a

fungus called Panama disease began infecting

the Big Mike harvest.

or in part without permission is prohibited.)

1 Use a web search to discover the options that

might be used to safeguard the banana industry.

2 How far fetched do you consider the risk of global

economic terrorism by deliberate spread of a

banana disease?

Can this fruit be saved? The banana as we know it is on a crash

course toward extinction

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vegetation has to be rooted in soil, which gives it physical support. The soil also serves

as a store of essential mineral nutrients and water that are extracted by the roots

during plant growth. Soil develops by the accumulation of ﬁnely divided mineral

products of rock weathering and decomposing organic residues from previous

vegetation. The characteristics of the soil under natural vegetation in any particular

climatic region and on any particular rock type depends on the balance between

these processes of accumulation and forces that degrade and remove the soil.

The formation and persistence of soil in a region depend on local natural

checks and balances. Soil may be lost by being washed or blown away, perhaps

to be redeposited as an accumulation of ﬁne-textured ‘loess’ somewhere else. Soil

is best protected when it contains organic matter, is always wholly covered with

vegetation, is ﬁnely interwoven with roots and rootlets and is on horizontal

ground. Natural soil systems are probably always too fragile to be fully sustained

when land is brought into cultivation. Dramatic evidence of unsustainable land

use came from the ‘dust bowl’ disaster in the Great Plains of the United States and

a similar disaster happening currently in China (Box 12.4).

Part IV Applied Issues in Ecology

408

soil forms...and is lost

12.4 HISTORICAL LANDMARKS

12.4 Historical landmarks

Large areas of southeastern Colorado, southwestern

Kansas and parts of Texas, Oklahoma and north-

eastern New Mexico were once used to support

rangeland management of livestock. The vegetation

consisted largely of native perennial grasses and had

been neither ploughed nor sown with seed.

At the time of the First World War, much of the land

was ploughed and annual crops of wheat were grown.

There were poor crops in the early 1930s due to severe

drought and the topsoil was exposed and carried

away by the wind. Black blizzards of windblown soil

blocked out the sun and piled the dirt in drifts. Occa-

sionally the dust storms swept completely across the

country to the East Coast. Thousands of families were

forced to leave the region at the height of the Great

Depression in the early and mid-1930s. The wind ero-

sion was gradually halted with federal aid: windbreaks

were planted and much of the grassland was restored.

By the early 1940s the area had largely recovered.

The story is being repeated today in northwest

China, where the need to feed 1.3 billion people has

led to the raising of too many cattle and sheep, and

the use of too many plows. This is more than the

land can stand and 2300 km

are turning to desert

each year. A huge dust storm blanketed areas from

Canada to Arizona in April 2001 – the dust originated

in China.

Soil erosion, America’s historical ‘dust bowl’ and

China’s current problem

Dust bowl ﬁeld and abandoned farm.

agriculture and forestry

requires soil

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