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These approaches have advantages in terms of reduced environmental hazard.

Even so, it is unreasonable to suppose that they will be adopted widely unless

they are also sound in economic terms. As we have already noted, in an industry

such as agriculture, practices that are economically unsustainable are, ultimately,

unsustainable overall. In this context, Figure 12.19 shows the yields of apples

from organic, conventional and integrated production systems in Washington

State from 1994 to 1999 (Reganold et al., 2001). Organic management excludes

such conventional inputs as synthetic pesticides and fertilizers whilst integrated

farming uses reduced amounts of chemicals by integrating organic and conven-

tional approaches. All three systems gave similar apple yields but the organic

and integrated systems had higher soil quality and potentially lower environ-

mental impacts. When compared with conventional and integrated systems, the

organic system produced sweeter apples, higher proﬁtability and greater energy

efﬁciency.

12.7 Forecasting agriculturally driven

global environmental change

Much attention has been focused on the predicted far-reaching consequences

of global climate change caused by human activities such as the burning of fossil

fuels. We deal with this in Chapter 13. However, very signiﬁcant threats are also

posed to ecosystems around the world by increasing agricultural development.

In this chapter we have considered the problems of the more-than-exponential

increase in the human population, and of the associated impacts of increased

erosion, unsustainability of water supply, salinization and desertiﬁcation, excess

plant nutrients ﬁnding their way into waterways, and unwanted consequences of

chemical pesticides. Model projections suggest that all these will increase over

the next 50 years as more land is converted to grow crops and pasture (Tilman

et al., 2001) (Figure 12.20). This can be expected to place biodiversity at high

risk, particularly because population increases are predicted to be greatest in

species-rich tropical areas. To control the environmental impacts of agricultural

expansion, we will need scientiﬁc and technological advances as well as the

implementation of effective government policies.

Chapter 12 Sustainability

419

environmental and economic

sustainability

Year

Cumulative

(1995–9)

300

250

200

150

100

Yield (tons ha

–1

)

1995 1996 1997 1998 1999

Organic

Conventional

Integrated

Figure 12.19

Fruit yields (metric tons per hectare) of three apple production

systems.

FROM REGANOLD ET AL., 2001

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Part IV Applied Issues in Ecology

420

Pasture

N fertilizers

P fertilizers

Irrigation

Pesticides

Cropland

150

100

Agricultural variable

Projected increase (%)

Figure 12.20

Projected increases in nitrogen (N) and phosphorus (P) fertilizers,

irrigated land, pesticide use and total areas under crops and pasture

by the years 2020 (maroon bars) and 2050 (green bars).

FROM LAURANCE 2001, BASED ON DATA IN TILMAN ET AL., 2001

The human population ‘problem’

Resource use by humans is deﬁned as sustainable if

it can be continued for the forseeable future. The root

of most environmental problems is the ‘population

problem’: a large human population that has been

growing at a more-than-exponential rate.

Three groups of nations can be recognized: those

that passed through the demographic transition ‘early’,

‘late’ or ‘not yet’. Even if it were possible instantane-

ously to bring about the transition in all remaining

countries of the world, the population problem would

not be solved, partly because population growth has

its own momentum.

The global carrying capacity for humans is variously

estimated at between 1 and 1000 billion, depending

mainly on what is deemed to constitute an acceptable

standard of living.

Harvesting living resources from the wild

Whenever a natural population is exploited by harvest-

ing there is a risk of overexploitation. But harvesters

also want to avoid underexploitation – when potential

consumers are deprived of resources and those who

do the harvesting are underemployed.

The concept of the maximum sustainable yield

(MSY) has been a guiding principle in the exploita-

tion of natural populations. There are two simple

ways of obtaining an MSY on a regular basis:

through a ‘ﬁxed quota’ and through a ‘ﬁxed effort’.

Two limitations of the MSY approach are: (i) that it

treats all individuals in the population as identical;

and (ii) that it treats the environment as unvarying.

Improved harvesting strategies correct both these

oversights. Lack of knowledge of most ﬁsheries

around the world means that management is often

based on the precautionary principle, often in the

absence of data.

The farming of monocultures

Increasingly, animals and plants have been domestic-

ated and managed in ways that allowed much larger

harvests – usually as monocultures. But a high price

may be paid to maintain these high rates of food pro-

duction. Monocultures offer ideal conditions for the

SUMMARY

Summary

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Chapter 12 Sustainability

421

epidemic spread of diseases and lead to widespread

degradation of land.

The sustainability of soil and of water supplies

In an ideal sustainable world, new soil would be formed

as fast as the old was lost, but in most agricultural

systems this is not achieved. When there is an over-

whelming reliance on artiﬁcial fertilizers, maintaining the

organic matter capital of the soil tends to be neglected

and this has declined worldwide.

Soil degradation can be slowed down by incorpor-

ating manures and residues, alternating cultivation and

fallow periods, or returning the land to grazed pasture.

In tropical regions, terracing is widely practiced over hilly

and mountainous terrain. In arid regions, overgrazing

and excessive cultivation can lead to desertization

and salinization.

Water is widely thought to be the resource that

future wars will be fought over. On a global scale,

agriculture is the largest consumer of fresh water.

Pumping water from underground aquifers is the

main cause of loss of agricultural land through

salinization.

Pest control

The aim of pest control is to reduce the pest popula-

tion to its economic injury level (EIL), but a so-called

economic threshold may be of more immediate

importance.

Pesticides may kill species other than their target

and may give rise to target pest resurgence or

secondary pest outbreaks. Pests may also evolve

pesticide resistance.

Biologists may also manipulate the natural

enemies of pests (biological control) via three forms

of biological control – importation, conservation or

inoculation – but even biocontrol agents can have

unwanted effects on non-target species.

Integrated farming systems

Integrated pest management (IPM) is a practical

philosophy of pest management that is ecologically

based but uses all methods of control where appro-

priate. It relies heavily on natural mortality factors and

calls for specialist pest managers or advisers.

Implicit in the philosophy of IPM is the idea that

pest control cannot be isolated from other aspects of

food production. A number of programs have been

initiated to develop and put into practice sustain-

able food production methods that incorporate IPM.

Evidence has been accumulating that this sustain-

able farming approach can yield improved economic

returns too.

Agriculturally induced global change

It is clear that very signiﬁcant threats are posed to

ecosystems around the world by the increasing human

population and concomitant increases to agricultural

development. These are expected to have a par-

ticularly damaging effect on biodiversity because

most agricultural growth is predicted to occur in the

species-rich tropics.

REVIEW QUESTIONS

Review questions

Asterisks indicate challenge questions

1* What is sustainability? Is it possible to have

sustainable population growth? Sustainable use

of fossil fuels? Sustainable use of forest trees?

Justify your answers.

2 Describe what is meant by ‘the demographic

transition’ in a human population. Explain why

it might be important, for future management of

human population growth, to discover whether

the demographic transition is an academic

ideal or a process through which all human

populations necessarily pass.

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Part IV Applied Issues in Ecology

422

3* The number of people that the Earth can

support depends on their standard of living.

Argue the case either for or against developing

nations having the right to expect standards

of living those in the developed world take for

granted.

4 Contrast the ways in which ‘ﬁxed quota’ and

‘ﬁxed effort’ harvesting strategies seek to

extract maximum sustainable yields from

natural populations.

5 Discuss the pros and cons of agricultural

monocultures.

6 One of the main bodies regulating the

production of organic food (food produced

without synthetic fertilizers or pesticides) in the

United Kingdom is the Soil Association. Explain

why you think it has adopted this name.

7 Explain the meaning and importance of the terms

economic injury level and economic threshold.

8 Weigh up the advantages and disadvantages of

the chemical and biological control of pests.

9 Explain why methods of pest control and

methods of soil fertility maintenance need

to be considered together in integrated

farming systems.

10* Hilborn and Walters (1992) have suggested

that there are three attitudes that ecologists

can take when they enter the public arena.

The ﬁrst is to claim that ecological interactions

are too complex, and our understanding and

our data too poor, for deﬁnite pronouncements

to be made (for fear of being wrong). The

second possibility is for ecologists to

concentrate exclusively on ecology and

arrive at a recommendation designed to

satisfy purely ecological criteria. The third

is for ecologists to make ecological

recommendations that are as accurate and

realistic as possible, but to accept that these

will be incorporated with a broader range

of factors when management decisions are

made – and may be rejected. Which of these

do you favor, and why?

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423

Chapter 13

Habitat degradation

CHAPTER CONTENTS

13.1 Introduction

13.2 Degradation via cultivation

13.3 Power generation and its diverse effects

13.4 Degradation in urban and industrial landscapes

13.5 Maintenance and restoration of ecosystem services

Chapter contents

KEY CONCEPTS

In this chapter you will:

realize that Homo sapiens is just one species among many whose

activities can reduce the quality of their environment – but to a

dramatically greater extent

understand that we have both physical effects (such as desertization

and changes to riverﬂow) and chemical impacts (pollution by nitrates,

carbon dioxide, chloroﬂuorocarbon, etc.)

recognize that most pollutants produced on land ultimately affect the

atmosphere or rivers, lakes and oceans

understand that power generation is responsible for the most far-

reaching environmental impacts when the carbon dioxide released

contributes to global climate change

appreciate the value to human welfare of ecosystem services lost when

we degrade habitats

Key concepts

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13.1 Introduction

13.1.1 Physical and chemical impacts of

human activities

People destroy or degrade natural ecosystems to make way for agricultural, urban

and industrial development. We physically damage the natural world when

mining for non-renewable resources such as gold and oil, and even exploitation

of a renewable resource can disrupt habitat when, for example, bottom trawling

for ﬁsh damages deep-sea coral communities. The worldwide scale of damage is

even greater as a result of chemical pollution produced by human activities such

as defecation, cultivation, power generation and industry.

Humans are not unique in degrading their environment. Feces, urine and

dead bodies of animals are sometimes sources of pollution in their immediate

environments – cattle avoid grass near their waste for several weeks, many birds

carry away the fecal sacs of their nestlings and the ‘undertaker’ caste of honeybees

removes dead bodies from the hive. Like us, many species also make profound

physical changes to their habitats. Among the ‘ecological engineers’ of the natural

world are beavers that construct dams, prairie dogs that build underground towns

and freshwater crayﬁsh that clear sediment from the riverbed. In each case,

other species in the community are affected. And there are even species that, like

farmers, increase plant nutrient concentrations in their habitats (leguminous

plants – see Section 8.4.6), and others that produce ‘pesticides’ (certain plants

produce allelochemicals, the function of which appears to be the inhibition of

growth of neighbors).

When population density was low, and prior to our harnessing of non-food

energy, humans probably had no greater impact than many other species. But now

the scale of human effects is proportional to our huge numbers and the advanced

technologies we employ.

Physical degradation of habitats includes soil loss and desertization caused by

intensive agriculture (discussed in Section 12.4.1) and changes to river discharge

as a result of water impoundment for hydropower generation or abstraction for

irrigation of crops (Section 13.2.5).

Chemical degradation has many causes. Pesticides are applied to land but

ﬁnd their way to places they were not intended to be – passing up food chains

Part IV Applied Issues in Ecology

424

Homo sapiens – just another

species?

As the human population has grown and new technologies have been developed,

we have had an ever-increasing impact on natural ecosystems. Physical

degradation and chemical pollution associated with cultivation, power generation,

urban life and industry have adversely affected human health and many

‘ecosystem services’ that were free and contributed greatly to human welfare.

Our environmental problems have ecological, economic and sociopolitical

dimensions, so a multidisciplinary approach will be needed to ﬁnd solutions.

the scale of human degradation

reﬂects our population density

and technology

physical degradation of habitats

chemical degradation – pollution

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(Box 13.1) and moving via ocean currents to the ends of the Earth. A plethora of

other exotic chemicals enter the natural environment from a variety of industrial

sources. But the most far-reaching kinds of chemical degradation result not from

our production of exotic chemicals but rather the augmentation of simple com-

pounds that already occur naturally. The heavy use on land of nitrogen fertilizer

spills into rivers, lakes and oceans, where raised levels of nitrate severely disrupt

ecosystem processes – with blooms of microscopic algae shading out waterweeds

and, when the algae die and decompose, reducing oxygen and killing animals.

Another pollutant route is via the atmosphere. Thus, hundreds of kilometers

downwind of large population centers, acid rain (caused by emission of oxides

of nitrogen and sulfur from power generation) kills trees and drives lake ﬁsh

Chapter 13 Habitat degradation

425

13.1 TOPICAL ECONCERNS

13.1 Topical ECOncerns

The peregrine falcon (Falco peregrinus) is a particu-

larly distinctive and beautiful bird of prey with an almost

worldwide range. Until the 1940s, about 500 pairs bred

regularly in the eastern states of the United States and

about 1000 pairs in the west and in Mexico. In the late

1940s their numbers started a rapid decline, and by

the mid-1970s the bird had disappeared from almost

all the eastern states and its numbers had fallen by

80–90% in the west. Similar dramatic declines were

occurring in Europe. Peregrine falcons were listed

as an endangered species (at risk of extinction). The

decline also occurred in many other birds of prey and

was traced to failure to hatch normal broods. There

was very high breakage of eggs in the nest.

The cause was eventually identiﬁed as the accumu-

lation of DDT (dichlorodiphenyltrichloroethane) in the

parent birds. The pesticide had apparently contamin-

ated seeds and insects that had then been eaten by

small birds and had accumulated in their tissues. In

turn these had been caught and eaten by birds of prey

and the pesticide interfered with their reproduction –

in particular causing the eggs to have thin shells and

be more likely to break.

The use of DDT was banned in the United States in

1972. Programs were developed to breed peregrines

in captivity and at least 4000 peregrines were bred

and released to the wild. Peregrines are now breed-

ing successfully over much of the United States and

are no longer considered an endangered species.

In Britain, recovery has been so successful that the

peregrine has become regarded as a pest by pigeon

fanciers and lovers of the smaller songbirds.

It was possible to identify DDT pollution as a cause

of eggshell thinning because eggshells had been col-

lected as dated specimens in museums and private

collections. A measure of eggshell thickness in col-

lections of eggs of the sparrow hawk (Accipiter nisus)

showed a sudden stepwise fall of 17% in 1947, when

DDT began to be used widely in agriculture, and a

steady increase in thickness after DDT was banned

(Figure 13.1).

Pollution and the thickness of birds’ eggshells

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extinct. And the biggest pollution problem of all involves the augmentation, via

the burning of fossil fuels, of carbon dioxide in the atmosphere. The consequent

global climate change has implications for every ecosystem in the world.

Our discussion of human degradation of habitats will ﬁrst consider the

consequences of cultivation (Section 13.2), before proceeding to an assessment

of damage associated with the generation of power (Section 13.3), and then the

ecological consequences of life in urban and industrial landscapes (Section 13.4).

But ﬁrst (Section 13.1.2) we will note how the cost of our activities can be

tallied in relation to the free ‘ecosystem services’ that are lost when habitats are

degraded. Discussion returns to this theme in the ﬁnal section (13.5), which

strikes a more optimistic note by discussing actions that can be taken to maintain

or restore ecosystem services.

Part IV Applied Issues in Ecology

426

It was a surprise to ornithologists in Britain to ﬁnd

evidence of a decline in eggshell thickness of 2–10%

in four species of thrush (Turdus) since the mid-19th

century (Green, 1998). This seemed to have started

long before the development of organic pesticides

and there was no sudden change when DDT was

introduced. Snails are an important part of the diet of

thrushes; thrushes derive much of the calcium for their

eggshells from snails. There is convincing evidence

that acid rain, caused by release to the atmosphere

of sulfur and nitrogen oxides from power generation

and industry, has acidiﬁed leaf litter and reduced

its calcium content, leading to a reduction in snail

populations and in the calcium content of their shells.

The shells of wild birds’ eggs have therefore recorded

two of the major, but quite different, forces of environ-

mental pollution: pesticides (Section 13.2.5) and acid

rain (Section 13.3.1).

1.80

1.70

1.60

1.50

1.40

1.30

1.20

1.10

1.0

0.9

0.8

1900 1910 1920 1930 1940 1950 1960 1970

Eggshell index

Year

Figure 13.1

Graph showing the changes in sparrowhawk eggshell thickness (museum specimens) in Britain.

FROM RATCLIFFE, 1970

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13.1.2 Economic costs of human impacts: lost

ecosystem services

Biodiversity has intrinsic value. But there is also a utilitarian view of nature that

focuses on the services that ecosystems provide for people to use and enjoy.

Provisioning services include wild foods such as ﬁsh from the ocean and berries

from the forest, medicinal herbs, ﬁber, fuel and drinking water, as well as the

products of cultivation in agroecosystems. Nature also contributes the cultural

services of esthetic fulﬁlment and educational and recreational opportunities.

Regulating services include the ecosystem’s ability to break down or ﬁlter out

pollutants, the moderation by forests and wetlands of disturbances such as ﬂoods,

and the ecosystem’s ability to regulate climate (via the capture or ‘sequestration’

by plants of the greenhouse gas carbon dioxide). Finally, and underlying all the

others, there are supporting services such as primary production, the nutrient

cycling upon which productivity is based, and soil formation.

In the case of three important provisioning services – production of crops,

livestock and aquaculture – human activities have had a positive effect. And because

of increased tree planting in some parts of the world there has even been a global

improvement in the sequestration of carbon by trees (a climate regulating service).

But we have degraded most of the other services (Millennium Ecosystem Assess-

ment 2005). As discussed in Chapter 12, many ﬁsheries are now overexploited

(a negative effect on this provisioning service), while intensive agriculture has

worked against the ecosystem’s ability to replace soil lost to erosion (a regulating

service). The continuing loss of forest in tropical regions has negative effects on

the ability of the terrestrial ecosystem to regulate riverﬂow – deforestation

increases ﬂow during ﬂooding and decreases it during dry periods. And, as we saw

in Section 1.3.3, deforestation (or even just the loss of riverside vegetation) can

diminish the terrestrial ecosystem’s capacity to hold and recycle nutrients (another

regulating service), releasing large quantities of nitrate and other plant nutrients

Chapter 13 Habitat degradation

427

Acid rain damage to spruce forest.

provisioning, cultural, regulating

and supporting services

a few positive human effects on

ecosystem services...

. . . but many negative effects

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into waterways. Note that the modiﬁcation of an ecosystem to enhance one service

(e.g. intensiﬁcation of agriculture to produce more crop per hectare – a provision-

ing service) generally comes at a cost to other services previously provided (loss

of regulating services such as nutrient uptake and of cultural services such as

sacred sites, streamside walks and valued biodiversity) (Townsend, 2007).

The concept of ecosystem services is important because it focuses on how eco-

systems contribute to human well-being, providing a counterpoint to the economic

reasons that justify our degradation of nature in the ﬁrst place (to produce food,

ﬁber, fuel, housing and luxury products for a burgeoning human population).

Economists can put a value on nature in a variety of ways. A provisioning service

for which there is a market is straightforward – values are easily ascribed to clean

water for drinking or irrigation, to ﬁsh from the ocean and medicinal products from

the forest. A more imaginative approach is required in other situations. Thus, the

travel cost that tourists are willing to pay to visit a natural area provides a minimum

value of the cultural service provided. To determine contingent valuation, surveys

of the public assess their willingness to pay for each of a set of alternative land use

scenarios; the answer is thus ‘contingent’ on a speciﬁc hypothetical scenario and

description of the environmental service concerned. Replacement cost estimates

how much would need to be spent to replace an ecosystem service with a man-

made alternative, for example by substituting the natural waste disposal capacity

of a wetland by building a treatment works. And when an ecosystem service has

already been lost, the real costs become apparent. Take, for example, the largely

deliberate burning of 50,000 km

of Indonesian vegetation in 1997 – the economic

cost comprised US$4.5 billion in lost forest products and agriculture, increased

greenhouse gas emissions, reductions to tourism and healthcare expenditure on

12 million people affected by the smoke (Balmford & Bond 2005).

Costanza et al. (1997) added up all ecosystem services worldwide, arriving

at an estimate of US$38 trillion (10

) – more than the gross domestic product

of all nations combined. This ‘new economics’ provides persuasive reasons for

taking greater care of ecosystems and the biodiversity they contain.

13.2 Degradation via cultivation

When intensive livestock production forces animals to live the equivalent of urban

life, their waste is produced faster than natural decomposers and detritivores can

handle it (see Chapter 11). All the problems of human urban overpopulation

then apply to domestic livestock. Intensive agriculture is also associated with an

increase in the level of the nitrate and phosphate that runs into rivers and lakes

(and into drinking water) and problems associated with the use of insecticides and

herbicides. As we have already seen in Section 12.7, the environmental threats posed

by agricultural intensiﬁcation are expected to increase in the coming decades.

13.2.1 Intensive livestock management

Pigs, cattle and poultry are the three major contributors to pollution in industrial-

ized agriculture feedlots. The waste from factory-farmed poultry is easily dried

and forms a transportable, inoffensive and valuable crop and garden fertilizer. In

contrast, the excreta from cattle and pigs are 90% water and have an unpleasant

Part IV Applied Issues in Ecology

428

a valuation of ecosystem

services...

. . . adding up to a global total of

$38 trillion

excreta from cattle and pigs is

bulky (and smelly) but poultry

waste is more acceptable

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