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

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smell. A commercial unit for fattening 10,000 pigs produces as much pollution

as a town of 18,000 inhabitants.

The law in many parts of the world increasingly restricts the discharge of agri-

cultural slurry into watercourses. The simplest and often the most economically

sound practice returns the material to the land as semisolid manure or as sprayed

slurry. This dilutes its concentration in the environment to what might have

occurred in a more primitive and sustainable type of agriculture and converts

pollutant into fertilizer. Soil microorganisms decompose the organic components

of sewage and slurry and most of the mineral nutrients become immobilized in

the soil, available to be absorbed again by the vegetation.

Nitrogen is a special case: nitrate ions are not adsorbed in the soil and rainfall

leaches them into drainage (and therefore potential drinking) water. The nitrate

becomes a new pollutant and one of the biggest culprits is farm specialization where

forage crops are grown in one area, but stock is fattened on the other side of the

country. This means that fertilizer must be used to make up the shortfall when plants

are reaped and transported to the stock, whose excreta can hardly be shipped

all the way back to the farm of origin. In the USA, for example, only 34% of the

nitrogen excreted in animal waste is returned to ﬁelds where the crops are grown

(Mosier et al., 2002). Much of the rest eventually ﬁnds its way into streams and

rivers. A change in practice to one where animal feed crops and stock fattening

occur in the same area would certainly reduce nutrient loss to waterways.

13.2.2 Intensive cropping

Some of the nitrogen used in agricultural fertilizer is obtained by mining potassium

nitrate in Chile and Peru, and some, as we have seen, comes from animal excreta,

but the majority comes from the energy-expensive industrial process of nitrogen

ﬁxation, in which nitrogen is catalytically combined with hydrogen under high

pressure to form ammonia and, in turn, nitrate. However, it is wrong to regard

artiﬁcial fertilization as the only practice that leads to nitrate pollution; nitrogen

ﬁxed by crops of legumes such as alfalfa, clover, peas and beans also ﬁnds its way

into nitrates that leach into drainage water.

Excess nitrates in drinking water can be a health hazard – the Environmental

Protection Agency in the United States recommends a maximum concentration of

10 mg l

−1

in drinking water. Nitrates may contribute to the formation of carcino-

genic nitrosamines and, in young children, may reduce the oxygen-carrying

capacity of the blood. Public water systems are required to be monitored regularly

and violations reported to the federal government. In 1998, for example, nearly

0.2% of children in the USA (117,000 children in all) lived in areas in which the

nitrate standard was exceeded.

There are a number of tools to minimize fertilizer loss from the land (thus saving

money) to the water (where a useful resource becomes an irritating pollutant).

Farmers might aim to maintain a ground cover of vegetation year-round, practice

mixed cropping rather than monoculture and take care to return organic matter

to the soil. The overriding objective should be to match nutrient supply to crop

demand. Modern ‘controlled release’ fertilizers hold much promise in this regard

(Mosier et al., 2002).

The excess input of nutrients, both nitrogen- and phosphorus-based, from

agricultural runoff (and human sewage) has caused many ‘healthy’ oligotrophic lakes

Chapter 13 Habitat degradation

429

most agricultural crops depend

on fertilizer nitrogen – or nitrogen

ﬁxation by legumes

nitrates in drinking water are

a hazard to health

tools to minimize fertilizer

loss from land

9781405156585_4_013.qxd 11/5/07 15:03 Page 429

(low nutrient concentrations, low plant productivity with abundant water weeds,

and clear water) to switch to a eutrophic condition where high nutrient inputs lead

to high phytoplankton productivity (sometimes dominated by bloom-forming

toxic species). This makes the water turbid, eliminates large plants and, in the

worst situations, leads to anoxia and ﬁsh kills: so-called cultural eutrophication.

Thus, important ecosystem services are lost, including the provisioning service of

wild-caught ﬁsh and the cultural services associated with recreation.

The process of cultural eutrophication of lakes has been understood for some

time. But only recently did scientists notice huge ‘dead zones’ in the oceans

near river outlets, particularly those draining large catchment areas such as the

Mississippi in North America and the Yangtze in China. The nutrient-enriched

water ﬂows through streams, rivers and lakes, and eventually to the estuary and

ocean where the ecological impact may be huge, killing virtually all invertebrates

and ﬁsh in areas up to 70,000 km

in extent. More than 150 sea areas worldwide

are now regularly starved of oxygen as a result of decomposition of algal blooms,

fueled particularly by nitrogen from agricultural runoff of fertilizers and sewage

from large cities (UNEP, 2003). Oceanic dead zones are typically associated with

industrialized nations and usually lie off countries that subsidize their agriculture,

encouraging farmers to increase productivity and use more fertilizer.

13.2.3 Managing eutrophication

Lake eutrophication, where phosphorus is often the principal culprit, can be

reversed by either chemical or biological means. Reduction of phosphorus inputs,

by better managing fertilizer use, may be combined with an intervention such

as chemical treatment to immobilize phosphorus in the sediment; recovery to

a more oligotrophic state can occur within 10–15 years (Jeppesen et al., 2005).

In essence, this is bottom-up control (see Section 9.5.1) of nutrient availability,

reducing phytoplankton productivity and increasing water quality.

The aim of biological control – known as biomanipulation – is also to reduce

phytoplankton density and increase water clarity, but via an increase in grazing

by zooplankton resulting from the active reduction of the biomass of zooplank-

tivorous ﬁsh (by ﬁshing them out or by increasing piscivorous ﬁsh biomass). The

outcome is the same, but the process is now top-down control of a cascade in the

food web.

Lathrop et al. (2002) biomanipulated Lake Mendota in Wisconsin, USA, by

increasing the density of two piscivorous ﬁsh: walleye (Stizostedion vitreum)

and northern pike (Esox lucius). More than 2 million ﬁngerlings of the two

species were stocked beginning in 1987 (Figure 13.2a) and total piscivore biomass

stabilized at 4–6 kg ha

−1

. The biomass of zooplanktivorous ﬁsh declined, as

a result of increased predation by the piscivores, from 300–600 kg ha

−1

prior to

biomanipulation to 20–40 kg ha

−1

in subsequent years. The consequent reduction

in predation pressure on zooplankton (Figure 13.2b) led, in turn, to a switch from

small zooplankton grazers (Daphnia galeata mendotae) to the larger and more

efﬁcient Daphnia pulicaria. The increased grazing pressure had the desired effect

of reducing phytoplankton density and increasing water clarity (Figure 13.2c).

The only way to alleviate problems in the world’s oceans is by careful manage-

ment of terrestrial catchment areas to reduce agricultural runoff of nutrients

and by treating sewage to remove nutrients before discharge (known as tertiary

treatment – Section 13.4.1). The vegetation zones between land and water, such

Part IV Applied Issues in Ecology

430

downstream problems of

fertilizer runoff

cultural eutrophication of lakes

and oceans

reversing cultural eutrophication

of lakes – ‘bottom up’ by

chemical means...

. . . or top down by

biomanipulation

constructing wetlands to manage

ocean water quality

9781405156585_4_013.qxd 11/5/07 15:03 Page 430

as wetland areas (consisting of swamps, ditches and ponds) and riparian forest

along the banks of streams, can be particularly beneﬁcial because the plants and

microorganisms remove some of the dissolved nutrients as they ﬁlter through the

soil. In this way, the riparian zone provides a regulating ecosystem service.

Chapter 13 Habitat degradation

431

Fingerlings stocked (1000s)

(b)

0.4

0.8

1.2

1.6

(a)

200

400

600

800

Biomanipulation

1976

78 80 82 84 86 88 90 92 94 96 98

Depth (Secchi)

(c)

Northern pike

Rate of zooplankton consumption

(g m

–2

day

–1

)

Year

Walleye

Figure 13.2

(a) Fingerlings of two piscivorous ﬁsh stocked in Lake Mendota;

the major biomanipulation effort started in 1987 (vertical

dashed line). (b) Estimates of zooplankton biomass consumed

by zooplanktivorous ﬁsh per unit area per day. The principal

zooplanktivore ﬁsh were Coregonus artedi, Perca ﬂavescens and

Morone chrysops. Note that the consumption of zooplankton was

reduced because the piscivorous ﬁsh reduced densities of the

zooplanktivorous ﬁsh. (c) Mean and range of the maximum depth

at which a Secchi disk is visible (a measure of water clarity) during

the summer from 1976 to 1999. The dotted vertical lines are for

periods when the large and efﬁcient grazer Daphnia pulicaria was

dominant. This grazing zooplankton species was much more

prominent after biomanipulation had allowed zooplankton to

increase in density; D. pulicaria plays a large role in reducing

the density of phytoplankton so that water clarity increases

(Secchi disk visible at greater depth).

FROM BEGON ET AL., 2006; AFTER LATHROP ET AL., 2002

9781405156585_4_013.qxd 11/5/07 15:03 Page 431

But riparian and wetland communities have often been destroyed to provide

a greater area for agricultural production. These ecosystems can sometimes be

restored to a seminatural state. An alternative is ‘treatment wetlands’, which are

constructed, planted and have water ﬂow controlled to maximize the removal of

pollutants from the water draining through them. Estimates for catchment areas

in southern Sweden, which are a major source of nitrate enrichment of the Baltic

Sea, indicate that to remove 40% of the nitrogen currently ﬁnding its way into

the sea, a system of wetlands covering about 5% of the total land area would

need to be recreated (Figure 13.3).

13.2.4 Pesticide pollution

Many of the manufactured chemicals that are used to kill pests have become import-

ant environmental pollutants. The most widely polluting pesticides are those used to

control pests and weeds that damage crops in agriculture, horticulture and forestry

or to kill pests that transmit diseases of livestock and humans. All are sprayed or

dusted onto the areas in which the pests live, but only a very small proportion hits

the target – most lands on the crop or on bare ground. Such pesticides are there-

fore used in much larger quantities than strictly necessary. The characteristics of

the most widely used pesticides were described in Chapter 12.

In the early industrial development of pesticides, manufacturers were not

much concerned with the speciﬁcity of their product. The potential for disaster

is illustrated by the occasion when massive doses of the insecticide dieldrin were

applied to large areas of Illinois farmland from 1954 to 1958 to ‘eradicate’ a

grassland pest, the Japanese beetle. Cattle and sheep on the farms were poisoned,

90% of the cats on the farms and a number of dogs were killed, and among

wildlife 12 species of mammals and 19 species of birds suffered losses (Luckman

& Decker, 1960).

Part IV Applied Issues in Ecology

432

10 km

Figure 13.3

The locations of 148 wetlands under construction along tributaries

of the Rönneå River in southern Sweden. If these are built to

occupy 5% of the total land area, a 40% reduction can be expected

in agricultural nitrogen input to the Baltic Sea.

FROM VERHOEVEN ET AL., 2006, BASED ON ARHEIMER & WITTGREN, 2002

9781405156585_4_013.qxd 11/5/07 15:03 Page 432

Chemical insecticides are generally intended to control particular target

pests at particular places and times. Problems arise when they are toxic to

many more species than just the target and particularly when they drift beyond

the target areas and persist in the environment beyond the target time. The

organochlorine insecticides have caused particularly severe problems because they

are biomagniﬁed. Biomagniﬁcation happens when a pesticide is present in an

organism that becomes the prey of another and the predator fails to excrete the

pesticide. It then accumulates in the body of the predator. The predator may itself

be eaten by a further predator, and the insecticide becomes more and more con-

centrated as it passes up the food chain. Top predators in aquatic and terrestrial

food chains, which were never intended as targets, can then accumulate extra-

ordinarily high doses (Figure 13.4; see also Box 13.1).

13.2.5 Physical degradation associated with cultivation

It hardly needs stating that one of the biggest impacts of cultivation is the phys-

ical loss of natural habitats, together with the species they contain. Sometimes,

however, the impact is more subtle. A large proportion of the world’s crops

depend on insect pollinators and bees play a leading role. Farmers often rely

on domesticated honeybees (Apis mellifera), importing hives when their crops

are in ﬂower. However, many wild bee species also pollinate crops (providing a

free provisioning ecosystem service) and these species are much less abundant in

landscapes that retain little natural vegetation.

Kremen et al. (2004) studied the role played by native bees in watermelon

(Citrullus lanatus) ﬁelds on Californian farms that varied in the proportion of

native and other habitats found nearby. Satellite imagery was used to quantify native

upland habitat (woodland and chaparral), riparian woodland and highly modiﬁed

land classes (agriculture, grassland dominated by non-native species, urban land)

in the vicinity of each ﬁeld. Kremen’s team found that the proportion of upland

native habitat within 1–2.4 km of the ﬁelds was strongly correlated with deposi-

tion of watermelon pollen by native bees, reﬂecting maximum ﬂight distances of

about 2.2 km for species that nest in these natural habitats. Next they calculated the

proportion of surrounding land that must consist of upland native habitat to yield

the 500–1000 pollen grains required per watermelon plant to produce marketable

fruit. It turns out that 40% of habitat within 2.4 km of a ﬁeld needs to be upland

native habitat to provide sufﬁciently for melon pollination needs, providing a strong

economic argument to conserve these natural habitats. For farms that are far from

natural habitat, active restoration with native plants in hedgerows and ditches and

around ﬁelds, barns and roads, might allow a target of about 10% native habitat

to be achieved (equating to 20–40% of watermelon pollination needs).

Increasing agricultural intensity is usually associated with the removal of

surface and ground water for irrigation. Coupled with impoundment of river

water behind dams, this abstraction for irrigation can have dramatic physical

consequences for patterns in riverﬂow. Thus, for example, the Nile in Africa,

the Yellow River in China and the Colorado River in North America dry up for

parts of the year before they reach the ocean. In many less dramatic cases, water

abstraction for agricultural, industrial and domestic use changes the hydrographs

(discharge patterns) of rivers both by reducing discharge (volume per unit time)

and by altering daily and seasonal patterns of ﬂow.

Chapter 13 Habitat degradation

433

pesticides are most polluting

when they are unselective,

persistent and if they

‘biomagnify’ in food chains

loss of natural habitat to

cultivation

changes to river discharge via

impoundment and irrigation

9781405156585_4_013.qxd 11/5/07 15:03 Page 433

The rare Colorado pikeminnow (Ptychocheilus lucius), which eats other ﬁsh,

is now restricted to the upper reaches of the Colorado River. Its present dis-

tribution is positively correlated with prey ﬁsh biomass, which in turn depends on

the biomass of invertebrates upon which the prey ﬁsh depend, and this, in its

turn, is positively correlated with algal biomass, the energy base of the food web

(Figure 13.5a–c). Osmundson et al. (2002) argue that the rarity of pikeminnows

can be traced to the accumulation of ﬁne sediment on the riverbed, where it

reduces algal productivity in downstream regions of the river. Historically, spring

snowmelt often produced ﬂushing discharges with the power to remove much of

Part IV Applied Issues in Ecology

434

Concentration

of PCBs

Concentration

of chlordanes

Copepods

Amphipods

Polar cod

Cod

Black guillemot

Glaucous gull

130,442

2188

205

108 76

100

292

5530

45 11.5

21.5

Figure 13.4

Organochlorines, applied as pesticides on land, are transported to the Arctic

through river runoff and oceanic and atmospheric circulation. A study in the

Barents Sea showed how two classes of pesticide are biomagniﬁed during

passage through the marine food chain. Concentrations in sea water are

very low. Herbivorous copepods (that feed on phytoplankton) have higher

concentrations (measured in nanograms per gram of lipid in the organisms),

and predatory amphipods higher concentrations still. The polar cod

(Boreogadus saida), which feeds on the invertebrates, and cod (Gadus

morhua) which also includes polar cod in its diet, show further evidence of

biomagniﬁcation. However, it is the higher steps in the food chain where

biomagniﬁcation is most marked, because the sea birds that feed on the ﬁsh

(black guillemots, Cepphus grylle) or on ﬁsh and other sea birds (glaucous

gull, Larus hyperboreus) have much less ability to eliminate the chemicals

than ﬁsh or invertebrates. Note how chlordanes are biomagniﬁed to a lesser

extent than polychlorinated biphenyls (PCBs). This results from the birds’

greater ability to metabolize and excrete the former class of pesticide.

BASED ON DATA IN BORGA ET AL., 2001

9781405156585_4_013.qxd 11/5/07 15:03 Page 434

the silt and sand that would otherwise accumulate. As a result of river regulation,

however, the mean recurrence interval of such discharges has increased from once

every 1.3–2.7 years to only once every 2.7–13.5 years (Figure 13.5d), extending

the period of silt accumulation. Managers must aim to incorporate ecologically

inﬂuential aspects of the natural hydrograph of a river into restoration efforts if

endangered (or valuable harvestable) species are to be sustained.

13.3 Power generation and its

diverse effects

Since the industrial revolution of the 18th century, our use of fossil fuels has

provided the power to transform much of the face of the planet through urban-

ization, industrial development, mining and highly intensive agriculture, forestry

and ﬁshing. In Section 13.3.1 we consider the far-reaching effects of chemical

pollutants from fossil fuel use. Because fossil fuels are exhaustible, increasingly

costly to extract, pollute the atmosphere and contribute to global warming, much

recent emphasis has been placed on developing alternative energy sources that do

not release carbon dioxide. The cleanest and safest technologies are expected

to derive from hydropower schemes (already at a technologically advanced state

in many parts of the globe), together with wind farms (rapidly developing) and

solar and wave power. Nuclear power, whose popularity had declined because

of concerns over security and radioactive waste disposal, is receiving renewed

consideration because it does not release greenhouse gases. We discuss nuclear

power in Section 13.3.2 and wind power in Section 13.3.3.

Chapter 13 Habitat degradation

435

FROM BEGON ET AL., 2006; AFTER OSMUNDSON ET AL., 2002

3.5

0 0.5

4.5

1.5

(a)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

1.0 2.0 2.5 3.0

FEDCBA

Downstream

(d)

Upstream

1908–1942

1966–2000

984

–4

(c)

–1

–2

–3

(b)

In(invertebrate biomass) (g m

–2

)

In(chlorophyll a) (mg m

–2

)

3.50 0.5 1.51.0 2.0 2.5 3.0

In(chlorophyll a) (mg m

–2

)

In(fish biomass) (g m

–2

)

In(pikeminnow density)

(no. km

–1

)

Recurrence interval (yr)

In(fish biomass) (

–2

)

Figure 13.5

Interrelationships among biological

parameters measured in a number

of reaches of the Colorado River to

determine the ultimate causes of the

declining distribution of Colorado

pikeminnows. (a) Invertebrate

biomass versus algal biomass

(chlorophyll a). (b) Prey ﬁsh

biomass versus algal biomass.

ﬁsh biomass (from catch rate per

minute of electroﬁshing). (d) Mean

recurrence intervals in six reaches

of the Colorado River (for which

historical data were available) of

discharges necessary to remove

silt and sand that would otherwise

accumulate, during recent

(1966–2000) and pre-regulation

periods (1908–1942). Lines above

the histograms show maximum

recurrence intervals.

9781405156585_4_013.qxd 11/5/07 15:03 Page 435

13.3.1 Fossil fuels and atmospheric pollution

The most profound and far-reaching consequences of the burning of fossil fuels,

principally coal and oil, are those of atmospheric pollution. Thus, the concentra-

tion of carbon dioxide in the atmosphere has increased from about 280 parts per

million (ppm) in 1750 to about 370 ppm today, and is projected to continue to

rise to 700 ppm by the year 2100 unless there are rather drastic changes in human

behavior. A remarkable census of atmospheric carbon dioxide was started in 1958

at Mauna Loa Observatory in Hawaii and this detected the extraordinary pattern

shown in Figure 13.6. The principal cause of this increase has been the burning

of fossil fuels, which in 1980, for example, released about 5.2 × 10

metric tons

of carbon into the atmosphere (Table 13.1).

Part IV Applied Issues in Ecology

436

360

380

340

320

(ppm)

Year

1960 1970 1980 1990 2000 2006

Figure 13.6

The concentration of atmospheric carbon dioxide measured at

the Mauna Loa Observatory, Hawaii showing the seasonal cycle

(dipping each northern summer when photosynthetic rates are

maximal in the northern hemisphere) and, more signiﬁcantly, the

long-term increase that is due largely to the burning of fossil fuels.

COURTESY OF THE CLIMATE MONITORING AND DIAGNOSTICS LABORATORY (CMDL) OF THE NATIONAL

OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA)

Table 13.1

Balancing the global carbon budget (in 10

metric tons per year) in 1980 to account for increases in

atmospheric carbon caused by human activities. In the row labeled ‘Missing’ the minus sign indicates

the need to identify an unknown uptake of carbon of the size shown. This has now been identiﬁed as

fertilization of terrestrial vegetation by atmospheric carbon dioxide so that an increase of the order of what

was estimated as ‘missing’ can be accounted for by an increase in the amount of carbon locked in extra

vegetation biomass (Kicklighter et al., 1999).

AFTER DETWILER & HALL, 1988

EXTREME MEDIAN EXTREME

LOW ESTIMATE ESTIMATE HIGH ESTIMATE

Release to atmosphere

Fossil fuel combustion 4.7 5.2 5.7

Cement production 0.1 0.1 0.1

Tropical forest clearance 0.4 1.0 1.6

Non-tropical forest clearance −0.1 0.0 0.1

Total release 5.1 6.3 7.5

Accounted for

Atmospheric increase −2.9 −2.9 −2.9

Ocean uptake −2.5 −2.2 −1.8

Missing? −0.3 +1.2 +2.8

9781405156585_4_013.qxd 11/5/07 15:03 Page 436

The clearing and burning of tropical forest to make way for agriculture or

timber production and the decay of the residues make a further contribution to

the increase in atmospheric carbon dioxide (Table 13.1). A considerable amount

of this is recaptured in photosynthesis by the replacement vegetation (Kicklighter

et al., 1999), but this is least when forest is converted to grassland, which has

a much lower biomass. In total about 1.0 × 10

metric tons per year has been

released through changes in tropical land use (Detwiler and Hall, 1988). This

calculation was made for 1980, and the ﬁgure for tropical forest clearance must

now be signiﬁcantly greater as a result of the uncontrollable spread of forest

ﬁres in Indonesia and in South America following the droughts associated with

the El Niño phenomenon of 1997/98.

The Earth’s atmosphere behaves like a greenhouse. Solar radiation warms

up the Earth’s surface, which reradiates energy outward, principally as infrared

radiation. Carbon dioxide – together with other gases whose concentrations

have increased as a result of human activity (nitrous oxide, methane, ozone,

chloroﬂuorocarbons) – absorbs infrared radiation. Like the glass of a greenhouse,

these gases (and water vapor) prevent some of the radiation from escaping and keep

the temperature high. The air temperature at the land surface is now 0.6 ± 0.2°C

warmer than in pre-industrial times. Given further predicted rises in greenhouse

gases, temperatures will continue to rise by a global average of between 2.0°C and

5.5°C by 2100 (IPCC, 2001; Millennium Ecosystem Assessment, 2005), but to

different extents in different places. Such changes will lead to a melting of glaciers

and icecaps, a consequent rise in sea level, and large changes to global patterns of

precipitation, winds, ocean currents and the timing and scale of storm events.

In response to these changes, we can expect latitudinal and altitudinal shifts

in species distributions and widespread extinctions as ﬂoras and faunas fail to

track and keep up with the rate of change in global temperatures (Hughes, 2000).

In addition, the global threats imposed by harmful invasive species will change.

Take, for example, the Argentine ant (Linepithema humile), a native of South

America. This is now established on every continent except Antarctica. It can

achieve extremely high densities and has adverse consequences for biodiversity

(eliminating native invertebrates) and for domestic life, swarming over human

foodstuffs and even sleeping babies. A distributional model was developed for

the ant, based on occurrences in its native and invaded ranges, and related both

to climatic data (e.g. maximum, minimum and mean temperatures, precipitation,

number of frost days, number of wet days) and topographic data (e.g. elevation,

slope, aspect). The model provided a good ﬁt with current distribution based

on current climate. Next, predicted climate change was used to model the ant’s

future distribution. Figure 13.7 indicates in red those areas predicted to improve

for the ant by 2050 (increased likelihood of ant occurrence) and in blue those

areas expected to worsen. The species will retract its range in tropical areas but

expand into higher latitudes. Ironically, the Argentine ant looks set to do less

well in its native South America than in North America and Europe.

Efforts to eradicate Argentine ants have mostly been unsuccessful. The

management response is therefore to increase biosecurity precautions in regions

expected to become progressively more invadable in future.

Of the pollutants that humans release into the atmosphere, most are returned

to Earth, about half as gases or particles and half dissolved or suspended in rain,

snow and fog. They may be carried in the wind for hundreds of kilometers across

Chapter 13 Habitat degradation

437

atmospheric pollution due to

the burning of fossil fuels

and deforestation

the greenhouse effect

acid rain

9781405156585_4_013.qxd 11/5/07 15:03 Page 437

state and national borders, and when they cause harm they can be the source

of bitter international dispute. Atmospheric pollutants sulfur dioxide (SO

) and

oxides of nitrogen (NO

), contributed particularly by the burning of fossil fuels,

interact with water and oxygen in the atmosphere to form dilute sulfuric and

nitric acids, which fall as acid rain.

Rain water has a pH of about 5.6, but pollutants lower it to below 5.0 and

values as low as 2.4 have been recorded in Britain, 2.8 in Scandinavia and even

2.1 in the United States. Many of the most visibly dramatic effects of acid rain

have been observed in the forests of central Europe where industry depended

on low-quality coal with a high sulfur content and forest dieback occurred on

a massive scale. Even in the United States, high-elevation spruce forests have

been affected, including the Shenandoah and Great Smoky Mountain national

parks.

Further effects have occurred in lakes and streams, especially when the com-

position of the underlying soil and rock does not help to neutralize the acidity.

A high concentration of hydrogen ions may itself be toxic, but changes in the

availability of nutrients and other toxins are usually more important. At a pH

below 4.0–4.5, the concentrations of aluminum (Al

), iron (Fe

) and manganese

(Mn

) become toxic to most plants and to aquatic animals that expose delicate

tissues directly to the water (such as the gills of ﬁsh). Acid rain is most damaging

in water that is already naturally acidic: it may then lower the pH so far that it

sterilizes the environment for many of the native species (e.g. Figure 13.8).

13.3.2 Nuclear power

When ﬁrst developed, nuclear power was viewed as an almost ideal, long-term

source of industrial and domestic power. However, the view that the release of

radiation could be readily controlled faded rapidly. Some leakage occurs from

nuclear power reactors, and it is doubtful whether the reprocessing of waste

Part IV Applied Issues in Ecology

438

Figure 13.7

Predicted changes to the distribution of the Argentine ant between now and 2050. Red areas are those predicted to improve for Argentine ants,

whereas blue areas are predicted to worsen for the species.

AFTER ROURA-PASCUAL ET AL., 2004

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