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

Подождите немного. Документ загружается.

nuclear fuel can ever be made completely clean. Moreover, the polluting power

of radioactive waste has a time scale that may be orders of magnitude greater

than that of other human pollutants. For example, plutonium-239 has a half-

life of about 25,000 years. Plutonium is separated and recovered from the

spent fuel in nuclear reactors and stocks are expected to have risen to more

than 100 metric tons by 2010. Ways have to be found to protect against risks

of leakage over this sort of time scale, perhaps by burial in deep mines after

incorporation in glass.

The radiation received by an organism arises from human activities (nuclear

warfare, leakage from and accidents at nuclear plants, and medical use) together

with a very similar sized contribution from ‘background radiation’ from cosmic

rays and produced during the radioactive decay of materials such as radium and

thorium in the Earth’s crust. It is a sobering thought that the total radiation given

to a cancer patient can be many thousand times greater than the total normal

exposure from the combined natural and artiﬁcial background radiation.

A major accident in 1986 at a nuclear power station at Chernobyl in the

Ukraine released 50–185 million curies of radionucleides into the atmosphere.

Close to the explosion, 32 deaths occurred within a very short time. Farther away,

individuals contracted radiation sickness and some died. Effects in the locality

have continued to appear – livestock have been born deformed, and thousands

of radiation-induced illnesses and deaths from cancer are expected in the longer

term. Farther aﬁeld, wind-dispersed atmospheric pollution from Chernobyl was

detected in Sweden 3 days after the accident. Fallout also reached the British

Chapter 13 Habitat degradation

439

AFTER FLOWER ET AL., 1994

Cymbella

perpusilla

Frustulia

rhomboides

Percent

Fragilaria

virescens

Brachysira

vitrea

1988

1969

1940

1903

5.2 5.4 5.6 5.8

Sediment depth (cm)

010 01020 0 10 20 30 0 10 20 30

Date AD

6.0

Figure 13.8

The history of the diatom ﬂora of an Irish lake (Lough Maam, County Donegal) can be traced by taking cores from the sediment at the bottom of the

lake. The percentage of various diatom species at different depths reﬂects the ﬂora present at various times in the past (four species are illustrated).

The age of layers of sediment can be determined by the radioactive decay of lead-210 (and other elements). We know the pH tolerance of the

diatom species from their present distribution and this can be used to reconstruct what the pH of the lake has been in the past. Note how the

waters have acidiﬁed since about 1900. The diatoms Fragilaria virescens and Brachysira vitrea have declined markedly during this period while

the acid-tolerant Cymbella perpusilla and Frustulia rhomboides increased after 1900.

natural background radiation

and that produced by human

activities are of similar magnitude

Chernobyl – the worst nuclear

pollution disaster so far

9781405156585_4_013.qxd 11/5/07 15:03 Page 439

Isles. Figure 13.9 shows the persistence of cesium-137 in the acid soils of the

northwest of Britain, where it was absorbed by plants and eaten by sheep. The sale

of sheep for food was still banned more than 10 years after the accident because

of persistence of the isotope at dangerous levels.

13.3.3 Wind power

In this time of global climate change, harnessing the power of the wind has

much to commend it. But while this form of power generation does not release

carbon dioxide, local communities often object to the massive structures that will

appear in their localities. (This conundrum parallels the situation for hydropower

stations, which produce clean power but at the cost of altered river discharge

patterns and lost recreational opportunities downstream.) Wind farms also pose

an ecological risk in terms of threats to migrating birds. On land, soaring birds

Part IV Applied Issues in Ecology

440

2000

3000

4000

100

500

250

500

1000

250

500

1000

2000

100

1000

100

Figure 13.9

An example of long-distance environmental pollution: the distribution

in 1988 in Great Britain of fallout of cesium-137 from the Chernobyl

nuclear accident in the Soviet Union in 1986 (measured in Becquerels

per kilogram). The contours show the persistence of the cesium on

acidic upland soils where it is recycled through soil, plants and

animals. On typical lowland soils, cesium does not persist in

food chains.

AFTER NERC, 1990

9781405156585_4_013.qxd 11/5/07 15:03 Page 440

such as falcons and vultures are at particular risk of colliding with the turbines (up

to 100 m above the ground), particularly because the engineers often select their

locations for the same wind-related reasons that birds select their routes (Barrios

& Rodriguez, 2004). Many wind farms are also planned for marine settings – in

Europe, for example, more than 100 applications have been submitted. Each may

consist of as many as 1000 turbines, up to 150 m tall, as far offshore as 100 km

and in water as deep as 40 m. The turbines may pose risks to migrating birds

(from the smallest of song birds to cranes and birds of prey) as well as sea birds

dispersing locally to ﬁnd food.

Thousands of square kilometres of the marine environment off the German

coast are planned for wind farming by 2030. To predict the possible consequences

for bird populations, Garthe and Huppop (2004) developed a species sensitivity

index (SSI) for 26 seabird species, combining their scores for a range of properties,

including ﬂight maneuverability (less agile species score highly because they are

more likely to collide with turbines), ﬂight altitude (species ﬂying at 50–200 m

score highly because they are more vulnerable to turbines than lower ﬂyers), per-

centage time spent ﬂying (those in the air for more of the time score highly) and

conservation status (‘vulnerable’ or ‘declining’ score highly). The most sensitive

species (highest SSI) include the non-maneuverable and ‘vulnerable’ black-

throated diver (Gavia arctica) and the maneuverable but ‘declining’ sandwich tern

(Sterna sandvicensis) that ﬂies almost constantly and at perilous altitudes. The

SSI for each species was then coupled with density distribution data (low-density

species score highly because their populations are more at risk) to produce

vulnerability maps (all bird species combined) for the German area of the North

Sea. Three classes of vulnerability were assigned – ‘major concern’ (combined wind-

farm sensitive data [WSI] > 43), ‘less concern’ (WSI < 24) and ‘concern’ (between

these extremes) (Figure 13.10). Such ecological information should be taken into

account when selecting wind farm locations.

Chapter 13 Habitat degradation

441

3° 4° 5° 6° 7° 8° 9°

55°

54°

55°

54°

Less concern

Concern

Major concern

Figure 13.10

Areas in the German sector of the North Sea (inset, right) where wind farm development is considered to be of ‘less concern’, ‘concern’ or

‘major concern’ on the basis of bird density patterns and species-speciﬁc sensitivity indexes (SSIs).

AFTER GARTHE & HUPPOP, 2004

9781405156585_4_013.qxd 11/5/07 15:03 Page 441

13.4 Degradation in urban and industrial

landscapes

A wide range of habitat degradation occurs as a result of human activity in

urban and industrial settings. Marked changes to riverﬂow result from the loss

of permeable surfaces – roofs, pavements and roads are impermeable, in contrast

to ﬁeld and forest. Our feces, urine and dead bodies create large disposal problems

in towns and cities because density is so high. Exotic industrial chemicals ﬁnd their

way into waterways and the atmosphere. And our mining activities, whether for

fossil fuels or valuable jewels or ores, cause physical and chemical degradation to

surrounding ecosystems. In this section, our examples encompass sewage disposal

(Section 13.4.1), the industrial production of ﬂuorocarbons and consequences

for the ozone layer (Section 13.4.2) and the ecological problems associated with

mining (Section 13.4.3).

13.4.1 Disposal of bodily waste

All human body products, but most notably feces and urine, can be regarded

as pollutants. The Greeks were probably the ﬁrst to control the accumulation

of pollution within towns, and a law of 320

BC forbade dumping of waste in

the streets. The Romans were also very pollution conscious, dumping city waste

in pits outside the city walls. When Roman and Greek civilizations perished,

their quite sophisticated control of urban pollution collapsed. Medieval castles,

for example, were often designed with latrines projecting from castle walls that

simply dumped waste at the base of the walls (the accumulated wastes give

archeologists a direct record of historical diets and infestation with intestinal

worms!). Until the 14th and 15th centuries the open streets again became the

main, and often the only, destination for human and animal feces and urine.

A special trade developed, that of the scavenger, who was paid to carry waste to

dumps outside the cities; in 1714 every city in England had an ofﬁcial scavenger

(the forerunner of the Environmental Protection Agency!). Even when water

closets (invented by Thomas Crapper) began to be installed in some countries

early in the 19th century, the underground reservoirs (cesspools) into which

they emptied often overﬂowed and contaminated drinking water. Outbreaks of

cholera in the middle of the 19th century were traced directly to this source of

contamination, a discovery that led to the connection of household waste directly

to sewers in both Britain and the United States.

At ﬁrst glance, the easiest way to cope with accumulated feces and urine might

appear to be to dilute them in large bodies of water. However, it is not easy to

dispose of human waste and at the same time provide healthy drinking water. In

addition to health issues, we have already seen in Section 13.2.2 how there can

be profound ecological effects of disposing of sewage in water bodies.

All natural ecosystems have an inherent capacity to decompose feces and

up to a point natural decomposition processes in rivers, lakes and oceans may

cope with increased organic matter from human sewage without obvious changes

to the nature of the biological communities they contain. However, problems

arise when the rate of sewage input exceeds this capacity. First, excessively high

rates of decomposition of dead organic matter in rivers and lakes can lead to

Part IV Applied Issues in Ecology

442

when natural ecosystems cannot

cope with human waste...

9781405156585_4_013.qxd 11/5/07 15:03 Page 442

anaerobic conditions (causing the death of ﬁsh and invertebrates). This happens

because oxygen is consumed by the decomposer microorganisms faster than it

is replenished from photosynthesis by aquatic plants and diffusion from the air.

Second, the supply of nutrients such as phosphate and nitrate that normally limit

plant growth in water bodies may be increased to a level where algal growth is

so great that it shades and kills other aquatic plants – the cultural eutrophication

discussed in Section 13.2.2.

Modern sewage systems were developed as ecological devices for pollution

management. They aim to capture pollutants from waste water and to clean it,

usually in a drainage system separate from the one that carries heavy ﬂows of

storm water. Ideally, a sewage system cleans polluted water to a state suitable

for drinking before discharging it back into rivers, lakes and the sea. The full

treatment of sewage has three stages (Figure 13.11), though in many places

only the ﬁrst or ﬁrst and second stages are actually used before discharge into

the environment.

After paper, rags and plastic have been removed by passing the sewage through

screens, primary treatment is a physical process in which much of the solid

Chapter 13 Habitat degradation

443

Business

Industry

Bar screen

Secondary treatment

Rivers, lakes, and sea

Sludge

Biosolids

Sludge

Digester

Chlorine

Ultraviolet light irradiation

Dewatering

Removal of paper,

rags and plastics

Homes

Primary settling tank

Secondary

sedimentation tank

Tertiary nutrient stripping

Pumping

station

Biosolids

land application

Solid

storage basin

Dedicated

land disposal

Waste water Waste water

Figure 13.11

The sequence of treatments

commonly applied to the sewage

waste from a modern urban

community.

. . . sewage treatment systems

are needed

9781405156585_4_013.qxd 11/5/07 15:03 Page 443

organic sewage waste is allowed to settle to the bottom of settlement tanks, from

which it is removed as sludge.

Secondary treatment is an engineered biological process designed to mimic (and

indeed enhance) natural decomposition. In its simplest version, the partly cleaned

water is sprayed onto a layer of crushed rock within which microorganisms have

been encouraged to grow; as the water trickles down through these percolating

or trickling ﬁlters, natural decomposition mineralizes much of the remaining

organic matter, releasing carbon dioxide to the atmosphere. A more sophisticated

and efﬁcient method of secondary treatment is the activated sludge method, in

which the sewage is passed into aerated tanks containing sludge that is activated,

or seeded, with microorganisms. After secondary treatment the remaining solids

are settled to yield more sludge. The waste water now appears clean, but it still

contains two types of impurity, namely disease organisms and high concentrations

of mineral nutrients, the latter having both health consequences (Section 13.2.2)

and causing eutrophication if released into rivers and lakes.

A ﬁnal ‘polishing’ stage usually includes chlorination, and sometimes ultra-

violet (UV) light irradiation to kill bacteria. Full tertiary treatment involves the

stripping of nutrients, largely by artiﬁcial and expensive chemical processes.

Untreated sewage is obviously a pollutant, with adverse health and ecological

consequences for water bodies into which it is discharged. However, discharge of

sewage that has only been subject to primary treatment is still likely to cause eutro-

phication because it remains rich in organic matter and nutrients. Moreover, even

secondary treatment removes only the organic matter, leaving waste water rich in

plant nutrients. The sludge that accumulates in settling tanks is itself a pollutant

that has to be disposed of, usually by dumping at sea or burying in landﬁll sites.

Buried sludge decomposes anaerobically, sometimes taking more than 20 years to

mineralize completely, and it produces methane, which is a greenhouse gas that

contributes to global climate change (Section 13.3.1). Sludge can be more appro-

priately used as a fertilizer, either dried or as a liquid sprayed onto the land; in this

way the nutrient cycle can be reconstituted by returning nutrients, assimilated

from crops by people, to agricultural land to be taken up by future crops.

13.4.2 Chloroﬂuorocarbon compounds and thinning of

the ozone layer

Ozone is produced by the inﬂuence of sunlight on oxygen and during the oxida-

tion of carbon monoxide and hydrocarbons such as methane. It has three very

different roles in environmental pollution. The ﬁrst two are negative, in the sense

that undesirable polluting consequences occur as the concentration of ozone

increases. First, in atmospheres polluted with methane, industrial hydrocarbons,

oxides of nitrogen and carbon monoxide, ozone can reach concentrations that are

toxic to plants and that contribute to smog. Second, ozone is also a greenhouse

gas, though it is not particularly signiﬁcant in this respect.

However, ozone also accumulates as a layer in the upper atmosphere. This

‘ozone layer’ is beneﬁcial because it absorbs most of the UV radiation (wavelength

200–300 nm) incident on the Earth’s upper atmosphere and so makes the Earth

habitable for plants and animals. The increasing frequency of skin cancer among

humans has focused attention on the damage caused by exposure to the sun and

on the importance of stability of the ozone layer.

Part IV Applied Issues in Ecology

444

products of sewage treatment

are themselves pollutants

ozone can have adverse

consequences locally...

. . . but in the upper atmosphere

it shields the Earth from

damaging UV radiation

9781405156585_4_013.qxd 11/5/07 15:03 Page 444

Evidence that nitric oxide produced by supersonic aircraft might contribute

massively to reduce the level of atmospheric ozone led to the halting of their

large-scale development. However, that has by no means been the end of the

story. Chloroﬂuorocarbon compounds (CFCs) had been developed as aerosols

and refrigerants and used on a very large, international scale. It became clear that

these posed the threat that their chlorine content could interact with and destroy

atmospheric ozone.

Ozone chemical processes are very complicated, and methane, nitrous oxide

and carbon monoxide may all play a part in its decomposition. The upper atmo-

sphere is not the easiest place in which to study the chemical characteristics of

gases! But pollution of the upper atmosphere poses questions of the greatest

signiﬁcance for environmental scientists, especially since the discovery that the

concentration of ozone in the atmosphere over Antarctica had started to decline by

1978 and was doing so very rapidly after 1982. The phenomenon happens at the

start of the southern hemisphere spring (August to October). The size of the ozone

hole over Antarctica on September 24, 2006 was one of the largest ever recorded,

equivalent to more than the surface area of North America (Figure 13.12). It is

Chapter 13 Habitat degradation

445

(a)

(b)

Area of North America

Area of Antarctica

Size of ozone hole (million km

)

1980 1985 1990 1995 2000 2005

Year

chlorine compounds and other

pollutants decompose ozone

in the atmosphere and need

to be phased out

Figure 13.12

(a) An image of the ozone hole over Antarctica for September 24,

2006; the blue and purple colors are where there is least ozone

(<220 Dobson units). (b) Average size of the ozone hole from

September 7 to October 13 each year from 1980 to 2006. The

vertical lines show the minimum and maximum areas during this

period each year.

COURTESY OF THE US NATIONAL AERONAUTICS AND SPACE ADMINISTRATION, WWW.NASA.GOV

9781405156585_4_013.qxd 11/5/07 15:03 Page 445

clearly in the interests of humans and probably most other organisms that ozone

concentrations should remain low close to the Earth’s surface (e.g. minimizing

smog) but high in the upper atmosphere, and that we should ﬁnd out how to

ensure this. International agreements to phase out CFCs are expected to lead to

recovery of the ozone hole by about 2050.

13.4.3 Mining

Our dependence on fossil fuels has effects that go beyond atmospheric pollution.

The extraction and transport of coal and especially oil can also cause physical

disruption of habitats. Thus, more than 1 million tonnes of oil enters the world’s

waterways every year from wells drilled into the seabed or from oil tankers. Oil

in and on the sea affects wildlife in many ways. It reduces the level of aeration

of the water, and it prevents light from penetrating the surface. Damage to

invertebrates can be widespread, affecting chitons, mussels, crustaceans and

bryozoans, as well as seaweeds and kelps. Feathers become choked with oil so

that sea birds cannot ﬂy and ﬁsh gills become coated and cease to function.

The largest accident in the United States occurred on March 24, 1989, when

the oil tanker Exxon Valdez ran aground in Prince William Sound, Alaska. It

spilled nearly 50,000 tons of crude oil, which spread along the coast for nearly

1000 km, contaminating the shores of a national forest, ﬁve state parks, four

state critical habitat areas and a state game sanctuary. The episode is believed

to have killed 300 harbor seals, 2800 sea otters, 250,000 birds and possibly

13 killer whales. Many commercial ﬁsheries were closed for a year or more

because of the concern that ﬁsh caught in the area might ﬁnd their way into the

human food chain. By 1996, 28 species and resources were still listed as having

failed to recover.

Metals were ﬁrst used by humans in the late Stone Age, about 6500 years

ago. Gold, silver and copper were the ﬁrst metals used; they are easy to extract

because they exist in nature as the metals themselves rather than as chemical com-

pounds. Nuggets of pure metallic gold were found in riverbeds and were beaten

and molded for decoration. Once such metals were valued it was an obvious step

to dig and mine for them, and from that point almost every phase in the extrac-

tion and industrial use of metals involves a sequence of phases of environmental

pollution.

Each type of metal has its own peculiarities. Here we use the mining and

puriﬁcation of copper to illustrate pollution through the extraction of metal.

Copper is present in deposits either as the metal or as copper sulﬁde or oxide.

Like most metal deposits, it usually exists in a mixture with other metals, some

of which may be worth saving (e.g. gold), whereas others are discarded in more

or less hazardous waste.

The mining industry may pollute at every stage of extraction, puriﬁcation and

disposal:

1 Mining and quarrying. Mining or quarrying exposes the metal and its ores.

Many of the world’s copper reserves are close to the surface and are easily

extracted by open cast mining: the copper mines of Bougainville (Solomon

Islands, Papua New Guinea) and of Utah are among the largest human scars

on the Earth’s surface (Figure 13.13).

Part IV Applied Issues in Ecology

446

physical disruption from the

mining of fossil fuels

the mining and puriﬁcation

of copper

9781405156585_4_013.qxd 11/5/07 15:03 Page 446

2 Processing. The ores are crushed and ﬁnely ground. This processing

immediately exposes ores to the elements, and even after the best has been

extracted the residues are copper-rich and the metal leaches as toxic waste

into rivers and lakes. Waters close to copper mines are commonly brilliantly

blue-green with copper salts and quite sterile.

3 Concentration. The ﬁnely ground ore is agitated in water, and the metal

becomes concentrated in the froth and dried to a cake. The remainder,

which is still rich in copper, may be further concentrated to recover more of

the metal. Ultimately water and solid ‘tailings’ have insufﬁcient copper to

warrant further extraction but contain sufﬁcient copper to form a hazardous

and polluting waste.

4 Puriﬁcation through heat. The concentrate is then roasted to 1230–1300°C,

polluting the atmosphere by the burning of the necessary fuel. The roasting

drives off a host of pollutants such as arsenic, mercury and sulfur into the

atmosphere.

5 Puriﬁcation through electrolysis. The copper can now be puriﬁed by

electrolysis, which leaves most of the other metals in a sludge that may be

further puriﬁed (to remove gold, for example) but ultimately contributes

yet more toxic waste.

The major role of some metals as environmental pollutants occurs after they have

been puriﬁed and used industrially and are then released into the environment as

industrial waste. Lead and mercury are particularly striking examples. Lead became

an environmental pollutant from the moment that the Romans started to use it

to make water pipes and so started to pollute their drinking water. It is ranked by

the US Environmental Protection Agency as number 1 in their list of 275 hazardous

substances, posing a particular risk for the development of the nervous system in

young children and in the fetus. It is being phased out of many commercial uses.

Chapter 13 Habitat degradation

447

Figure 13.13

Binyon Canyon Mine, Utah. A toxic and sterile

environment created by the world’s largest

excavation.

lead and mercury can be

especially dangerous pollutants

9781405156585_4_013.qxd 11/5/07 15:03 Page 447

It is not clear whether lead pollution has signiﬁcant consequences for wildlife on

the land or in aquatic environments, but it does not appear to become concentrated

along food chains. This is a major contrast with mercury.

Mercury is used in a variety of specialized applications in industry and

medicine – in electric switches, batteries, ﬂuorescent and mercury vapor lights,

thermometers, barometers and dental amalgams. The main culprits in releasing

mercury to the atmosphere are, in order of importance, coal-ﬁred power plants,

medical waste incinerators, municipal waste incinerators and industrial boilers.

In the natural environment mercury can be converted by microbial activity to

methylmercury, a form that is readily absorbed and accumulated up food chains,

especially in lakes and estuaries. Fish, the top predators, may accumulate con-

centrations of mercury 10,000 to 100,000 times that in the surrounding water

(Bowles et al., 2001). Native peoples who hunt and eat wildlife can accumulate

even higher concentrations. Mercury is a serious poison that can cause permanent

damage to the human brain and kidneys, and particularly to the developing fetus.

It may also damage the immune system.

Land that has been damaged by mining is usually unstable, liable to erosion

and devoid of vegetation. The simplest solution to land reclamation is the

re-establishment of vegetation cover, because this will stabilize the surface, be

visually attractive and self-sustaining (Bradshaw, 2002). Candidate plants for

reclamation are those that are tolerant of the toxic heavy metals present. Of

particular value are ecotypes – different genotypes, within a species, that ﬁll

different niches (see Section 2.3.1) – that have evolved resistance in mined areas.

Thus, certain metal-tolerant grass genotypes (or cultivars) have been selected

for commercial production in the UK for use on neutral to alkaline soils con-

taminated by acidic copper wastes (Agrostis capillaris cultivar ‘Parys’) or lead or

zinc (Festuca rubra cultivar ‘Merlin’) (Baker, 2002).

In addition, many species characteristic of naturally metal-rich soils have

evolved biochemical systems for nutrient acquisition, detoxiﬁcation and the

control of local geochemical conditions. Phytoremediation of metal-contaminated

sites can take a variety of forms (Susarla et al., 2002). Phytoaccumulation occurs

when the contaminant is taken up by the plants but is not degraded rapidly or

completely; these plants, such as the zinc-accumulating herb Thlaspi caerulescens,

are harvested to remove the contaminant and then replaced. Phytostabilization,

on the other hand, takes advantage of the ability of root exudates to precipitate

heavy metals and render them biologically harmless. Finally, phytotransformation

involves elimination of a contaminant by the action of plant enzymes; for example,

hybrid poplar trees Populus deltoides × nigra have the remarkable ability to

degrade TNT (trinitrotoluene) and show promise for the restoration of munition

dump areas.

13.5 Maintenance and restoration of

ecosystem services

We have now considered a range of examples of the many impacts of human

activities on ecosystems, noting that these can often be measured in terms of lost

‘ecosystem services’ (Section 13.1.2). The concept of ecosystem services brings

Part IV Applied Issues in Ecology

448

prospecting for plant species to

restore contaminated sites

a triple bottom-line approach to

natural resource management

9781405156585_4_013.qxd 11/5/07 15:03 Page 448