Townsend C.R., Begon M., Harper J.L. Essentials of Ecology
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Chapter 12
Sustainability
CHAPTER CONTENTS
12.1 Introduction
12.2 The human population ‘problem’
12.3 Harvesting living resources from the wild
12.4 The farming of monocultures
12.5 Pest control
12.6 Integrated farming systems
12.7 Forecasting agriculturally driven global environmental change
Chapter contents
KEY CONCEPTS
In this chapter you will:
l
appreciate the underlying dynamics of human population growth and
its relationship to the sustainable (or unsustainable) use of resources
l
understand the biological basis of sustainable harvesting of wild
populations – particularly in fisheries
l
recognize the benefits and costs of farming monocultures
l
understand that much agricultural practice has not been sustainable
because of loss and degradation of soil
l
appreciate that water may be the least sustainable of global resources
l
recognize the benefits and costs of different methods of pest control
and the importance of devising integrated management practices
Key concepts
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12.1 Introduction
To call an activity ‘sustainable’ means that it can be continued or repeated for
the foreseeable future. Concern has arisen, therefore, precisely because so much
human activity is clearly unsustainable. We cannot go on increasing the size of
the global human population; we cannot (if we wish to have fish to eat in future)
continue to remove fish from the sea faster than the remaining fish can replace
their lost companions; we cannot continue to harvest agricultural crops or forests
if the quality and quantity of the soil deteriorates or water resources become
inadequate; we cannot continue to use the same pesticides if increasing numbers
of pests become resistant to them; we cannot maintain the diversity of nature if
we continue to drive species to extinction.
Sustainability has thus become one of the core concepts – perhaps the core
concept – in an ever-broadening concern for the fate of the Earth and the
ecological communities that occupy it. In defining sustainability we used the
words ‘foreseeable future’. We did so because, when an activity is described as
sustainable, it is on the basis of what is known at the time. But many factors
remain unknown or unpredictable. Things may take a turn for the worse (as
when adverse oceanographic conditions damage a fishery already threatened by
overexploitation), or some unforeseen additional problem may be discovered
(resistance may appear to some previously irresistible pesticide). On the other hand,
technological advances may allow an activity to be sustained that previously
seemed unsustainable (new types of pesticide may be discovered that are more
finely targeted on the pest itself rather than species that are innocent bystanders).
However, there is a real danger that we observe the many technological and
scientific advances that have been made in the past and act on the faith that there
will always be a technological ‘fix’ coming along to solve our present problems,
too. Unsustainable practices cannot be accepted simply from faith that future
advances will make them sustainable after all.
The recognition of sustainability’s importance as a unifying idea in applied
ecology has grown gradually, but there is something to be said for the claim
that sustainability really came of age in 1991. This was when the Ecological
Society of America published ‘The sustainable biosphere initiative: an ecological
research agenda’, ‘a call-to-arms for all ecologists’ with a list of 16 co-authors
(Lubchenco et al., 1991). And in the same year, the World Conservation Union,
the United Nations Environment Program and the World Wide Fund for
Nature jointly published Caring for the Earth. A Strategy for Sustainable Living
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what is ‘sustainability’?
The sustainability of human activities, and of the size and distribution of the
human population, have increasingly become preoccupations of the general public
and of the politicians who represent them. But attaining or even approaching
sustainability requires more than a will to do so – it requires ecological
understanding, carefully acquired and even more carefully applied.
sustainability ‘comes of age’
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(IUCN/UNEP/WWF, 1991). The detailed contents of these documents are less
important than their existence. They indicate a growing preoccupation with
sustainability, shared by scientists and pressure groups, and recognition that
much of what we do is not sustainable.
The emphasis shifted more recently from a purely ecological perspective to
one that incorporates economic and social conditions that influence sustainability
(Milner-Gulland & Mace, 1998), a theme that has gathered pace in the new
millennium. Thus, the Millennium Ecosystem Assessment, based on contributions
from a large number of natural and social scientists, has as its aim providing both
the general public and decision-makers with ‘a scientific evaluation of the con-
sequences of current and projected changes in ecosystems for human well-being’
(Balmford & Bond, 2005; Millennium Ecosystem Assessment, 2005).
In this chapter, we first consider the size and rate of growth of the human
population, a primary driver of the environmental problems that confront us
(Section 12.2). Then we deal with two areas of applied ecology where sustain-
ability is a particularly pressing issue – the harvesting of living resources from the
wild (Section 12.3) and the production, in unnatural agroecosystems, of the food
and fiber needs of humankind (Sections 12.4–12.7).
12.2 The human population ‘problem’
12.2.1 Introduction
The root of most, if not all of the environmental problems facing us is the
‘population problem’, the effects of a large and growing population of humans.
More people means an increased requirement for energy, a greater drain on non-
renewable resources like oil and minerals, more pressure on renewable resources
like fish and forests (Section 12.3), more need for food production through agri-
culture (Section 12.4) and so on. The issue is undoubtedly one of sustainability:
things cannot go on the way they are. Yet it is not clear exactly what ‘the problem’
is (Box 12.1). Here, therefore, we examine first the size and growth rate of
the global human population and how we reached our current state, then how
successful we can expect to be in projecting forward into the future, before finally
addressing ‘the problem’ more directly by asking the question, ‘How many people
can the Earth support?’
12.2.2 Population growth up to the present
When the finger is pointed at human population growth as the key issue, it
is often said that what is wrong is that the global population has been growing
‘exponentially’. But in an exponentially growing population (see Chapter 5) the
rate of increase per individual is constant. The population as a whole grows at
an accelerating rate (a plot of numbers against time sweeps upwards), because
the population growth rate is a product of the individual rate (constant) and the
accelerating number of individuals. In Chapter 5, such exponential growth was
contrasted with a population limited by intraspecific competition (such as one
described by the ‘logistic’ equation), where the rate of increase per individual
decreases as population size increases. In the case of the global human population
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what is the human population
problem?
past population growth:
‘more than exponential’
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12.1 TOPICAL ECONCERNS
12.1 Topical ECOncerns
What is ‘the human population problem’? This is not
an easy question to pin down, but what follows are
some possible versions of the answer (Cohen, 1995,
2003, 2005). The real problem, of course, may be a
combination of these – or of these and others. There
is little doubt, though, that there is a problem, and that
the problem is ‘ours’, collectively.
l
The present size of the global human population
is unsustainably high. Around
AD 200, when
there were about a quarter of a billion people
on Earth, Quintus Septimus Florens Tertullianus
wrote that ‘we are burdensome to the world, the
resources are scarcely adequate to us’. By 2005
the total had risen to an estimated 6.5 billion
(United Nations 2005).
l
It is not the size but the distribution over the Earth
of the human population that is unsustainable.
The fraction of the population living, highly
concentrated, in an urban environment has risen
from around 3% in 1800 to 29% in 1950 and 47%
in 2000. Each agricultural worker today has to feed
her- or himself plus one city dweller; by 2050 that
will have risen to two urbanites (Cohen 2005).
l
The present rate of growth in size of the global
human population is unsustainably high. Prior to
the widespread agricultural revolution of the 18th
century, the human population, very roughly, had
taken 1000 years to double in size. The most
recent doubling took just 39 years (Cohen 2001).
l
It is not the size but the age distribution of the global
human population that is unsustainable. In the
‘developed’ regions of the world, the percentage
of the population that was elderly (over 65) rose
from 7.6% in 1950 to 12.1% in 1990. This proportion
will jump dramatically after 2010 when the large
cohorts born after World War II pass 65.
The human population problem
1950
2000
2050
?!
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(Box 12.2), however, the rate of increase per individual (and also therefore the
annual percentage increase in size: the rate of increase per 100 individuals) has
certainly not been decreasing – but neither has it remained constant (Cohen,
1995). Rather, the individual rate has itself been accelerating. Even exponential
growth would be unsustainable; but the more-than-exponential growth that we
have witnessed would, if continued, become unsustainable even sooner.
Chapter 12 Sustainability
393
l
It is not the size but the uneven distribution of
resources within the global population that is
unsustainable. In 1992, the 830 million people of
the world’s richest countries enjoyed an average
income equivalent to US$22,000 per annum. The
2.6 billion people in the middle income countries
received $1600. But the 2 billion in the poorest
countries got just $400. These averages
themselves hide enormous inequalities.
1 What role or responsibility does the individual, as
opposed to government, have in responding to
the human population problem?
2 Which of the variants of the problem, above,
pose particular questions of the relationship
between the developed and the developing
parts of the world or between the ‘haves’ and
the ‘have nots’?
Rich world
Poor world
12.2 QUANTITATIVE ASPECTS
12.2 Quantitative aspects
Figure 12.1 shows estimates of the size of the global
human population from 2000 years ago to the present.
Apart from the occasional hesitation and even rarer
downturn (such as that caused by the ravages of the
Black Death towards the end of the 14th century) the
overall picture has clearly been one of ever more rapid
population growth: the slope of the curve gets steeper
and steeper.
But is this exponential growth? The answer is a
conclusive ‘No’. Figure 12.1b shows this same graph
(black line), but also shows: (i) what an exponentially
growing population would have looked like that started
at the same point 2000 years ago and finished at the
present population size; and (ii) for the sake of contrast,
a population anchored at the same start and finish
points but growing according to the logistic equation.
Disregarding the logistic as utterly unrealistic, it
is also clear that exponential growth is much more
‘gradual’ than what has actually been observed. The
crux of the difference between these three graphs is
The growth of human populations
s
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12.2.3 Predicting the future
It is interesting to see what has happened to the total human population in the
past – and to do so alerts us to the scale of the problem we face – but the major
practical importance of such a survey lies in the opportunity it might provide to
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0
1
2
3
4
5
6
Population (billions)
(a)
0 200
AD
400 600 800 1000 1200 1400 1600 1800 2000
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Population (billions)Growth rate per individual
(b)
(c)
Year
Logistic
Exponential
Actual
shown in Figure 12.1c, which uses the same informa-
tion, but this time plots the changing growth rate
per individual against time: the per capita rate. This
parameter was introduced in Box 5.4, where it was
described, formally, as dN/dt • (1/N) or, in words, as the
rate of population growth, dN/dt, divided by the num-
ber of individuals. For the logistic, under the influence
of increasingly intense intraspecific competition, the
growth rate per individual declines in a straight line
down to zero – as it always does for the logistic. For
exponential growth, the rate is constant – again ‘by
definition’. But the actual growth curve gives rise to
an individual rate which not only increases with time
as the global population has increased – it increases
more than linearly – it accelerates. The historical
pattern of growth has been more than exponential.
Figure 12.1
See text for details.
s
prediction is more than projection
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predict future population sizes and rates of growth. There is an enormous differ-
ence, however, between projection and prediction. Simply to project forwards
would be to make the almost certainly false assumption that things will go on in
the future just as they have in the past.
Prediction, by contrast, requires an understanding of what has happened in
the past, as well as how the present differs from the past, and finally how these
differences might translate into future patterns of population growth. In particular,
it is essential to recognize that the global population of humans is a collection of
smaller populations, each with its own often very different characteristics. Like
all ecological populations, the human population is heterogeneous.
One common way in which subpopulations have been distinguished has been
in terms of the ‘demographic transition’. Three groups of nations can be recognized:
those that passed through the demographic transition ‘early’ (pre-1945), ‘late’
(since 1945) or ‘not yet’ (pre-transition countries). The pattern, illustrated for the
combined ‘early transition’ populations of Europe in Figure 12.2, is as follows.
Initially, both the birth rate and the death rate are high, but the former is only
slightly greater than the latter, so the overall rate of population increase is only
moderate or small. (This is presumed to have been the case in all human popula-
tions at some time in the past.) Next, the death rate declines while the birth rate
remains high, so the population growth rate increases. Subsequently, however, the
birth rate also declines until it is similar to or perhaps even lower than the death
rate. The population growth rate therefore declines again eventually (sometimes
to become negative with the death rate higher than the birth rate), though at a
population size far greater than before the transition began.
The hypothesis commonly proposed to explain this transition, put simply, is
that it is an inevitable consequence of industrialization, education and general
modernization leading, first, through medical advances, to the drop in death rates,
and then, through the choices people make (delaying having children and so on)
to the drop in birth rates.
Certainly, when all the regional populations of the world are considered
together, there has been a dramatic decline from the peak population growth
rate of about 2.1% per year in 1965–1970 to around 1.1–1.2% per year today
(Figure 12.3). And, as Cohen (2005) points out, while population growth rate has
Chapter 12 Sustainability
395
5
4
3
2
1
0
1850 1870 1890 1910 1930 1950 1970 1990
Rate (percentage)
Crude birth rate (percentage)
Crude death rate (percentage)
Year
Figure 12.2
The decline in the annual rate of
population growth in Europe since
1850 has been associated with a
decline in the death rate, followed
by a decline in the birth rate, and an
overall narrowing of the gap between
the two.
AFTER COHEN, 1995
the global population is
heterogeneous
early, late and future
demographic transitions
global population growth rate
peaked before 1970 and has
declined since then
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fallen at times in the past (during the plague and great wars), never before the
20th century has a fall in the global population growth rate been ‘voluntary’.
The decade we are now passing through (2000–2010) has a very special place
in human history because it will encompass three unique transitions:
1 Until now, young people (e.g. the 0–4 years class) have always
outnumbered old people (e.g. the 60+ years class), but from 2000 the old
will outnumber the young.
2 Until now, rural people have always outnumbered urban people, but from
approximately 2007 urban people will predominate.
3 From 2003 onward, women, on average, worldwide have had, and will
continue to have, too few or just enough children during their lifetime to
replace themselves and the children’s father in the next generation
(Cohen 2005).
The first two transitions must be considered problematic from the point of
view of sustainability – will the small population of workers be able to sustain
a large body of senior citizens? And will the small population of agricultural
workers be able to provide food for the rest of us? The third transition gives cause
for some optimism – but the dramatic drop in population growth rate by no
means provides an immediate fix to the population problem, as we will see in
the next section.
12.2.4 Two future inevitabilities
Even if it were possible to effect some kind of demographic transition in all
countries of the world, so that the birth rates were no more than the death rates
(zero growth), would the ‘population problem’ be solved? The answer, sadly, is
‘No’, for at least two important reasons. First, there is a big difference in age
structure between a population with equal birth and death rates in which both those
rates are high and one in which both are low. When life tables were described
in Chapter 5, we made the point that the net reproductive rate of a population
was a reflection of the age-related patterns of survival and birth. A given net
reproductive rate, though, can be arrived at through a literally infinite number
of different birth and death patterns, and these different combinations them-
selves give rise to different age structures within the population. If birth rates are
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Percent increase per year
2.5
2.0
1950 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050
1.5
1.0
0.5
0
Year
Figure 12.3
Population growth rate
averaged for the world
as a whole from 1950
to 2050.
AFTER COHEN 2001, BASED ON US CENSUS BUREAU DATA
the current decade is unique
in the history of human
population dynamics
unsustainable age structures?
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high but survival rates low (‘pre-transition’) then there will be many young, and
relatively few old individuals in the population. But if birth rates are low and
survival rates high – the ‘ideal’ to which we might aspire ‘post-transition’ – then
relatively few young, productive individuals will be called upon to support the many
who are old, unproductive and dependent (see Box 12.1). The size and growth
rates of the human population are not the only problems: the age structure of a
population adds yet another (Figure 12.4).
Moreover, suppose that our understanding was so sophisticated, and our
power so complete, that we could establish equal birth and death rates tomorrow.
Would the human population stop growing? The answer, once again, is ‘No’.
Population growth has its own momentum, which would still have to be con-
tended with. Even with a birth rate matched to the death rate, there would be
many years before a stable age structure was established, and in the mean time
there would be considerable further population growth before numbers leveled
off. According to a population projection prepared by the United Nations (the
‘medium fertility variant’), the world’s population is expected to grow from
6.3 billion today to peak at 8.9 billion in 2050 (Cohen, 2003). The reason,
simply, is that there are, for example, many more babies in the world now than
there were 25 years ago, and so even if birth rate per capita drops considerably
now, there will still be many more births in 25 years’ time, when these babies grow
up, than at present; and these children, in turn, will continue the momentum
effect before an approximately stable age structure is eventually established.
As can be gauged from Figure 12.4 it is the populations in the developing regions
of the world, dominated by young individuals, that will provide most of the
momentum for further population growth.
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397
AFTER COHEN 2005
Number of people (millions)
Age (years)
100200 200100
0–4
5–9
10–14
15–19
20–24
25–29
30–34
35–39
40–44
45–49
50–54
55–59
60–64
65–69
70–74
75–79
80–84
85+
0
Less developed countries
More developed countries
Figure 12.4
Predicted population size and age structure in 2050 for the less
developed and more developed countries of the world. The
horizontal scale is in millions of people (males to the left and
females to the right), and the vertical scale shows age groups in
5-year increments. In the two centuries prior to 1950, Europe and
the New World experienced the most rapid population growth,
while the populations of most of Asia and Africa grew very slowly.
But since 1950, rapid population growth has shifted from Western
countries to Africa, the Middle East and Asia. Note the way the
population of more developed countries becomes strongly biased
towards older people, while that of less developed countries
demonstrates a very much stronger representation of young people.
China and the USA are excluded from the graph because they are
exceptions in their categories: China’s long-standing one-child
policy will produce an age structure more like developed countries
and the USA will retain a ‘younger’ age profile because of
substantial immigration.
the momentum of population
growth
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12.2.5 A global carrying capacity?
The current rate of increase in the size of the global population is unsustain-
able even though it is lower now than it has been: in a finite space and with
finite resources, no population can continue to grow forever. What is an appro-
priate response to this? To suggest an answer, it is necessary to have some sense
of a target, and thus it is interesting, and may be important, to know how large
a population of humans could be sustained on the Earth. What is the global
carrying capacity?
There is astonishing variation in the estimates that have been proposed
over the last 300 or so years and even the estimates since 1970 span three orders
of magnitude – from 1 to 1000 billion. To illustrate the difficulty in arriving at
an estimate of global carrying capacity, a few examples are described here (see
Cohen, 1995, 2005 for further details of the authors mentioned below).
In 1679, van Leeuwenhoek estimated that the inhabited area of the Earth was
13,385 times larger than his home nation of Holland, whose population then
was about 1 million people. He then assumed that all this area could be populated
as densely as Holland, yielding an upper limit of roughly 13.4 billion.
In 1967, De Wit asked the question ‘How many people can live on Earth if
photosynthesis is the limiting process?’ The answer he arrived at was roughly
1000 billion. He built into his calculation the fact that the length of the poten-
tial growing season varies with latitude, but assumed, amongst other things, that
neither water nor minerals were limiting. He acknowledged that if people wanted
to eat meat, or wanted what most of us consider a reasonable amount of living
space, and so on, then the estimate would be much less.
By contrast, Hulett in 1970 assumed that levels of affluence and consumption
in the United States were ‘optimal’ for the whole world, and that not only food
but requirements for renewable resources like wood and non-renewable resources
like steel and aluminum needed to be brought into the calculations. The figure he
came up with was no more than 1 billion. Kates and others, in a series of reports
from 1988, made similar assumptions but worked from global rather than United
States averages and estimated a global carrying capacity of 5.9 billion people
on a basic diet (principally vegetarian), or 3.9 billion on an ‘improved’ diet (about
15% of calories from animal products) or 2.9 billion on a diet with 25% of
calories from animal products.
More recently, Wackernagel and his colleagues in 2002 sought to quantify the
amount of land humans use to supply resources and to absorb wastes (embodied
in their ‘ecological footprint’ concept). Their preliminary assessment was that
people were using 70% of the biosphere’s capacity in 1961 and 120% by 1999.
They reasoned, in other words, that global carrying capacity was exceeded before
the turn of the millennium – when our population was about 6 billion.
As Cohen (2005) has pointed out, many estimates have been based on (or
rely heavily on) a single dimension – biologically productive land area, water,
energy, food and so on – and a difficulty with them all is the reality that the
impact of one factor depends on the value of others. Thus, for example, if water
is scarce and energy is abundant, water can be desalinated and transported to
where it is in short supply, a solution that is not available if energy is expensive.
Moreover, it is clear from the examples above that there is a difference between
the number the Earth can support and the number that can be supported with an
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some estimates of ‘the global
carrying capacity’
defining global carrying capacity
is far from straightforward
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