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Cambridge University Press
978-0-521-78727-7 - Environmental Biology
Michael Reiss and Jenny Chapman
Excerpt
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CHAPTER 1
Introduction to
environmental biology
By the end of this chapter you should be able to:
1 explain the meanings of the terms habitat, microhabitat, niche and population;
2 distinguish between a community and an ecosystem;
3 describe similarities and differences between the growth in numbers of the human
population and other animal populations;
4 outline the effects of human activity on the natural environment;
5 explain the value of monitoring the environment.
What is ecology? amount that falls in a year, how it is distributed
throughout the seasons, and significant informa-
This book is about ecology and conservation. tion about the temperature, sunlight and soil
Ecology is the study of organisms in their natural type. The jigsaw pieces interlock with one another
surroundings. The word ecology comes from two in numerous, subtle ways.
Greek words – oikos meaning home and logos In many ways ecology is a relatively new
meaning understanding. So ecology is all about science. Indeed, the word was only coined by the
understanding the homes of animals, plants and German biologist Ernst Haeckel in 1869, fully ten
other organisms. The surroundings of an organism years after Charles Darwin published his theory of
are known as its environment. Environments con-
sist of many components including both physical
features, such as climate and soil type, and
biological features, such as predators and prey. The
term environmental biology has wider connota-
tions than ecology because it includes the study of
humans in the environment, so you will find such
subjects as agriculture, pollution and the unnatu-
ral surroundings we create in this book too.
Understanding the ecology of an area is like try-
ing to put together a gigantic, multidimensional
jigsaw. Some pieces are the individual species in
the area. In an oak wood, for example (figure 1.1),
the species might include bluebells, oak trees,
earthworms, snails, hedgehogs, wood ants and
tawny owls. Other pieces in the jigsaw are the ● Figure 1.1 Stoneywell Wood, Leicestershire, in
important aspects of the physical environment, spring. The leaves on the oak trees are just
for example the pH of the rainwater, the total emerging above the carpet of bluebells.
© Cambridge University Press www.cambridge.org
Cambridge University Press
978-0-521-78727-7 - Environmental Biology
Michael Reiss and Jenny Chapman
Excerpt
More information
2 Introduction to environmental biology
natural selection. Yet, in little over a century
ecology has grown to become one of the most
important disciplines within biology. Like all
branches of science, it has its own language. This
includes the terms habitat, population, commu- 10
nity and ecosystem, which you will have already 97
8
met in Biology 1.
A habitat is the place where an organism lives. 96 98
6
The word is Latin and literally means ‘it dwells’.
Actually, organisms from a single species can live 4 95
in a number of habitats. For example, the common 99
rat (Rattus norvegicus) is typically found associated
Height above ground (m)2 99.5
with farms, refuse tips, sewers and warehouses.
However, it also occurs in hedgerows close to 0
cereal crops or sugar beet, and in salt marshes. On 2 4 6 8 10 12 14
islands (e.g. the Isle of Man, Rhum and Lundy) rats Prey length (mm)
also occupy grassland and the sea shore. ● Figure 1.2 The feeding niche of the blue-grey
With small organisms, especially those living in gnatcatcher (Polioptila caerulea). The contours show
a restricted area such as in the soil or on a single the feeding frequencies for adult birds during the
plant or animal, it is worth being more precise nesting period in July and August in oak
about exactly where they live. The term woodlands in California. 95% of their diet is taken
microhabitat – ‘a small habitat’ – is used to within the contour marked 95, 96% within the
contour marked 96, and so on.
describe this. A single habitat may have many
microhabitats. For example, if you are an insect Polioptila caerulea, a North American bird. This is
living on an oak tree, life is very different depend- an insectivore and the horizontal axis shows the
ing on whether you live on the upper surface of length of the insects on which it feeds. The verti-
the leaves, the lower surface of the leaves or inside cal axis shows the height above ground at which
them. It is even more different if you live under it forages. The contour lines with numbers
the bark, next to the roots or inside an acorn. Each indicate the frequency with which the birds feed
of these different places is a microhabitat. at a particular height and on a particular length
A niche is a complete description of how the of prey. You can see that the birds concentrate on
organism relates to its physical and biological prey 4mm in length, which they catch about
environment. Just as in a jigsaw puzzle each piece 3–6m off the ground.
has its own unique shape and pattern, and only However, there are many other aspects to an
fits in one place, so each species has a unique organism’s niche in addition to its feeding niche.
niche – the way it fits into its environment. In theory, other axes could be added at right-
Consider a particular species, the grey heron angles to those in figure 1.2. Temperature could be
(Ardea cinerea). Its habitats are water meadows, shown on a third axis, risk of predation at differ-
rivers, lakes and the sea shore. A complete ent times of the year on a fourth, height above
account of its niche would include a description ground of the bird’s nest on a fifth, and so on. In
both of its physical environment (such as the type practice, though, no more than two or three axes
of water it needs, the temperature range in which can be shown on a graph. Computers, however,
it can survive and reproduce) and of its biological can store and compute data for many more.
environment (such as the prey it eats, its competi- The ecological principle that each species has
tors and the vegetation it needs for its nest). its own unique niche and that no two species can
It is difficult to provide a quantitative descrip- coexist if they occupy the same niche is known as
tion of an organism’s niche. Figure 1.2 shows the Gause’s competitive exclusion principle. The
feeding niche of the blue-grey gnatcatcher, biologist G. F. Gause gets the credit because of his
© Cambridge University Press www.cambridge.org
Cambridge University Press
978-0-521-78727-7 - Environmental Biology
Michael Reiss and Jenny Chapman
Excerpt
More information
Introduction to environmental biology 3
research on single-celled ciliates in the genus The term ‘community’ is a valuable one in ecol-
Paramecium. ogy. However, in 1935 Sir Arthur Tansley invented
A population is a group of individuals within a the term ecosystem because he realised that the
species that have the opportunity to breed with organisms that make up a community cannot
one another because they live in the same area at realistically be considered independently of their
the same time. It follows from this definition that physical environment. The term ecosystem, there-
individuals from two different species cannot fore, applies to a community of organisms and its
belong to the same population. This is because, associated physical environment.
with occasional exceptions, species are reproduc- There is one other feature of ecosystems and
tively isolated from one another. Tawny owls do their associated communities worth stressing.
not breed with short-eared owls, for example. This is that ecosystems are dynamic. Indeed, some
Most species are divided into many populations ecosystems change as new species invade and
that are geographically separated. Bluebells in one others die out. A grassland invaded by shrubs and
wood, for example, will belong to a different trees will change gradually as scrubland and then
population from the bluebells in another wood woodland develops. In a mature ecosystem, such
several kilometres away. Indeed, in a large wood as oak woodland, the population sizes and activi-
there may be several populations of bluebells, ties of the different species will alter from season
though the boundaries between populations may to season and year to year. The bluebells in figure
be somewhat arbitrary. 1.1 flower so beautifully in spring, but by late
A communityis an association of species that summer they have set seed, the leaves have died
live together in some common environment or back and the bluebell bulbs are ready to lie
habitat. Most communities are composed of a mix- dormant until the next spring.
ture of prokaryotes, protoctists, fungi, plants and
animals. The organisms in a community interact SAQ 1.1
with one another in all sorts of ways. For a start, Arrange the following terms in a hierarchy of
there will be feeding relationships. In most com- descending size and complexity: community, habitat,
munities, autotrophs (also known as producers ecosystem, microhabitat.
and comprising green plants, photosynthetic
algae, photosynthetic bacteria and chemosynthetic
bacteria) provide food for herbivores (also known SAQ 1.2
as primary consumers). In turn, herbivores are What parts of an ecosystem are also found in its
eaten by first-level carnivores (also known as community?
secondary consumers), and these may be eaten by
second-level carnivores (or tertiary consumers).
Eventually organisms die and their remains are Humans in the environment
broken down by decomposers. These feeding
relationships can be represented by food chains or We have given ourselves a Latin binomial, Homo
by food webs that show the interrelationships sapiens, just like all the other species we have
between the various food chains in a community. classified. However, it is obvious that the impact
The species in a community also interact with humans have on the environment is unlike that
one another in other ways. They may rely on one of any other species. Ancient humans evolved in
another for reproduction, as is the case in insect- Africa and migrated out into Asia and Europe a
pollinated plants. Or one species may act as a million or more years ago. A second wave of
home for another, as a humpback whale carries migration of modern humans spread out of Africa
barnacles. Or the interaction may be more subtle about 130000 years ago resulting in the colonisa-
– all the species in a woodland, for example, rely tion of every continent.
on the activities of the various soil organisms Before humans evolved, of course, all the com-
which recycle nutrients. munities in the world were natural. In Britain,
© Cambridge University Press www.cambridge.org
Cambridge University Press
978-0-521-78727-7 - Environmental Biology
Michael Reiss and Jenny Chapman
Excerpt
More information
4 Introduction to environmental biology
a b
overshoot
carrying carrying
capacity capacity
Number in populationexponential growth exponential growth
Log of number in population
0 Time 0 Time
● Figure 1.3 Population growth. a Normal plot of population increasing exponentially
until the carrying capacity is reached, when the population stabilises. b A log-normal
plot of the same data as a. With population plotted on a log scale an exponential curve is
represented by a straight line.
natural vegetation during the Ice Ages was tree- species, the population initially grows at a rate of
less Arctic grassland called tundra; during increase related to the reproductive rate of the
warmer interglacials, after the ice sheets melted, species. Plotting the log of the numbers of
trees invaded. In the south, forests of oaks, ash, individuals in the population against a linear plot
lime and hornbeam grew; in the Scottish high- of time gives a straight line (figure 1.3b). Eventually
lands, the main vegetation was Scots pine conifer there will be competition for resources that are in
forest. These mature forests are called the climax limited supply. This competition is intraspecific
vegetation, but such vegetation is now rare due to because it occurs between individuals belonging
human activities. to the one species. The result of this increasing
Humans learned to make and use fire early in competition is that the population growth slows
their history, about half a million years ago – very down. Eventually the population should reach the
useful during Ice Ages! Before this, only lightning maximum size that the environment can sustain,
started wildfires that had the potential to damage a figure known as the environment’s carrying
vegetation. About 10000 years ago humans also capacity. The population may overshoot the
began to change the natural vegetation by cultivat- numbers the environment can support, but will
ing crops. Animals were domesticated at about the then fall to stabilise at the carrying capacity.
same time. Captive animals graze areas of vegeta- However, human population biology is more
tion in much greater densities than natural animal complex, and seems to have gone through differ-
populations do. The practice of burning and ent phases of growth. As you can see from figure
grazing led to the vegetation in many areas of the 1.4, anthropologists and archaeologists think that
world developing into grasslands (see page 10). As the world’s human population was stable, or only
human populations grew, their dwellings – in rising very slowly, up to about 10000 years ago
villages, towns and then cities – also restructured and that it was rather small – somewhere between
or even destroyed the vegetation. 5 and 10 million. Archaeological evidence
As humans are animals, human population indicates that about 10000 years ago the popula-
biology might be expected to follow the same tion started to rise more rapidly; there was a
rules as those of other animal populations (figure change in the rate of increase of population and
1.3a and Biology 2, chapter 3). In other animal in the carrying capacity. This reflects the change
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