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4
Ecology
ESSENTIAL IDEAS
n
The continued survival of living organisms including humans depends on sustainable
communities.
n Ecosystems require a continuous supply of energy to fuel life processes and to replace
energy lost as heat.
n Continued availability of carbon in ecosystems depends on carbon cycling.
n Concentrations of gases in the atmosphere affect the climates experienced at the Earth’s
surface.
Ecology is the study of living things in their environment. It is an essential component of
modern biology. Understanding the relationships between organisms and their environment
is just as important as knowing about the structure and physiology of animals and plants.
Environment is a term we commonly use to mean ‘surroundings’. In biology, we talk about the
environment of cells in an organism, or the environment in which the whole organism lives.
So ‘environment’ is a rather general, unspecific term – but useful, nonetheless.
4.1 Species, communities and ecosystems
– the
continued survival of living organisms including humans depends on sustainable
communities
■■ Species
There are vast numbers of different types of living organism in the world – almost unlimited
diversity, in fact. Up to now, about 2 million species have been described and named in total.
But what we mean by ‘species’?
By the term ‘species’ we refer scientifically to a particular type of living thing. A species is
a group of individuals of common ancestry that closely resemble each other and are normally
capable of interbreeding to produce fertile offspring.
There are three issues to bear in mind about this definition.
n Some (very successful) species reproduce asexually, without any interbreeding at all.
Organisms that reproduce asexually are very similar in structure, showing little variation
between individuals.
n Occasionally, members of different species breed together. However, where such crossbreeding occurs, the offspring are almost always infertile.
n Species change with time; new species evolve from other species. The fact that species do change
means they are not constant always easy to define. However, evolutionary change takes place
over a long period of time. On a day-to-day basis, the term ‘species’ is satisfactory and useful.
So, a species is a group of organisms that is reproductively isolated, interbreeding to produce
fertile offspring. Organisms belonging to a species have morphological (structural) similarities,
which are often used to define the species.
■■ Populations, communities and ecosystems
Members of a species may be reproductively isolated in separate populations. A population
consists of all the individuals of the same species in a habitat at any one time. The members of a
population have the chance to interbreed, assuming the species concerned reproduces sexually.
The boundaries of populations are often hard to define, but those of aquatic organisms living in
a small pond are clearly limited by the boundary of the pond (Figure 4.1).
A community consists of all the living things in a habitat – the total of all the populations,
in fact. So, for example, the community of a well-stocked pond would include the populations of
rooted, floating and submerged plants, the populations of bottom-living animals, the populations
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4.1 Species, communities and ecosystems 185
of fish and non-vertebrates of the open water, and the populations of surface-living organisms –
a very large number of organisms, in fact.
Finally, a community forms an ecosystem by its interactions with the non-living (abiotic)
environment. An ecosystem is defined as a community of organisms and their surroundings,
the environment in which they live and with which they interact. An ecosystem is a basic
functional unit of ecology, since the organisms that make up a community cannot realistically
be considered apart from their physical environment.
Examples such as a woodland or a lake illustrate two important features of an ecosystem,
namely that it is:
n a largely self-contained unit, since most organisms of the ecosystem spend their entire lives
there and their essential nutrients are endlessly recycled around and through it
nan interactive system, in that the kinds of organism that live there are largely decided by the
physical environment and in that the physical environment is constantly altered by the organisms.
The organisms of an ecosystem are called the biotic component, and the physical environment
is known as the abiotic component.
Within any ecosystem, organisms are normally found in a particular part or habitat. The
habitat is the locality in which an organism occurs. So, for example, within a woodland, the
tree canopy is the habitat of some species of insects and birds, while other organisms occur in
the soil. Within a lake, habitats might include a reed swap and open water. Incidentally, if the
occupied area is extremely small, we call it a microhabitat. The insects that inhabit the crevices
in the bark of a tree are in their own microhabitat. Conditions in a microhabitat are likely to be
very different from conditions in the surrounding habitat.
■■ The pond or lake as an ecosystem
Figure 4.1 represents a transect through a fresh water ecosystem – a pond or lake. Notice that
a range of habitats within the ecosystem are identified, and that the feeding relationships of
the community of different organisms are highlighted. We will consider feeding relationships
between organisms next.
■■ Figure 4.1
A pond or lake as
an ecosystem
energy from sunlight
green
plants
are
producers
submerged
stems provide a
microhabitat for
algae growing
on them
rooted plants
floating
plants
plankton
herbivores
eat plants
carnivores
consumers eat animals
detritivores
eat dead
organic
matter
reed swamp
of margin
examples
of habitats
open surface
water
mud deposited
on pond bottom
decomposers on
surface of mud
sediment
containing
nutrient
reserve
detritus
feeding
fish on
pond mud
1 Apply one or more of the terms shown below to describe each of the listed features of a fresh water lake.
population ecosystem habitat abiotic factor community biomass
a the whole lake
b all the frogs of the lake
c the flow of water through the lake
d all the plants and animals present
e the total mass of vegetation growing in the lake
f the mud of the lake
g the temperature variations in the lake
04_01 Biology for the IB Diploma Second edition
Barking Dog Art
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186 4 Ecology
■■ Feeding relationships – producers, consumers and decomposers
Think of an ecosystem with which you are very familiar. Perhaps it is one near your home,
school or college. It might be savannah, a forest, a lake, woodland or meadow. Whatever you
have in mind, it will certainly contain a community of plants, animals and microorganisms, all
engaged in their characteristic activities. Some of these organisms will be much easier to observe
than others, possibly because of their size, or the times of day (or night) at which they feed, for
example.
The essence of survival of organisms is their activity. To carry out their activities organisms
need energy. We have already seen that the immediate source of energy in cells is the molecule
ATP (page 115), which is produced by respiration. The energy of ATP has been transferred
from sugar and other organic molecules – the respiratory substrates. These organic molecules are
obtained from nutrients as a result of the organism’s mode of nutrition.
We know that green plants make their own organic nutrients from an external supply of
inorganic molecules, using energy from sunlight in photosynthesis (page 121). The nutrition of
a typical green plant is described as autotrophic (meaning ‘self-feeding’) and, in ecology, green
plants are known as producers. There are a very few exceptions to this (Figure 4.2).
An autotroph is an organism that synthesizes its organic molecules from
simple inorganic substances
Nature of Science
Looking for patterns, trends and discrepancies
Broomrape (Orobanche sp.) is a ‘root parasite’, attaching to
the root systems of its various host plants, below ground.
Above ground, the shoots are virtually colourless
(chlorophyll-free), and the leaves reduced to small bracts.
Why ?
Once established, the plant is seen to concentrate on
reproduction, seed production and seed dispersal. This
suggests that the task of reaching fresh hosts is a major
challenge in the life-cycle of a parasite. Which of these
features are evident in the plant shown here?
■■ Figure 4.2 Not quite all green plants are autotrophic
■■ Classifying a species from a knowledge of its mode of nutrition
1 Autotrophs versus consumers
So, the great majority of green plants are entirely autotrophic in their nutrition. In this they play
a key part in food chains, as we shall shortly see. In contrast to green plants, animals and most
other types of organism use only existing nutrients, which they obtain by digestion and then
absorption into their cells and tissues for use. Consequently, animal nutrition is dependent on
plant nutrition, either directly or indirectly. In ecology, animals are known as consumers and
animal nutrition is described as heterotrophic (meaning ‘other nutrition’).
n A heterotroph is an organism that obtains organic molecules from other organisms.
n A consumer is an organism that ingests other organic matter that is living or recently killed.
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4.1 Species, communities and ecosystems 187
■■ Figure 4.3
False-colour
micrograph of
Euglena, a species
that is both
autotrophic and
heterotrophic
In heterotrophic nutrition, bacteria are taken into food
vacuoles by phagocytosis and the contents digested by
hydrolytic enzymes from lysosomes. Find the Golgi
apparatus and lysosomes in the cytosol.
In autotrophic nutrition, photosynthesis occurs in the
chloroplasts. There is a light-sensitive ‘eyespot’ present
which enables Euglena to detect the light source.
Notice the plasma membrane has a ridged appearance
here – this arrangement is supported by a system of
microtubules below.
Note that some of the consumers, known as herbivores, feed directly and exclusively on
plants. Herbivores are primary consumers. Animals that feed exclusively on other animals are
carnivores. Carnivores that feed on primary consumers are known as secondary consumers.
Carnivores that feed on secondary consumers are called tertiary consumers, and so on.
2 Detritivores and saprotrophs
2 Construct a
dichotomous key in
the form of a flow
chart, classifying
species on the basis
of their alternative
modes of nutrition.
Eventually, all producers and consumers die and decay. Organisms that feed on dead plants and
animals, and on the waste matter of animals, are described as saprotrophs (meaning ‘putrid
feeding’) and, in ecology, these feeders are known as detritivores or decomposers.
n A saprotroph is an organism that lives on or in dead organic matter, secreting digestive
enzymes into it and absorbing the products of digestion.
n A detritivore or a decomposer is an organism that ingests dead organic matter.
Feeding by saprotrophs releases inorganic nutrients from the dead organic matter, including
carbon dioxide, water, ammonia, and ions such as nitrates and phosphates. Sooner or later, these
inorganic nutrients are absorbed by green plants and reused. We will look in more detail at the
cycling of nutrients in the biosphere later in this chapter.
■■ Practical ecology: Testing for associations between species
The distribution of two or more species in a habitat may be entirely random. Alternatively, factors
such as specific abiotic conditions may bring about close association of some species – plant
A may tend to grow close to plant B. For example, soils rich in calcium ions typically support
distinctively different populations from those found on dry acid soils. If we want to discover
whether there is a particular association between two species in a habitat, we need reliable data
on their distribution; this is obtained by random sampling. In this way, every individual in the
community has an equal chance of being selected and so a representative sample is assured.
Quadrats are commonly used to study populations and communities. A quadrat is a square
frame which outlines a known area for the purpose of sampling. The choice of size of quadrat
varies depending on the size of the individuals of the population being analysed. For example,
a 10 cm² quadrat is ideal for assessing epiphytic Pleurococcus, a single-celled alga, commonly
found growing on damp walls and tree trunks. Alternatively, a 1 m² quadrat is far more useful
for analysing the size of two herbaceous plant populations observed in grassland, or of the
earthworms and the slugs that can be extracted from between the plants or from the soil below.
Quadrats are placed according to random numbers, after the area has been divided into a grid
of numbered sampling squares (Figure 4.4). The presence or absence in each quadrat of the two
species under investigations is then recorded. The data is then subjected to statistical test. The
chi-squared (χ2) test is used to examine data that falls into discrete categories – as it does in this
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188 4 Ecology
case. It tests the significance of the deviations between numbers observed (O) in an investigation
and the number expected (E). The measure of deviation, known as chi-squared, is converted into
a probability value using a chi-squared table. In this way, we can decide whether the differences
observed between our sets of data are likely to be real or, alternatively, obtained just by chance.
■■ Figure 4.4 Random
locating of quadrats
100
90
1 A map of the habitat (e.g. meadowland) is
marked out with gridlines along two edges
of the area to be analysed.
80
70
y axis
60
50
40
30
20
10
2 Coordinates for placing quadrats are
obtained as sequences of random numbers,
using computer software, or a calculator,
or published tables.
0 10 20 30 40 50 60 70 80 90 100
x axis
100
90
80
70
y axis
60
3 Within each quadrat, the individual species
are identified, and then the density,
frequency, cover or abundance of each
species is estimated.
50
40
30
20
10
0 10 20 30 40 50 60 70 80 90 100
x axis
4 Density, frequency, cover, or abundance
estimates are then quantified by measuring
the total area of the habitat (the area
occupied by the population) in square metres.
The mean density, frequency, cover or
abundance can be calculated, using the
equation:
mean density (etc.) per quadrat × total area
population size =
area of each quadrat
■■ Recognizing and interpreting statistical significance
Two moorland species and the chi-squared test
This example examines whether the moorland species bell heather (Erica onerea) and common
heather, also known as ling (Calluna vulgaris), tend to occur together. Moorlands are upland
areas with acidic and low-nutrient soils, where heather plants dominate. Heathers have long
woody stems and grow in dense clumps. They have colourful, bright flowers. The question here
is whether there is a statistically significant association between ling and bell heather on an
area of moorland. As scientists we would carry out a statistical test to work out the probability of
getting results that indicate there is no association between the two species – indicating the null
hypothesis is true. The null hypothesis in this example would be that there is no statistically
significant association between bell heather and ling in an area of moorland; that is, their
distributions are independent of each other. If our results do not support the null hypothesis,
then there is an association.
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4.1 Species, communities and ecosystems 189
Ling Calluna vulgaris
Bell heather Erica tetralix
■■ Figure 4.5 A moorland ecosystem and two common plants found there
1 The measurements and results
In order to sample the two species, the presence or absence of each species was recorded in each
of 200 quadrats. The quadrats were located at random on a 100 m by 100 m area of moorland
(Table 4.1).
■■ Table 4.1
Observed
results – the
distribution of ling
and bell heather
Ling present
Ling absent
Bell heather present
 89
 31
120
Bell heather absent
45
35
80
Total
134
 66
200
2 The calculations
aExpected results: assuming that the two species are randomly distributed with respect to
each other, the probability of ling being present in a quadrat is:
column total/total number of quadrats
134
= 200
= 0.67
Similarly, the probability of bell heather being present in a quadrat is:
120
200
= 0.60
The probability of both species occurring together, assuming random distribution between
each species, is: 0.60 × 0.67 = 0.40. The number of quadrats in which both species can be
expected is therefore 0.40 × 200 = 80.
Having worked out the number of expected quadrats where the species are found together,
other expected values can be calculated by subtracting from the totals. For example, the
expected number of quadrats with bell heather but no ling is 120 − 80 = 40. Expected
values follow the assumption that totals for each row and column do not change, because
the relationship shown by the data is assumed to represent the true relative frequency of
each species (Table 4.2).
■■ Table 4.2 The full
expected results
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Ling present
Ling absent
Bell heather present
 80
 40
120
Bell heather absent
54
26
80
Total
134
 66
200
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190 4 Ecology
Now the calculated values can be checked by using the ratios represented in the table of
observed results (Table 4.1).
For example, the expected number of quadrats where there is no ling and no bell heather
can be calculated as follows:
66
Probability of no ling in a quadrat =
= 0.33
200
80
Probability of no bell heather in a quadrat =
= 0.40
200
Probability of neither species in a quadrat = 0.33 × 0.40 = 0.13
Number of expected quadrats with neither species present = 0.13 × 200 = 26
(Note that this figure agrees with the estimated value in Table 4.2).
b Statistical test: the observed and expected results are recorded in Table 4.3.
■■ Table 4.3
Observed (O)
and expected (E)
distribution of ling
and bell heather
Ling present
Ling absent
O
E
O
E
Bell heather present
 89
 80
 31
 40
120
Bell heather absent
45
54
35
26
80
Total
134
 66
200
Then, chi squared is calculated from the formula:
χ2 = ∑
(O – E)2
E
So, chi squared in this example
(89 – 80)2 (45 – 54)2 (31 – 40)2 (35 – 26)2
=
+
+
+
80
54
40
26
= 1.01 + 1.50 + 2.03 + 3.11
= 7.65
To find whether this result is statistically significant or not, the value must be compared to
a critical value (Table 4.4). To locate the critical value, the appropriate degrees of freedom
need to be calculated.
Degrees of freedom = (number of columns – 1) × (number of rows – 1)
In this case, degrees of freedom = (2 – 1) × (2 – 1) = 1
■■ Table 4.4
Critical values for
the χ2 test
Degrees of freedom
1
2
3
4
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0.05 level of significance
3.84
5.99
7.81
9.49
The chi-squared value of 7.65 is larger than the critical value of 3.84 for 1 degree of freedom
at the probability level of p = 0.05 (the 5% probability level). The null hypothesis is therefore
rejected; there is a statistically significant association between bell heather and ling in this area
of moorland. So, the distributions of the two species are not independent of each other – the
distribution of the two species is associated.
c The value of chi squared may also be obtained using a programmed pocket calculator or a
computer program such as:
www.socscistatistics.com/tests/chisquare/Default2.aspx
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4.1 Species, communities and ecosystems 191
3 Carry out a χ2 test to see if there is an association between bell heather and bilberry from the observed
results shown in the table.
From your calculations, deduce whether the two species are associated or whether they tend to occupy
different microhabitats on this moorland.
Bilberry present
Bilberry absent
Bell heather present
 12
 88
100
Bell heather absent
 55
 45
100
Total
 67
133
200
■■ The need for sustainability in human activities
We have noted that ecosystems are largely self-contained and self-sustaining units. They
have the potential to maintain sustainability over long periods of time. Most organisms of
an ecosystem spend their entire lives there. Here, their essential nutrients will be endlessly
recycled around and through them. This illustrates a key feature of environments – that they
naturally self-regulate. The basis of sustainability is the flow of energy through ecosystems
and the endless recycling of nutrients. This is summarized in Figure 4.6, and is the focus of
Section 4.2.
Unfortunately, we humans often destabilize ecosystems. This is a result of our presence
in large numbers over much of the globe, and our profligate use of space and resources. Our
demands for food for expanding populations, and for materials and minerals for homes and
industries tend to destroy ecosystems.
Today, the impact of humans on the environment is very great indeed. Conservation
attempts to manage the environment so that, despite human activities, a balance is maintained.
The aims are to preserve and promote habitats and wildlife, and to ensure natural resources are
used in a way that provides a sustainable yield. Conservation is an active process, not simply
a case of preservation, and there are many different approaches to it. More effective family
planning and population control in human communities could be a highly significant factor in
some areas of the world.
4 For an ecosystem near your home, school or college, list the ways in which you feel the human
community has adversely changed the environment.
Can you suggest a practical way that this harm could be reduced or reversed?
Investigating the self-sustainability of ecosystems – using mesocosms
The sustainability of an ecosystem may change when an external ‘disturbing’ factor that
disrupts the natural balance is applied. Investigation of this may be attempted in natural
habitats or in experimental, enclosed systems. Both approaches have advantages and
drawbacks (Table 4.5).
■■ Table 4.5
Alternative
approaches to
investigating
ecosystem
sustainability
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Advantages
Disadvantages
A natural ecosystem, for example an
entire pond or lake
realistic – actual environmental conditions
are experienced
variable conditions – minimum or nonexistent control over ‘controlled variables’
A small-scale laboratory model aquatic
system (a mesocosm)
able to control variables – opportunity to
measure the degree of stability or extent of
change in a community, and to investigate
the precise impact of a disturbing factor
unrealistic – possibly of disputed relevance
and applicability to natural ecosystems
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192 4 Ecology
Case study: An investigation of eutrophication
In water enriched with inorganic ions (such as from raw sewage or fertilizer ‘run-off’ from
surrounding land), plant growth is typically luxuriant. The increase in concentration of
ammonium, nitrate and phosphate ions particularly increases plant growth. When seasonal
water temperature rises, the aquatic algae undergo a population explosion – causing an algal
bloom, for example. This process is known as eutrophication.
Later, when the algal bloom has died back, the organic remains of the plants are decayed by
saprotrophic aerobic bacteria. The water becomes deoxygenated, and anaerobic decay occurs
with hydrogen sulfide. A few organisms can survive in these conditions and prosper, but the
death of many aquatic organisms results.
Can mesocosms be set up to investigate eutrophication, so avoiding the destruction of a
natural ecosystem?
■■ Figure 4.6
An experimental
mesocosm apparatus
data logging/
recording device
port for sampling for algal
density (as a dependent
variable) and point of entry of
additional phosphate ions (in
solution) (independent variable)
port for probes to
measure temperature,
light (controlled variables)
and O2 concentration (as
a dependent variable)
Possible steps to the investigation
– what ‘control’ flask is required?:
1 Set up of mesocosms A (experiment) and B
(control) with identical cultures of algal
suspensions in pond water. Allowed to stabilize,
and give evidence of normal algal growth
2 Addition of a quantity of concentrated
phosphate solution to A
?
light source/identical
light/dark regimes
magnetic stirrer
A Experiment mesocosm
B Control mesocosm
What would the control flask require?
3 Regular monitoring of change in algal cell
density and O2 concentration in A and B
mesocosms
Issues: Does an algal bloom develop?
How do the patterns of algal cell densities and O2
concentration change with time?
Look at the apparatus in Figure 4.6.
n Does the figure show an appropriate practical investigation of eutrophication under
controlled laboratory conditions? What changes might be made?
n Here, two dependent variables have been proposed. Why?
n If the additional phosphate ions added to mesocosm (A) resulted in an algal bloom, how
04_06 Biology for the IB Diploma Second
couldedition
the control (B) be arranged to establish that influx of phosphate ions caused it?
Barking Dog Art
n How would you expect the oxygen concentrations to change over an extended period in both
mesocosms (A) and (B)?
■■ Cycling of nutrients
Nutrients provide the chemical elements that make up the molecules of cells and organisms. We
recognize that all organisms are made of carbon, hydrogen and oxygen, together with mineral
elements nitrogen, calcium, phosphorus, sulfur and potassium, and several others, in increasingly
small amounts. Plants obtain their essential nutrients as carbon dioxide and water, from which
they manufacture sugar. With the addition of mineral elements, absorbed as ions from the soil
solution, they build up the complex organic molecules they require (Figure 2.65, page 121).
Animals, on the other hand, obtain nutrients as complex organic molecules of food which they
digest, absorb and assimilate into their own cells and tissues.
Recycling of nutrients is essential for the survival of living things, because the available
resources of many elements are limited. When organisms die, their bodies are broken down to
simpler substances (for example, CO2, H2O, NH3 and various ions), as illustrated in Figure 4.7.
Nutrients are released.
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5 Explain how it
is that animal life
is dependent on
the actions of
saprotrophs.
dead animal
4.1 Species, communities and ecosystems 193
Scavenging actions of detritivores often begin the process of breakdown and decay, but
saprotrophic bacteria and fungi always complete the breakdown. Elements that are released may
become part of the soil solution, and some may react with chemicals of soil or rock particles,
before becoming part of living things again through reabsorption by plants. Ultimately, both
plants and animals depend on the activities of saprotrophic microorganisms to release matter
from dead organisms for reuse.
The complete range of recycling processes by which essential elements are released and
reused involve both living things (the biota) and the non-living (abiotic) environment. The
latter consists of the atmosphere, hydrosphere (oceans, rivers and lakes) and the lithosphere
(rocks and soil). All the essential elements take part in such cycles. One example is the carbon
cycle (page 201).
In summary, the supply of nutrients in an ecosystem is finite and limited. By contrast, there
is a continuous, but variable, supply of energy in the form of sunlight. This we focus on next.
1 break up
of animal body by
scavengers and detritivores,
e.g. carrion crow, magpie, fox
1 break up
of plant body by detritivores,
e.g. slugs and snails, earthworms,
wood-boring insects
2 succession of microorganisms
– mainly bacteria, feeding:
• firstly on simple nutrients
such as sugars, amino acids,
fatty acids
• secondly on polysaccharides,
proteins, lipids
• thirdly on resistant molecules
of the body, such as keratin
and collagen
2 succession of microorganisms
– mainly fungi, feeding:
• firstly on simple nutrients
such as sugars, amino acids,
fatty acids
• secondly on polysaccharides,
proteins, lipids
• thirdly on resistant molecules
such as cellulose and lignin
3 release of simple
inorganic molecules
such as CO2, H2O, NH3,
ions such as Na+, K+,
Ca2+, NO3–, PO4–,
all available to be
reabsorbed by plant
roots for reuse
dead plant
■■ Figure 4.7 The sequence of organisms involved in decay
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194 4 Ecology
food chain as pyramid of biomass showing energy flow
(Note: materials are recycled)
cycling of materials
flow of chemical energy
ap
rot
rop
hs
■■ Figure 4.8
Cycling of nutrients
and the flow of
energy within
an ecosystem – a
summary
Energy enters the food chain from sunlight
and leaves as heat energy lost to space.
de
tri
tiv
ore
sa
nd
s
flow of energy as light or heat
secondary
consumers
carnivores
primary consumers
herbivores
sunlight
primary producers
green plants
inorganic matter
4.2 Energy flow – ecosystems require a continuous supply of energy to fuel
life processes and to replace energy lost as heat
■■ Most ecosystems rely on a supply of energy from sunlight
We can demonstrate the dependence of ecosystems on sunlight by drawing up food chains
Drawing up a food chain
Look at Figure 4.9. A feeding relationship in which a carnivore eats a herbivore, which itself has
eaten plant matter, is called a food chain. In Figure 4.9, light is the initial energy source, as it is
in most other food chains. Note that, in a food chain, the arrows point to the consumers and
so indicate the direction of energy flow. Food chains from contrasting ecosystems are shown in
Figure 4.10.
■■ Figure 4.9
A food chain
oak
Quercus robur
oak beauty caterpillar
Biston strataria
caterpillar-hunting beetle
Carabus nemoralis
common shrew
Sorex araneus
red fox
Vulpes vulpes
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