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10
Ecosystems:
CHAPTER
● Earth
● Biosphere
● Biome
● Ecosystem
● Community
● Population
● Organism
● Systems
● Organs
● Tissuses
● Cells
● Organelles
● Molecules
● Atoms
relationships
and populations
Chapter 10 Relationships
and populations
Key knowledge
Components of ecosystems: communities of living organisms, ecological groupings; ecological niche
● Relationships between organisms: feeding including parasite/host, predator/prey; of mutual benefit
including mutualism and symbiosis
● Population dynamics: carrying capacity of ecosystems; factors affecting distribution and abundance of
●
organisms, including birth and death rates, migration
● Techniques for monitoring and maintaining ecosystems
A satellite image of the Earth shows an orb with swirls of white covering misty
continents. Shining through is the intense blue of the oceans. This is our Earth,
our place in the universe.
Organisms find their place to live where water and ice (hydrosphere), land
(lithosphere) and air (atmosphere) interact. This dynamic, ever-changing layer around
the Earth is about 20 km thick, from the bottom of the oceans to the highest peaks of
the mountains. It is called the biosphere. Nowhere is there conclusive evidence that
living things exist elsewhere in the universe.
Enhanced views of Earth from space present
patterns of browns and blues that identify
large terrestrial and aquatic biomes.
Each biome has features that
distinguish it from another.
Terrestrial biomes
roughly correspond to
major variations in
vegetation types,
climate, topography
(surface features,
altitude) and soil
type. Tropical
rainforests,
temperate
deciduous forests,
deserts, grasslands
and tundra are
examples of major
terrestrial biomes.
Figure 10.1 The Earth
from space.
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80°N
60°N
40°N
Tropic of Cancer
Equator
Tropic of Capricorn
40°S
Tundra
Temperate forest
Chaparral and/or
evergreen hardwood (Mediterranean)
Desert
Temperate grassland
Tropical rainforest
Taiga (coniferous forest)
STU
Figure 10.2 Distribution of
the world’s major terrestrial
biomes.
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Terrestrial biomes
bioTERMS
biodiversity
the variety of different living
things: the different plants,
animals and microorganisms,
their genes and the ecosystems
of which they are a part
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Savanna grassland
Other biomes (ice, mountains, semi-arid)
Just as the biosphere is composed of biomes that are different from each other,
patterns of ecosystems in the whole of Australia can be distinguished. Biogeographic
areas, or bioregions, are distinguished by ecological characteristics of the landscape or
seascape. They can also be related to the patterns of use of land and sea.
Lakes, oceans and rocky shores are examples of aquatic biomes. How permanent
the body of water is, its salinity, depth and availability of nutrients all influence the
kinds of organisms that inhabit them and the relationships between them.
The national terrestrial bioregions have been broken down into 21 Victorian
terrestrial bioregions as shown in Figure 10.3. Another five bioregions occur
in Victorian waters. Bioregional networks are being established to facilitate
partnerships between the wide range of land managers who share responsibilities for
biodiversity conservation. The land managers are rather like the managers of a chain
of supermarkets.
Chapter 10 Relationships
and populations
LM
Lowan Mallee
GO
Goldfields
OR
Otway Ranges
MM
Murray Mallee
GVU
Central Victorian Uplands
WP
Warrnambool Plain
WI
Wimmera
NIS
Northern Inland Slopes
OP
Otway Plain
VP
Victorian Volcanic Plain
VR
Victorian Riverina
GIP
Gippsland Plain
GP
Glenelg Plain
VA
Victorian Alps
WPR
Wilsons Promontory
DT
Dundas Tablelands
HS
Highlands – Southern Fall
EGU
East Gippsland Uplands
GG
Greater Grampians
HN
Highlands – Northern Fall
EGL
East Gippsland Lowlands
OTW
Otway (Marine)
CV
Central Victoria (Marine)
VE
Victorian Embayments (Marine)
FLI
Finders (Marine)
TWO
Twofold Shelf (Marine)
Figure 10.3 Bioregions
of Victoria.
Ecologists study the relationships between living things and their surroundings. This
is called ecology (from the Greek oikos for home and logos for study), a term first used
by Ernst Heinrich Heinkel in 1869. In the early days, ecology was largely descriptive,
providing qualitative data based on observational studies. It has become a more exact
science since measurements providing quantitative data have contributed to the
development of models and theories that help us understand relationships better.
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Ecology
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Bioregions
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Figure 10.4 Ernst Heinrich Heinkel
(1888–1958).
For nearly two centuries of European settlement people’s ideas of relationships
between themselves, their surroundings and other living things tended to be restricted to a
European view. The rich knowledge that indigenous peoples had of interactions between
the environment and the living things that inhabit their world was largely ignored.
Fortunately this is changing. There is increased recognition of the need to integrate both
perspectives and areas of knowledge to understand our unique environment better.
Today, with increased urbanisation, many people have little opportunity to develop
first-hand understanding of relationships ‘in the wild’. Our relationships tend to be
with our immediate surroundings, each other and, if we have them, our pets. Most of
these relationships, direct and indirect, are to do with our wellbeing.
Many people obtain what they need from a local store or supermarket. They learn
where various items are; they may be annoyed if they cannot find something but at
least an assistant can help. Some of you may work at packing shelves or at a checkout.
Everyone expects facilities to support them; they don’t expect the refrigerators to fail,
the doors to get stuck or suppliers to fail to deliver the goods.
A supermarket is an example of a system in which there are inputs, processing and
outputs. Complex interactions occur between the people who are part of the system.
They make sure that the supermarket runs smoothly, that supplies meet demand and
people obtain what they need.
Ecosystem
bioTERMS
ecosystem
a self-sustaining unit made up
of living things (a community)
interacting with and within a
particular habitat
interactions
interplay or association
between organisms of the
same or different species
community
the sum of all the living
organisms living in a habitat
at a particular time
It is much the same with an ecosystem. An ecosystem is not really a place, although
we tend to use the word in that way. The concept of an ecosystem is useful; it provides
a framework for studying the interactions between living things and their non-living
surroundings, usually referred to as their environment. For example, a forest ecosystem
includes not only all the organisms that make up the community living there, but also
aspects of the physical environment, such as rain, the inorganic minerals of the soil,
sunlight and atmospheric oxygen and carbon dioxide. All these factors interact with
each other in a system we call an ecosystem.
habitat + community = ecosystem
The English ecologist Sir Arthur Tansley first used the term ‘ecosystem’ in 1935.
He realised that organisms and their immediate environment need to be considered
together as a functional unit. For example, it is easier to understand why a platypus
lives the way it does if it is studied in relation to the conditions in the stream that
it lives in, the organisms that it eats and its relationships with the plants and other
animals in its immediate surroundings.
REVIEW
1 Distinguish between the following terms:
a biosphere and biome
b ecology and ecosystem
c habitat and community
d biotic and abiotic factors.
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2 Draw a concept map that shows the relationship between the
following: biosphere, habitat, ecosystem, community, biome,
environmental factors.
3 Distinguish between qualitative and quantitative data.
Chapter 10 Relationships
and populations
Describing and naming ecosystems
An ecosystem is usually named after the most dominant species in the community,
together with its overall appearance or ‘layout’. Examples of individual ecosystems
include river red gum woodland, mangrove swamp, alpine herb field, wetlands,
spinifex grassland, coral reefs, heath land, rock pools and caves. All ecosystems of the
same type have essentially the same features – rather like the recognisable stores of a
particular chain of supermarkets. A coral reef, for example, will have more features in
common with every other coral reef than it does with a woodland.
As with biomes, the distribution of ecosystems depends on factors such as soil type
and climate, particularly temperature and rainfall. Can you see why the ecosystem
types roughly correspond to the vegetation types shown in Figure 10.6.
Vegetation is classified according to:
• the percentage of ground shaded or covered by the tallest layer of vegetation
• the form (tree, shrub, grass) of the tallest layer
Open forests have between 30 and 70% of the ground shaded by trees between
10 m and 30 m tall. If the trees are taller, the forest is described as a tall open forest.
If the percentage cover is between 10 and 30%, it is a woodland.
Figure 10.5 Some Australian ecosystems.
bioTERMS
dominant species
the most common or most
obvious species in a particular
community
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Rainforest
Heath
Wet sclerophyll forest
Shrubland
Dry sclerophyll forest
Grassland
Woodland
STU
Figure 10.6 Distribution
of vegetation types in
Australia.
DE
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Vegetation types
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Ecosystems are relatively self-contained. An ecosystem tends to support itself by
cycling or exchanging materials within that ecosystem. In a forest, for instance, leaves
fall and decompose and their nutrients are returned to the soil. In turn, the plants
remove these nutrients from the soil and use them in growth. But this is a fairly simple
explanation of how an ecosystem operates.
If you travel from one ecosystem to the next it may not be easy to notice where
one ecosystem stops and another begins. In ecosystems next to each other, physical
conditions, such as soil type or temperature, gradually change or merge; the
ecosystems overlap. Ecosystems are seldom closed. Some of the animals, such as birds,
will be a part of two or more different communities as they move from one area to
another; the ecosystems are open. Despite this difficulty, the concept of the ecosystem
has proved very useful to ecologists.
Chapter 10 Relationships
and populations
Table 10.1 The ground cover features of the major Australian ecosystems
Ecosystem
Ground cover
Desert
In patches, widely spaced
Grasslands
In arid regions typically hummocks and
tussocks with a very low cover of <30%
Scrublands
Foliage cover 30–70%
Woodlands
Widely spaced canopy cover 10–30%;
well-developed shrubs and grasses
Alpine
Low, fairly continuous cover
Open (sclerophyll) forest
Fairly open canopy cover 30–70%; good
understorey and ground cover
Closed (rainforest) forest
Dense canopy cover 70–100%; distinct layers or
storeys within forest
Reefs and marshes
Dense growth
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How does an ecosystem work?
In an earlier chapter we investigated the factors in the environment that organisms
have to deal with if they are to survive. In this section we are looking more closely at
how relationships between organisms can affect survival.
The community
Analysing an ecosystem involves studying how different organisms interact in a
community. Finding out the number of different kinds of organisms in an ecosystem
gives an indication of its biological diversity or biodiversity. Organisms can be
classified in a number of ways, depending on the purpose of the classification:
according to their structure, how they obtain their nutrients, the role they have in the
ecosystem, and how they can be used to give information about an ecosystem.
Table 10.2 Species types in ecosystems.
Species
Description
Endemic
Those that are native to a particular area
Exotic
Those that have come from or been introduced from elsewhere
Indicator
A particular species that give clues about the health of an ecosystem
Keystone
A species that is critical to the survival of an ecosystem because so many other
species depend on it
BIOBOX 10.1
FROGS IN DECLINE
ure
Fig
10.7
aw b
T he b
is endangered.
aw frog
No one seems to know why frogs and other amphibians
around the world are dying – not even the scientists
of 60 nations who contributed to the Global
Amphibian Assessment.
Amphibians are believed to be the best
indicators of the health of ecosystems. They are
regarded as ‘canaries in the coalmine’. Canaries
used to be taken down coalmines to detect the
quality of the air. If they died, it indicated that the
air was not breathable for the miners.
The sensitive skin of the amphibians detects
slight changes in water and air quality. If numbers
decline, then it is a signal that all is not well. Australia and
New Zealand species seem to have been hit particularly hard.
Australia has 214 species of frogs: 15 are classified as endangered,
12 as vulnerable and four as extinct. Worldwide nearly 1900 species – almost one-third of global species – are
threatened with extinction.
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Chapter 10 Relationships
and populations
REVIEW
4 Distinguish between an open and a closed ecosystem.
5 How are ecosystems named?
6 Explain the significance of a keystone species.
Relationships and interactions
between living things
A necklace of parasites for a carpet python? A karri forest that depends on a fungus?
A fish living in the mouth of another fish? The living world is full of interesting and
even bizarre examples of relationships.
Every living thing is profoundly affected by the presence or even absence of other
living things. All the living factors that affect an organism are referred to collectively
as its biotic environment and this, in turn, can shape or be shaped by the abiotic
environment. Organisms of a soil community, for example, are affected by the texture,
mineral and water content of the medium in which they live. But the properties of
the soil itself are affected by the activities of burrowing worms and decomposers. The
burrows of the worms allow air and water to reach into the soil and the decomposers
increase the fertility of the soils by recycling organic material.
Human exploitation, disease and climate change have been put forward to account for the decline in species.
Even when a suitable habitat remains, numbers often still decline, so the cause is still a mystery. In Australia the
fungal disease of amphibians, chytridiomycosis, a disease linked to drought and climate change, has contributed
to the decline.
So how do we find out what kinds of organisms are in an ecosystem? Keys and field guides can help you
identify the organism, once you have found it. Keys help you to put the organism into a classification group and
give it its individual or species name. But to do this you need to be able to identify parts of organisms and compare
similarities and differences.
a
b
bell-like flower
blue bell
narrow
leaf
plants
trumpet-like flower
bell-like flower
wild daffodil
dead nettle
broad
leaf
top petal does
not overhang
lower petal
1. narrow leaf
broad leaf
2. bell-like leaf
trumpet-like flower
go to 2
go to 3
blue bell
wild daffodil
heart-shaped leaf
lesser
celandine
club-shaped leaf
primrose
3. top petal overhangs
lower petal
top petal does not
overhang lower petal
4. heart-shaped
leaf
club-shaped leaf
dead nettle
go to 4
lesser
celandine
primrose
Figure 10.8 (a) Keys and (b) field guides.
309
Figure 10.9 Sea anemones
compete for food sources.
A wallaby doesn’t exist on its own. It interacts not only with other wallabies but
also with the vegetation it eats, the ticks, mites, flies and other parasites that pester it,
and the wedge-tailed eagle or dingo that may attack its young.
Understanding relationships between organisms and their interactions with each
other can help us understand how an ecosystem works. Some interactions can be
straightforward but others are extremely complex. It is only by experimentation and
painstaking study of behaviour (ethology) that we can come near to unravelling them.
Competitor or collaborator?
Communities are complex interactions of different populations
and the individuals of which they are composed. Many are in
competition with each other; they require the same resources to
fulfil their needs for survival. Competition within and between
species is a common feature of all communities.
Seemingly harmless sea anemones, for example, compete for
the same food source. They can detect slight genetic variations
in intruders of the same species. Both rivals discharge a battery
of stinging cells, normally used to paralyse and catch prey. In
the end, one will admit defeat, close up and creep away.
Members of some species solve the problem of catching
their prey by collaborating with each other as wolves do in
hunting for their prey or dolphins when herding schools of fish
(see Chapter 9). These are examples of intraspecific interactions
– relationships between members of the same species.
On the other hand, the association the wolves and dolphins
have with their prey is interspecific – a relationship between
members of different species.
Predator and prey
An obvious feeding relationship is a predator–prey relationship
in which one organism kills another or consumes part of it
for its food. Predators can be classified by the way they obtain
prey. Searchers, such as wrens, bream and mice, spend much of
their time foraging for small invertebrate prey. Pursuers spend
time and energy stalking large vertebrate prey before the chase.
They can afford to wait for a long time between meals. Some
predators just lie in wait for their meal to pass by. Animals
employ a huge range of strategies that enable them to locate,
capture and consume their prey.
Although there is usually a preferred prey species, it is
unusual for a predator to depend on only one species. It is
an advantage for a predator to be a member of a network of
food chains. If one prey species becomes in short supply, the
predator can turn to others.
A dynamic relationship exists between predator and prey
that is usually balanced but sometimes conditions can change
and upset this balance.
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Unit 2
Under favourable conditions, with increasing
availability of prey, the number of predators can
increase, although it usually remains less than that
of the prey. During a period of adverse conditions,
the prey population can decrease. When this occurs,
there is increased intraspecific competition. Predators
turn to alternative prey species and the effect on
them can be so severe that some may become
endangered or extinct.
The tale of the fox and others
In the 1950s in New South Wales when poison
was used to kill their preferred food (rabbits), foxes
turned to the more vulnerable bettongs. It was a
similar story in other parts of Australia where rabbits
were prolific; predators such as wedge-tailed eagles,
foxes and dingoes resorted to other prey.
Foxes are responsible for devastating many smallto medium-sized Australian mammal populations
including the rock wallaby and the tammar wallaby,
but particularly the bettong. What remains of these
populations survives in remote places or those
inaccessible to foxes.
Before the release of the calicivirus in 1995,
rabbits were found in the stomachs of 63% of foxes
in an area surveyed in the Flinders Ranges of South
Australia. After the release the percentage fell to less
than 16%. It was not quite the same for feral cats
– they tended not to turn to native animals for food.
Many died of starvation or failed to breed.
So what did the foxes eat? They turned to carrion
(dead animals) and small mammals and invertebrates
such as locusts, grasshoppers and centipedes. Though
the impact was initially quite severe, there is some,
but not conclusive, evidence of recovery. At least the
drop in rabbit numbers allowed native vegetation
to recover.
Size of population
Chapter 10 Relationships
and populations
prey
predator
Too much of a good thing
The predator and prey relationship can go wrong. Foxes, for example, go on the
rampage and kill more prey than they need for survival. This kind of surplus killing
is uncommon in predator and prey species that have coevolved over thousands or
millions of years, yet it does happen. In these cases localised surplus killing is triggered
by some unusual event. One example of such a killing is when over 80 Thomson’s
gazelles were killed by predators in Africa on a very dark night with unusually
heavy rain.
Dingoes have been known to surplus kill. They are pursuers rather than pouncers
but they attack domestic stock or kangaroos around waterholes. It is less exhausting.
Time
Figure 10.10 The predator–prey
relationship.
Figure 10.11 The bettong
became the foxes’ prey.
bioTERMS
coevolve
the evolving and adapting
together of species
311
ACTICA
PR
.1
10
CTIVITY
LA
bioTERMS
coexistence
living together
Coexistence – peaceful and otherwise
An interesting aspect of predation is that in some cases it can affect the coexistence of
a number of other species. The starfish is a natural predator of mussels in the intertidal
zones of shores. Removal of the starfish in experiments resulted in the expansion
of the population of mussels. They displaced the other sessile (fixed) organisms,
such as barnacles, as they spread. When the starfish were returned, the mussels were
again preyed upon and the barnacles were able to re-occupy the space. The predator
– the starfish – allowed the coexistence of two species with the same
requirement for space.
REVIEW
7 Compare, using examples, interspecific and intraspecific relationsh
ips.
10 What is meant by a dynamic relationship between predator
and prey?
8 What is the distinctive feature of a predator–prey relationship?
Give three examples of predator–prey relationships.
9 Explain, by using an example, why a prey population is usually
larger than the predator population.
11 Explain the advantage to a predator of having more than one
prey species.
12 Distinguish between coexistence and coevolution of species. Give
examples to demonstrate the difference.
13 Account for surplus killing by some species.
Countering attack
Figure 10.12 This butterfly has
fake eyes and fake antennae.
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Unit 2
Why is it that prey species are not wiped out by their
predators? Prey species have evolved counteradaptations – structural,
physiological and behavioural features – which enable them to avoid their predators.
We saw some examples of these in an earlier chapter.
Bats are very successful survivors and echolocation may be the key. It allows
them to detect their prey at night when many other predators cannot forage or hunt.
Certain insects have evolved counteradaptations. The listening membranes of some
species of moth detect the sonar pulses of the bat. This advanced warning gives the
moths the opportunity to escape.
Other moths even produce their
own sonar, confusing the bats.
Large fake eyes on the wings
of butterflies and fins of fish seem
to distract the predator from the
important end of the animal’s
body. This gives the potential
prey a chance to get away, largely
undamaged. Other animals, such
as tortoises, have tough armour,
which proves to be a barrier to all
but the most determined predator.
Some animals deter predators
by chemical defence – some
squids, skunks and insects squirt
nasty fluids.
Chapter 10 Relationships
and populations
BIOBOX 10.2
MIXED MESSAGES
Figure 10.13 Colours and patterns as warning signals.
Keep off!
The monarch butterfly is coloured
and patterned in a way characteristic
of many animals that are highly
poisonous to others. This visual
signal is somehow understood by
animals of all kinds as a warning. A
strong pattern of black and yellow or
orange is one of the most common
and is displayed, for example,
by arrow-poison frogs, monarch
butterflies, bees, wasps and hornets.
This sharing of a common pattern
by different poisonous animals is
known as Mullerian mimicry and
is advantageous to them all by
deterring predators.
Fooling others – Batesian mimicry
Some animals, such as hoverflies, clear-winged moths and
some beetles, display a danger patterning too, but they are
frauds – they have no stings nor are they poisonous. This kind
of mimicry – Batesian mimicry – benefits the mimic because
its potential predator avoids it. The model – the animal being
copied – could be affected because a few successful meals of a
mimic may lead the predator to try the real thing.
Batesian mimicry is known in the plant world, too.
Different species have evolved to resemble each other to their
mutual advantage.
Find me if you can – camouflage and disguises
Animals can avoid being preyed upon by disguising or
camouflaging themselves. This is known as crypsis. Many
insects can hardly be distinguished from thorns or leaves
and a Costa Rican caterpillar looks like a viper.
Some predators use disguise. Floating ‘seaweed’,
in reality a seahorse, can snap up a passing shrimp. A
praying mantis is indistinguishable until it moves and
catches its unsuspecting victim.
Figure 10.14 A viper or an elephant
hawkmoth caterpillar?
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Preying on plants
The term ‘predator–prey’ is usually associated with predators and animal prey
but it is often used in relation to animals that feed voraciously on plants. In
one year the leaf-chewing insects of the snow gums, Eucalyptus pauciflora, in
the Snowy Mountains, removed 38% of the leaf area and 76% of shoots.
Up to 50% of the eucalypt leaf area is regularly chewed by stick insects, cup
moths and other kinds of insects and their larvae. Christmas beetles can strip
a tree in a very short time.
Although insects and other herbivores can stunt the growth of trees
and alter the species composition of forests, there is one advantage to this
activity – the continuous downpour of frass, the faeces of the insects and their
larvae. Frass provides a rich source of nutrients for the decomposers in the forest
litter. This process enables nutrients to be recycled more rapidly than would
happen otherwise.
Some insects, particularly ants, remove up to 70% of seeds after they fall.
These animals are referred to as seed predators and they affect the rate of
germination and consequently the renewal of forests.
aly
Euc
.15
0
1
re
Figu
pt
g ed
ma
a
d
es
leav
s
by
a
y la
wfl
Figure 10.16 Venus flytrap.
e.
rva
A chemical interaction
In many species of trees, chemical
defence can be switched on by the
attack of leaf-cutting insects – an
induced defence. The damage to
the tissues causes the release of a
chemical that is thought to diffuse
from the leaf to neighbouring
leaves. These respond by increasing
their levels of toxic or repellent
compounds, with the effect of
deterring further feeding and
damage to the plant.
bioBYTE
The mystery of the suicidal Norwegian lemmings
can possibly be explained by a kind of plant
chemical lemming interaction. When lemmings
start to feed on sedges and cotton grass in the Arctic
tundra, the plants respond by producing a toxin that
neutralises the lemmings’ digestive juices. Plants
stop doing this if grazing is light but, if heavy, as
when populations of lemmings explode, the toxins
accumulate, so the lemmings cannot digest their
food and the more they eat the hungrier they get.
Having stripped the land of food resources, on
reaching a lake or sea edge, it could be that the
lemmings attempt to swim to other areas in a
desperate search for more food.
Plants preying on animals
Animal-eating plants? This sounds rather odd but there are many examples of plants
relying on animals to supplement their diet. In nutrient-poor environments pitcher
plants, often called carnivorous plants, trap the insects and detritus that fall into their
specially adapted pitchers. The action of enzymes produced by the pitcher plant and
the activity of microorganisms break down the organic matter, thereby making it
available to the plant. Pitcher plants may have lodgers, too – small frogs who wait for
unwary insects to fall in.
Pitcher plants capture their prey passively but sundews and Venus flytraps are more
active in how they go about it.
REVIEW
14 What are counteradaptations? Give an example.
15 Do you think herbivores should be referred to as predators? Justify
your answer.
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Unit 2
16 Describe a chemical interaction between a plant and an animal
that feeds on it. Explain any advantages and disadvantages in such
a relationship.
Chapter 10 Relationships
and populations
Symbiotic relationships
Symbiosis is the general term used to describe the relationship in which individuals
of two or more different species live and in which at least one of the species benefits.
There are three main types:
1 parasitism: one organism benefits at the expense of the other
2 mutualism: both species in the relationship benefit and neither is harmed
3 commensalism: one organism benefits and the other neither benefits nor is harmed.
Parasitism!
Parasites can be fascinating – for some. Most species, including
humans, harbour parasites in every conceivable part of their body.
a
The parasites are extraordinarily well adapted in life cycle, structure
and physiology to find their host and survive the hazards of being
dependent. The hosts themselves have coevolved strategies for
surviving the effects of their unwelcome invaders.
In a predator–prey relationship, one animal benefits at the entire
expense of the other, but in other exploitative relationships, it is
different. In parasitism, the parasite benefits and the host is harmed.
But it is not in the interest of the parasite to damage the host to the
extent of killing it.
An ectoparasite is an organism
b
that lives on the surface of another
and obtains its food from the host,
for example a tick on a lizard or an
aphid on a leaf. An endoparasite
lives inside the tissues of its host,
for example a tapeworm in a sheep,
malaria parasites in blood or a leaf
miner that burrows in a leaf.
Parasitic flatworms live in the
gut and absorb nutrients. Blood
flukes feed on blood in the tissues,
whereas leeches prefer to suck it
from the outside. Worms of many
kinds inhabit the insides of their
hosts. Nematodes worm their way into plant roots, while some inhabit animals.
Mites are ectoparasites. They have been called ‘terrestrial plankton’ because they
are so numerous and exist everywhere – even on us. About 2 million dust mites keep
us company in bed, living off the skin we shed! Colonies of mites inhabit a single
listening membrane of a moth, using different parts to lay their eggs, discard their
wastes and feed on the moth’s blood.
The relatives of the mites, the ticks, are also bloodthirsty creatures living on a
variety of hosts, particularly warm-blooded vertebrates. Ticks are common on cattle,
numbats, bandicoots, kangaroos and birds. About 70 species live in Australia but only
about 13 bite humans. A few can cause serious harm to their host. The Australian
paralysis tick, Ixodes holocyclus, has the unenviable reputation of being the most toxic
tick in the world.
Figure 10.17 A parasitic way of
life. (a) The top left red blood cell
is infected with a malarial parasite,
Plasmodium, an endoparasite.
(b) This grass skink has red mites,
which are ectoparasites, in its armpit.
315
Parasites of plants
Plants suffer from parasites, too. Different fungi infect nearly every
kind of plant either through the roots or through the leaves. Dodder
plants have thin stems that wind around the host plant. Fine suckers
grow out, penetrate the plant stems and absorb the nutrients. The
cherry ballart (native cherry) is not obviously a parasite, being a tree,
but it obtains most of its nutrients from the roots of nearby host trees
such as eucalypts.
REVIEW
17 Why are predation and parasitism regarded as exploitative relationships?
18 Use examples, including both animals and plants, to distinguish between ectoparasites
and endoparasites.
19 Explain why parasites seldom kill their host.
Figure 10.18 Parasite of plants:
dodder plant.
bioTERMS
mutualism
a relationship that exists
between organisms of different
species and that benefits both
and harms neither
association
a relationship or interaction
between species
Mutualism
Not all relationships cause harm. There are all possible grades of mutualism, from a
rather loose association in which the partners seem to gain little from each other, to
ones that are so intimate that the two partners can be regarded as a single organism.
Pollinators
Pollination is essential for plants to reproduce. Some plants are pollinated by the wind
but many rely on insects, birds, small mammals and even reptiles to transfer pollen
from one flower to another. Usually they are rewarded for their efforts: pollen and
nectar are rich in nutrients.
Figure 10.19 These
pygmy possums, while
collecting nectar from
eucalyptus blossom, are
also acting as pollinators.
Generalist pollinators pollinate a range of different species, whereas specialists will
pollinate only the kind of plant with which they have coevolved. In these partnerships,
both members depend so much on each other that if one disappears the other would
remain sterile or starve.
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Figure 10.20 The rare
spider orchid, Caladenia
robinsonii.
bioBYTE
The rare spider orchid is on
the brink of extinction because
of destruction of its habitat. It
is thought that only 20 plants
remain in the wild. The orchid
relies on a wasp to pollinate
it. The orchid has coevolved to
resemble the wasp, fooling it into
attempting to mate with it. In the
process, the orchid is pollinated.
BIOBOX 10.3
THE PLIGHT OF POLLINATORS
The decline of some plant species has been traced to the decline of their pollinators. An epidemic of parasitic varroa
mites in North America and Europe is thought to be responsible for the massive drop in honeybee colonies. The
bees are the major pollinators for over a hundred commercial crops and, without them, production costs of crops,
particularly of California almonds, have risen to over $6 billion per year.
Other pollinators – bats, birds, butterflies and other insects – are in worldwide decline, largely due to the
overuse of pesticides. Declining bat populations affect the survival of eucalyptus and neem trees in tropical areas. In
Central America, the decline in hummingbirds has threatened the survival of many plants. In Britain, a quarter of the
250 native bee species are endangered and these are responsible for most of the pollination of wild plants and fruit
crops, such as strawberries, apples and pears.
Figure 10.21 Pollinators: reward for effort.
In some instances, the loss of a single pollinator species – a keystone species – can cause the collapse of entire
ecosystems. The loss of flying foxes in some Pacific islands could have repercussions along food chains because
mammals depend on the fruit of trees pollinated by the flying foxes.
To halt the decline, scientists have suggested constructing ‘nectar corridors’ along which pollinators can
migrate from one habitat to another.
317
Seed dispersers – sometimes a sticky business
An interesting relationship involves parasitism
and a seed disperser. The mistletoe bird relies
almost exclusively on mistletoe berries for its
food. Even having passed through the bird’s
digestive system, the berry retains its stickiness
and remains attached to the bird’s rear end.
The mistletoe bird rubs on, say, the eucalypt
branch to remove the annoying berry. In the
process, the berry becomes attached to the
branch. It grows out small roots that penetrate
the eucalypt. Eventually they reach the
transport vessels and absorb the sap.
Figure 10.22 Making
the most of mistletoe.
Lichens – do one and one make one?
Lichens were a mystery for a long time. They result from
the close physical and metabolic interaction between an alga
or cyanobacterium and a fungus. The partnership has been
successful as lichens can survive in extreme conditions of
altitude, low and high temperatures and desiccation (drying
out). Lichens are used as indicator species because of their
sensitivity to atmospheric pollutants. Many kinds have
become extinct and more are endangered because of changes
in the atmosphere.
bioTERMS
soredia
(unit of reproduction)
seed disperser
an animal that forages and
disperses seeds
facultative
describes an association that
may be obligatory for one
species, but not another
Figure 10.24 (below) Lichens show
many different forms, such as blisters,
streamers, colourful nodules and these
crusty leaf-like structures.
algal layer
fungal layers
fungal hyphae (threads)
‘rootlets’ for attachment
to substrate
Figure 10.23 Section through a lichen, showing fungal hyphae (absorbing threads) among the algal cells.
The fungus is nearly always the dominant organism
in a lichen but it is unable to survive without its alga or
cyanobacterium partner. The fungus relies on the products of
photosynthesis taking place in the alga. This is an obligatory
association for the fungus. The algae or cyanobacteria, on the
other hand, are protected from desiccation though they can
usually survive on their own. As the host is not essential for
them, the association for the alga or cyanobacterium is described
as facultative.
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It is difficult to classify lichens because of the complex fungus–algae relationship.
Are they one organism or two? Are they prokaryotes, protists or fungi? The present
taxonomic solution is to put the 20 000 or so different lichens into a group of their
own – the too hard basket.
Symbiotic digesters
Complex communities of microorganisms live in the gut of many animals. Without
them, cows, sheep, koalas, termites and many others would not be able to digest their
food. The microorganisms secrete enzymes that break down cellulose. In return, the
host provides a relatively safe environment for them – unless they get moved along and
are themselves digested!
Termites are voracious feeders. They can attack a eucalypt tree, feed on the central
heartwood, which is dead, and hollow it out rapidly. In doing so, they make shelter
available for other animals. The tree is not usually killed, as the living part of the trunk
is not attacked. What kinds of relationships exist between the bacteria, the termites,
the tree and the animals that live in the hollows?
Relationships of the enclosing kind
The ultimate intimate association is achieved when
one of the partners lives inside the cells of the other.
Many coral polyps, jellyfish, clams and sea slugs
have algae living in their tissues. The algae need
nitrates and phosphates for their metabolism. These
are made available in the waste material of their
animal partner. The partner is careful to ensure that
its algae are always adequately exposed to light as
they benefit from the organic compounds produced
in photosynthesis.
Some sea slugs from the Great Barrier Reef
actually stimulate the colonies of algae to reproduce.
The sea slugs grow out tentacles along their sides and
the algae become squatters. The sea slugs come to rely
entirely on the food produced by the colonies. But
how did they get there in the first place? The algae
were present in the coral the slugs fed on; they
passed from the slug’s gut into the tentacles.
Figure 10.25 Ultimate intimacy
can be seen in the relationship
between the Hydra and the
green algae in its cells.
Figure 10.26 (a) The remora and
shark (b) Sea anemones and fish.
b
a
Commensalism
Commensalism is a one-sided affair. Only one
of the two organisms involved, the commensal,
benefits from the association. Some relationships
are easy to identify as commensal, such as the
relationship between the remora and the shark. The
remora gets a free ride – and possibly a free meal of
leftovers – by attaching the suction pad it has on
the back of its head to the shark. Many fish gain
protection by swimming between the tentacles of
sea anemones. The tentacles have stinging cells that
paralyse and capture prey but do not affect the fish.
319
BIOBOX 10.4
MORE MUTUAL MATTERS
There are countless examples of other interesting
relationships. One such is between a crab and its
carefully placed anemones. The anemones
scavenge on the food particles that escape
from the crab. Perhaps they also benefit
by being taken into areas they could
not otherwise go. The anemone
can stand on rocks and shells
but not on soft sand or mud. The
anemones themselves perhaps
give some kind of protection to
the crabs. Why do we think so? The
claws of crabs with anemones on
them tend to be smaller than those
of species that do not carry anemones.
Interestingly, the claws of hermit crabs that
have the anemone perched on the back of their
borrowed shell are large.
ure
Fig
7Ac
10.2
h its anemones – a beneficial relationship
rab wit
.
Travelling companions and a cleaning service
Some animals act as carriers for their smaller partners. A little mouse in the forests of Costa Rica carries beetles
clinging to its fur round its eyes and neck. The beetles do not feed on the mice but on the fleas that occupy the
mice’s nests. Because the beetles keep down the number of fleas, the
mice benefit.
Ox-pecker or tick birds are seen perched on the back of
buffalos, zebras, rhinoceroses and other large herbivores.
Coevolution has ensured that the birds are able to
probe for the ticks with flattened beaks and hang
on to the beast when it is moving with long
claws. In addition, they warn their carriers of
approaching predators. Sometimes, the oxpeckers take more than the ticks – they like
to sample the blood in an open wound.
Galapagos finches have a similar
relationship with giant tortoises, which
signal their need to be cleaned by rising up
on their legs and remaining immobile. The finches
Figure 10.28 Us
eful travelling companions.
gain access to the softer parts of the tortoise, which are irritated
by parasites.
Large fish and whales have a similar cleaning service provided by smaller fish or shrimps. Dead bits
of tissue, fish lice and fungi are all removed, sometimes at recognised ‘cleaning stations’.
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and populations
Epiphytes
Climbing plants, such as lianas, use trees to support them in their reach for light.
Seeds germinate on the forest floor and the rapidly growing shoots spread out. If they
reach a vertical surface they take hold of it. Other seeds, such as those of orchids, are
wafted high up on the branches and take hold there. In these instances, the tree offers
support without apparently gaining anything in return.
BIOBOX 10.5
LIFE SUPPORT
Figure 10.29 Bromeliad ponds.
Some of the commonest epiphytes in forests are the bromeliads.
They germinate high up in the canopy and anchor themselves
tightly by wrapping their roots around a branch. The leaves grow
in such a way that they channel rain water and form a miniature
pond, which becomes home to an assortment of small animals.
Dragonflies and mosquitoes lay their eggs here, tiny frogs pass
through their life cycles here and slugs, worms, beetles and
small reptiles also form part of the community. Even birds come
to visit and drink from the ‘pond’, leaving their droppings for
the microorganisms.
The bromeliad benefits from this arrangement by being
able to extract nutrients from the decaying organic matter in the
‘pond’, which otherwise would not be available. This relationship
is therefore mutualistic, but what of the relationship between the
bromeliad and the tree?
As the name suggests, the strangler fig does not have such a kindly relationship with its supporting tree. The
relationship starts well enough but ends disastrously. As the lattice of roots grows downwards, the roots are able
to use the nutrients from the forest floor, leaving less for the supporting tree. This, together with the fig’s upwardly
growing bushy branches that compete successfully for light, eventually results in the death of the supporting tree.
Strange, hollow lattices are left standing once the trunk rots away. Commensalism begins the relationship, but at
what point does it end?
REVIEW
20 Describe three relationships or interactions between plants
and their pollinators.
21 Identify the components of the ‘mistletoe’ relationships
described in the text.
22 What kind of relationship is demonstrated by lichens?
Explain your answer.
23 Identify the relationships between termites and the
microorganisms of their gut, termites and the tree that they
hollow out, and termites and the possums that shelter in the
hollowed-out tree.
24 Draw up a table that compares the characteristics of
parasitism, mutualism and commensalism. Include examples
of each kind of relationship. What general description is used
for these relationships? Explain.
321
Population studies
Relationships between organisms can change. We have seen how the
balance between interacting species can be upset. Studying populations can
help us predict such changes and their consequences; it can give us an idea
of biodiversity within ecosystems and, on a much larger scale, the biosphere.
Ecologists describe the total number of a particular species in a
particular place at a particular time as the population. For example, the
number of manna gums (Eucalyptus viminalis) in a heath-land community
or the number of straw-necked ibis (Threskiornis spinicollis) in a wetland in
a particular month; or even the number of foxes in an urban area.
Populations in an ecosystem are dynamic. Maybe you have noticed
changes in the number and kind of plants or insects from one season to
another or from year to year. Even the ratio of males to females can change.
After mating, for example, male Antechinus die from stress and disappear
from the population. Some female spiders eat their partners!
Figure 10.30 Reproducing
dangerously – Antechinus.
bioTERMS
open ecosystems
ecosystems in which there is
immigration and emigration
closed ecosystems
ecosystems in which there is
little or no immigration and
emigration
Growth of populations
In open ecosystems migration between populations of animals and plants (dispersal)
can occur. This will affect overall numbers in the ecosystems involved, their
distribution and, in the longer term, birth and death rates. In closed ecosystems the
growth rate depends only on birth and death rates.
A population is increasing if the birth rate (br) and immigration rate (ir) exceed the
death rate (dr) and emigration rate (er). Rate refers to the number of individuals per
hundred, thousand or whatever unit is appropriate.
growth rate = (br + ir) – (dr + er)
For example:
growth rate = (100 per thousand + 65 per thousand) –
(37 per thousand + 25 per thousand)
= 103
bioBYTE
Therefore, the population has grown by 103 individuals per 1000. Growth rates
can also be expressed as a percentage: +10.3% if there is an increase or –10.3% if there
is a decrease.
CALM’s Foxglove team
(Conservation And Land
Management) in Western Australia
monitored changes in population
of native species. This followed
control of foxes in designated areas.
Woylies (brush-tailed bettongs)
were trapped, fitted with radio
transmitters and released. The
information the team obtained
helped to develop strategies for
controlling foxes on a much larger
scale elsewhere.
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Unit 2
Quantifying risk
Information that is provided by population studies is useful for monitoring
ecosystems. Knowing the age structure of a population is significant in
predicting future growth trends. Figure 10.31 shows population pyramids for
two different human populations. Population A has a greater proportion of
children than does population B. What is the significance of this?
Scientific data can help in the management of ecosystems. It can be used
to plan how to ensure sufficient numbers of young live to reach reproductive
age, how to develop strategies for protecting endangered species and
maintaining ecosystems, and how to restore degraded ones.
Chapter 10 Relationships
and populations
90
80
Age (years)
70
60
50
40
30
20
Males
Females
Males
Females
10
50
40
30
20
10
0
10
20
30
40
50
12 10 8 6 4 2 0 2 4 6 8 10 12
Population A (millions)
Population B (millions)
Figure 10.31
Population pyramids.
Distribution
Understanding how an ecosystem operates takes more than knowing what kinds
of species are part of the system or how they relate to each other. It is important to
know their distribution – exactly where in the physical space members of the different
species are found.
Members of a species are seldom spread evenly throughout the entire ecosystem.
There are patterns in the way populations are distributed.
• random distribution: organisms are spaced irregularly – the location of an organism
does not affect the location of another (more common for plants than for animals)
• uniform or continuous distribution: organisms are evenly spaced – the presence
of one determines how close or distant another will be. It is common in relatively
high-density populations of some animals that set up breeding territories
• clumped or grouped distribution: a number of individuals is grouped together and
the groups make up the population as a whole. This is sometimes to do with social
behaviour as in schools of fish, or clumping of vegetation in favourable mini-habitats.
clumped
uniform
Figure 10.32 Distribution patterns:
clumped, uniform and random.
random
323
Knowing the distribution and abundance (how many) of a species can help keep
track of populations of significance. Knowledge of particular plant species can give
clues about the distribution and abundance of animals that depend on them. The
forestry industry needs to know about the distribution and abundance of valuable tree
species and the fishing industry about fish stocks. Keeping track of pest and plague
species, such as mice and locusts, gives forewarning of potential outbreaks that would
require management.
Measuring distribution
Figure 10.33 Differences in
physical conditions result in vertical
stratification in a tropical rainforest.
The line transect (trans across, sect section) method is best to use if environmental
factors such as land surface change along the distance to be sampled. Gradual changes
like this are referred to as environmental gradients.
Vertical transects can show vertical distribution of species. Different conditions at
different levels result in stratification, for example in forests.
Light:
70%
Wind:
15 km/h
Humidity: 67%
Light:
50%
Wind:
12 km/h
Humidity: 75%
Light:
12%
Wind:
9 km/h
Humidity: 80%
Light:
6%
Wind:
5 km/h
Humidity: 85%
Light:
1%
Wind:
3 km/h
Humidity: 90%
Light:
0%
Wind:
0 km/h
Humidity: 98%
Source: Biozone Learning Media, Year 11 Biology Student Resource and Activity Manual
Environmental factors change across a tidal mudflat or a marine rock platform. In
these cases a profile can be drawn.
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Chapter 10 Relationships
and populations
BIOBOX 10.6
LOCUSTS
Figure 10.34 Australian plague locust.
Australia has about 700 endemic species of grasshoppers and
locusts but the one that usually hits the headlines and the news
screens is the Australian plague locust, Chortoicetes terminifera.
Locust populations increase enormously in favourable climatic
conditions – usually following periods of rainfall. Many minor
plagues occur regularly but there have been four major plagues
in the past 50 years.
Although plagues of this locust have been experienced
in inland areas of New South Wales, South Australia and even
Victoria, most have their origin in the Channel Country of
south Queensland.
Can you imagine eating about one-third to half of your
body weight each day of your life? Plague locusts do. No
wonder farmers and others want advance warning of their
approach. After hatching, the hoppers – the first of several
stages in the locust’s life cycle – move away in dense marching
bands that can cover several kilometres.
Densities of 1000/m2 have been recorded. It takes
little imagination to realise that these eating machines
can strip crops, grazing land and native vegetation in a
matter of hours. It is not surprising that the young locusts,
called instars, have to split their ‘skins’ several times before
adulthood to make room for expansion!
The adult locusts form swarms of varying density and
can contain up to several million individuals. The swarms
are capable of being carried up to 50 m high and of flying
10–20 km per day. Long-distance migration takes place at
night, as the locusts travel with the help of the night breezes.
Plagues such as these have economic and
environmental implications. It is understandable why
chemicals are used to control the locusts. However, it is
reported that these are moderately toxic for birds and fish
and highly toxic for aquatic and terrestrial arthropods.
The use of natural bioinsecticides is increasingly
common. An oil-based formulation containing Metarhizium,
a fungus, has been developed by the CSIRO to control
plague locusts. The spores are easy and cheap to produce,
and can be used in a spray or distributed as granules.
They can be stored readily and are extremely effective.
This method of control depends on the interaction
between the locust and the fungus.
persistent
intermittent
Figure 10.35 Distribution of Australian plague locusts in Australia.
325
Vertical height (m)
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
rocky ledge
supralittoral
zone
intermittent splashing
periwinkles
algae visible in winter
splash zone
highest tide
level
usual tidal
range
lowest tide range
sea anenome, starfish, octopus,
brown algae with floats
periodic wetting and drying out
algae (leafy green)
periwinkles, limpets, chitons,
rock crabs
wave pounding
kelps (brown algae)
with holdfast,
barnacles, sea squirts
rock pools
sublittoral
zone
Figure 10.36 Transect profile of a marine rock platform.
Measuring abundance
It is not always possible to determine the population of a species. Birds and insects fly
here and there, and some animals move too fast to count. It may be difficult to work
out what actually makes up an individual organism. How do you count individual
plants of grass?
bioTERMS
sampling techniques
ways of estimating populations
of species
quadrat
a frame of known dimensions
used to estimate populations of
species in a given area
Figure 10.37 Quadrat sampling.
Studies of the small native daisy,
Rutidosis leptorrhynchoides, are
aiming to establish what population
size is needed to avoid in-breeding and
sustain the population.
Direct observation
Making direct observations and recording sightings at particular intervals might be
possible but it is time-consuming and, in the case of male fur seals in the breeding
season on windswept shores, quite dangerous! Satellite images have been used to
determine land cover in relatively inaccessible regions. In aquatic ecosystems, plankton
nets are used to ‘sweep’ or sample the organisms and aircraft traverse areas
to count kangaroos.
Even when it is possible it may not be necessary to count all members of a given
population. Various sampling techniques can provide estimates of a population.
A sample is a small group of organisms selected from the total population in a given
area or volume. This sample represents the whole population.
Choosing a particular site because it is easy to get to or is more interesting, or
selecting only two sample specimens, reduces the reliability of the data obtained. It
does not give a true picture of the whole population. To represent the population as a
whole reliably, the samples must be collected in an unbiased way.
Quadrat
For organisms that are fixed or do not move
very much the quadrat method of sampling
can be used to estimate distribution and
abundance. It can also be used to calculate
density of a population.
Several quadrats representative of the area
are sampled at random. For each quadrat:
• the number of individuals of each species is
counted and recorded, or
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Unit 2
Chapter 10 Relationships
and populations
• the relative numbers of each species is estimated using a scale from abundant (3) to
absent (0), or
• percentage cover is estimated, and
• the totals of the quadrats are averaged.
A simple mathematical calculation can give the total number or percentage cover
for each species in the whole area. The density can be calculated too.
average density of members of species (estimated) 
total number of individuals counted  area of each quadrat  number of quadrats
Figure 10.38 Playing tag – tagging
a turtle.
population
N=?
CTIVITY
LA
.2
10
The capture–mark–recapture method is commonly
used to sample motile species. A random sample
of individuals of a species is taken and an overall
estimation of the abundance of the species is made.
Step 1: capture – animals are caught randomly
and in such a way that they are unhurt. Small
animals are trapped in cages or pitfalls in the
ground, birds are trapped in fine nets and some
animals are caught easily when they ‘freeze’ in
spotlights. Flying insects are attracted to light traps.
Step 2: mark – each captured animal is marked so
that the mark is not obvious to predators or harmful
to the organism. Insects are usually marked with a
blob of paint, whereas birds are tagged on the leg
or wing.
The animals are returned to their habitat and left
to mix with the unmarked individuals.
Step 3: recapture – later, a random sample is taken and the number of marked
individuals counted. From this information the total population can be estimated.
The procedure has to be planned carefully so that the chances of each individual being
caught are equal.
ACTICA
PR
Capture–mark–recapture
sample 1
26 captured
All marked
individuals are
returned to the
population.
All captured
individuals
are marked.
M = 26
sample 2
21 captured (n)
3 marked (m)
Figure 10.39 Catch me if you can.
M = 26
N=?
Marked individuals have spread
throughout the population.
327
Total population (N ) = no. marked in first sample (M)  total number
of animals recaptured (n)  no. of recaptured animals
that are marked (m)
Mn
N  _______
m
20  50
For example, total population ________
10
1000
 _____
10
 100
Population density
Studying the density of populations in an ecosystem is useful. Density refers to the
number of individuals in a given area, such as the number of kangaroos per hectare.
Sometimes it is difficult to distinguish individual plants, as with grass. In that case,
density is the amount of biomass (dry) per unit area (for example, 0.6 kg of grass
per m 2). In the case of diatoms in a pond it is the number per unit volume, for
example 300/mL.
bioBYTE
A Greening Australia program aims to return
small woodland birds to the landscape where
agriculture has affected populations of birds.
But this depends on a sufficient amount of
native vegetation to support them. It has been
estimated that to reoccupy an area, birds need a
minimum of 10–21 hectares of native vegetation
with a dense understorey. To support breeding
and sustain the population, 100 hectares is
needed. Birds are useful bioindicators and they
provide free pest control for woodland trees.
Carrying capacity
Knowing the density of populations can help assess an
ecosystem’s ability to provide sufficient resources to support
populations. This is the carrying capacity of the ecosystem
and it can change from time to time as environmental
conditions change.
It has been estimated, for example, that 100 km 2 of moist tall
eucalypt forest is the minimum area possible to support 10 000
sugar gliders as a viable population. Information such as this is a
guide to the minimum size a park or any closed ecosystem needs
to be to conserve animals.
REVIEW
25 Ecosystems can be open or closed to particular species.
What effect does this have on growth of a population?
26 What characteristics of populations are usually studied when
analysing ecosystems?
27 A particular population of kangaroos has 1000 births during
the year; 72 individuals also join the population while 108 leave.
There are 345 deaths. Work out the growth rate for this
population for the year.
28 Describe three basic patterns of distribution of populations
of organisms.
328
Unit 2
29 Draw up and complete the table below summarising methods
of estimating distribution and abundance of populations of
organisms. State what each method is best used for.
Method
Brief description
Used for …
30 Explain the meaning of ‘carrying capacity’ of an ecosystem.
Chapter 10 Relationships
and populations
Dynamic populations
Population regulation
Number of individuals in the population
When a few members of
a species colonise a new
and favourable habitat, its
Population
Population
Population
growing slowly
growing
growth
population increases rapidly.
exponentially
decelerating
This population growth cannot
be sustained – resources are
used. The population begins
to level off. Despite minor
fluctuations in populations,
there tend to be upper and
lower limits. For a given species
in a particular habitat, there is a
certain equilibrium population
that the ecosystem can support.
Factors in the environment,
During this phase population growth increases
as population gets under way. Often starts
collectively referred to as
slowly because initially there is a shortage of
environmental resistance, act on
reproducing individuals which may be widely
dispersed.
a population. If the population
rises above the equilibrium
or set point, competition for
resources such as food and space begins to take effect. The increased ability of diseasecausing organisms and parasites to spread also increases deaths and possibly reduces
breeding. This could be to such an extent that the population falls. If it falls below the
set point, there is less competition and the population begins to rise again. This kind
of negative feedback process, or homeostatic control, keeps the population more or
less constant.
increase in
population
equilibrium
population (near
to the carrying
capacity of the
environment)
fall in
population
raised
environmental
resistance,
e.g. increased
competition for
food or increased
predation
lowered
environmental
resistance, e.g.
less competition
for food or less
predation
negative
increase in
population
Population more
or less constant
Birth rate and death rate balance
each other, resulting in equilibrium.
Environmental resistance sets in,
increasing the death rate and/or
decreasing the birth rate.
Time
This phase represents the maximum growth
rate under optimal environmental conditions
– no environmental resistance, birth rate
exceeds death rate.
bioTERMS
equilibrium population
the population at set point
environmental resistance
environmental factors
that affect population;
density-dependent and
density-independent
set point
the level of equilibrium a
population reaches
feedback
fall in
population
Figure 10.40 The S-curve: generalised
graph of population growth.
population returns
to norm near to the
carrying capacity of
the environment
homeostatic control
a negative feedback process
that, overall, maintains
equilibrium
negative
feedback
Figure 10.41 Homeostatic mechanism of population control.
329
The factors mentioned above are density-dependent factors. The greater the
density of a population, the more individuals die or fail to reproduce. Densityindependent factors, such as severe weather conditions, volcanic activity or habitat
destruction by clearing or fire, are those that affect all individuals in a population
regardless of age or stage.
Figure 10.42 Populations
are regulated by
density-dependent and
density-independent factors.
Density-dependent
factors
Competition
Disease
Parasites
Predation
Food supply
Density-independent
factors
Indirectly affect
the food supply
Population density
influences the
effects of these
factors
These factors are influenced
by the kangaroo density
Factors affect
individuals to
the same extent
regardless of
population density
Organisms crowded
together:
• compete more for
resources
• are more easily found by
predators
• spread disease and
parasites more readily
Physical factors
Acidity
Rainfall
Humidity
Temperature
Salinity
Catastrophic events
Volcanic eruptions
Tsunami
Fire
Drought
Earthquake
Flood
Can cause ill
health or death
Influences the
individual’s ability
to reproduce
Source: Biozone Learning Media, Year 11 Biology Student Resource and Activity Manual
Percentage of population surviving
Figure 10.43 Survivorship curves.
In some cases, populations of species are not able to recover from such extreme
environmental resistance and become extinct in that ecosystem.
A combination of factors, working together, determines the survival of populations.
If we examine survivorship of various species, patterns emerge. Species can be
classified according to their probability of survival. This can be represented in a graph.
100%
10%
1%
Table 10.3 Probability of survival of species.
0.1%
Age
Classification
Description
Examples
Early loss
Species that produce many offspring. Many,
even thousands, of young are produced but
the probability of survival is low in early life.
Probability increases with age.
Fish, frogs and many plants
Constant loss
Species whose probability of survival of
individuals does not change with age. There is a
fairly constant loss at all ages.
Many bird species
Late loss
Survival of individuals decreases with age.
Humans, elephants
bioTERMS
survivorship
the probability of survival
of species
330
Unit 2
Chapter 10 Relationships
and populations
Population distribution
and abundance
Density
The number of organisms
per unit area
Distribution
The location of individuals
within an area
Total abundance
The total number of organisms
Population growth rate
The change in the total population
per unit over time
Population composition
Migration, immigration and
emigration effect
• density and distribution
• population composition
• dynamic (overall change) in
the population
Sex ratios
The number of organisms
of each sex
Population fertility
The reproductive capacity of
the females
Age structure
The number of organisms of
different ages
Population
Males, females, young
=
Birth rate
The number of organisms born
per unit over time
–
Mortality rate
The number of organisms
dying per unit over time
Figure 10.44 Features
of populations.
Source: Biozone Learning Media, Year 11 Biology Student Resource and Activity Manual
REVIEW
31 Draw a simple annotated diagram that summarises homeostatic control of populations.
32 What does environmental resistance mean? Distinguish between density-dependent and
density-independent factors.
33 What do survivorship curves show about the survival of populations?
Restoring populations
Although the rate of species loss worldwide is alarming, there are many examples
of endangered populations of species being restored. Some of the recovery is due to
natural cycles of population change but many are the result of careful management
and legal protection of species.
Whale species, for example, are staging extraordinary recoveries after being hunted to
the verge of extinction. By 2050 it is predicted that the population of southern right whales
in Australian waters may approach their original population. Populations of humpback
whales are expanding in the north Atlantic, north Pacific and off Western Australia.
Controlling populations
When does a living thing become a pest? Sometimes populations of species increase
enormously at the expense of others when environmental conditions change.
The development of particular agricultural practices, the introduction of species
accidentally or deliberately, and the concentration of people in urban environments
have created excellent conditions for certain plants and animals to exploit.
Many fungi and insects, without natural predators, have become a problem, wreaking
havoc on crops of all kinds. Think of the mouse plagues common in Australian since
European settlement or the invasive weed, Paterson’s curse, which lives up to its name.
331
Chemical and biological control
Figure 10.45 Biological control
agents: (a) cane toad (b) green tree
ants.
a
b
bioTERMS
culling
reducing the size of a population
by killing some members of the
population
332
Unit 2
The use of chemical pesticides (chemical control) is a quick and effective method
of getting rid of pests, but there is a downside – the pesticides can be ecologically
damaging as well as costly. Nowadays scientists favour the use of biological control
agents, which exploit relationships between organisms, or an integrated approach
when rapid response is needed in the early stages to manage a problem.
There are four kinds of biological control agents:
1 general predators
2 specialised predators
3 parasites
4 microbial diseases.
Almost all ecological disasters resulting from biological control have been due
to the first method. For example, cane toads were introduced in north-eastern
Queensland to control sugar pests. They are now in Kakadu, wreaking havoc on
wildlife, and are expected to reach Sydney in 20 years!
Introduced plant weeds, some escaped from aquaria, choke many waterways.
A small South American weevil, the natural predator of the water weed Salvinia, was
released in 1980 with great success in many areas. Moths and flea beetles, introduced
in 1977, are reducing the impact of the alligator weed. Another beetle from South
America is controlling water hyacinth. There tends to be a balanced relationship now
between the control agents and the plants.
Wasps are being used to control the native stem-girdler moths, which can decimate
macadamia and pecan crops in Queensland. The wasps lay their eggs in the moth
eggs, which are consumed by the wasp larvae. Many other wasps have been harnessed
to control a range of pests, such as the Heliothis moth, which feeds on cotton plants.
Recent research has shown that green ants are a cost-effective method of controlling
most pests that attack mangoes. Growers are being encouraged to introduce the
ferocious predators into their mango orchards!
Nematodes, spread by their host, successfully combated the Sirex infestations of the
South Australian softwood plantations, which had suffered a loss of nearly 5 million trees.
‘Myco-insecticides’ are showing promise as control agents. Many species of fungi in
the soil attack grubs (larvae), which gnaw away at the roots of many plants, including
pasture grasses, sugar cane, potatoes and other crops. Selected fungi are now being
used as biological control agents of scarab beetles and other target insects.
Culling
The thought of culling populations of particular species that are pushing their
ecosystem beyond its carrying capacity can cause strong reactions in people. Populations
of some species of kangaroos have increased and expanded enormously since Europeans
arrived. Land use was changed in such a way that it affected many endemic species of
plants and animals. Six kangaroo species, for example, have become extinct, seven are
endangered but the remaining 35 species are viable in their rangelands.
The Australian red and grey kangaroos have thrived in those areas where
permanent water is made available for cattle. As populations of kangaroos have
increased there has been increased competition for resources. Each year over 2 million
kangaroos are culled under strict regulations. The kangaroo industry is worth over
$200 million each year. A small proportion of the meat is processed for human
consumption, most of it for export.
Chapter 10 Relationships
and populations
To cull or not to cull?
The population of flying foxes in the Melbourne Botanical Gardens grew to such an
extent that the valuable collections of plants were being destroyed. Because of stringent
regulations nowadays concerning the introduction of exotic species many of the plants
would not be able to be replaced. Rather than cull, attempts were made to move
flying foxes to other areas, eventually with success. Will this solve the problem for the
Botanic Gardens and what will happen in the flying foxes’ new home?
Reintroducing populations
Increased understanding has been gained through research and through indigenous
knowledge of relationships between organisms. This has led to a deeper recognition
of the part biodiversity plays in maintaining ecosystems. There are many examples
of efforts to conserve remnant populations of native animals and vegetation and
to reintroduce species into their preferred habitats. The ‘worth’ of these species lies
not only in the part they play in sustaining ecosystems but also, in many cases, in
sustaining agricultural systems.
The only green grasses left on many farming properties throughout prolonged
drought were remnant populations of native species. Some estimates say Australia
has been left with a mere 2% of the grasses it once had. Farmers are recognising the
benefit of planting these but seeds are scarce and collection for the establishment of
seed banks is costly. Indigenous knowledge of the use of many native plants is no
longer as widespread as formerly.
bioTERMS
remnant populations
small, isolated groups of a
species surviving after the
rest of the population has not
survived
Figure 10.46 The niche of
(a) river red gums and (b) a platypus.
Putting it together – the
ecological niche
a
Every ecosystem has a series of niches occupied by species
characteristic of that ecosystem. The concept of niche has
changed as ecologists have come to understand more about
the relationships between an organism, its environment
and other organisms. The first simple definition of
niche described it as an organism’s role or position in
its community.
Since then the definition has been
b
broadened. It includes a species’
requirements, the sets of physical conditions
and resources in its environment and its
adaptations (including tolerance ranges)
to meet those requirements and utilise
the resources. Its interactions with other
organisms of the same or different species,
where, how and when it feeds, and its
patterns of activity also contribute to the
description of an organism’s niche.
333
bioTERMS
fundamental niche
the ideal niche that an organism
would occupy if there were no
limiting factors
realised niche
the actual niche an organism
occupies subject to limiting
factors
An American zoologist, G. E. Hutchinson, distinguished between the fundamental
niche and the realised niche. The fundamental niche is the ‘ideal’ niche a species
would occupy if there were no competitors, predators or parasites. The realised niche
is narrower. It results from an organism’s inability to exploit the resources of its habitat
because of restrictions. A species may not, therefore, be distributed evenly throughout
its potential geographic range.
Abiotic factors suitable for the laughing kookaburra extend virtually all the way
down the eastern coast of Australia and include Tasmania. But the species is not
distributed evenly throughout this geographical range; successful competitors occupy
the kookaburra’s niche in certain areas.
As conditions and resources differ from place to place within an ecosystem, many
niches are open to different species. Animal species that share the same ecosystem
generally differ in their food, use of space and even timing of activities. Different forest
birds feed at different heights above the ground; different animals feed at night and
during the day. Species of waders, with different lengths of leg and different-shaped
beaks, can exploit different parts of the mud flats and reduce competition. This kind
of use of resources is called resource partitioning.
Figure 10.47 Resource partitioning:
feeding heights of birds in an eastern
Australian eucalypt forest.
spine-tailed swift
Canopy
striated
thornbill
leaden
flycatcher
white-throated
treecreeper
Secondary tree layer
brown
thornbill
rufous
fantail
Understorey
yellow-throated
scrubwren
Ground
334
Unit 2
ground
thrush
Chapter 10 Relationships
and populations
Between them, the 13 species of Darwin’s finches on the Galapagos Islands have
evolved to exploit a wide range of ecological niches. The finches can be grouped into
six main types, each having a beak specially adapted for dealing with a particular
kind of food. Some are ground dwellers, some live in trees; some are herbivores and
some insectivores.
Survival of individual organisms within an ecosystem depends on a number
of factors: their relationship with other members of the same species, and their
interactions with other species and with other factors operating in their environment.
In Chapter 11 we will see how ecosystems as a whole are maintained.
REVIEW
34 a Under what circumstances does an organism become a pest?
b Give advantages and disadvantages of the use of chemicals to control pests.
35 Draw up a table that summarises methods of biological control. Include examples of
control relationships.
36 Use an annotated diagram to summarise ways to restore the balance of populations of
species in an ecosystem.
37 a Describe the ‘ecological niche’ of an organism. A concept map may be useful.
b What is the difference between a fundamental niche and a realised niche?
335
Visual summary
commensalism
chemical
mutualism
biological
parasitism
competitor
symbiotic
relationships
collaborator
control
culling
Community
predator
living things
Carrying
Capacity
Biological
Diversity
prey
Ecosystems
counteradaptation
Biomes
biosphere
ecology
Naming
Habitat
surroundings
dominant species
growth rate
Populations of Species
quadrat
random
sampling
techniques
uniform
capture – mark
– recapture
distribution
density
observation
grouped
transect
336
food supply
density
dependent
predation
competition
disease
parasites
Unit 2
density
independent
physical
factors
catastrophic
events
Chapter 10 Relationships
and populations
Key terms
association
dominant species
open ecosystems
biodiversity
ecology
parasitism
biological control
ecosystem
pollinators
carrying capacity
ectoparasite
population
chemical control
endoparasite
quadrat
closed ecosystems
environmental gradients
random distribution
clumped or group distribution
environmental resistance
realised niche
coevolution
equilibrium population
remnant population
coevolve
facultative association
resource partitioning
coexistence
frass
sampling techniques
commensalism
fundamental niche
seed disperser
community
homeostatic control
seed predator
competition
interaction
set point
counteradaptations
interspecific
surplus killing
culling
intimate association
survivorship
density
intraspecific
symbiosis
density-dependent
mutualism
system
density-independent
obligatory association
uniform or continuous distribution
Apply understandings
 Copy and complete the table overleaf, which
summarises the main kinds of relationships
and interactions between living things. You
may need to refer to the information in
the Bioboxes for additional information.
Give at least two examples of each kind of
relationship. Some of the interactions are
subsets of others. Identify these.
 Using the information given in this chapter
and an atlas, suggest the names of the
ecosystems that are labelled A–D on the
map in Figure 10.48. Identify rainfall and
temperatures that would be experienced in
each labelled area.
B
C
A
D
Figure 10.48
337
Relationship or interaction
Description
Example
Colour patterns of a non-poisonous animal similar
to those of a poisonous one
Coevolution
Different species living together sharing the same
resource
Commensalism
Rivalry between species for particular resources
Crypsis
Relationship in which an organism lives in the
tissues of its host
Ecto-parasitism
Mullerian mimicry
Mutualism
Transfers pollen from anther of stamens to stigma
An animal that kills for food
Seed dispersal
Microorganisms living in the gut of a host
 a Draw a graph to show what would happen to a population of rabbits if there
were no predators in the area where the rabbits lived.
b Introduce a predator such as a fox into your population and show on your
graph what may be expected to happen to the rabbit population.
 Refer to Figure 10.49, the surface map (aerial view) of a sample plot in an area
where eucalypts grow.
a Calculate the total surface area covered by species A. What percentage cover is
this? How does it compare with species C?
b How would you classify the area represented by the plot?
c What limitations are there in using one sample plot to represent a larger area
such as a forest?
d Draw a transect profile across XY. What particular information is missing that
would be helpful in drawing a more accurate profile?
e Describe how the information presented would have been obtained.
f Suggest reasons for the difference in distribution of the different species.
 Elephant dung contains a great deal of fibrous matter, including seeds. Some
of their plant food species have evolved to produce seeds that have a coating
of rind to protect them from the elephants’ digestive juices. Unless the seeds
have passed through the elephants’ digestive system first, they are unable to
germinate. Analyse and explain the relationship described and use a visual medium
for communicating your analysis.
 A brown moth, so small it is barely visible and with a preference for chardonnay,
threatened to become the Victorian wine industry’s greatest scourge. The
light brown apple moth can devour up to $2500 worth of grapes per hectare.
338
Unit 2
Chapter 10 Relationships
and populations
X
N
A
D
Scale 1 cm = 2 m
B
Species
Height
Eucalypt species A
40 m
Eucalypt species B
26 m
Eucalypt species C
15 m
C
A
C
Species D
shrubs
Figure 10.49
y
Winemakers in the past resorted to chemical spraying. Now thousands of
Trichogramma wasps, native to Australia, are released. The wasps lay their
own eggs inside the moth eggs and, as they hatch, the wasp larvae eat the
moth caterpillars.
a What are the advantages and disadvantages of chemical spraying?
b What kind of relationship is being made use of to control the moths? Identify
the partners.
c Draw a simple graph to show what happens to the population of apple moths
and the population of Trichogramma wasps over time. Label carefully.
d Suggest why the moths favour chardonnay grapes over others.
Investigate and inquire
 Work in a group of about four. Each person is to choose a different category
of relationship or interaction between living things (for example mutualism,
parasitism) and is to find two Australian examples to investigate. Identify the
components of the relationship or interaction and include which organism is
harmed/benefited/not affected. Decide how your group will collate and present
its findings.
 Investigate an Australian example of pest management incorporating biological
control. Make sure you identify the reason for control, the pest to be controlled
and how it is to be managed. Include a bibliography in your report.
 Different species of ants have an amazing range of relationships and interactions
with their own kind and with other organisms. They milk caterpillars, store
honeydew and nectar in specialised workers and use their own grubs to weave
threads to bind their nests, to give just a few examples.
Analyse some ant interactions and prepare a poster to present your findings.
Use a range of references – electronic and print – and maintain a bibliography.
339
 A student is to estimate the population size and density of beetles in an area shown in
the grid. She collected data from 10 randomly placed quadrats each 10 cm  10 cm
(Figure 10.50).
Figure 10.50
9
8
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9
Table of results
Quadrat coordinate
Number of beetles
Quadrat coordinate
Number of beetles
[0.0]
10
[5.9]
14
[1.4]
0
[6.7]
3
[2.3]
45
[7.3]
6
[3.8]
32
[8.2]
12
[4.5]
48
[9.6]
16
a Calculate the population size and the population density of the beetles in the total area.
b Investigate two methods that could have been used to obtain the count of beetles.
 Report on a web search for examples of remnant populations of vegetation. Frame at least
three focus questions to help you with your investigation.
 Report on an investigation into examples of reintroduction into habitats of endangered or
extinct populations. Frame at least three focus questions to help you with your investigation.
 There is increasing interest in basing an agricultural system on farming native species rather
than on introduced European species. Present arguments for and against this proposal.
340
Unit 2