Contextual outline Common plants and animals have many interestingadaptations, and many relationships among organisms

can be studied in a local environment.

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CHAPTER

1Ecosystems

21.1 Organisms are adapted to their environment

Students learn to: identify some adaptations of living things to

factors in their environment identify and describe in detail adaptations of a

plant and an animal from the local ecosystem define the term adaptation and discuss

problems associated with inferringcharacteristics of organisms as adaptationsfor living in a particular habitat.

AdaptationsTake a look at yourself; you are a well adapted organ-ism.You have jointed legs, which allow you to move, towalk, to leap and to run. Consider your hands. They aremarvellous instruments capable of delicate work,manipulating objects, grasping, catching, turning. Youhave adaptations that help you to keep warm. Youshiver and dress in warm clothes. You have a digestivesystem, which enables you to eat and get nutrientsfrom a variety of foods.You are a well adapted creature.You have lungs, which allow you to breathe. Yethumans live in such a variety of environments that it isdifficult to argue that you are adapted best for aparticular land environment. Certainly you have manyadaptations for a life on land and are poorly adapted forlife in water. Certainly you would find it easier tosurvive in a mild temperate environment that is rich inresources, but your kind survives in every land environ-ment. Humans even successfully venture into seem-ingly impossible habitats for short periods of time,under the sea and in space. In these habitats we takeartificial environments with us; we live within acapsule, submarine or space station. A human, how-ever, is a little peculiar. What allows you to do this? Yourmost extraordinary adaptation, your braina brainunlike that of other animals because you have fewinnate behaviours. Your brain is capable of learningthroughout your lifetime. This is your greatest adapt-ation because it allows you to survive in a changingworld and in many habitats. But what is an adaptation?

It is easy to identify some adaptations. The hair onour head keeps our head warm. It is an adaptation.When we are cold our hands turn blue as our bloodvessels leading to our extremities such as our feet, noseand hands contract to reduce the flow of blood to theseareas. This reduces heat loss and it is an adaptation.When we are hot we may sit in the shade to cool down;this is also an adaptation. Adaptations are character-istics of organisms that help the species to survive.

Organisms possess a variety of adaptations thattake advantage of beneficial aspects of living in partic-ular environments as well as adaptations to cope withthe difficulties. Adaptations are often classified asstructural, physiological or behavioural. The terms arelargely self-explanatory. If the adaptation is a physicalfeature, it is a structural adaptation. Ears, the stream-lined shape of a sperm cell, and the flattened body of acockroach are structural adaptations. Physiologicaladaptations include processes such as the kangaroosdigestive processes, which allow it to gain nutrientsfrom tough dry grasses, and shivering to keep ourselveswarm. Behavioural adaptations are simply behaviourssuch as putting on a jumper, a snail coming out to feedwhen it is wet, or the nocturnal activity of a possum.Often a behaviour, a structure, and physiology allcombine to provide a survival strategy. A spider spins aweb (behavioural), has spinnerets from which the webis secreted (a structure), and produces the silk withinspecialised tissues as a result of a sequence of chemicalreactions (physiology). As you read through this unit,try to identify each adaptation as either physiological,behavioural or structural.

Adaptations of organisms in your environmentWatching wildlife programs or reading books aboutexotic animals and plants could make you think thatanimals with interesting adaptations can only be foundin outback Australia, Africa or the jungles of SouthAmerica. You share your environment with manyorganisms with fascinating adaptations.

Sometimes the environment has changed a greatdeal from that to which the animals and plants wereoriginally adapted. Sudden changes can be caused bycatastrophic events such as earthquakes, volcaniceruptions and meteors. Humans can also changeenvironments enormously and quickly.Yet, your houseis inhabited by many organisms. Your bed is probably

Biology in Context: The Spectrum of Life

Figure 1.1.1 Bacteria on skin

3the home of bed mites. Your kitchen is visited bycockroaches. Bacteria dwell on your skin despite yourfrequent washing (see Figure 1.1.1) and dust mites liveoff the dust in your home, which is mainly made up ofhuman skin cells that you and your family have lost.Even birds and mammals may use your house as asubstitute tree.

Animal adaptation: spidersIn order to analyse the adaptations of organisms inenvironments, we will consider one group of organismscommon in environments inhabited by humansspiders.You may have spiders that weave their webs onyour window ledges and ceilings. You may have had ahuntsman spider invade your bedroom. Most of thespiders around your home feed on insects. Part of aspiders environment includes its food source. Differentspiders are adapted to capture different insects, orinsects as they move through different parts of theenvironment. Many spiders, such as the orb spinnerspider, spin a web and mainly catch and feed on flyinginsects. Others such as the huntsman spider lie in wait,flattened against the bark of trees. They use theirexcellent eyesight to spot prey and their well muscledlimbs allow them to pounce on their prey. Their preyincludes the many insects that crawl over the treesbark. Other spiders have still more elaborate adapt-ations to capture their prey. The netcasting spider isanother common spider. It weaves a web but unlike theorb spinner it holds this web as a net between its fourfront limbs. Hanging from a thread it waits for insectsto crawl below and hurls its net onto them to trapthem. Like the huntsman it requires, and has, excellenteyesight, which allows it to cast its net with accuracy.

A less common spider is the magnificent spider.The magnificent spider is one of the two Australianspiders that use a chemical-baited sex trap to captureits preymale moths. Female moths produce a scent toattract males. The male moths can detect tiny amountsof this scent and they fly towards it in order to mate.The magnificent spider also produces droplets of thechemical with this same scent. To catch a moth, thespider dangles a droplet of scent on a silk thread. Whena male moth approaches, the spider twirls the thread ina circular motion. The moth flies towards the sex scentwith amorous intent and is trapped in the movingthread.

All of the spiders described above are well adaptedto their environments. They have many adaptations incommon. They are all well camouflaged and havesimilar body structure. They have eight jointed legs,which allow them to move quickly and nimbly. Theyalso have jaws to grasp their prey and a poisonous bite.They all digest their prey by secreting a digestive fluidover their prey. Their prey is digested outside the bodyand the spider then sucks it dry to obtain its food.

Spiders, like most animals, have a range of adaptations,some peculiar to their environment. Others enablesurvival across a range of habitats.These spiders may allinhabit the same environment but each inhabits adifferent part of it. They use different feeding strategiesto catch different prey in different ways.The special partof an environment occupied by a species is called itsniche.

Varied adaptationsMany species of plants and animals have adaptations inwhich they mimic features of other organisms. Someplants attract flies, as pollinators, by smelling likerotting flesh. Needless to say such foul smelling plantsare not often favoured by home gardeners. These typesof chemical mimicry are less common than visual formsof mimicry. The Australian ichnumen orchid, forexample, is shaped like a large female wasp, and malestry to mate with the flower. In attempting to mate, theyspread pollen from one flower to another. You haveprobably noticed large spots on the wings of somebutterflies, which mimic the eyes of a larger organism.It is thought that these may be an adaptation tofrighten their predator. While such a display may workfor some species, others are adapted to be difficult tosee. Some insects are shaped so like a leaf or twig thatthey are almost impossible to see.

Plant adaptation: old man banksiaMany animals are part of our environment. We alsoshare our environment with many plants. If you live inan area with well maintained gardens you may find itdifficult to determine how each plants characteristicsare adaptations to their environment. One way to studyadaptations of these organisms is to find out about thenatural habitats of plants. If you have access to naturalbushland you will find it easier to identify plant adapt-ations and determine how these enable the plants tosurvive. Many Australian plants can be found in yourenvironment, in suburban gardens, city parks, in thecountry, in reserves or national parks.

The adaptations of one plant will be consideredhere as an example of plant adaptations. If you saw anold man banksia growing in a suburban garden youmight wonder at its flower and bark. Some of thesefeatures seem to provide no adaptive advantage andthe production of the huge flower and thick bark wouldbe expensive in the consumption of energy andmaterials. However, the huge flower, in any habitat,seems easy to explain as a reproductive adaptation. Theflower produces large amounts of nectar. The floweradvertises the nectar to birds, insects and some smallmammals that come to feed. As they feed theyinadvertently collect pollen, which is transferred toother banksias on which they feed. Thus the flower is areproductive adaptation. If you saw this same banksia

Ecosystems

4growing in its habitat of poor sandy soil, just after abushfire, then you would see that its thick corky barkhas protected the banksia so that after its leaves andmany of its branches have been burnt, buds beneaththe bark have sprung to life and allowed the banksia togrow back after fire.The woody fruit of the banksia alsoprotect the seeds from fire and after fire they open upto release the seeds. The seeds fall to the ground andgerminate in the ash-enriched soil to produce a newgeneration of banksias. If you looked at the banksiaroots, you would find a dense network of fine roots(proteoid roots), which are capable of obtainingnutrients from the very poor soils it inhabits. Thus theold man banksia is well adapted to its environment,which includes fire and poor soil. However, if you hadseen this banksia as it struggled to survive in an over-watered suburban garden with rich soil you might haveincorrectly concluded that it was a poorly adaptedspecies. We humans sometimes have peculiar ideasabout what is an ideal environment. We sometimesthink that for plants an ideal environment is plenty ofwater, rich soils and ample sunlight. Yet, an idealenvironment is usually the environment to which anorganism is adapted even when this environmentmight seem harsh to us.

Defining adaptationOrganisms live in and are part of an environment.Adaptations can be defined as characteristics that makean organism suited to its environment. This view ofadaptation is useful to a biologist questioning whethera certain characteristic helps an organism to survive.The biologist needs to study the organisms environ-ment to see how the adaptation is suited to thisenvironment. Sometimes there is a clear matchbetween the characteristic and the environment. Thestick insect is well camouflaged so that it can lie hiddenamong the dead twigs of trees it inhabits. Thecentipedes flat body shape and many legs allow it topush through the soil and leaf litter as it hunts smallprey. However, it is not always easy to infer that acharacteristic has evolved as an adaptation to aparticular habitat. Organisms are products of evolution.This means that a present-day organisms character-istics are the products of millions of years of change.During these millions of years, the ancestors ofpresent-day organisms have survived in differenthabitats to which they were adapted. Hence thecharacteristics of organisms are not all adaptations totheir current environments but may have been in-herited from ancestors. This is most obvious amongorganisms that have drastically changed their habitatsover their evolutionary history. Dolphins and whales,for example, are thought to have evolved from land-dwelling mammals that gradually became adapted to

aquatic existence. Whales and dolphins are verysuccessful animals. They are well adapted to life inwater. Their fins and flukes combine with their stream-lined shape to propel them at great speeds through theviscous medium they inhabit. Their lungs, though,seem an odd adaptation. Lungs are characteristics ofair-breathing land vertebrates. It is likely that the lungsof whale and dolphins do not indicate a particularadaptation for their aquatic environment. Rather, theyhave lungs because they have evolved from land-dwelling air-breathing ancestors and lungs were anadaptation to their ancestors environment. Therefore,in studying organisms, we need to be careful not toassume that all characteristics are adaptations to theorganisms present environment. Some characteristicsmay be leftoveradaptations to environments inhabitedby ancestors. Other characteristics may have noadaptive advantage at all but may be features thatprovide no significant advantage or disadvantage.

The environmentIn this unit you have seen how organisms are adaptedto their environment. The organisms environmentincludes both the living things with which it shares itsenvironment, such as the predators and prey, and alsoincludes its non-living surroundings, such as thequality of the soil and the frequency of bush fires. Theenvironment is a product of the interactions betweenthe many organisms and the non-living aspects of theenvironment that exist together. In the next unit, webegin the study of these relationships so thatenvironments and the organisms that inhabit them canbe better understood. In Chapters 2, 4 and 5 moreadaptations of a variety of Australian organisms to theirenvironment are discussed in detail.

Summary 1.1 Adaptations are inherited characteristics of organisms

that increase the chance of survival of the species. Adaptations are also often described as character-

istics of organisms that are suited to the organismshabitats.

It is sometimes difficult to infer that the character-istic of an organism is an adaptation to its particularhabitat because: the organism may be observed outside the

habitat in which it evolved, for example, in asuburban garden

the characteristic may provide no particularadvantage in a particular habitat but has beeninherited from ancestral organisms thatinhabited different habitats

it may simply be difficult to be certain how aparticular characteristic helps a species tosurvive.

Biology in Context: The Spectrum of Life

5Ecosystems

Organisms have a range of adaptations to theirhabitats. Plants and animals in your environmenthave a range of adaptations. Many are well adaptedto life in a house and its surroundings.

Closely related organisms, such as spiders, sharesome adaptations such as body structure, limbs forlocomotion and external digestion, but they havespecialised adaptations that suit them to theirparticular habitat, such as the strategies they use tocapture prey.

Questions 1.11 What is an adaptation?2 Give an example of an adaptation you possess. Explain why this is an

adaptation.3 Name one organism that can be found in your home. Identify one of its

adaptations.4 Describe three adaptations of animals from this unit. Identify each as

structural, behavioural or physiological.5 Describe three adaptations of plants from this unit. How does each

increase the chance of survival?6 Figure 1.1.2 shows a cup-moth caterpillar. Cup-moth caterpillars live

on eucalyptus. When disturbed, the cup-moth caterpillar sends outbunches of poisonous spikes, which give a severe sting. From theinformation provided, list two adaptations of the cup-moth caterpillar.Why is it advantageous for the cup-moth caterpillar to be brightlycoloured?

7 Why is it difficult to infer that some characteristics of organisms areadaptations to living in a particular habitat?

8 (a) A puffball fungus releases thousands of spores simultaneously.What would be the adaptive advantages of this reproductive strategy?What is one disadvantage of this strategy?(b) Figure 1.1.3 shows a human fetus inside the womb. What is one

advantage of this type of reproductive strategy? What are twodisadvantages?

9 The cicada (see Figure 1.1.4) is a commonly seen organism in Australiaduring summer. Biologists think it lives underground for about 5 to 7

Figure 1.1.2 A cup-moth caterpillar

Figure 1.1.3 A human fetus inside the womb

6 Biology in Context: The Spectrum of Life

years before coming out of the ground to change form and mate. Duringsummer males produce a spectacular drumming noise. Two studentsthought that the drumming was an adaptation. Look carefully at thecicada shown and use your knowledge from observations of cicadas toanswer the following:(a) One student inferred that the drumming noise was used to scare

away birds and other predators. Another student thought it mightbe to attract other cicadas.(i) Why do you think cicadas drum? Explain your reasoning.(ii) How would you test these different ideas?

(b) Most cicadas come out of their holes at night when they changefrom their underground form to their flying, tree-dwelling form.Why do you think they do this at night?

(c) One type of cicada, commonly called a double drummer, comesout in the day-time but usually in great numbers at the same time.How might this be an adaptation?

(d) A cicada has a thin pointed tube, going from the mouth regionalong the centre at the front of the body, which is visible outsidethe body. Cicadas feed on trees and when underground on treeroots. Why might this tube be a feeding adaptation?

(e) Suggest three other adaptations cicadas have.

10 Is school an adaptation? If it is an adaptation, would you classify it asstructural, behavioural or physiological?

1.2 Ecosystems: environments, ecology and communities

Students learn to: identify the factors that determine the

distribution and abundance of a species ineach environment.

Your environment and ecologyTake a look at your environment. Your environmentincludes rocks and soil; the air you breathe and wateryou drink, as well as the buildings in which you live.And you are not alone. You share your environmentwith a variety of plants and animals. Some are too smallto see, such as the bacteria on your skin and the virusesthat give you a cold. Others, such as trees, can be huge.These plants, animals and other parts of yourenvironment do not just exist together. They interactwith each other.

Plants provide food and oxygen for animals.Animals provide carbon dioxide for plants. Treesprovide shelter and nesting sites. Animals pollinateflowers and disperse seeds. Plants and animals needthe suns light and warmth. Soil is held in place by rootsand enriched by leaf litter and animal droppings.

Ecology is the study of such interrelationshipsbetween organisms and the interrelationships betweenorganisms and their non-living (physical) sur-roundings. Therefore ecology can be simply defined asthe study of how organisms interact with otherorganisms and their physical surroundings.

Consider one local environment inhabited by aschool student. A description of the school environmentmight include details about buildings and classroominteriors, climate and topography, gymnasium facilitiesand the size of the asphalt playgrounds, the types andnumbers of students and teachers, the availability ofshade trees and lawns, the pigeons that feast on left-overs between recess and lunch, and the availability ofjunk food at the tuckshop. In short, a completeinventory of the environment would include both theabiotic (non-living things) and biotic (living things).

An ecologist would not be satisfied with such adescription as an ecological study of the environmentbecause the fundamental purpose of ecology is tounderstand the way in which these things interact. Forexample, what influence does the climate have on thegrowth of the shade trees? How do these trees affectthe distribution of students during lunch? Whatinfluence do school holidays have on the availability offood scraps for pigeons and how do the pigeonsrespond to fluctuating food supplies? That is, anecologist is interested not merely in a description of theenvironment but wants to observe and explain, firstly,

Figure 1.1.4 A cicada emerging from its shell

7Ecosystems

Farmlandecosystem

Woodlandecosystem

Heathecosystem

Forestecosystem

clouds

rain

sun

eucalypts

ferns

grasses

shrubs

eucalypts

grasses

grasses

sedges shrubs

the way in which organisms affect and are affected byother organisms and, secondly, the way in whichorganisms affect and are affected by their non-livingsurroundings.

Ecosystems and communitiesThe basic unit of study in ecology is an ecosystem. Theset of interacting organisms in an area together withtheir non-living surroundings makes up an ecosystem.It can be defined as a self-sustaining group of organ-isms interacting with its environment. Ecosystems mayvary enormously in size and complexity. An ecosystemcould be an ocean or a coral reef, a creek or anaquarium, a forest or a rotting log, the earth itself or aPetri dish culture of microbes.

Ecosystems may vary in size, but they all have aboundary within which the processes occurring can bestudied and across which the energy and materialinputs and outputs can be examined. The boundary isusually set for the convenience of the study. In this way,within an area of bushland one ecologist might study aforest ecosystem, another, the neighbouring woodlandecosystem, a third, the nearby heath ecosystem, andstill another may choose to study the whole area as asingle ecosystem (see Figure 1.2.1). Each has clearboundaries set for the purpose of the study.

An ecosystem can be thought of as being made upof two interrelated partsthe biotic and abioticcomponents.The set of interacting organisms within anecosystem is called a community. In ecology the termcommunity does not usually refer to a single species

but refers to all the many different interacting plantsand animals in the area.

When studying a specific ecosystem, it is convenientto name it. Hence the ecosystem that is composed ofthe blue gum forest community and its abiotic environ-ment can be called the blue gum forest ecosystem.

Naming ecosystems and communities is usefulbecause it allows a biologist to communicate some-thing about an environment simply with a name.

Summary 1.2 Ecology is the study of how organisms interact with

other organisms and their physical surroundings. The non-living aspects of the environment are

called the physical or abiotic components. The organisms or living things in the environment

are the biological or biotic components of theenvironment.

An ecosystem is the basic unit of study in ecology. An ecosystem is a self-sustaining group of organ-

isms interacting with its environment. An ecosystem consists of the biotic (living) and

abiotic (non-living) components of an area. Ecosystems vary in size and complexity. The interacting organisms within an ecosystem are

called a community. Communities are made up of different plants and

animals. A specific community or ecosystem is often named

by stating its dominant species and its vegetationstructure (e.g. blue gum forest community and bluegum forest ecosystem).

Figure 1.2.1 Profile of a bushland ecosystem

8Questions 1.21 Name two plants and two animals, other than humans, that are part of

your environment.2 On a map that shows your school or home, identify at least two eco-

systems.3 Use a common name to identify an ecosystem near where you live or

near your school that you could study.4 What are each of the following:

(a) ecology(b) ecosystem(c) community?

5 What are the two components that make up an ecosystem?6 What two groups of components make up an organisms environment?7 Give one example from this unit and one example from your own

experience for each of the following:(a) an interaction between organisms(b) an interaction between organisms and their physical surroundings.

8 What is the basic unit of study in ecology?9 Is mainland Australia one ecosystem or many? Explain.10 What two features of a community or ecosystem are often included in

their name?

11 (a) Copy Figure 1.2.2 into your book.(b) Draw lines to indicate the boundaries of three ecosystems.

Suggest possible names for each.

1.3 The distribution and abundance oforganisms

Students learn to: examine trends in population estimates for

some plant and animal species within a localecosystem.

In units 1.3 and 1.4 you will learn how abiotic factorsinfluence the distribution of plants and animals.

When you study an organism in an ecosystem, thereare two questions that often spring to mind: how manyof them are there, and where are they? The region wherean organism is found is its distribution. The number ofindividuals in an ecosystem is its abundance.

DistributionThe distribution of an organism usually shows thelocations in which it can be found. Distributions on alarge scale, such as the distribution of an organism inAustralia, can be determined by such methods astrapping, personal sightings, the observation of tracksor traces, etc.The data are collected and the distributionis then often shown on a map (see Figure 1.3.1).

When studying a smaller ecosystem, the distri-bution of a particular organism is sometimes describedon maps of the area (see Figure 1.3.2). The methodused to determine the distribution usually depends onthe nature of the ecosystem itself, the characteristics ofthe organism under study, and the resources availableto the researcher. The methods used to study the distri-bution of feral cats in a city like Sydney, for example,might differ considerably from the method used tostudy the distribution of banksias in a coastal reserve.

Biology in Context: The Spectrum of Life

Key:

tall red gums

stunted grey gums

grass

Figure 1.2.2 An aerial view of three ecosystems

Figure 1.3.1 Satin bower-bird distribution in Australia

9Ecosystems

TransectsThe distribution of plants can generally be determinedby identifying individual plants and describing theirlocation in an area. This is usually done by marking outa straight line across an area, noting the types of plantspresent, and plotting their position along this line on adiagram. This indicates the distribution of plants alonga cross-section of the ecosystem. This cross-section iscalled a transect (see Figures 1.2.1 and 1.3.3). Althoughplants are most often the subject of transect studies, thedistribution of animals that tend to stay in the sameplace, such as those on a rock platform, can beexamined with the aid of transects.

On a transect, it is common practice to sketch thetopography of the cross-section as a single line and

sketch the plants as they occur along the line to showtheir distribution. Typically, a vertical scale indicates theheight of the plants and the distance across the area isindicated along a horizontal scale. Sometimesimportant factors that may influence the distribution,such as changes in soil type or depth, can also beindicated on the transect (see Figure 1.3.3).

If a different perspective is desired, a series of tran-sects across an ecosystem can be combined to producean aerial view or surface map of the vegetation distri-bution. Figure 1.3.2 could have been produced this way.

Estimating abundanceThe abundance of an organism is the number ofindividuals belonging to the same species in an area.This can most accurately be determined by simplycounting every individual in an area. The abundance ofpeople in Australia is determined by counting everyperson in a census. However, it is often impossible tolocate every individual in an ecosystem and even whenpossible, it is usually too time-consuming and costly.Imagine the time it would take to identify and counteach ant in an ant nest, let alone a forest ecosystem.

QuadratsThe abundance of a species in an ecosystem is usuallyfound by taking small samples of the community andusing the data obtained from them to estimate thepopulation in the ecosystem as a whole. As with distri-bution, it is generally easier to find the abundance of aplant than of an animal. The abundance of a plantspecies is usually determined by marking out a numberof small, randomly selected square areas in theecosystem. These squares are called quadrats. Theindividuals within the quadrats are counted. Theaverage number per area (density) is calculated and

residential

eucalypts

privet

lantana

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tea-tree

grass

1 km

residential

oval

industry

creek

Figure 1.3.2 Vegetation distribution in a suburban reserve

Heig

ht (m

)

Distance (m)

80

60

40

20

050 100 150 200 250 300 350 400 450

grasses

lantana

privet

eucalypts

tea-tree

key:

Figure 1.3.3 Transect through a suburban reserve

10 Biology in Context: The Spectrum of Life

this can be used to estimate the abundance in thewhole ecosystem (see Figure 1.3.4).

The size of the quadrats used depends on thecharacteristics of the organism being studied. A largeorganism requires a larger quadrat than a small organ-ism. An organism whose distribution is even and con-sistent requires fewer quadrats than one whose distri-bution is scattered and erratic. A few small quadratswould suffice to estimate the population of grass on anoval whereas many large quadrats would be required toestimate the abundance of the sparsely scattered cedarsin an Australian rainforest.

When estimating abundance, a number of quadratsare always selected at random so that any chance vari-ations within the quadrats will even out. This can beexplained if we compare the sampling procedures withtossing a coin.To estimate the number of times a tossedcoin will land heads, if tossed a million times, youcould take a very small sample, of only two tosses. Iftossed twice, a coin might land heads twice. Thisparticular sample would result in an estimate of amillion heads in a million tosses. If a sample of 100tosses were used, a result closer to 50/50 wouldprobably be obtained. The estimate from this largersample is likely to be more representative of the actualoutcome. The greater the number of quadrats sampled,the better the abundance estimate is likely to be.

The abundance of animals can also sometimes beestimated by using quadrats.The abundance of animalsthat remain in one place, such as barnacles, sea-squirtsor oysters, can be estimated by using quadrats. So toocan be abundance of large animals in herds on openplains, and animals that can be easily flushed fromdefinable areas of undergrowth. Quadrats have beensuccessfully used to estimate the abundance of zebras,kangaroos, tigers, and ground parrots, for example.Often, however, the fact that animals move makes theuse of quadrats difficult.

The capturemarkingrecapture techniqueA less widely used method of estimating abundance isthe capturemarkingrecapture technique. In this

procedure, traps are set in the study area. The animalsare captured, marked and released. Traps are reset inexactly the same way and under the same conditions asecond time. By comparing the number of markedindividuals with the number captured, the populationof the animals in the area can be estimated (see Table1.3.1).

The capturemarkingrecapture procedure is basedon the assumption that the number of markedindividuals in the second catch is proportional to thenumber of marked individuals in the whole population.Using this procedure, let us say that traps were set and15 individuals were captured, marked and released.Then traps were reset and 10 individuals werecaptured. If 5 of these 10 were marked, it is reasonableto assume that half of the total population is marked.Altogether, 15 individuals were marked, therefore thetotal population in the area is 30. That is, twice thenumber originally marked. More accurate estimatescan be obtained by repeating the exercise a number oftimes and calculating an average.

The capturemarkingrecapture technique soundsgreat in principle but has problems when put intopractice. The technique is based on the assumption thatanimals are no more or less likely to be captured in thefirst trapping than in the second or subsequenttrappings. This assumption often does not hold true.Some small rodents, for example, seem to enjoy the food

Key:

heathland

quadrats

density = 13 Christmas bells 500 m 2,i.e. 2.6/100 m 2

abundance = density x total area= 2.6/100 x 5000= 130 Christmas bells

Christmas bells

10 m

10 m

Figure 1.3.4 Quadrats used to estimate the abundance of Christmas bells in a heath

Capture (nI) Recapture (n2)

Number caught 15 10Number marked 5 (m2)Abundance = Number captured x Number recaptured

Number marked in recapturei.e. A = n1 x n2

m2A = 15 x 10

5A = 30

Table 1.3.1 Capturemarkingrecapture data on bush rats

11Ecosystems

they get when first trapped and seem only too willing tobe recaptured. Other animals may be badly frightenedby the experience and avoid the traps in future. Thisproblem results in the capturemarkingrecapturetechnique sometimes producing unreliable data.

Summary 1.3 The distribution of an organism is the region that it

inhabits. The distribution of an organism on a large scale is

usually described by plotting on a map the places inwhich it is found.

Transects are often used to determine the distri-bution of plants.

A diagram of a transect is often used to show thedistribution of plants.

The distribution of animals can often be determinedby personal sightings, trapping, or the observationof tracks and droppings.

The population of an organism is the group ofindividuals of the same species in an area.

The abundance of plants and some animals can beestimated by counting the number of individuals inrandomly selected quadrats.

The abundance of animals can sometimes be esti-mated by the capturemarkingrecapture technique.

It is usually easier to determine the distribution andabundance of plants than of animals becauseanimals move and may hide.

Estimates are usually used to determine theabundance and distribution of organisms because itis too difficult or too expensive to find every organ-ism in an environment.

Questions 1.31 Distinguish between the terms population and distribution.2 Describe how you could determine the distribution of plants in an

ecosystem.3 How could you estimate the number of weeds in a local area, for

example your garden or backyard, a local park or pasture?4 How could you estimate the population of an animal in your local area,

for example the population of cockroaches in your kitchen, snails inyour garden, dogs in your street or barnacles on a rock platform?

5 Describe how the information in Figure 1.3.5 on the distribution ofkoalas might have been determined.

6 Describe how you would estimate the abundance of tea-trees on acoastal sand dune.

7 What factors would influence the size and number of quadrats thatwould be used to estimate the abundance of an organism?

8 Describe in detail how the abundance of wombats in an ecosystemcould be estimated.

9 Figure 1.3.6 shows the distribution of saltbush shrubs in an arid eco-system. Using quadrats, answer the following questions.(a) Estimate the saltbush abundance.(b) What is the saltbush density per 100 square metres?

(c) Feral camels and donkeys are thought to be damaging the eco-system by destabilising sand dunes and feeding on saltbush.Suggest one way in which the extent of their impact could bemeasured.

10 A group of biology students, with the help of a park warden, obtainedthe following data when using the capturemarkingrecapture tech-nique to estimate the abundance of Antechinus stuartii in a woodlandon the New South Wales south coast (see Table 1.3.2).(a) Estimate the Antechinus abundance in the woodland.(b) How could the accuracy of the estimate be improved?(c) Why might an ecologist be reluctant to use the capturemarking

recapture technique?

Figure 1.3.5 Koala distribution in Australia

key: saltbush Scale: 1 cm = 10 mm

Figure 1.3.6 Distribution of saltbush in an arid ecosystem

Day Capture Number marked Released

1 (capture) 8 8 82 (recapture) 6 3 6

Table 1.3.2 Capturemarkingrecapture data for Antechinusstuartii in a woodland ecosystem

12 Biology in Context: The Spectrum of Life

1.4 Factors determining distribution and abundance

Students learn to: identify factors determining the distribution

and abundance of species in environments.

The abundance and distribution of organisms areinfluenced by a complex range of physical and bio-logical factors interacting in the environment. Theparticular combination of influences that determine theabundance and distribution of particular organisms isusually unique. However, four major interrelationshipscan be identified. These are the organisms interactionwith or influence by: abiotic environmental factors,availability of resources, other members of the samespecies, and other organisms of different species.

Abiotic factorsOne of the most important aspects of the abioticenvironment for terrestrial organisms is climate.Typically, the climate and soil characteristics of an eco-system interact to determine the types of plants thatgrow in a region. Since plants generally provide foodand shelter for animals, the vegetation in the eco-system in turn determines the distribution andabundance of animals. Although all of these (animals,plants, soil and climate) interact to maintain the eco-system, it is ultimately the physical environment thathas the most profound influence on the long-termdistribution and abundance of organisms.

Whilst the roles of climate and soil in determiningthe character of an ecosystem and community are ofcritical importance, it must be remembered that theyare not single factors but represent sets of interactingphysical factors. Climate includes such things as theamount and pattern of rainfall, average temperatureand temperature range, humidity, and all the regularand irregular atmospheric phenomena that constitutethe weather. Soil characteristics include the texture,depth, drainage, quantity and dispersal of humus(decaying material), moisture content and water-holding ability, and the availability of salts such asnitrates, phosphates and sulfates. And these are justsome of the non-living factors that combine to providethe fundamental characteristics of the environment towhich organisms are adapted.

Within a particular ecosystem, the measurement ofthese physical factors in the area can often provideevidence to explain and predict the characteristicpatterns of distribution. Table 1.4.1 lists some of theabiotic factors that could be examined when studyingan ecosystem and briefly states how they can bemeasured. Further details are given in Investigation 1.

* For more detail on measurement techniques see Investigation 1 at theend of this chapter.

ResourcesAnything an organism uses is a resource. Plants have afundamental role in the community as a resource foranimals. Resource needs of animals include such thingsas food, living space, shelter, nesting sites, nestingmaterials, oxygen, and water. The availability of thesecan play a critical role in the abundance and distri-bution of organisms.

Sometimes the availability of a single resource maybe the single factor that determines the maximumpopulation of a particular species in an area. Such afactor is called a limiting factor (see Figure 1.4.1). Forexample, many Australian birds, bats, possums andgliders nest on roofs in the hollow branches ofeucalypts. These hollows typically begin to occur in thedead branches of trees that are about 100 years old.Large hollows suitable for the possums and parrotsmay not develop until trees are 150 to 200 years old.

Abiotic factor Measurement*

light intensity light meterair temperature thermometerdaily temperature range max./min. thermometerrelative humidity wet and dry bulb thermometerrainfall rain gaugewind anemometersoil temperature soil thermometersoil depth digging to expose soil profile and measuringsoil moisture comparison of wet and dry weightssoil porosity rate of water flow through samplehumus estimating leaf litter depth and comparing

burnt and unburnt soil weightspH indicator and pH chartsoil mineral nutrient soil test kit

Table 1.4.1 Abiotic environmental factors and their measurement

Figure 1.4.1 The abundance of parrots and the influence of nesting sites

Time

Abun

danc

e

no additionalnesting boxes

additional nestingboxes provided

13Ecosystems

If these large old trees are logged from forests, thepopulation of many birds and mammals can bereduced rapidly in a single breeding season. Even ifsimilar eucalypt species are replanted to produce ayoung robust forest, it may be hundreds of years beforesuitable hollows reappear. Nevertheless, the popu-lations can to some extent be preserved if artificialnesting boxes are provided and reserves containingstands of mature trees are established throughout theforest. Similarly, the provision of nesting boxes insuburban gardens and reserves can help to maintainand increase the abundance of many parrots andpossums.

Summary 1.4 The distribution and abundance of organisms can

be affected by a variety of factors including abioticenvironmental factors, availability of resources,other members of the same species, organisms ofdifferent species.

An examination of variations in these factors withinan ecosystem can often provide an explanation forthe distribution and abundance of organisms withinthat ecosystem.

The distribution and abundance of a particularorganism is usually determined by a number ofinteracting factors.

A resource is anything that is used by an organism. When a single resource, such as breeding sites or

food, is the factor that limits the abundance of anorganism, it is called a limiting factor.

Questions 1.41 List the four main interrelationships that influence the distribution and

abundance of organisms.2 List some of the main abiotic factors that may influence the distribution

and abundance of an organism.3 Explain why it can be said that the physical environment ultimately

determines the distribution and abundance of organisms.4 (a) Describe the distribution of one plant and animal in your local area.

For both the plant and animal answer the following questions:(b) Why do you think it is found in the areas shown?(c) Is it more common in some places than others? Explain.

5 Carnivorous plants, which capture and digest insects, often dominateimpoverished, water-logged soils but are rare where soils are rich andwell drained.(a) What factors influence their distribution?(b) Why can they survive in soils where few other plants can?

6 In terms of the abundance of organisms, what is meant by a limitingfactor? Is there a single limiting factor controlling the abundance of anyorganisms in your area?

7 Suggest the main abiotic factors you would examine in a terrestrialecosystem and state briefly how you could measure them.

8 Suggest some of the main abiotic factors you would examine in anaquatic ecosystem.

1.5 Comparing the abiotic factors ofterrestrial and aquatic environments

Students learn to: compare the abiotic factors of aquatic and

terrestrial environments.

In unit 1.4 you saw how the abundance and distri-bution of organisms could be explained in terms ofabiotic factors and resources. In this unit, the abioticfactors of aquatic (water) and terrestrial (land)environments are compared to illustrate the differentabiotic factors and resources that exist in these two verydifferent environments.

From a human perspective, it is all too easy toregard the aquatic environment as hostile and unfor-giving.Yet water provides an environment in which it ismuch easier for life to exist than on land. It is thoughtthat life first evolved in water and water is the majorcomponent of all living things.

Physical characteristicsViscosityViscosity is a measure of how difficult it is to movethrough a substance. For example, lets compare theviscosity of water and honey. If you dropped a ball-bearing into a glass of honey and a glass of water, theball-bearing would fall much more slowly through thehoney than through the water. Honey is more viscousthan water. If we extend the experiment a little furtherby dropping the ball-bearing into a glass containing air,then the ball will fall faster through the air thanthrough the water. Water is more viscous than air.

Viscosity is an important feature of the aquatic andterrestrial environments and it is one area in which aterrestrial existence provides an advantage over theaquatic environment. It is much easier for animals tomove through air than through water. Many aquaticanimals have a streamlined shape, which allows themto move more easily in water.

BuoyancyBuoyancy is a measure of a substances ability tosupport or hold up an object. For example, if you placea cork in water it is easily supported by the water, butthe same cork falls through air. Water provides greaterbuoyancy than air. Air appears to offer no support at all.However, this is not true because air does provide somesupport. If you drop a sheet of paper, the buoyancyprovided by air can be observed, but highly specialisedadaptations, such as wings, are required to make anyuse of it.

14 Biology in Context: The Spectrum of Life

In this way water provides an advantage over theterrestrial environment because it provides greatersupport for an organism and this support is more thana mere upward thrust. Organisms are surrounded bywater, which not only helps to hold them up but, insome cases, also maintains their very shape. A jelly fish,for example, quickly collapses into a deformed blobwhen removed from water.

Temperature variationThe terrestrial environment can experience huge vari-ations in temperature in very short periods. Within asingle day, variations of 15 to 20C are not uncommon,and far greater temperature fluctuations occur, in desertenvironments. Even in relatively small bodies of water,such temperature variations do not occur. Water temp-eratures change much more slowly and this can be veryfrustrating for early summer beach-goers who swelterin high temperatures on land but find the water still toocold for anything more than a quick dip.

Except at the very edges, the temperature of theoceans remains constant from year to year. Indeed, anyslight variation in oceanic temperature could have quitedisastrous effects. For example, an increase in oceantemperatures of only a few degrees could change globalweather patterns, melt Arctic and Antarctic ice, andexpand the volume of the ocean, enough to floodcoastal cities.

Since it is much easier to adapt to a constantenvironment than to varying conditions, the constanttemperatures of the aquatic environment are muchmore conducive to life than the varied temperaturesexperienced on land.

Conduction of heatAlthough temperatures remain more constant in waterthan on land, organisms tend to lose heat more rapidlyin water. This is because water conducts heat betterthan air. People lost at sea probably die more often fromheat loss (hypothermia) than drowning because heatis so quickly lost to the surrounding water. Aquaticbirds and mammals whose body temperatures arehigher than that of the water they inhabit must beadapted to prevent this heat loss by conduction.

Availability of gasesOrganisms need oxygen for respiration, and plantsneed carbon dioxide for photosynthesis. As almost20 per cent of air is oxygen, it is available in abundanceon land except at very high altitudes. Much less carbondioxide is available since only about .03 per cent of airis carbon dioxide, but this appears to adequatelyprovide the needs of photosynthesis in plants.

Oxygen and carbon dioxide dissolve in water.Where water is in close contact with the air, both arereadily available. In particular, these gases are most

abundant where turbulent water is tossed through theair in places such as river rapids, water falls andbreaking ocean waves. The dissolved gases are thengradually mixed throughout the water by slow con-vection currents. Nevertheless, as water depthincreases, the availability of both gases decreases.Stagnant ponds and pools, too, often lack sufficientoxygen for the survival of most organisms.

The availability of gases in water is also affected bytemperature. When you heat water, you will notice thesmall bubbles that form well before the water boils.These are bubbles of air that come out of solution as thewater heats up. As temperature increases, the solubilityof gases in water decreases.This means that there is lessoxygen and carbon dioxide in warm tropical seas thanin Arctic and Antarctic oceans. However, the abundanceand variety of life in the tropical marine environmentshows that enough of both gases is available.

Diffusion of gasesThe movement of gases through water and air can beinfluenced by wind and currents, but their movementinto and out of cells depends on diffusion. Thediffusion of gases is about 10 000 times faster throughair than through water. As a consequence, air provides atremendous advantage for the rapid movement ofgases. However, to gain entry to cells, they must dissolvein water to pass through the cell membrane. This is whyany surface used for gas exchange must be moist.

A Freshwatercell wall

cell membrane

water in

water out

water diffuses in and out of thecell across the cell membrane

More water enters the cell by osmosis than leaves the cellthe concentration of substances is greater inside the cell than outside

B Salt water

cell wallcell membrane

water in

water out

water diffuses in and out of thecell across the cell membrane

More water leaves the cell by osmosis than enters the cellthe concentration of substances is greater outside the cell than inside

Figure 1.5.1 Osmosis in fresh water and salt water

15Ecosystems

Availability of waterOn land, water is at a premium. It is quickly lost fromorganisms by evaporation and must be replaced con-stantly. In an aquatic environment, water surroundsorganisms and yet it may not be as readily available asyou might imagine. In a freshwater environment, watertends to constantly diffuse into organisms. This isbecause cells contain more ions and organic substancesthan the surrounding water. This causes a net move-ment of water into the cells. By contrast, in the marineenvironment, cells often have a lower concentration ofsalts than the surrounding water. Under these con-ditions, there is a net movement of water out of the cell(see Figure 1.5.1). (For more information about thismovement of water into and out of cells see Unit 2.5)

Availability of ionsOn land, ions (salts) are available in soil water. Plantsabsorb these through their roots and animals obtainthem when they feed off plants or other animals. Somesoils lack essential ions and few plants will grow undersuch conditions. Conversely, some soils, particularly inWestern Australia and Victoria, contain excessive saltsand this prevents plant growth because water diffusesfrom the roots into the soil rather than from the soilinto the plant. In the marine environment, most ionsare available in abundance. Just as convection currentscarry oxygen and carbon dioxide to the ocean depths,so, too, these same currents return ions from de-composed organisms to the surface. Nevertheless,

some ions such as those of calcium are in demand by somany organisms for the production of calciumcarbonate shells that its availability may limit theabundance of some animals (see Figure 1.5.2).

LightOn land, light is available in abundance. It is generallyonly scarce on the floors of dense forests and caves. Inwater, light is often at a premium. The surface of waterreflects light. This means that only about 70 per cent ofthe light that strikes the surface penetrates. Further-more, water absorbs light.Therefore as depth increases,light availability decreases. On the ocean floors, there isno light for photosynthesis or vision and both plantsand animals have become adapted to cope with thedifficulties this presents (see Figure 1.5.2).

Pressure variationsOn land, there are frequent fluctuations in pressure.Typically, these are measured regularly and included indaily weather reports. However, these variations aresmall and have little direct impact on organisms. Inwater, by contrast, although pressures do not fluctuate,there is considerable variation. As water depthincreases, pressure increases (see Figure 1.5.2).Pressures are so great on the floors of the deepestoceans that they can crush submarines.Yet despite thetremendous pressures, specially adapted animals doinhabit the ocean depths.

These organisms were first studied by dragging nets

light light reflected

light penetrateslow pressure

light absorbed

no light

Deep waters

high pressure

ionsabundant

ionsfrom deadorganisms

gases dissolve

gasesabundant

Surfacewaters

Deepwaters

Figure 1.5.2 Comparing surface water and deep water in an ocean

16 Biology in Context: The Spectrum of Life

along the ocean floor behind surface ships, but theanimals captured were sometimes so distorted whenbrought to the low pressures on the surface that theiroriginal appearance was altered. More recently,specially designed vessels have permitted some explor-ation of this environment. However, the difficultiesfaced by such research have prompted some scientiststo argue that more is known about outer space thanabout the ocean depths.

Summary 1.5 Life probably first evolved in water. Water provides

an environment in which it is easier for life to existthan on land.

Water is more viscous than air. Therefore it is moredifficult to move through water than through air.

Water is more buoyant than air. Therefore waterprovides greater support for organisms than air.

Temperatures vary less in water than on land.Ocean temperatures are fairly constant. Constanttemperatures are easier to adapt to than varyingtemperatures.

Water is a better conductor of heat than air. There-fore a body immersed in water will rapidly lose heatto its surroundings.

Gases (e.g. oxygen and carbon dioxide) are availablein greater abundance on land than in water.

The availability of gases decreases with altitude onland and decreases with depth in water.

Gases diffuse more quickly through air thanthrough water.

Water can be lost quickly by evaporation fromexposed surfaces on land.

In the freshwater environment, the concentrationgradient favours the movement of water out of mostcells.

In the marine environment, most ions (salts) arereadily available. In freshwater ions are in very lowconcentrations. On land, most ions are readilyavailable in solution in soil water, though some soilscontain an excess of salts while others contain toolittle.

Light availability in water decreases with depth. On land, air pressure may fluctuate quickly but it

has little direct effect on organisms. Air pressure decreases with altitude. Water pressure

increases with depth.

Questions 1.51 List three advantages and three disadvantages for life in the terrestrial

and aquatic environments that are related to their abiotic character-istics.

2 Why do most fish have a streamlined body shape?3 Why do land animals need larger muscles and bones for support than

aquatic animals?

4 Why is the concentration of oxygen higher near the surface than on thebottom of the oceans?

5 Why is the salt, calcium carbonate, in high demand in the marineenvironment?

6 (a) Why do both marine and terrestrial organisms need to be adaptedto avoid excessive water loss?(b) Why dont freshwater organisms require similar adaptations?

7 Why is it eternally dark on the ocean floor?8 Why dont many aquatic organisms require mechanisms to regulate

their body temperatures?9 The terrestrial environment is sometimes described as a two-phase

environment whereas the aquatic habitat is sometimes said to consistof a single phase.(a) Explain what is meant by this statement.(b) To what extent do you agree?

1.6 The distribution and abundance oforganisms: the influence of light

Students learn to: identify factors determining the distribution

and abundance of organisms in aquaticenvironments.

In Unit 1.5, you compared the abiotic factors ofterrestrial and aquatic environments. This comparisonrevealed the different challenges faced by life on landand life in water.You learnt in Unit 1.1 that an ecologistwould be dissatisfied with this comparison. An eco-logist would want to know about the relationshipsbetween the abiotic factors and the organisms that livein the environment. To consider all the abiotic factorsand their varied influences would be too large a task inthis book. So, in the next two units the effects ofselected aquatic and terrestrial abiotic factors will beanalysed. First, the influence of light availability onaquatic and terrestrial environments will be consideredin this unit. Then, in Unit 1.7 the distribution of comm-unities across Australia will be explained in terms of avariety of terrestrial abiotic factors including rainfall,temperature and soil quality.

Light in waterLight provides the energy requirements of virtually allliving things. On land, it is generally readily available inabundance. In water, useful amounts of light are onlyavailable to a depth of about 100 m depending on thewater clarity. The lack of light is brought about by twofactors: firstly, about 30 per cent of the light that strikesthe waters surface is reflected and, secondly, waterabsorbs light.

17Ecosystems

Water does not absorb all wavelengths of lightequally. The different wavelengths of light, which wesee as colours, make up the colour spectrum of whitelight. If you have been snorkelling or skin-diving, youwill realise that underwater things take on a green orblueish tinge. This is because the red and orange wave-lengths of light are quickly absorbed by water. Thedegree to which water absorbs the different wave-lengths of light is called the absorbance spectrum ofwater (see Figure 1.6.1).

Green, red and brown algaeFigure 1.6.1 also shows the absorbance spectrum ofchlorophyll. A comparison of the two graphs reveals amajor problem for aquatic plants. Water absorbs thevery wavelengths of light that are used most bychlorophyll for photosynthesis. This means that asdepth increases, not only the quantity but also thequality of light decreases.

Plants have evolved a variety of adaptations tomake the best possible use of the light available inwater. Red and brown algae contain coloured materials

(pigments), which absorb the light that penetrates tothe greatest depth in water (see Figure 1.6.2). The redpigment (phycoerythrin) and the brown pigment(fucoxanthin) absorb the blue and green wavelengthsof light. The energy is then transferred to chlorophyll,which carries out photosynthesis. This allows red andbrown algae to live at a much greater depth than greenalgae. If you visit a rock platform at low tide, it is easyto observe a green band of algae exposed on the rocksurface, a red band of algae lower on the rock platform,and brown kelp at the greatest depth, usually coveredby water even at low tide.

Animal adaptationsThe lack of light in deep water also presents majordifficulties for aquatic animals. We humans rely heavilyon sight as our main means of obtaining informationabout our surroundings, but this is not true of allanimals. Dogs, for example, live largely in a world ofsmells because they rely mainly on their sense of smell.Since sight may be of little consequence to them, thelack of light in the ocean depths poses few problems forthe animals that dwell there.

Many aquatic animals rely heavily on smell andsound rather than sight. A few even produce their ownlight, which is called bioluminescence. This may helpthem attract a mate or lure unsuspecting smalleranimals to a predators wide-set jaws. In this blindworld, some animals use senses that humans do notpossess. Electric eels, for example, are sensitive to theminute electrical impulses given off by the nerves in themuscles of other animals. The platypuss bill is similarlysensitive and it detects prey, such as yabbies, in thesame way. Such adaptations are not only advantageousin deep oceans but may be equally useful in shallow,murky freshwater.

Finally, the problems related to lack of light are notlimited to aquatic habitats. At night, there is little lightin the terrestrial environment. Yet many animals arenocturnal. In Australia the vast majority of mammalsare most active at night. Many bats, with the exceptionof flying foxes, see poorly and yet they are one of themost successful mammals. In short, it is only ourpeculiar human perspective that makes us see darknessand dim light as a hostile environment. Had thischapter been written by a bat, it may have discussedadaptations to cope with the bright light of day!

In this unit, the adaptations of organisms to oneabiotic factor, light, have been considered and the wayin which light availability influences the distribution ofalgae described.Variations in many other abiotic factorsalso influence the distribution of organisms in aquaticenvironments. These include: the salt concentration of the water, ranging from the

extremely salty Dead Sea to freshwater with almostno salt

Abso

rban

ce

violet blue green yellow orange red

water(5 m deep)

Light

phycoerythrin

fucoxanthin

chlorophyll-a

Figure 1.6.1 Light absorbance by water

Figure 1.6.2 Absorption of light by pigments in green (chlorophyll),red (phycoerythrin) and brown (fucoxanthin) algae

solar energy reaching ocean surface

ultravioletvisible light

infrared

Sea surface

increasing depth

1 m

10 m

100 m

300500 6

00 700

400only 45% of light energyreaches 1 metre

about 16% reaches10 metres

only 1% remains at100 metres

redorange

yellow

green

blueviolet

10

00

Wavelength (n

anometers)

18 Biology in Context: The Spectrum of Life

temperature variations, which range from hotsprings and geysers through warm tropical seas tofreezing Antarctic oceans

pressure differences, ranging from low pressures insurface waters to extreme pressure in deep oceantrenches

variation in available gases, ranging from little inwarm, stagnant ponds to the plentiful gases in turb-ulent waters of oceans and cold streams

the availability of sulfates for chemosyntheticbacteria near hot gas outlets on the ocean floor.

These and other factors influence the distribution ofvarious organisms, since different organisms areadapted to different conditions.

Summary 1.6 Light provides the energy needs of virtually all

organisms. Water reflects and absorbs light. Therefore as depth

increases, the amount of available light decreases. The colours of light absorbed most by water are

similar to those absorbed most by chlorophyll (seeFigure 1.6.1). The wavelengths needed for photo-synthesis are quickly absorbed by water.

Red and brown algae contain red and brown pig-ments, which absorb the light that penetrates to thegreatest depth.

Red and brown algae are more abundant in deeperwater than green algae.

Algae are distributed near the water surface and notat great depths in water.

Life is abundant near hot gas outlets on the oceanfloor where chemosynthetic bacteria are theproducers.

Questions 1.61 Why is light generally more abundant on land than in water?2 Describe the main problems for photosynthesis in water that are

related to light.3 Why do objects collected under water sometimes have a different

colour when they are brought to the surface?4 (a) Explain why red and brown algae can survive at greater depths of

water than green algae.(b) How does this explain the distribution of redbrown and greenalgae on a rock platform?

5 Do red and brown algae contain chlorophyll? Explain. How does lightinfluence distribution and abundance on land?

6 What is the advantage for a terrestrial plant in growing tall?7 Describe how aquatic animals are adapted to environments with no

light. How does light influence their distribution and abundance?8 Use library resources to investigate how abiotic factors influence the

distribution and adaptations of organisms in an aquatic environment.Two of the most interesting ecosystems you could investigate include:(a) coral reefs, such as the Great Barrier Reef(b) ecosystems found near hot-gas outlets and near mid-ocean

ridges on the ocean floor.

1.7 The flow of energy and matter in an ecosystem

Students learn to: describe the roles of photosynthesis and

respiration in the transformation of energy inecosystems

identify the general equation for aerobiccellular respiration and outline this as asummary of a chain of biochemical reactions

identify the uses of energy by organisms describe the flow of energy through a natural

ecosystem describe the role of decomposure in an

ecosystem.

Uses of energy by organismsThe energy available to organisms in an ecosystem isused in a variety of ways, including movement, makingsound, carrying out chemical reactions as part ofcellular metabolism, producing heat and, in someorganisms, producing light. You are probably familiarwith using energy for sound, movement and heat asyou, talk, walk and maintain your body temperature,but energy is also used by some organisms to producelight. Glow worms, flash-light fish and fireflies, forexample, all use chemical energy to produce light. Thisprocess is bioluminescence, which is a spectacularlyefficient process because, unlike all human systemsdevised to provide light, when organisms convertchemical energy into light energy almost no heat isproduced. Thus organisms again demonstrate thatbiology and evolution have succeeded in developingefficient systems beyond the capacity of humaninvention. The efficiency of this light-producing systemis one reason for their extensive study by biologists.Another is the sheer beauty of the biochemical systemsand the extraordinariness of the organisms.

Energy transfer and lossIt is a fundamental law of science that energy cannot becreated or destroyed. It can, however, be changed fromone form to another. In an electric toaster, for example,electrical energy is converted into mainly heat energy.When a match burns, chemical energy is converted intoheat and light energy. When you shout, some of thechemical energy in glucose is converted into soundenergy. (This actually involves a series of energychangesyou might like to try to draw up a list ofthem.) However, these energy transfers are not perfect.Whenever energy is changed from one form to another,some energy is lost. In car engines, for example, a lot of

19Ecosystems

the energy we would like to see transformed intomoving the car is lost as waste heat and sound.

Energy transfer through ecosystemsIn ecosystems the initial source of energy for thecommunity is light. Plants absorb some of the lightenergy from the sun. Some of this light energy is con-verted, through photosynthesis in the chloroplasts,into chemical energy in glucose molecules.This glucosecan then be transported to other parts of the plant.Typically, about half of it is broken down in respirationto make energy available for cellular processes. The restof the glucose is converted into larger carbohydratesand other organic compounds (see Figure 1.7.1).

Photosynthesis and respiration are both processesmade up of a chain of chemical reactions, which arecontrolled in cells by many enzymes and factors. Twoequations can be used to summarise these complexprocesses.

Photosynthesis is often summarised as:

light carbon dioxide + water glucose + oxygen

This summary shows the reactants in photo-synthesis (carbon dioxide and water), the energy source(light) and the products (glucose and oxygen). It doesnot show the many steps involved in the process. Nordoes it show the role of enzymes and other factors.

Respiration is often summarised as:

glucose + oxygen water + carbon dioxide + energy

Again this reaction tells nothing of the many steps orenzymes or factors in the process but it does summarisethe equation, showing the reactants and the products,and indicates that energy is made available as a result ofthe process. In living systems, photosynthesis convertslight energy into chemical energy, and respiration servesto make this energy available for cellular functions.Respiration and photosynthesis are not oppositereactions, the steps in the reactions are very different.The reactions are very different biochemically.

When animals (herbivores) eat and digest plants,the complex carbohydrates are converted back intoglucose. This glucose can be broken down byrespiration in the animal cells to provide the animalsenergy requirements (see Figure 1.7.2). Similarly, whenanimals (carnivores) eat other animals, they can makeuse of the chemical energy stored in the substances ofthe dead animals body.

The cycling of matter in ecosystemsIn photosynthesis, the carbon dioxide obtained from airand the water absorbed from the soil are used toproduce glucose. From this, other carbohydrates can bemanufactured within the plant cells. Some of thecarbohydrates are used, together with the nitrates, sul-

Figure 1.7.1 Energy transfer from light to plant cells

light energy

chlorophyll photosynthesis glucosetransportedthroughout the plant

respirationenergyfor cell

processes

converted into othercarbohydrates

(starch, sugar etc.)

Figure 1.7.2 Energy transfer from plants to animals

plant eaten by animalcarbohydrates

digested toglucose

respirationenergy

for animal cellprocesses

converted into other

substances

20 Biology in Context: The Spectrum of Life

fates and phosphates, which have been absorbed fromthe soil, to produce proteins and nucleic acids. Otherelements obtained from the soil and incorporated intoplant tissues include calcium, magnesium, iodine,cobalt, molybdenum, and many others. In this waymatter from the physical surroundings, in the form ofsimple salts and gases, is absorbed by the plant andconverted into complex organic substances, some ofwhich are used to produce the plants tissues. When ananimal eats a plant, some of the digested material isused to produce the animals tissues.

The role of decomposersWhen a plant or animal respires, water and carbondioxide are returned to the atmosphere. As animalsrelease waste urine or faeces, materials are returned totheir surroundings. When plants and animals die, theyare decomposed, returning their remaining nutrients totheir physical surroundings. The decomposers, mainlybacteria and fungi, recycle matter. They decomposedead plant and animal material making nutrientsavailable to plants (see Figure 1.7.3). The result is atransfer of matter from organisms to the physicalenvironment. These materials can then be taken inagain by plants to continue the cycle. Unlike energy,matter moves in a cycle through the ecosystem. It is trans-ferred from the physical surroundings to plants; fromplants to animals; and from plants and animals back tothe physical surroundings.

This matter cycle is actually made up of a numberof cycles including, among others, the carbon/oxygenand nitrogen cycles and the water cycle. These cyclesare described in Figures 1.7.4, 1.7.5 and 1.7.6.

Although, in time, all the matter is eventuallyrecycled through ecosystems, within a specific eco-system matter can enter and be lost across ecosystemboundaries. Nevertheless no ecosystem can sustain along-term net loss of matter. In time, a balance must beachieved between the gain and loss of matter.

Summary 1.7 Light energy from the sun provides the original

source of energy for ecosystems. Plants convert light energy to chemical energy in

glucose through photosynthesis. Animals obtain energy in the form of chemical

energy in food. This is mainly in the form of carbo-hydrates.

The chemical energy in glucose is made available toplant and animal cells through cellular respiration.

Organisms never obtain all the energy available intheir food source because energy is constantly beingused and lost by organisms as waste heat; andenergy is lost in every energy transfer.

fungirespire

animalsrespire

oxygen used inrespiration producescarbon dioxide as it

releases energy fromsugar

plantsrespire

eaten byanimals

photosynthesisuses carbon dioxide

to make sugarand releases

oxygen

carbon dioxide in the air

decay ofdead plantsand animals

bacteria respireburning fuels:wood, coal, gas and petrol are usedin these processesthey are theremains of plants and animalswhich lived millions of years ago.

oxygen in the air oxygen used inburning

Figure 1.7.4 The carbon/oxygen cycle (simplified)

carbondioxide

water

salts

plants

respiration

deathand

decay

animals

urine faeces

Figure 1.7.3 How matter is cycled through an ecosystem

21Ecosystems

Energy is lost from plants and animals mainly aswaste heat energy. This results in a loss of energyfrom the ecosystem.

Energy is not recycled in an ecosystem.

Matter is recycled in an ecosystem. The recycling of matter in an ecosystem occurs

through a number of interconnected cycles (seeFigures 1.7.4, 1.7.5 and 1.7.6).

denitrifying bacteriain waterlogged soil

dead animalsand plants animal

waste

nitrifying bacteria

plant proteinmade with nitratesand absorbedby-plant roots

nitrogen gasin the air

lightningproduces nitrates

nitrogen-fixingbacteria in rootnodules of acaciaspeas, beansand clover

nitrates inthe soil

nitrogen-fixingbacteria inthe soil

animals eat plants,obtaining protein

animalprotein

Figure 1.7.5 The nitrogen cycle (simplified)

underground water

evaporationfrom the sea

evaporationfrom plants and

animalsevaporationfrom rivers

evaporationfrom land

rain

evaporationfrom lakes

Figure 1.7.6 The water cycle (simplified)

22 Biology in Context: The Spectrum of Life

Questions 1.71 What is the original source of energy in ecosystems?2 (a) In what form do cells obtain energy for use in cellular processes?

(b) By what process is light energy converted into this?(c) By what process is the energy made available within cells?

3 What substances in plants are the main source of energy for animals?4 When an animal eats plants, it never obtains all the energy in the plants

it eats. Why?5 In the physical environment, what substances provide the initial source

of material for living things?6 (a) What substances in plants provide the main sources of materials

for animals?(b) How do plants obtain these substances?

7 How do plants and animals release matter to the physical environment?8 What eventually happens to all the matter in a community?9 People sometimes remove dead logs from forests for use in fire places,

and leaf litter for use as garden mulch. Explain the long-term con-sequences of these actions in relation to the flow of matter and energywithin the ecosystem.

10 When areas of natural vegetation are cleared for agriculture, the landsometimes turns out to be impoverished and inadequate for crops.(a) Where have all the nutrients gone?(b) How was the original ecosystem maintained?

11 A group of people are isolated and have only grain and chickens asfood sources. Should they(a) feed the grain to the chickens and eat the chickens?(b) feed the grain to the chickens and eat the eggs the chickens lay?(c) eat the grain and the chickens?

Explain your answer.

1.8 Interrelationships among organisms

Students learn to: identify examples of allelopathy, parasitism,

mutualism and commensalism in the eco-system and the role of organisms in eachtype of relationship

outline factors that affect numbers inpredator and prey populations in the areastudied

describe and explain the short-term andlong-term consequences on the ecosystemof species competing for resources.

Interrelationships between members ofdifferent speciesWithin a community, two organisms sometimes haveno observable effect on each other. However, differentspecies within an ecosystem often influence one

another. Some of the main types of interrelationshipsare considered in this unit.

MutualismA relationship between two organisms in which bothbenefit is called mutualism. Examples of mutualisminclude the alga and fungus that make up a lichen, thealga and polyp that make up coral acacias with theirnitrogen-fixing bacteria, and the bacteria in the digestivesystems of many herbivores that digest cellulose.

No large animals can digest cellulose. All grazinganimals must rely on symbiotic bacteria or protozoa intheir digestive systems to break down cellulose.Kangaroos have an additional stomach near thebeginning of the digestive tract. This contains thebacteria and protozoa that break down the cellulose ingrass. Both the kangaroo and the bacteria benefit fromthe relationship. The kangaroo obtains access to anadditional food source and the bacteria have a habitatwith a constant environment and an ample supply offood. All herbivores have symbiotic protozoa andbacteria in the gut, but few can match the efficiency ofthe kangaroos digestion.

CommensalismCommensalism is a relationship between two organ-isms in which only one benefits and the other isunaffected. Some examples of commensalism includethe anemone fish and the sea anemone, and theremora fish and the shark. The anemone fish livesamong the tentacles of sea anemones, gaining pro-tection from predators.The anemone appears to receiveno benefit.The remora hitches a ride on sharks. It gainsa free ride and feeds on scraps from the sharksfood butappears to provide no service to the sharks.

ParasitismParasitism is a relationship in which one organismlives in or on another organism and feeds from it. Theorganism in, on or off which a parasite lives is called itshost. Well adapted parasites cause little harm to theirhost. Their host remains healthy and able to providethem with a habitat and food. Many tapeworms liveattached to the lining of the digestive system of theirhost animal and absorb digested food without causingany serious harm. Some less well adapted parasitescause discomfort, which irritates the host and triggersresponses aimed at getting rid of the parasite.Ticks andfleas, for example, feed off dogs, who scratch and gnawat their coats in an attempt to remove them. Someparasites, such as disease-causing bacteria, bring aboutillness and can kill their host. These disease-causingparasites are called pathogens.

23Ecosystems

AllelopathyAllelopathy is a relationship in which one organismdirectly hinders the growth or development of anotherby releasing toxins. Some plants and fungi produceantibiotics that prevent the growth of bacteria. SirAlexander Flemings discovery that bacteria did notgrow around the fungus Penicillium notatum led to thedevelopment of the antibiotic penicillin.

Plants may also release substances that inhibit thegrowth of other plants. Sometimes substances aresecreted by the roots. Lantana is an introduced plantthat has become a pest in the Australian bush. It not onlycrowds out native species by competing for soil nutrientsand light but also appears to release substances into thesoil that inhibit the growth of some native species.Plants may also indirectly inhibit the growth of otherplants. The decomposition of pine needles can result insoils too acidic for the germination and growth of manyplants. Inhibition is not limited to exotic plants. Thedecay of eucalyptus leaves, for example, can render soilsunsuitable for some introduced plants. In each of thesecases, the plants chance of survival has been increasedby reduced competition for resources.

PredationA relationship in which one organism eats another iscalled a predator/prey relationship, or predation.The term is usually only applied to relationships inwhich one animal eats another. Dingoes and wallabies,lions and zebras, orb spinner spiders and beetles are allexamples of predator/prey relationships.

Predator/prey relationships often have a majorimpact on the abundance of organisms. Indeed, preyand predator populations are sometimes so closelyrelated that graphs of their abundance may look verysimilar. Figure 1.8.1 shows the effects of a predator/preyrelationship between two mites that were studiedunder laboratory conditions.

The shape of the graphs can be explained in thefollowing way: The predator mite eats the mite of a

different species, which is its prey. When the preypopulation increases, there is more food for thepredator and therefore the predator populationincreases. As the predator population increases, moreprey is consumed. The predator population falls againbecause there is less food, and the cycle begins oncemore. This causes the populations of both organisms tofluctuate in the same pattern. In these graphs both thepredator and the prey graphs have a similar shape, butthe predator population change always lags behind that of the prey and the predator population is usuallyless than the prey population.

Such obvious relationships are seldom observedunder natural conditions because many variables inter-act to influence the abundance of both predators andprey. In particular, where predators have a variety offood sources, such simple patterns are not observed.

CompetitionCompetition is a relationship in which two organismscompete for a limited resource. Competition betweenorganisms in the same place for the same set of resourcesusually results in the elimination of the less successfulone.The introduction of dingoes and, more recently, feralcats and foxes has been blamed for the reduced popu-lation of some native carnivores in parts of Australia.

Sometimes organisms are fairly evenly matched intheir competition for resources. Such organisms may co-exist indefinitely. In rainforest, for example, the avail-ability of light is often at a premium for seedlings. There-fore there is constant competition for light. Nevertheless,no single species dominates and the rainforest remainsan exceptionally diverse community. Competition is mostintense within a single species population because all theindividuals require the same resources.

Occasionally one species is more successful thananother and yet both continue to coexist. On the rockplatform, the black periwinkle (Nerita) competes for foodwith the limpet (Cellana). Both feed on the algae growingon the rocks. The periwinkle moves faster, but feeds less

1200

1000

800

600

400

200

0

25

20

15

10

5

Pred

ator

abu

ndan

ce

Prey

abu

ndan

ce

1 5 10 15 20 25 30 35 40 45 50 55 60

prey

predator

Time (weeks)

Figure 1.8.1 The predator/prey relationship of two mites

24 Biology in Context: The Spectrum of Life

efficiently than the limpet. If the periwinkles areremoved, the limpet population increases. Where thereare many periwinkles, there are few limpets. Never-theless, some algae are always left behind by theperiwinkles and this ensures the continued survival of thelimpets.

Consequences of competitionWhen two species compete for the same resources, oneof the species usually loses. In the short term, thisresults in a decrease in the abundance of one of thespecies. The effects of competition on the population oforganisms can be observed under laboratory con-ditions. In such experiments, the grain beetle Calandrais more successful than Rhizopeatha. Where they co-exist, this results in a decrease in the Rhizopeatha popu-lation. If competition between species continues in anecosystem, one of the species can be eliminated fromthe area. In the long term, this can result in the extinct-ion of the less successful species.

Over long periods, organisms evolve and adapt totheir environment. Competition is one pressure in theenvironment that influences the evolution of organ-isms. Partly as a result of competition, organisms evolveto occupy a particular niche within each ecosystem. Anorganisms niche in an ecosystem results from acombination of the abiotic and biotic factors the speciesuses in its habitat. As a result of competition and evo-lution, organisms of different species do not occupy thesame niche in the same ecosystem. The black peri-winkle (Nerita) and the limpet (Callana) feed on thesame food resource and share the same environment inthe same ecosystem but they occupy different nichesbecause they feed in different ways.

Summary 1.8 The distribution and abundance of organisms is

influenced by a range of factors, which include theabiotic environment; the availability of resources;interaction with other species; and interaction withmembers of the same species.

An examination of variations in these factors within

an ecosystem often provides an explanation for thedistribution and populations of organisms withinthe ecosystem.

Members of different species within an ecosystemmay have no significant impact on each other (seeTable 1.8.1).

Competition between species may result inelimination of one species or the species adapting tooccupy distinct niches. In the short term, theabundance and distribution of at least one of thespecies are reduced.

Questions 1.81 List the four main interrelationships that influence the distribution and

abundance of organisms.2 State two types of relationship in which the organisms are not harmed

and give an example of each.3 State two types of relationship in which an organism is harmed and

give an example of each.4 Use two specific examples to explain how relationships within species

can influence their distribution and abundance.5 In some predator/prey relationships, predators tend to prey more

heavily on the young, weak and sick than on the strong and healthy.How might such a relationship benefit the prey in the long-term?

6 State whether the situations described below are(i) allelopathy(ii) mutualism(iii) commensalism(iv) competition(v) parasitism(vi) predator/prey relationships.(More than one answer may be chosen.)

(a) The pollination of orchids by bees as they search for nectar.(b) The killing of lyre birds by feral cats.(c) The digestion of wood in the gut of termites by micro-

organisms.(d) The infection of humans by the malaria plasmodium.(e) The building of nests in trees by magpies.(f) Water moccasin snakes d