I

lntroductionEcosystems require a continuous supply ofenergy to fuel life processes and to replaceenergy lost as heat. Continued availabilityof carbon and other chemical elements inecosystems depends on cycles. The future

survival of living organisms including humansdepends on sustainable ecological communities'Concentrations of gases in the atmosphere havesignificant effects on climates experienced at theEarth's surface.

@ srius+ Classifging species as autotroPhs, consumers,

detritivores or saprotrophs from a knowledge oftheir mode of nutrition.

Ð Testing for association between two speciesusing the chi-squared test w¡th data obtainedbg quadrat samPling.

à Recognizing and ¡nterpret¡ng statisticalsignificance.

> Setting up sealed mesocosms t0 trg t0establish sustainabilitg. IPractical 5)

@ n.ture of scienceà Looking for patterns, trends and discrepancies:

plants and algae are mostlg autotrophic butsome are not.

Understandingà Species are groups of organisms that can

potentiallV interbreed to produce fertile offspring.

à Members of a species mag be reproductivelgisolated In separate populations.

à Species have either an autotrophic or

heterotrophic method of nutrition [a fewspecies have both methods].

t Consumers are heterotrophs that feed on livingorganisms bg ingestion.

) Detritivores are heterotrophs that obtain organicnutrients from detritus bg internal digestion.

à Saprotrophs are heterotrophs that obtainorganic nutrients from dead organic matter bg

external digestion.à A communitg is formed bg populations

of different species living together andinteracting with each other.

à A communitg forms an ecosustem bg itsinteractions with the abiotic environment.

à Autotrophs and heterotrophs obtain inorganicnutrients from the abiotic environment'

à The supplg of inorganic nutr¡ents is maintainedbg nutrient cacling.

à Ecosgstems have the potentialto be

sustainable over long periods of time.

À Figure 1 A bird of paradise in PapuaNew Guinea

SpeciesSpecies are groups of organisms that can potentiallginterbreed to produce fertile offspring.Birds of paradise inhabit papua New Guinea and other Australasianislands. In the breeding season the males do elaborate and distinctivecourtship dances, repeatedly carrying out a series of movementsto display their exotic plumage. one reason for this is to show to afemaÌe that they are flt and would be a suitable partner. Anotherreason is to show that they are the same type of bird of paradise asthe female.

There are forty-one different types of bird of paradise. Each ofthese usually only reproduces with others of its type and hybridsbetween the different types are rarely produced. For this reasoneach of the forty-one types of bird of paradise remains distinct, withcharacters that are different to those of other types. Biologists calltypes of organism such as these species. AÌthough few species haveas elaborate courtship rituals as birds of paraclise, most species havesome method of trying to ensure that they reproduce with othermembers of their species.

when two members of the same species mate and produce offspringthey are interbreeding. occasionally members of different species breedtogether. This is caÌled cross-breeding. It happens occasionally with birdsof paradise. However, the offspring produced by cross-breeding betweenspecies are almost always infertile, which prevents the genes of twospecies becoming mixed.The reproductive separation between species is the reason for eachspecies being a recognizable type of organism with characters thatdistinguish it from even the most closely related other species. Insummary/ a species is a group of organisms that interbreed to producefertile offspring.

PopulationsMembers of a species mag be reproductivelg isorated inseparate populations.A population is a group of organisms of the same species who live in thesame area at the same time. If two populations live in different areasthey are unlikely to interbreed with each other. This does not mean thatthey are different species. If they potentially could interbreed, they arestill members of the same species.

If two populations of a species never interbreed then they may graduallydevelop differences in their characters. Even if there are recognizable

different species.

?02

4.1 SPECIES, COMMUNITIES AND ECOSYSTEMS

AutotroPhic a nd heterotrophic nutritionSpecieshaveeitheranautotrophicorheterotrophicmethod of nutrition [a few species have both methods).All organisms need a supply of organic nutrients' such as glucose and

amino acids. They ur" ,t."d"d for growth and reproduction' Methods of

obtaining these cãrbon compounds can be divided into two types:

o some organisms make their own carbon compounds from carbon

dioxide and other simple substances - they are autotrophic' whichmeans self-feeding;

. some organisms obtain their carbon compounds fromotherorganisms - they are heterotrophic' which means feeding on others'

Some unicellular organisms use both methods oT nutrition' Euglena

gracilßfor example ñas chloroplasts and carries out photosynthesis when

there is sufficient light, but can also feed on detritus or smaller organisms

by endocytosis' Orgãnisms that are rot exclusively autotrophic or

heterotroPhic are mixotroPhic'

r, Figure 3ArobidoPsisthationo -the autotroPhthat molecular biologistsuse as a model Plant

À Figure 4 Humming birdsare heterotroPhic; the Plantsfrom which theg obtainnectar are autotroPhic

r, Figure 5Eugleno - an

unusual organismas it can feed bothautotrophicallU andheterotroPhica I lg

Galápagos tortoises

The tortoises that live on

the Galápagos islands are

the largest in the world.Theg have sometimes beengrouped together into one

species, Chelinoidis nigro,but more recentlg have beensplit into sePerate sPecles'

Discuss whether eachofthese observationsindicates that PoPulationson the various islands are

separate sPecies:

The GaláPagos tortolsesare poor swimmers andcannottravel from oneisland to another sotheg do not naturallginterbreed.

Tortoises fromdifferent islands haverecognizable differencesin their characters,including shell size andshape.

Tortoises from differentislands have beenmated in zoos andhgbrid offsPring havebeen produced buttheghave lower fertilitg andhigher mortalitg thanthe offsPring of tortoisesfrom the same island.

taJ <' 'lt

@ ft.nds in Plant and algal nutritionLooking for patterns, trends and discrepanc¡es: plants

and algae are mostlg autotrophic but some are not'Almost all plants and algae are autotrophic - they make their owncomplex organic .ompo"trttds using carbon dioxide and other simple

substances. A supply àf ..t"'gy is needed to do this' which plants and

algae obtain by absorbing lighì. Their method of autotrophic nutritionis therefore photosyntheiis and they carry it out in chloroplasts'

This trend for plants and algae to make their own carbon compounds

by photosynthesis in chloroplasts is followed by the majority of species'

However there are small numbers of both plants and algae that do not fitthe trend, because although they are recognizably plants or algae' they

Figure 2 GaláPagos tortolse

203

Data-based questions: Unexpected d¡etsAlthough we usually expect plants to be autotrophs andanimals to be consumers, living organisms are very variedand do not always conform to our expectations. Fþures óto 9 show four organisms with diets that are unexpected.I Which of the organisms is autotrophic? tll2 Which of the organisms is heterotrophic? ttl3 Of the organisms that are heterotrophic, deduce which is aconsumer, which a detritivore and which a saprotroph. t4l

do not contain chloroprasts and they do not ca,'y out photoslmthesis.These species grow on other plants, obtain carbon .o_porrrrdrfromthem and cause them harm. They are therefore parasitic.To decide whetheralgae are g.orrp, of andand insignincant ¿i t-ÏT:tthere arã and how they evolved. specres

o lhe number of parasitic pranrs and argae is relativery sma'- onryabout L% of all plant and algal species.

' It is almost certain lhat the originar ancestrar species of prant andalga were autotrophic and thai the parasiti. ,pà.i., evoived fromthem. chloroplasts can quite easilyie lost f¡om cells, but cannoreasily be developed. Arso, parasitic species are diverse and occur inmany different families. This pattern suggests that parasitic prantshave evolved repeatedly from photosynthetic specfus.Because of this evidence, ecologists regard prants and argae as groups ofautotrophs, with a smalr number of exceptionar species îrrut u.ä parasitic.

r Figure 6 Venus flg trap: grows inswamps, with green leaves thatcarru out photosgnthesis and alsocatch and digest insects, to providea supplg of nítrogen

A Figure 7 Ghost orchid: growsunderground in woodland, feedingoff dead organic matter, occasionallggrowing a stem with flowers aboveground

r Figure I Eugleno: unicellthat lives in ponds, using itschloroplasts for photosgnthesis,but also ingesting dead organicmatter bg endocgtosis

A Figure 9 Dodder: grows parasiticallg0n gorse bushes, using small root-likestructures to obtain sugars, amino acidsand other substances it requires, fromthe gorse

4.1 SPECIES, COMMUNITIES AND ECOSYSTEMS

ConsumersConsumers are heterotrophs that feed on living organismsbg ingestion.Heterotrophs are divided into groups by ecologists according to thesource of organic molecules that they use and the method of takingthem in. One group of heterotrophs is called consumers.

Consumers feed off other organisms. These other organisms are eitherstill alive or have only been dead for a relatively short time. A mosquitosucking blood from a larger animal is a consumer that feeds on anorganism that is still alive. A lion feeding off a gazelle that it has killed is

a consumer.

Consumers ingest their food. This means that they take in undigestedmaterial from other organisms. They digest it and absorb the products ofdigestion. unicellular consumels such as Paramecium take the food in byendocytosis and digest it inside vacuoles. Multicellulal consumers suchas lions take food into their digestive system by swallowing it'Consumers are sometimes divided up into trophic groups accordingto what other organisms they consume. Primary consumers feed onautotrophs; secondary consumers feed on primary consumers and so on.ln practice, most consumers do not fit neatly into any one of these groupsbecause their diet includes material from a variety of trophic groups.

r. Figure 10 Red kite [Milvus milvusJ is a

consumer that feeds on live preg but alsoon dead animal remains Icarrion]

Ä Figure ll Yellow-necked mouse (Apodemus

ftovicollisJ is a consumer that feeds mostlU onliving plant matter, especiallg seeds, but alsoon living invertebrates

DetritivoresDetritivores are heterotrophs that obtainorgan¡c nutr¡ents from detritus bginternal digestion.Organisms discard large quantities of organicmatter, for example:

o dead leaves and other parts of plants

o feathers, hairs and other dead parts of animalbodies

o feces from animals.

This dead organic matter rarely accumulatesin ecosystems and instead is used as a sourceof nutrition by two groups of heterotroph -detritivores and saprotroPhs.

Detritivores ingest dead organic matter and thendigest it internally and absorb the products ofdigestion. Large multicellular detritivores such as

earthworms ingest the dead matter into their gut.Unicellular organisms ingest it into food vacuoles.The larvae of dung beetles feed by ingestion offeces rolled into a ball by their parent.

SaprotrophsSaprotrophs are heterotrophs that obtainorgan¡c nutr¡ents from deadorgenic matter bg external digestion.Saprotrophs secrete digestive enzymes into the deadorganic matter and digest it externally. They thenabsorb the products of digestion. Many types ofbacteria and fungi are saprotrophic. They are alsoknown as decomposers because they break downcarbon compounds in dead organic matter andrelease elements such as nitrogen into the ecosystemso that they can be used again by other organisms.

a Figure 12 Saprotrophic fungi growing over the surfaces of dead

leaves and decomposing them bg secreting digestive enzumes

TOK

To what extent do the classificationsgstems (labels and categories] weuse set limits to what we perceive?There are ínf inite waus to divide upour observations. 0rganisms can beorganized in a number of wags bgscientists: bg morphologg Iphgsicalsimilaritg to other organismsJ,phglogeng Ievolutionarg historgJ andniche [ecological roleJ. ln evergdaglanguage, we classifg organisms suchas domestícated orwild; dangerous orharmless; edible or toxic.

Clearcutting

@ lOentifging modes of nutr¡t¡onclassifg i ng species as a utotrop hs, co nsu mers, detritivo resor saprotrophs from a knowledge of their mode of nutrition.By answering a series of sirnple questions about an organism's mode ofnutrition it is usually possible to deduce what trophic group it is in. Thesequestions are presented here as a dichotomous key, which consists of aseries of pairs of choices. The key works for unicellular and multicellularorganisms but does not work for parasites such as tapeworms orfungi that cause diseases in plants. All multicellular autotrophs arephotosynthetic and have chloroplasts containing chlorophyll.

Feeds on living or recentlg Feeds on dead organicmatter: DETRITIVORESkilled organisms : CONSU¡/ERS

Either ingests organic mater bg endocatosis Ino cellwalls] or bg taking it into its gut.

START HERE

Cell walls present. No ingestion of organic matter. No gut

Secretes enzVmes intoits environment to digestdead organic matter: SAPROTROPHS

Enzgmes not secreted.0nlg requires simpleions and compoundssuch as C02: AUTOTROPHS

CommunitiesA communitg ¡s formed bg populations of differentspec¡es living together and ¡nteracting with each other.An important part of ecology is research into relationships betweenorganisms. These relationships are complex and varied. In some casesthe interaction between two species is of benefit to one species andharms the other, for example the relationship between a parasite and itshost. In other cases both species benefit, as when a hummingbird feedson nectar from a flower and helps the plant by pollinating it.All species are dependent on relationships with other species for theirlong-term survival. For this reason a population of one species cannever live in isolation. Groups of pclpulations live together. A group

Figure 14

ln a classic essag written in 1972, thephgsicist Philip Anderson srated this:The obilitg to reduce evergthing tosimplefundamental Iows does notimplg the obilitg to startfrom thoselows ond reconstructthe universe. Ateoch level of complexitg entirelg newproperties opPeor.

Clearcutting is the most commonand economicallg profitable form oflogging. l1 involves clearing everU treein an area so that no cenopg remains.With reference to the concept ofemergent properties, suggest whg theecological communitg often fails torecover after clearcutting.

206

4.1 SPECIES, COMMUNITIES AND ECOSYSTEMS

of populations living together in an area and interacting with each otheris known in ecology as a community. \pical communities consist ofhundreds or even thousands of species living together in an area.

^r Figure 13 A coral reef is a complex communitg wilh mang interactions between thepopulations. Most corals have photosgnthetic unicellular algae called zooxanthellae livinginside their cells

@ rield work - associations between spec¡esTesting for association between two species using the chi-squared test with dataobtained bg quadrat sampl¡ng.Quadrats are square sample areas, usually marked o The quadrat is placed precisely at the distancesout using a quadrat frame. Quadrat sampling determined by the two random numbers.involves repeatedly placing a quadrat frame atrandom positions in a habitat and recording thenumbers of organisms present each time.The usual procedure for randomly positioningquadrats is this:o A base line is marked out along the edge of the

habitat using a measuring tape. It must extendall the way along the edge of the habitat.

o Random numbers are obtained using eithera table or a random number generator on acalculator.

o A first random number is used to determinea distance along the measuring tape. Alldistances along the tape must be equally likely.

o A second random number is used to determinea distance out across the habitat at right anglesto the tape. All distances across the habitatmust be equally likely.

If this procedure is followed correctlf with a largeenough number of replicates, reliable estimates of

A. Figure 15 Ouadrat sampling of seaweed populations on a

rockg shore

population sizes are obtained. The method is onlysuitable for plants and other organisms that arenot motile. Quadrat sampling is not suitable forpopulations of most animals, for obvious reasons.

If the presence or absence of more than onespecies is recorded in every quadrat duringsampling of a habitat, it is possible to test for anassociation between species. Populations are oftenunevenly distributed because some parts of thehabitat are more suitable for a species than others.If two species occur in the same parts of a habitat,they will tend to be found in the same quadrats.This is known as a positive association. There canalso be negative associations, or the distribution oftwo species can be independent.

There are two possible hypotheses:

Hu: two species are distributed independently(the null hypothesis).

H,: two species are associated (either positivelyso they tend to occur together or negatively sothey tend to occur apart).

We can test these hypotheses using a statisticalprocedure - the chi-squared test.

The chi-squared test is only valid if all theexpected frequencies are 5 or larger and thesarnple was taken at random from the population.Method for chi-squared testI Draw up a contingency table of observed

frequencies, which are the numbers of quadratscontaining or not containing the two species.

Species A

presentSpecies A

absentRow

totals

Species B present

Species B absent

Column totals

Calculate the row and column totals. Addingthe row totals or the column totals should givethe same grand total in the lower right cell.

2 Calculate the expected frequencies,assuming independent distribution, foreach of the four species combinations.Each expected frequency is calculated fromvalues on the contingency table using thisequation:

expected _ row total x column totalfrequency grand total

3 Calculate the number of degrees of freedomusing this equation.

degrees of freedom: (m - l)(n - l)where m and n are the number of rclwsand number of columns in the contingencytable.

4 Find the critical region for chi-squared from atable of chi-squared values, using the degreesof freedom that you have calculated and asigniflcance level (p) of 0.05 (5%). The criricalregion is any value of chi-squared larger thanthe value in the table.

5 Calculate chi-squared using this equation:

y2_F(f"-f")tA - Z_r--Í"

where / is the observed frequency

f, is the expected frequency and

E is the sum of.

6 Compare the calculated value of chi-squaredwith the critical region.o If the calculated value is in the critical

region. there is evidence at the 5% levelfor an association between the two species.We can reject the hypothesis Ho.

o If the calculated value is not in the criticalregion, because it is equal or below thevalue obtained from the table of chi-squared values, Ho is not rejected. Thereis no evidence at the 5% level for anassociation between the two species.

208

4.1 SPECIES, COMMUNITIES AND ECOSYSTEMS

: Data-based questions: Chi-squared testingFigure l6 shows an area on the summit of CaerCarad.oc, a hill in Shropshire, England'

The area \s grazed by sheep in summer andhill walkers cross it on grassy paths. There areraised hummocks with heather (Calluna vulgaris)growing in them. A visual survey of this sitesuggested that Rhytidiadelphus squarrlsus, a speciesof moss growing in this area, was associatedwith these heather hummocks. The presence orabsence of the heather and the moss was recordedin a sample of I00 quadrats, positioned randomly'

Species Frequencg

Heather onlg IMoss onlg 7

Both species 57

Neither species 27

0uestionsI Construct a contingency table of observed

values. t4l2 Calculate the expected values, assuming no

association between the species. t4l

3 Calculate the number of degrees of freedom' [2]

4 Find the critical region for chi-squared at asignificance level of 5%. l2l

5 Calculate chi-squared. l4ló State the two alternative hypotheses, Ho and

H,, and evaluate them using the calculatedvalue for chi-squared. l4l

7 Suggest ecological reasons for an associationbetween the heather and the moss. Í41

8 Explain the methods that should have beenused to position quadrats randomly in thearea of study. t3l

t Figure 16 Caer Caradoc, Shropshire

Results

@ Statistical significanceRecognizi ng a nd i nterpreti ng stat¡stica I signif ica nce'Biologists often use the phrase "statistically that it is false. A statistic is calculated using the

signiñcant,, when discussing results of an results of the research and is compared witheiperiment. This refers to the outcome of a range of possible values called the criticala statistical hypothesis test. There are two region. If the calculated statistic exceeds the

alternarive types of hypothesis: critical region, the null hypothesis is considered

. Ho is rhe null hypothesis and is the belief that ::jt-"ttt and is therefore rejected' though

there is no relarionship, for.*"rrnt.;h", *; ::.fT""t say that this has been proved

means are equal or that there is "" ";;;;;;i;" with certaintv'

or correlation between two variables. When a biologist states that results were

. Hr is the alternative hyporhesis and is the stæisticallv significant it means that if the null

belief thar rhere is a relarionrtrip, tor'.Iäpf. h^yp*othesis (Ho) was true' the probability of getting

rhat two means are different or that rff."*it:" Ï:lt:.:t extreme as the observed results would

association between two variables. LrrLrv ¡r q¡r

be very small' A decision has to be made abouthow small this probability needs to be' This is

The usual procedure is to test the null known as the significance level. It is the cut-offhypothesis, with the expectation of showing point for the probability of rejecting the null

209

hypothesis when in fact it was true. A level of5% is often chosen, so the probabitity is less thanone in twenty. That is the minimum acceptablesigniflcance level in published research.o If there is a difference between the mean

results for the two treatments in anexperiment, a statistical test will showwhether the difference is signiflcant at the 5%level. If it is, there is a less tharr 5Vo probabilityof such a large difference between the samplemeans arising by chance, even when thepopulation means are equal. We say that thereis statistically significant evidence that thepopulation means differ.

o In the example of testing for an associationbetween two species, described on previouspages, the chi-squared test shows whetherthere is a less than 5% probability of thedifference between the observed and theexpected results being as large as it iswithout the species being either positively ornegatively associated.

When results of biological research are displayedon a bar chart, letters are often used to indicatestatistical significance. TWo different letters,usually a and D, indicate mean results with astatistically significant difference. TVvo of the sameletter such as a and ¿ indicates that any differenceis not statistically significant.

 Figure 17 Grasses in an area ofdeveloping

EcosgstemsA communitg forms an ecosustem bg its interactionsw¡th the abiotic environment.A community is composed of all organisms riving in an area. Theseorganisms could not Ìive in isolation - they depend on their non-living surroundings of air, water, soil or rock. Ecologists refer to thesesurroundings as the abiotic environment.In some cases the abiotic environment exerts a powerful influence over theorgspethe t.There are also many cases where living organisms influence the abioticenvironment. Sand dunes are an example of longcoasts where sand is blown up the shore and ow inthe loose wind-blown sand. The roots of thes sandand their leaves break the wind and encourage more sand to be deposited.so, not only are there complex interactions within communities, there arealso many interactions between organisms and dre abiotic environment.

lnorganic nutr¡entsAutotrophs and heterotrophs obtain inorganic nutrientsfrom the abiotic environment.Living organisms need a supply of chemical elements:o carbon, hydrogen and oxygen are needed to mar<e carbohydrates,

lipids and other carbon compounds on which life is based.sand dunes

4.1 SPECIES, COMMUNITIES AND ECOSYSTEMS

. Nitrogen and phosphorus are also needed to make many of thesecompounds.

o Approximately flfteen other elements are needed by livingorganisms. Some of them are used in minute traces only, but theyare nonetheless essential.

Autotrophs obtain all of the elements that they need as inorganicnutrients from the abiotic environment, including carbon and nitrogenHeterotrophs on the other hand obtain these two elements andseveral others as part of the carbon compounds in their food. They dohowever obtain other elements as inorganic nutrients from the abioticenvironment, including sodium, potassium and calcium.

Nutrient cgclesThe supplg of inorganic nutrients is maintained bgnutr¡ent cgcling.There are limited supplies on Earth of chemical elements. Althoughliving organisms have been using the supplies for three billion years,they have not run out. This is because chemical elements can beendlessly recycled. Organisms absorb the elements that they require asinorganic nutrients from the abiotic environment, use them and thenreturn them to the environment with the atoms unchanged.

Recycling of chemical elements is rarely as simple as shown in thisdiagram and often an element is passed from organism to organismbefore it is released back into the abiotic environment. The detailsvary from element to element. The carbon cycle is different from thenitrogen cycle for example. Ecologists refer to these schemes collectivelyas nutrient cycles. The word nutrient is often ambiguous in biology butin this context it simply means an element that an organism needs.The carbon cycle is described as an example of a nutrient cycle in sub-topic 4.2 and the nitrogen cycle in Option C.

Sustainabi litg of ecosgstemsEcosgstems have the potential to be sustaineb¡e overlong periods of time.The concept of sustainability has risen to prominence recently becauseit is clear that some current human uses of resources are unsustainableSomething is sustainable if it can continue indefinitely. Human use offossil fuels is an example of an unsustainable activity. Supplies of fossilfuels are flnite, are not currently being renewed and cannot thereforecarry on indefinitely.Natural ecosystems can teach us how to live in a sustainable way, sothat our children and grandchildren can live as we do. There are threerequirements for sustainability in ecosystems:

o nutrientavailabilityo detoxification of waste productso energy availability.

^l, Figure 18 Living organisms have been recgclingfor billions of gears

Reserves of anelement in the

abiotic environment

Element formingpart of a living

organrsm

?1.t

Ä Figure 19 Sunlight supplies energU to a forestecosgstem and nutrients are recgcled

ActivitgCave ecosgstems

0rganisms have been foundliving in total darkness incaves, including egelessfish. Discuss whetherecosgstems in dark cavesare sustainable.

Figure 20 shows a

small ecosgstem withphotosUnthesizing plantsnear artif¡cial lighting in acave that is open to visitorsin Cheddar Gorge. Discusswhetherthis is more orless sustainable thanecosgstems in dark caves.

Nutrients can be recycled indefinitely and if this is done there shouldnot be a lacÌ< of the chemical elements on which life is based. The wasteproducts of one species are usually exploited as a resource by anotherspecies. For example, ammonium ions released by decomposers areabsorbed and used for an energy source by Nitrosomonas bacleria in thesoil. Ammonium is potentially toxic but because of the action of thesebacteria it does not accumulate.

Energy cannot be recycled, so sustainability depends on continuedenergy supply to ecosystems. Most energy is supplied to ecosystemsas light from the sun. The importance of this supply can be illustratedby the consequences of the eruption of Mount Tambora in t 8I 5.Dust in the atmosphere reduced the intensity of sunlight for somemonths afterwards, causing crop failures globally and deaths due tostarvation. This was only a temporary phenomenon, however, andenergy supplies to ecosystems in the form of sunlight will continuefor billions of years.

@ uesocosmsSetting up sealed mesocosms to trg to establishsusta¡nebilitg. IPractical 5)Mesocosms are small experimental areas that are set up asecological experiments. Fenced-off enclosures in grassland orforest could be used as terrestrial mesocosms; tanks set up inthe laboratory can be used as aquatic mesocosms. Ecologicalexperiments can be done in replicate mesocosms, to find out theeffects of varying one or more conditions. For example, tanks couldbe set up with and without fish, to investigate the effects of fish onaquatic ecosystems.

Another possible use of mesocosms is to test what types of ecosystemsare sustainable. This involves sealing up a conìmunity of organismstogether with afu and soil or water inside a container.You should consider these questions before setting up either aquaticor terrestrial mesocosms:

o Large glass jars are ideal but transparent plastic containers couldalso be used. Should the sides of the container be transparent oropaque?

o Which of these groups of organisms must be included to make upa sustainable community: autotrophs, consumers, saprotrophs anddetritivores?

o How can we ensure that the oxygen supply is sufficient for all theorganisms in the mesocosm as once it is sealed, no more oxygenwill be able to enter.

o How can we prevent any organisms suffering as a result of beingplaced in the mesocosm?

À Figure 20

4.2 ENERGY FLOW

@ srcrrcÐ Quantitative representations of energg flow

using pgramids of energg.

Understanding-) Most ecosgstems relg on a supplg of energg

from sunlight.à Light energg is converted to chemical energg in

carbon compounds bg photosgnthesis.à Chemical energg in carbon compounds flows

through food chains bg means of feeding.-) Energg released bg respiration is used in living

organisms and converted to heat.t Living organisms cannot convert heat to other

forms of energg.à Heat is lost from ecosgstems.-) Energg losses between trophic levels restrict

the length of food chains and the biomass ofhigher trophic levels.

@ nature of sc¡ence) Use theories to explain natural phenomena:

the concept of energg flow explains the limitedlength of food chains.

Sunlight and ecosgstemsMost ecosustems relg on a supplg of energu fromsunl¡ght.For most biological communities, the initial source of energy issunlight. Living organisms can harvest this energy by photosynthesis.Three groups of autotroph carry out photosynthesis: plants,eukaryotic algae including seaweeds that grow on rocky shores, andcyanobacteria. These organisms are often referred to by ecologistsas producers.

Heterotrophs do not use light energy directly, but they are indirectlydependent on it. There are several groups of heterotroph in ecosystems:consumers, saprotrophs and detritivores. All of them use carboncompounds in their food as a source of energy. In most ecosystems allor almost all energy in the carbon compounds will originally have beenharvested by photosynthesis in producers.

The amount of energy supplied to ecosystems in sunlight varies aroundthe world. The percentage of this energy that is harvested by producersand therefore available to other organisms also varies. In the SaharaDesert, for example, the intensity of sunlight is very high but little of itbecomes available to organisms because there are very few producers.In the redwood forests of California the intensity of sunlight is less thanin the Sahara but much more energy becomes available to organismsbecause producers are abundant.

ActivitgCganobacteria in caves

Cganobacteria arephotosgnthetic bacteria thatare often verg abundantin marine and freshwaterecosUstems. Figure 1

shows an area ofgreencganobacteria on an areaof wall in a cave that isilluminated bg artificial light.The surrounding areas arenormallg dark. lf the artificiallight was not present, whatother energg sources couldbe used bg bacteria in caves?

r Figure 1

Data-based questions: I nsolationInsolation is a measure of solar radiation The two maps in figure 2show annual mean insolation at the top of the Earth's atmosphere(upper map) and at the Earth's surface (lower map).

0uestionsI State the relationship between distance from the equator and

insolation at the top of the Earth's atmosphere. tll2 State the mean annual insolation in Watts per square metre

for the most northerly part of Australiaa) at the top of the atmosphere tllb) at the Earth's surface. tll

3 Suggest reasons for differences in insolation at the Earth'ssurface between places that are at the same distance fromthe equator. t2l

4 Tropical rainforests are found in equatoriaÌ regions of allcontinents. They have very high rates of photosynthesis.Evaluate the hypothesis that this is due to very highinsolation. Include named parts of the world in youranswer. t5l

0 40 B0 I2O 160 200 24O 280 320 360 400 w/m

^r Figure 2

4.2 ENERGY FLOW

Energg conversionLight energg is converted to chemical energg in carboncompounds bg photosgnthesis.Producers absorb sunlight using chlorophyll and other photosyntheticpigments. This converts the light energy to chemical energy, which is used tomake carbohydrates, lipids and all the other carbon compounds in producers

Producers can release energy from their carbon compounds by cellrespiration and then use it for cell activities. Energy released in this wayis eventually lost to the environment as waste heat. However, only someof the carbon compounds in producers are used in this way and thelargest part remains in the cells and tissues of producers' The energy inthese carbon compounds is available to heterotrophs.

Energg in food chainsChemical energg in carbon compounds flows through foodchains bg means of feeding.A lood chain is a sequence of organisms, each of which feeds on the previousone. There are usually between two and five organisms in a food chain. It israre for there to be more organisms in the chain. As they do not obtain foodfrom other organisms, producers are always the first organisms in a foodchain. The subsequent organisms are consumers. Primary consumers feedon producers; secondary consumers feed on primary consumers; tertiaryconsumers feed on secondary consumers, and so on. No consumers feed onthe last organism in a food chain. Consumers obtain energy from the carboncompounds in the organisms on which they feed. The arrows in a lood chaintherefore indicate the direction of energy flow.

Figure 4 is an example of a food chain from the forests around Iguazufalls in northern Argentina.IrF

+ +

A Figure 4

Respiration and energg releaseEnergg released bg respiration is used in living organ¡smsand converted to heat.Living organisms need energy for cell activities such as these:

o Synthesizinglarge molecules like DNA, RNA and proteins.

o Pumping molecules or ions across membranes by active transport.

o Moving things around inside the cell, such as chromosomes or vesicles,or in muscle cells the protein fibres that cause muscle contraction.

ATP supplies energy for these activities. Every cell produces its ownATP supply.

ActivitgBush and forest fires

r Figure 3

Figure 3 shows a bush fire inAustralia.

What energg conversion ishappening in a bush f ire?

Bush and forest firesoccur naturallg in someec0suslems.

Suggesttwo reasons forthishgpothesis: There are fewerheterotrophs in ecosgstemswhere f ires are commoncompared to ecosgstemswhere f ires are not common.

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All cells can produce ATP by cell respiration. In this process carboncompounds such as carbohydrates and lipids are oxidized. Theseoxidation reactions are exothermic and the energy released is usedin endothermic reactions to make ATp. so cell respiration transferschemical energy from glucose and other carbon compounds to ATp. Thereason for doing this is that the chemical energy in carbon compoundssuch as glucose is not immediately usable by the cell, but the chemicalenergy in z\TP can be used directly for many different activities.The second law of thermodynamics states that energy transformationsare never 100% efficient. Not all of the energy from the oxidationof carbon compounds in cell respiration is transferred to ATp. Theremainder is converted to heat. Some heat is also produced when ATp isused in cell activities. Muscles warm up when they contract for exampleEnergy from ATP may reside for a time in large molecules when theyhave been synthesized, such as DNA and proteins, but when thesemolecules are eventually digested the energy is released as heat,

Data.based questionsFigure 5 shows the results of an experiment inwhich yellow-billed magpies (Pica nuttalli) wereput in a cage in which the temperature couldbe controlled. The birds' rate of respirationwas measured at seven different temperatures,from -10"C to +40'C. Between -lO'C and30'C the magpies maintained constant bodytemperature, but above 30'C body temperatureincreased.

a) Describe the relationship between externaltemperature and respiration rate in yellow-billed magpies. t3l

b) Explain the change in respiration rate astemperature drops from +10"C to -10'C. t3l

c) Suggest a reason for the change inrespiration rate as temperature increasedfrom 30'C to 40'C. t2l

10 20temperature ['C]

Ä Figure 5 Cell respiration rates at different temperatures ingellow-billed magpies

d) Suggest two reasons for the variation inrespiration rate between the birds at eachtemperature.

20t

fro, 15

=Eo910c,po.=oE5

-10 0 30 40 50

t2l

Heat energg in ecosgstemsLiving organ¡sms cannot convert heat to other formsof energg.Living organisms can perform various energy conversions:o Light energy to chemical energy in photosynthesis.o Chemical energy to kinetic energy in muscle contraction.o Chemical energy to electrical energy in nerve cells.

o Chemical energy to heat energy in heat-generating adipose tissueThey cannot convert heat energy into any other form of energy.

4.2 ENERGY FLOW

Heat losses from ecosgstemsHeat is lost from ecosustems.Heat resulting from cell respiration makes living organisms warmer'Thisheatcanbeusefulinmakingcold-bloodedanimalsmoreactive.Birds and mammals increase their rate of heat generation if necessary to

maintain their constant body temperatures'

According to the laws of thermodynamics in physics' heat passes fromhotter to cooler bodies, so heat produced in living organisms is all eventually

lost to the abiotic environment. The heat may remain in the ecosystem for

awhile,butultimatelyislost,forexamplewhenheatisradiatedintotheatmosphere. Ecologisti assume that all energy released by respiration for use

in cell activities will uitimately be lost from an ecosystem'

@ f*plainingthe length of food chainsUse theories to expla¡n natural phenomena: theconcept of energg flow explains the limited lengthof food chains.If we consider the diet of a top carnivore that is at the end of a foodchain, we can work out how many stages there are in the food chainleading up to it. For example, if an osprey feeds on fish such-as salmon

that fed on shrimps, whicñ fed on phytoplankton' there are fourstages in the food chain.

There are rarely more than four or five stages in a food chain' We

might expect fáod chains to be limitless, with one species being eaten

Uyänotftãr ad infinitum. This does not happen' In ecology' as in allbranches of science, we try to explain natural phenomena such as the

restricted length of food chains using scientific theories. In this case itis the concepi of energy flow along food chains and the energy losses

that occur between ttópni. levels that can provide an explanation'

Energg losses and ecosgstemsEnergg losses between trophic levels restr¡ct the length

of foõd chains and the biomass of higher trophic levels.

year per square metre of ecosystem than in primary consumers'

The reason for this trend is loss of enelgy between trophic levels'

oMostoftheenergyinfoodthatisdigestedandabsorbedbyorganisms in a tr*ophic level is released by them in respiration for

r Figure 6 An infrared camera image of an

Afiican greg parrot fPsittocus erithocus)shows how much heat is being released to the

environment bg different parts of its bodg

ActivitgThinking about energgchanges

What energg conversionsare required to shoot abasketball?

What is the final form of theenergg?

f ish-eating toP carnlvore

Salmon and sog

Most salmon eaten bVhumans is produced in f ishfarms. The salmon havetraditionallg been fed onfish meal, mostlg based onanchovies harvested off thecoast ofSouth America. Thesehave become scarce andexpensive. Feeds based onplant products such as sogbeans are increasinglg beingused. ln terms of energg flow,which ofthese human diets ismost and least efticient?1 Salmon fed on fish meal

2 Salmon fed on sog beans

3 Sog beans.

decomposers secondarg consumeÍ

tlc,oob ¡jl t r,-r; [2oo ¡1 t-2 u' t1

r,-,'pnmargconsumer, , .,. , [2,500 kJ m-2 gr 1]

plankton[15o,oookJm 2gr-t¡

r Figure 8 An energg pgramid for an aquatlcecosUstem Inot to scale]

secondarg consumer[],ooo MJ rrr-2 9r 1]

pflmaru consumerIz,oootvJm 2gr-1¡

producers[5o,ooo y3 ¡-z gr-t¡

r Figure 9 Pgramid of energg for grassland

@ Pgramids of energg0uantitative representations of energg flow usingpgram¡ds of energg.

2t8