Tackling Data Interpretation Questions - Ecology

Bio FactsheetSeptember 1997 Number 6

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It is important that A-level students understand the abilities which they are expected to have developed during their study of Biology sincethese will be tested in their final examinations. In this regard, the actual syllabus which is being pursued is irrelevant since theassessment objectives of all of the boards are essentially the same (Table 1).

One mark for choosing appropriate scale -the candidate should use at least half the paperin each direction.

One mark for correctly labelling the axes. One mark for correctly plotting the points. One mark for labelling the lines or providing

a key.

Commonly, candidates are then asked to:

(i) Suggest which line represents the predatorand which line represents the prey.

The prey usually reach higher population peaksthan predators.

(ii) Describe and explain the fluctuations shown.

The basic principles are simple. An increase inthe number of prey represents an increase infood availability for the predator, whose numberssubsequently increase. As the number ofpredators increase, the number of prey decreasesbecause they are being eaten. Eventually, thenumber of prey will decline to the point wheresome of the predators are unable to find or catchfood and they die. It is also likely to be the casethat the few remaining prey are the fastest orhealthiest and those best able to escape from thepredators. The predator population thus falls,predation decrease and the prey population mayincrease again.

However, Examiners require candidates to focusin a little more closely. In describe questionscandidates should comment on:

(a) The periodicity of the predator andprey curves i.e. the average timebetween peaks (or troughs) for eachof the organisms.

(b) The magnitude of the population boomsof each of the predator and prey.

Candidates frequently imply that the recoveryof the number of prey is an instantaneousconsequence of the decline of the predators. Butthis is not the whole story. The prey populationwill not increase merely as a result of lesspredation; rather it will stop decreasing so quickly.Prey populations will only increase after mating,gestation and birth have occurred and this explains- for both the predator and the prey - why thereis a time lag between population decline andascent.

Ecology is an essential component of all A-levelsyllabuses and provides Chief Examiners withan easy opportunity to set questions testing allof these assessment objectives. This Factsheetwill illustrate how these assessment objectivesare tested in ecological questions and will suggesttechniques which candidates can use to maximisetheir marks on such questions. For illustration,this Factsheet will focus on four types ofcommonly examined topics:

1. Predator-Prey graphs2. Sigmoid growth curves3. Questions about the flow of energy4. Questions about the factors affecting the

distribution of organisms

1. Predator-Prey graphsCandidates are often asked to draw or interpretpredator-prey graphs. Table 2 shows thepopulations of two species, x and y, which formpart of a grassland community in Canada. Oftencandidates will be asked to plot the datagraphically. Such questions usually attract fourmarks, as follows :

Ability Explanation

Recall Facts, concepts, principles and relationships. For example,Can the candidate define the term "community"

Apply Can the candidate apply this knowledge to new situations?

Describe, interpret Can the candidate describe graphs accurately, interpretand translate information presented in graphs, kite diagrams and tables etc. and

translate information e.g. draw a graph using information in atable or vice versa?

Formulate hypotheses Can the candidate formulate a hypothesis and devise anand plan suitable investigation if they are told that lichen species change as theinvestigations distance from urban centres increases?

Analyse and evaluate Can the candidate analyse data from investigations and assess itsvalidity?

Select and present Can the candidate choose ideas, facts and arguments whichsupport a particular case and present them in an appropriatemanner?

Table 1. Examination Board Assessment Objectives

Table 2. Population of species x and y

Year 1955 1960 1965 1970 19801975

Species x 1200 800 920 1170 10001310

Species y 170 150 155 160 160180

Tackling Data Interpretation Questions - Ecology Bio Factsheet

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In questions which focus on analysis, the strong-est candidates will mention that predators veryrarely feed on just one prey species so decline inone prey may not result in the dramatic fluctua-tion of the predator at all.

2. The sigmoid growth curveSigmoid curves crop up everywhere in A-levelBiology! The sigmoid curve is shown in Fig 1.The graph describes:

Candidates may also incorrectly describe the re-lationship between birth rate and death rate atthe plateau stage. Even though the population isnot increasing at the plateau the birth rate isunlikely to be zero. It is however the case thatthe birth rate = death rate.

The interpretion of such curves also requires care.Part a of the curve represents slow populationgrowth but the explanation of this must betailored to the organism concerned. In yeast, it isquite acceptable to state that slow populationgrowth is a result of the need of the organism tosynthesise appropriate enzymes for the substrateit finds itself in. This is not an appropriateexplanation however for the slow growth inpopulation of, for example, rabbits.

3. The flow of energyAll A-level syllabuses require candidates tounderstand how energy flows throughecosystems. By far the most common type ofquestion focuses on the loss of energy at eachtrophic level since this limits the size of foodchains and explains why numbers and biomassof organisms at each trophic level usuallydecreases. This concept can be asked in a numberof different ways, thus:

Why are there usually no more than fivestages in the food chain?Why do the number of organisms decrease ateach stage of food chain?Why does the biomass of organisms decreaseat each stage of a food chain?Why are big, fierce animals rare?

At every stage in the food chain, energy is lostbecause:

1. Energy is lost as heat during respirationand is therefore unavailable to the nextorganism.

2. Energy is lost in faeces which the nextorganism does not eat.

3. Not all of the preceding organism is eaten.

Thus, the amount of available energy decreasesat each trophic level. This limits the number ofstages which can be sustained in the food chainand usually means that the number and biomassof organisms at each trophic level declines.

Frequently, this topic also generates questionswhich test candidates' ability to demonstratetheir critical awareness of social, economic andenvironmental issues.

Farmers attempt to minimise respiratory lossesby minimising the movement of their animals(eg. battery farms) and by ensuring thathomeotherms (birds and mammals which useenergy trying to maintain a constant bodytemperature) do not have to waste energy doingthis.

Modern agriculture is the cause of many envi-ronmental problems - nitrate pollution,eutrophication, organic pollution, pesticidecontamination, habitat destruction, soil erosionetc. Many of these problems are made worse bylivestock production systems and this topic maybe tested in an application question. Annually,millions of human beings die because they areunable to satisfy energy requirements. In termsof the energy efficiency of our farming system,humans would be much better off trying tosatisfy their energy requirements by eatingvegetables and crops, rather than feeding themto cattle and eating the meat. By introducinganother stage into the food chain (crop cattle humans, rather than crop humans) a hugeamount of energy is lost.

The second type of question on energy flowconcerns the relationship between Gross PrimaryProductivity and Net Primary Productivity andfrequently involves simple calculations. The keyequation which candidates must be able to recalland use is:

GPP = NPP + R

Fig 1. The sigmoid growth curve

(i) The growth of yeast or bacterial cultures.

(ii) The growth in the average size of leaves on atree.

(iii) The growth of an animal populationintroduced into a new habitat.

Commonly, questions on the sigmoid growthcurve will test candidates' ability to describe,interpret and translate data on populationgrowth. Most candidates understand thesignificance of the shape of the curve and areable to explain each of the stages:

(a) Population increasing slowly - adaptingto new environment.

(b) Population increasing exponentially - nolimiting factors.

(c) Population growth slows and then stops.birth rate = death rate at the plateau.

(d) Death rate exceeds birth rate.

However, many candidates provide inaccuratestatements in their descriptions. Exponentialgrowth is only being achieved when the timetaken for the population to double is the sameover a number of generations. Exponential growthis unsustainable because of density dependentlimiting factors such as food supply, space andpredation. Often however, intraspecificcompetition is more significant then interspecificcompetition in limiting population size and thisis largely because members of the same speciesare competing for exactly the same factors.

Total amount ofenergy fixed by

green plants

Net primaryproductivity

Respiration

NPP is the most useful concept since this repre-sents the amount of energy or organic matterwhich is actually available to the next trophiclevel. Commonly, candidates are asked tocalculate one of the three variables when theyare given the other two. For example, Fig 2 belowshow the energy flow in a grassland ecosystem.

Fig 2. Energy flow in a grassland

Incident light

Reflectio

n

eg. most ofthe greenlight

Cow ManGrass

a

b

c

d

Num

ber/s

ize

Time

Calculate:

(i) The respiratory loss of the grass plants.

Since GPP = NPP + R GPP - NPP = R

2.2 x 104 - 1.4 x 104 = 0.8 x 104kjm-2yr-1(Dont forget the units)

3 x 106

GPP = 2.2 x 104NPP = 1.4 x 104

Tackling Data Interpretation Questions - Ecology Bio Factsheet

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Acknowledgements:This Factsheet was researched and written byBrian Taylor

Curriculum Press, Unit 305B, The Big Peg,120 Vyse Street, Birmingham. B18 6NF

Biopress Factsheets may be copied free of chargeby teaching staff or students, provided that theirschool is a registered subscriber.ISSN 1351-5136

(ii) The percentage of sunlight energy whichis fixed as GPP by the grass plants.

Candidates who are terrified of percentagecalculations must grit their teeth, practise andthink.

Not all of the light energy will be fixed - somewill be reflected so the percentage must be lessthan 100%, so:

GPP = 2.2x 104Total available = 3 x 106

So the percentage is 2.2 x104 x 100 % = 0.7%

3 x106

The factors affecting productivity confuse manycandidates and sometimes it helps merely tosubstitute in the word photosynthesis. Hence"Why is tropical rainforest more productivethan temperate coniferous woodland?"becomes "Why is the rate of photosynthesisfaster in the tropics than in temperateareas?"

Productivity and photosynthesis are influencedby temperature, carbon dioxide concentration,water and nutrient availability, light intensity andwavelength and pollution. The tropics are hotand wet, hence productivity is high.

4. Factors affecting the distribution oforganismsQuestions are frequently set on the biotic andabiotic conditions which affect the distributionof organisms in or between different habitats.Zonation along rocky shores, the distribution oforganisms in freshwater lakes or betweenpolluted and non-polluted areas are commonexamples. In such questions, candidates are oftenasked to suggest explanations for a pattern ordistribution which is often set in an unusualcontext. Many candidates omit to write anyanswers for such questions believing that theyshould be able to recall factual responses toeverything. The examiners however will usuallycredit any sensible suggestions and the list ofacceptable alternatives on mark schemes may belong. Candidates should try to stay calm andthink about the underlying principles.For example:

Fig 3. shows the distribution of seedlings of thecapulin tree (Prunus capuli) on a wooded hillsidein Mexico. Part of the hillside has been deforested.

Suggest two reasons to account for thedistribution of P.capulin seedlings.

Candidates should not be concerned that:

(a) They have not studied Mexico.(b) They know nothing about P.capulin.

The distribution of species is affected by bioticand abiotic factors. The deforested area has thehighest number of seedlings. What biotic andabiotic factors could account for this? Even ifcandidates cannot recall the changes whichdeforestation causes to the immediatemicroclimate, it is relatively simple to suggestthings.

For example, the soil in the clearing will receivemore direct sunlight and therefore may be drieror warmer than soils beneath the canopy on eitherside. However, it could be argued that the soilson the top of the slope beneath the canopy mightalso be drier as they lose water through drainageso perhaps temperature - or the temperature range- may be more important than water availability.Then again, the soils in the cleared area will besubject to increased rain splash and erosion,increased leaching and increased frosts. Thesefactors may all affect the distribution of P.capulin.We do not know, the Chief Examiner probablydoes not know and the key is to apply thebiological basics to this new situation and tosuggest things.

Fig 3. Distribution of P. capulin seedlings

No.

of s

eedl

ings

Distance

Distance

Vert

ical

heig

ht