ECOLOGY
4.1 Communities and ecosystems
Notes: The definitions could be set in the context of field visits to local habitats and the term ‘Ecology’ defined in relationship between organisms, communities and ecosystems.
4.1.1 Define ecology.
4.1.2 Define ecosystem.
4.1.3 Define population.
4.1.4 Define community.
4.1.5 Define species.
4.1.6 Define habitat.
4.1.7 Explain what is meant by the biosphere.
4.1.8 Describe what is meant by a food chain giving three examples, each with at least three linkages (four organisms).Food chains are best determined using real examples. A à B indicates that A is being ‘eaten’ by B. the different classifications of food chains (grazer, decomposer, etc.) will not be required.
4.1.9 Describe what is meant by a food web.
4.1.10 Define a trophic level.
4.1.11 Deduce a trophic level(s) of organisms in a food web.The student should be able to place an organism at the level of producer, primary consumer, secondary consumer, etc. as the terms herbivore and carnivore are not always applicable.
4.1.12 Draw a food web given appropriate information, containing up to 10 organisms.
The comment made in 4.1.8 also applies here.
4.1.13 Define autotroph (producer).
4.1.14 Define heterotroph (consumer).
4.1.15 Define detritivore.
4.1.16 Define saprotroph (decomposer).
4.2 Photosynthesis, respiration and energy relationships
Notes: The importance of the role of plants can be emphasised and prominence given to the concept of inter-relationships between organisms. It should be noted that there are many assessment statements that are objective 1. The depth can be guided by AHL topic 9 in that what is expected for that topic is not expected here.
In statements 4.2.1 – 4.2.9 the programme puts accent on the building up of organic compounds from inorganic material. In statements 4.2.10 – 4.2.12 these same molecules are being broken down, permitting the introduction of the carbon cycle in 4.2.19.
4.2.1 State that light is the initial energy source for almost all communities.
Reference to communities that start with chemical energy is not required.
4.2.2 Describe the fact that photosynthesis involves energy conversion in which light energy is converted to chemical energy.
The over-simplified summary equation 6CO2 + 6H2O à C6H12O6 + 6O2 will not be used since it implies carbohydrate as the only product of photosynthesis.
Reference to actual wavelengths or frequencies is not expected, nor the relative proportions of the colours.
4.2.4 State that chlorophylls are the main photosynthetic pigments.
The fact that more than one form commonly exist may be mentioned, although the differences in structure between chlorophyll is not required.
It is important for students to appreciate that pigments actively absorb certain colours of light due to their structure. The remaining colours of light are reflected and give rise to the colour perceived by the brain of the observer. It is not necessary to mention wavelengths or structure responsible for the absorption.
4.2.6 State that light energy is used to split water molecules to produce oxygen and hydrogen, and to produce ATP.
4.2.7 State that ATP and hydrogen, (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.
4.2.8 Explain that photosynthesis can be monotored by the production of oxygen, the uptake of carbon dioxide or the increase in biomass.The recall of details of specific laboratory experiments to indicate that photosynthesis has occurred or to measure the rate of photosynthesis will not be expected. The principles of measuring input/output of gases, changes associated with these (e.g., pH) as a direct means and increase in biomass as an indirect means are the important points. Details concerning the use of radioisotopes are not required.
The concept of limiting factors is not expected here. The shape of the graphs is expected.
4.2.10 State that respiration involves the breakdown of organic molecules to release energy stored by photosynthesis.
4.2.11 State that the carbon dioxide fixed by photosynthesis is released by respiration.Details concerning the use of radioisotopes are not required.
4.2.12 State that the energy released during breakdown/respiration of complex compounds in an organism is used within an organism to do work or is lost as heat.
4.2.13 Define biomass.The term ‘standing crop’ will not be used.
Qualitative treatment only.
Reference to the Second Law of Thermodynamics is not expected.
4.2.16 Explain what is meant by a pyramid of energy and reasons for its shape.
4.2.17 Design a pyramid of energy given appropriate information.4.2.16- All ecological pyramids can be criticised as to their value in helping
4.2.17 understanding ecological relationships. Pyramids of numbers and biomass are the most problematical and so are not required. The lowest bar of the pyramid of energy represents gross primary productivity, the next bar the energy ingested as food by primary consumers, and so on. The units being measured in energy per unit area per unit time. Reasons for the shape could include various ways in which energy is lost between trophic levels.
4.2.18 Explain that energy enters and leaves an ecosystem, but nutrients must be recycled.
4.2.19 Draw the carbon cycle to show the process including photosynthesis, respiration, combustion and fossilization.The details of the carbon cycle should involve the interaction of living organisms and the biosphere through the process of photosynthesis, respiration, fossilization and combustion. Recall of specific quantitative data is not required.
4.3 Populations, natural selection and evolution.Specific names of decomposer organisms are not required.
Notes: There is an opportunity here to do some work and plot and interpret graphs.
Two themes contribute: ‘structure and function’ and ‘evolution’.
4.3.1 Outline how population size can be affected by natality, immigration, mortality and emigration.
4.3.2 Explains reasons for the exponential growth phase, the plateau phase and the transitional phase between these two phases.A ‘formula’ relating these is not required; simply that the first two increase population size whilst the latter two decrease it.
4.3.2- The emphasis should be placed upon the factors affecting population
4.3.3 growth rate. The terms exponential growth phase, transitional phase and plateau phase will be used. The names lag, log, stationary, decline and death, sometimes used to describe the different phases of growth will not be required.
4.3.4 Define carrying capacity.
4.3.5 List three factors which set limits to population increase.
4.3.6 State that populations tend to produce more offspring than the environment can support.It is important that the Theme of Evolution is placed firmly in the context of ecology and an understanding of natural selection is crucial. Overt references to Malthus, Wallace and Darwin (et al) in the development of the theory of evolution by natural selection is not required, since the option on evolution allows this aspect and the whole background to evolution to be developed.
4.3.7 Explain that the consequences of the potential overproduction of offspring is a struggle for survival.
4.3.8 State that the members of a species show variation (cross reference 3.3.3).
4.3.9 Explain how, by natural selection, the best adapted will survive to breed.
4.3.10 Discuss the theory that species evolve by natural selection.
4.3.11 Discuss the need for evolution in response to environmental change.
4.4 Human impact
4.4.1 Outline two examples of local or global issues of human impact causing damage to an ecosystem or the biosphere, one of which must be increased greenhouse effect.Notes: Studies of pollutants can become complicated. Examples should be chosen with care, e.g. in the case of ozone students should be aware that its influence in the lower atmosphere (the troposphere) is negative. Human activities are producing ozone as a pollutant, whereas in the upper atmosphere (the stratosphere) its effect is positive in that it acts as a shield against UV light. CFCs, etc. are depleting it.
In studying the greenhouse effect, the students should be made aware that it is a perfectly natural phenomenon and without it, organisms may have evolved differently. The problem lies in its enhancement by certain human activities. Knowledge that gases other than carbon dioxide exert a greenhouse effect is required.
4.4.2 Explain the causes and effects of the two issues in 4.4.1, supported with data.
4.4.3 Discuss measures which could be taken to contain or reduce the impact of these issues, with reference to the functioning of the ecosystem.
4.5 Ecological techniques
4.5.1 Describe one method used to measure each of three abiotic characteristics of a habitat including light.Notes: Obviously techniques could be taught but this would be to miss an opportunity to do fieldwork. Students gain considerably from working in local habitat. This sub-topic may be taught almost completely by practical work, in which case the 2h allocated into it may be added into the practical component for the SSC or combined with that for the ecology and conservation option. The importance of statistical analysis should be obvious from the assessment statements.
Light includes intensity, quality and duration and should include importance for both animals as well as plants.
4.5.2 Define random sample.
4.5.3 Describe one technique used to estimate population size of one animal species based on a capture-mark-release-recapture method.Various mark and recapture methods exist. The simplest form, the Lincoln index, which involves one mark, release and recapture cycle is required.
Population size = n1 x n2
n3
n1 = number initially caught, marked and released
n2 = total number of individuals caught in the second sample
n3 = number of marked individuals in the second sample
Although simulations can be carried out (e.g., sampling beans in sawdust) it is much more valuable if this is accompanied by a real exercise on population of animals. The limitations and difficulties of the method can be fully appreciated and some notion of the importance of sample size can be explained.
It is important that students appreciate the importance of choosing an appropriate method for marking organisms – sometimes their life expectancy can be drastically reduced by the over application of some labelling substance.
4.5.4 Describe one method of random sampling used to compare the population numbers of two plant species based on quadrat methods.
4.5.5 Evaluate graphical presentations of ecological data.
4.5.6 Define mean.
4.5.7 Define mode.
4.5.8 Define median.
4.5.9 State that the term standard deviation is used to summarise the spread of variables around the mean and that 68% of the values fall within one standard deviation of the mean (plus and minus).
4.5.10 Calculate the means and standard deviation of two different frequency distributions.x = å x¦
n
x = mean
x = class value
¦ = frequency
n = number of values
å x2¦ - (å x¦ )2
s = n 1
n - 1
s = standard deviation
4.5.11 Describe how standard deviation is useful in comparing the means and the spread of ecological data between two or more sites.
4.5.10- Statement 4.5.9 made the point that for normally distributed data
about 68% of all values lie with the range of the mean plus or minus two standard deviation (s.d. or s or s ). This rises to about 95% for plus or minus two standard deviations. A small s.d. indicates that the data is clustered closely around the mean value. Conversely, a large s.d. indicates a wider spread around the mean. The size of a s.d. might be the result of the genetic or environmental factors (or both). When comparing, two samples are drawn from a similar (the same) population. The bigger the difference, the less likely this is so. This is dependent on sample size – larger samples make more reliable results. Comparing two samples of fictitious populations: population A has three plants 10 cm, 20 cm and 30 cm high respectively, and population B has three plants all 20 cm high – when compared, the means are identical but the standard deviations are widely different. – This could reinforce the point that means ‘do not tell the whole story’ and stress the importance of standard deviation. It might also be used to show that very small samples are unreliable!
Details of statistical tests to quantify variations between populations, such as Standard Error, or details about confidence limits are not required.
