Hull's School1

Hull's SchoolUnit 3 Revision Questions

380 min255 marks

1.The diagram below shows a system that can be used to produce biogas from waste organic matter.

(a)Name the main component of biogas.(1)(b)Suggest how biogas is produced from the organic matter in the digester.(2)

(c)State one use of biogas.(1)(d)Suggest one agricultural use of the sludge removed from the digester after it is dried.(1)(Total 5 marks)

2.The table below refers to processes and bacteria in the nitrogen cycle.Complete the table by filling in the most appropriate word or words in the empty boxes.Name of processChemical changeName of one genus ofbacterium responsiblefor the process

Ammonia to nitrate

Nitrogen to ammonia.RhizobiumDenitrification

(Total 5 marks)

3.The table below gives descriptions of some of the characteristic features of mammalian herbivores related to their nutrition.Complete the table by stating the name of the structure to which each description refers.DescriptionStructureAllows continual growth of teeth

Permits efficient grinding and crushing of plant food

Allows manipulation of food in the mouth during chewing

Is the site of cellulose digestion by microorganisms

(Total 4 marks)

4.The diagram shows a pyramid of numbers, which is a diagrammatic way of representing the feeding relationships within an ecosystem.

(a)Complete the diagram by writing the names of the second and third trophic levels in the spaces provided.(2)(b)Describe how you would collect data to enable a pyramid of numbers to be drawn..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(4)(c)Explain why the number of individuals usually decreases along a food chain.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(Total 8 marks)

5.The diagram below shows the energy flow for part of a salt marsh ecosystem. All values are givenin kJ m2 yr1.

(a)(i)Explain what is meant by primary production.............(2)(ii)Calculate the percentage of the energy from sunlight which is fixed as GPP by the ricegrass. Show your working.

Answer ..(2)(b)Give two ways in which energy is lost between the rice grass and the insects.1 .....2 .....(2)(Total 6 marks)

6.The diagram below shows the quantity of energy flowing through a food chain in a terrestrial ecosystem. The figures given are kJ m2 yr1

(a)Calculate the percentage of the incident energy which becomes available as the net primary production (NPP) of green plants. Show your working.Answer ....................................(2)(b)Give two reasons why not all the energy of the incident sunlight is incorporated into biomass of green plants.1 ....................................................................................................................................................................................................................................................................2 ....................................................................................................................................................................................................................................................................(2)(c)Using the information shown in the diagram, explain why the biomass of insectivorous birds is usually very much less than the biomass of caterpillars.............................................................................................................................................................................................................................................................................................................................................................................................................(2)(Total 6 marks)

7.The photograph below shows the effect of deforestation on an area of tropical rainforest.

(a)State two reasons for deforestation.1. .....................................................................................................................................................................................................................................................................2. .....................................................................................................................................................................................................................................................................(2)(b)Describe the effect of deforestation on the soil.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(c)Distinguish between desertification and deforestation................................................................................................................................................................................................................................................................................................................................................................................................................(2)(Total 6 marks)

8.The diagram below outlines the events that take place when fertiliser enters a river.

(a)(i)Name one component of fertiliser that will cause an increase in the number of algae in the water....(1)

(ii)The increased number of algae causes other plants in the water to die. Explain why this happens.............(2)(b)Explain why there is an increase in bacteria in the water...(1)(c)Describe the consequences of the increased number of bacteria in the river.....(2)(Total 6 marks)

9.An experiment was carried out to compare the uptake of nitrogen in soyabean seedlings grown in an atmosphere enriched with carbon dioxide with that of seedlings grown in a normal atmosphere (control plants). Soyabeans belong to Papilionaceae (legumes) and all the experimental plants had root nodules containing Rhizobium.At the beginning of the experiment, the seedlings were 25 days old. The total amount of nitrogen incorporated into compounds in the plants was then measured at intervals until the plants were 100 days old.

The results of the experiment are shown in the graph below.

(a)(i)Of the nitrogen incorporated into compounds in the control plants, 75% was taken up from the soil. State the form in which this nitrogen was taken up by the plants...........................................................................................................................(1)(ii)Explain how the control plants obtained the remaining 25% of their nitrogen...............................................................................................................................................................................................................................................................................................................................................................................(2)

(b)(i)Compare the effect of the atmosphere enriched with carbon dioxide with that of the normal atmosphere on the mass of nitrogen incorporated into the seedlings...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(ii)Suggest one reason for any differences you observe.....................................................................................................................................................................................................................................................(1)(c)A possible application of gene technology would be to incorporate genes for nitrogen fixation into cereal plants.Suggest the possible benefits of such an application.............................................................................................................................................................................................................................................................................................................................................................................................................(2)(Total 9 marks)

10.The seas around Britain consist of tidal waters fed by the estuaries of many rivers. These waters are generally shallow, in comparison to open seas, and are often relatively close to towns and industries.Figure I illustrates the water circulation pattern within an estuary. The natural recycling of nutrients and detritus within these waters means that there is a great capacity for high productivity.Figure 1 Movement of plankton, detritus and nutrients in an estuary

The convenience of having relatively large bodies of water close to towns and industries has meant that the seas and estuaries around Britain have long been used for waste disposal. Much of this waste is the biodegradable components of sewage from domestic and industrial sources. However, some of the discharges may contain non-biodegradable and toxic compounds.In addition to this, many of the estuaries receive the drainage from agricultural land that may contain the residues of fertilisers and pesticides.Figure 2 shows the effects of biodegradable wastes on the numbers of individual organisms, and on the numbers of species, living on the sea bed at different distances from the point of discharge.Figure 2 Effect of discharge of biodegradable waste on organisms living on thebed of an estuary

Figures 1 and 2 adapted from Biological Sciences Review Vol 2 Num 1 Sept 1989(a)Explain what is meant by the term productivity.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(b)Explain why there is a great capacity for high productivity in the water in estuaries.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)

(c)With reference to Figure 1, suggest why water in estuaries may be particularly affected by the residues of fertilisers and pesticides from agricultural land...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(d)Explain what is meant by the term biodegradable.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(e)Using the information in Figure 2, describe the effects of the discharge of biodegradable waste upon the organisms living on the bed of the estuary...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(f)Explain these effects...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)

(g)Explain why releasing raw sewage into the waters of estuaries can pose a threat to human health...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(Total 18 marks)

11.A student investigated the decomposition of leaves from a sweet chestnut tree. In November 1988, she placed an equal number of leaves in nylon bags with three different mesh sizes. The bags were then buried in garden soil. A fourth sample was placed in a mesh bag and suspended in the air. She examined the bags at intervals of six months and made estimates of the percentage of leaves that remained before reburying the bags. Her results are shown in Figure 1.Figure 1 The percentage of sweet chestnut leaves remaining in nylon bags of differingmesh size over a period of two and a half years.

Percentage of sweet chestnut leaves remaining in the bagsSampleNovember1988May1989November1989May1990November1990May19910.05 mm meshbag in soil10062605142381 mm meshbag in soil10062383030295 mm meshbag in soil10033101010100.05 mm meshbag in air1009692888480

In another investigation, the nitrogen content of leaves from six species of tree was measured. Some fresh leaves from these trees were then placed in separate nylon bags with a 5 mm mesh and buried in garden soil in July. The bags were examined after four months and the percentage of the leaves remaining was recorded. The results of this investigation are shown in Figure 2.Figure 2 A scattergraph showing the relationship between the percentage of leaves remaining after four months and the nitrogen content of the leaves.

(a)(i)Using the information in Figure 1, compare the breakdown of the sweet chestnut leaves in the four bags between November 1988 and May 1991.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(4)(ii)Suggest an explanation for the differences that you have described in (a)(i).................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)

(b)Using the information in Figure 2, describe the relationship between the nitrogen content of the leaves and the decomposition of the leaves. Suggest a reason for this relationship.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(c)Coniferous forests in Northern Europe have long cold winters and short warm summers. Tropical rain forests are found in the hot and wet regions of the world. Coniferous forests have a thick layer of pine needles (leaves) lying on the forest floor. Tropical rain forests have relatively few leaves lying on the forest floor. Suggest reasons for this difference...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(d)The organic nitrogen in fallen leaves is eventually converted into nitrates. Describe the role of soil microorganisms in this process...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(Total 14 marks)

12.Scientists estimate that the atmosphere holds 755 gigatonnes (Gt) of carbon, mostly as carbon dioxide. This value is increasing each year as a result of human activities, such as the burning of fossil fuels and of timber. The diagram below shows a simplified balance sheet of the carbon cycle for one year.

(a)Calculate the increase in atmospheric carbon during this year. Show your working.

Answer ..............................................................(3)(b)Describe and explain how carbon can be removed from the terrestrial (land based) part of the cycle for long lengths of time.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(4)

(c)By reference to the information in the cycle, suggest reasons for the net effect of oceanic processes on the carbon content of the atmosphere.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(4)(d)Suggest how the quantities of carbon dioxide released by the combustion of fossil fuels could be reduced.........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(Total 14 marks)

13.Weasels and stoats are small carnivorous mammals found in Britain. They are able to prey upon many of the small herbivorous mammals found in the countryside. Their diet includes mammals such as mice, voles, rabbits and hares and also worms and other invertebrates. On country estates they are considered to be pests because they eat the eggs and chicks of the game birds that are bred for shooting.A weasel

Ecoscene / Robin RedfernAlthough their quick reactions and long, slender bodies enable them to have some chance of escape, many weasels and stoats become the prey of larger carnivores, such as foxes and cats.Figure 1, Figure 2 and Figure 3 show data concerning the weasel and stoat populations in Britain.Figure 1 - Analysis of the prey eaten by weasels and stoats on game estates in Britain

Figure 2 - An estimate of the numbers of individuals and biomassfor weasels and stoats in BritainSpeciesNumberBiomass / kg 103Weasels450 00034Stoats462 000116Figure 3 - Mean number of weasels and stoats trapped per hectare on game estates in Britain

Figures 1 and 3 adapted from Biologist Vol 17 Num 3 June 2000Figure 2 adapted from Biological Sciences Review Vol 12 Num 4 March 2000(a)Name the trophic level in a pyramid of biomass that would include weasels and stoats. Explain your answer............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(b)Explain what the data in Figure 2 indicate about the relative size of weasels and stoats.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)

(c)With reference to Figure 1, suggest why it is better to record the prey animals eaten as percentage of diet by biomass rather than as numbers eaten.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(d)Compare the diet of the weasels with that of the stoats.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(e)With reference to Figure 3, describe the trend in the populations of weasels and stoats since 1975. Suggest what the effects of this trend might be on the other animals within the ecosystem...........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(Total 12 marks)

14.One of the features of a rainforest is the large expanse of trees growing closely together. The leaves and branches of the trees form a canopy over the forest which reduces the quantity of light reaching the forest floor. Only shade-loving plants can survive below the canopy.The clearance of large areas of forest for timber or farmland results in the forest being divided into fragments which get smaller and smaller as more trees are felled. This is called fragmentation. Studies have been carried out to determine whether the fragmentation of a rainforest affects its biodiversity.The fragments of rainforest are surrounded by land that may be used for grazing or growing crops. This means that there is an abrupt change from forest to farmland, creating an edge. In time, the farmland may be abandoned and revert to secondary forest. The canopy of a secondary forest is more open than that of the original forest and there are more climbing plants such as lianas.

The abundance of seedlings of shade-tolerant trees within a continuous (unfragmented) rainforest in Brazil was compared with that of a fragmented rainforest. A seedling was classed as a young tree between 5 and 100 cm in height. The fragments in which data were collected were approximately rectangular and had areas of 1, 10 and 100 hectares.Figure 1 shows the numbers of seedlings found in the continuous forest and in the 1 hectare fragment, compared with the centres, edges and corners of the 100 and 10 hectare forest fragments.In another study, the numbers of dead trees in the centre and at the edge of an area of rainforest were counted. The diameter of each dead tree was measured. The results of this study are shown in Figure 2.

Figure 1Bar chart showing the mean number of seedlings per m2 in the continuous forest and in the1 hectare fragment, and in different positions within the 100 and 10 hectare fragments of forest.

Figure 2Table showing the number of dead trees in the centre and at the edge of a forest.

(a)Define the term biodiversity...(1)(b)(i)Describe the effects of fragmentation on the mean number of seedlings per m2 as shown in Figure 1...................(3)(ii)Suggest reasons for the effects you have described in (b)(i)...................(3)

(iii)Suggest how these effects would change the forest in the future............(2)(c)(i)Use the data provided in Figure 2 to calculate the percentage difference between the total number of dead trees at the centre and the total number of dead trees at the edge of the forest. Show all your working.

Answer .(3)(ii)Would you expect the numbers of dead trees to increase or decrease as the fragments of forest become smaller? Explain your answer.......(1)(d)Using the data in Figure 2, explain which size of tree was affected most by growing at the edge rather than in the centre of the forest.....(2)(e)The large trees with a diameter of more than 60 cm represented almost a quarter of all the above-ground plant biomass. Explain the meaning of the term plant biomass.....(2)

(f)Describe the effects of a decrease in plant biomass on the food web in the forest.......(3)(Total 20 marks)

15.In the dusty desert margins of northern Nigeria, the population density has increased rapidly. Now approximately 85 per cent of the land is farmed. The annual rainfall has decreased. Itmight be expected that the land would have become desertified, but the opposite is true. Thefields are green and the roads are lined with fruit and vegetable stalls.

The local farmers have changed from their traditional ways of farming. In the past they allowed their goats and sheep to roam free over the semi-arid grasslands. Now they keep them in small fields and even in their backyards so that they can collect the manure to put on their fields. They have no need for chemical fertilisers. They grow crops such as cow peas, sorghum, millet and ground nuts. Now this region is the most productive in Nigeria. The farmers are producing yields that experts once believed were only possible with chemical fertilisers and irrigation.

(a)Explain why the land was expected to have become desertified.....(2)(b)Describe how an area of semi-arid grassland may become desertified.......(3)(c)Manure is rich in nitrogen-containing organic compounds. Explain how the nitrogen in the manure is converted into nitrates that can be used by plants.........(4)

(d)Plants such as cow peas and ground nuts are members of the Papilionaceae.Explain how growing these crops can help to increase yields of other crops suchasmillet.......(3)(Total 12 marks)

16.Study the passage and data below and then answer the questions that follow.During the mid 1970s there was concern in Germany over the health of the silver fir trees (a type of conifer). The older trees had dying branches and they were losing their needles (leaves). By 1980 the Norway spruce trees were also dying. Similar symptoms were reported across Europe including the United Kingdom.This decline in the health of European forests was linked to acid rain. The acid rain was thought to increase the acidity of soil, particularly that of naturally acidic soil which is found in areas where the underlying rock is granite.Since 1984, annual surveys of forest health have been carried out in most European countries. The state of a trees health is measured by estimating the degree of needle loss (Figure 1).Figure 2 shows the results of the surveys for the years 1984-1986 in Germany and the United Kingdom. Figure 3 shows the differences in health of three species of conifer: the Sitka spruce, Scots pine and Norway spruce. This study was carried out on stands of trees between 30 and 50 years old.Further surveys were carried out between 1996 and 1999, to determine the percentage of forests with moderate or severe needle damage (Figure 4).Figure 1Table showing the categories of tree health basedon the percentage of needles that have been lost bythe trees.Percentage of needles lostStatusKey to bar charts61 or moreSevere damage

2660Moderate damage

1125Slight damage

10 or lessHealthy

Figure 2Bar charts showing the results of surveys for the years 1984-1986.

Figure 3Bar charts showing differences in health of three species of conifer:Sitka spruce, Scots pine and Norway spruce.

Figure 4Results of surveys between 1996 and 1999, showing the percentage offorests with moderate or severe needle damage.

1996199719981999Germany20202122United Kingdom14192121(a)Define the term acid rain(1)

(b)Name two gases that cause the formation of acid rain.1 ......................................................................................2 ......................................................................................(2)(c)Using the information provided in Figures 1 and 2, compare the results of the tree surveys in Germany and the United Kingdom for the years 1984, 1985 and 1986.......(3)(d)Use the information presented in Figure 3 to determine which species has been least affected by the acid rain. Explain your answer.Name of tree ...............................................Explanation .....................(2)(e)Describe the effects of acid rain on conifer trees........(4)(f)(i)Using Figure 2, determine the total percentage of trees in the United Kingdom showing moderate and severe damage in 1986...(1)

(ii)Using Figure 4, describe how the health of the forests in the United Kingdom has changed since 1986.........(2)(iii)Suggest reasons for these changes........(2)(g)Explain why the loss of primary producers such as trees can affect other organisms in the same ecosystem.......(3)(Total 20 marks)

17.Study the passage and data below and then answer the questions that follow.

A study of river pollution in South WalesThe map in Figure 1 shows the course of a river that flows through a moderately-sized industrial town in South Wales. At the lower part of the town, the river flows through a tunnel that passes under a large tin plating works.Figure 1Map showing the course of the river through the town

In the 19th century, the effluent (waste water) from the tin plating works was allowed to pass untreated into the river as it flowed through the tunnel. In more recent times, the effluent has been treated to reduce the level of pollutants entering the river.A study was carried out to assess the water quality of the river. Two sites were chosen as shown in Figure 1. Site 1 was just above the entry to the tunnel. Site 2, was just below the point at which the river emerges above ground as its the tunnel.The degree of pollution was assessed by identifying the species of aquatic invertebrates that were present. The number of different species within each of 14 families was counted and recorded in the table shown in Figure 2.Each family was given an index number according to the quality of the water in which it is usually found. This number was multiplied by the number of species to give a score for each family.The scores for the two sites are shown in Figure 2. The degree of pollution and river quality was then assessed by reference to the table shown in Figure 3.

The invertebrates were collected using a kick sampling technique. This involves disturbing the rocks and debris at the bottom of the river to dislodge the invertebrates. They are then caught in a net that is held open downstream.Figure 2Data for the two sites

Site 1Site 2InvertebrategroupSpeciesindexnumberNumber ofspecies foundGroupscoreNumber ofspecies foundGroupscoreEcdyonuridae105500

Ephemeridae102200

Perlidae102200

Leuctridae102200

Corixidae52100

Dytiscidae54202

Helodidae53152

Tipulidae56305

Baetidae44163

Glossiphoniidae3002

Erpobdellidae3263

Limnaeidae3262

Chironomidae2483

Hydrobiidae2362

Total scorefor Site 1227Total scorefor Site 2

Figure 3Scoring table for assessment of pollutionTotal site scoreDegree of pollutionRiver quality0100Very severeVery poor101200SeverePoor201300ModerateModerate301400SlightGood401500NoneExcellent(a)Complete the table in Figure 2 by calculating the scores for each invertebrate group at Site 2, and the total score for the site.(2)(b)Using the information in Figure 2 and Figure 3, compare the results for Site 1 with those for Site 2.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(4)(c)Suggest two reasons why the results of kick sampling may be unreliable.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(d)Suggest why aquatic invertebrates can be used as indicators of pollution.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)

(e)Suggest and explain how the water may be affected as it flows past the agricultural land above the town.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(4)(f)Suggest why the concentration of dissolved phosphate in the water may increase as the river flows through the town.................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)(Total 16 marks)

18.Graph A below shows the global fossil fuel consumption, between 1860 and 1990

Graph B shows how the mean global temperature, between 1860 and 1990, varied from that in 1970.

(a)Calculate the percentage increase in the use of fossil fuels between 1970 and 1990.Show your working.

Answer......................................(2)(b)State two reasons for the increased use of fossil fuels.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(2)

(c)To what extent do the graphs support the theory that rising global temperature is due to the increased use of fossil fuels?...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(d)Suggest how the use of fast-growing plants, such as willow, as a fuel could help to slow down the rate at which global temperatures are rising.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(Total 10 marks)

19.Acid rain is a matter of serious environmental concern. Sulphuric acid is present in acid rain and has adverse effects on both plants and animals.(a)(i)Name two acidic components of acid rain other than sulphuric acid.1 ...............................................................................................................2. ..............................................................................................................(2)(ii)Describe how acid rain is formed......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)

(b)An experiment was carried out to investigate the effect of dilute sulphuric acid on the growth of cress seedlings. Batches of seeds were sown in glass dishes on filter paper to which dilute sulphuric acid was added. The dishes were then incubated. The root and shoot lengths were measured after 65 hours. The results are shown in the table below.Sulphuric acid concentration/ mol dm3Mean root length/ mmMean shoot length/ mm01 103 104 106 107 108 109 101 10255.563.46.52.02.81.51.31.31.025.218.49.54.60.80.50.30.00.0Adapted from Fleet, Jones and Petter, J.B.E.1987(i)Describe the relationship between the concentration of sulphuric acid and the growth of roots as shown by the results in the table....................................................................................................................................................................................................................................................................................................................................................(2)(ii)Compare the effects of sulphuric acid on the growth of roots and shoots......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)

(iii)Suggest two reasons why cress seedlings are suitable for investigating the effect of acid rain on plants.1 .................................................................................................................................................................................................................................2 ................................................................................................................................................................................................................................(2)(Total 12 marks)

20.Study the passage and data below and then answer the questions that follow.Acid rain and its effect on fresh water and fisheriesIt is difficult to obtain direct evidence that, as a result of the increased acidity of precipitation, certain lakes and streams with fishery problems are now more acid that they were in the past.Nevertheless, indirect evidence that surface water acidification has occurred comes from the remains of diatoms in lake sediments.Historical records of pH are rare and subject to a great deal of uncertainty, since analytical methods have changed over the years. Moreover, pH is naturally variable on a daily and annual basis and occasional spot measurements can give a misleading impression. Diatom analysis provides a way round these difficulties. Diatoms are microscopic algae which live free floating in the lake water. They have skeletons made of silica which are resistant to decay. When a diatom dies it sinks to the bottom of the lake where the remains accumulate as sediment.Different diatom species have different pH preferences. Those preferring the least acid water are called alkalibiontic. Acidibiontic species prefer more acidic water. If a core of the lake sediment is taken and it is discovered that alkalibiontic diatoms predominate in the deepest layers and they are replaced by more acid-loving species towards the surface of the sediment, that is a good evidence that the lake has acidified over historical time. Moreover, if the sediments can be dated, by radiotracer methods for example, then the acidification can be correlated in time to events such as the Industrial Revolution and forest clearance. Studies were made of the diatoms in the sediments of Loch Fleet in Galloway, an acid lake.It may take a long time, following the reduction in the acidity of the precipitation, before the recovery of the surface water quality is sufficient to allow restoration of fish stocks. Recovery can be speeded up by the application of calcium to neutralise the acid and suppress the release of aluminium. Experiments were conducted involving the application of calcium in the form of 300 tonnes of limestone to soils surrounding the lake during April 1986. The subsequent changes in the levels of aluminium and calcium in the water were monitored over the next year.

Figure 1The pH reconstruction for diatoms from Loch Fleet.

Figures 2a and 2b. The effect of varying levels of calcium application on the survival of brown trout fry (newly hatched fish) in a range of pH, aluminium and calcium concentrations.

Text and data adapted from Freshwater acidification and fisheries decline by D.J.A Brown, CEGB Research (1987).(a)How is acid rain formed?........................................................................(4)

(b)Describe the changes in the acidity of the water of Loch Fleet over the last 100 years.Explain why these changes occurred.........................................................................(4)(c)The use of diatoms may not be a fool-proof method of studying the changes in pH.Suggest ONE way in which the data could be misleading...................(1)(d)Describe the effect of aluminium ions on fresh water organisms.......................................................(3)

(e)Study the information provided in Figures 2a and 2b. Describe the effect of the calcium treatment on the survival of the brown trout fry.........................................................................(4)(f)Suggest why it is preferable to add the limestone to the surrounding soils rather than directly to the surface of the water.....................................(2)(g)Describe TWO methods used by European countries to reduce the production of acid rain.1 ..2 ..(2)(Total 20 marks)

21.Between 1882 and 1952 the proportion of the Earth's land surface classified as desert increased from 9.4% to 23.3%. The shift to desert is still going on. The most affected areas are the semi-arid lands around the fringes of deserts, such as the Sahel, the belt of land lying to the south of the Sahara Desert in Africa (Figure 1). Rainfall records from the Sahel (Figure 2) show that rainfall has been below average for the last 20 years and the decline is getting worse. The degradation of semi-arid land into desert is called desertification.Figure 1Map of Africa and the Middle East showing the extent of desert and arid zones,with areas under threat from desertification.

Figure 2The rainfall pattern in the Sahel. The bars show the departure from the normalrainfall.

Diagrams from Atlas of living world, Weidenfeld and Nicolson.

(a)Describe the effects of overgrazing and the collecting of wood for fuel on semi-arid land.......................................................(3)(b)People living in the Sahel have kept herds of sheep and goats for hundreds of years. Suggest one reason why overgrazing has become a problem in the last 20 years...................(1)(c)Figure 2 shows that the rainfall in the Sahel between 1950 and 1968 was above average. How would the increased rainfall have affected farming in the Sahel?....................................(2)(d)Describe the effects of the drought which occurred during the 1970s.....................................(2)(e)How might the changes to the global climate have affected desertification in the Sahel?....................................(2)

(f)Describe how people can protect their land from desertification.....................................(2)(Total 12 marks)

22.Study the passage and data below and then answer the questions that follow.Grazing in the Serengeti grasslandThe Serengeti is a huge area of tropical grassland in Tanzania. Herds of grazing mammals, such as wildebeest, gazelle and zebra, roam freely. Every year, these herds migrate across the Serengeti, in search of fresh grassland. The grazing mammals affect the primary productivity of the grassland. Long-term research has found that the rate of primary production is linked to both the rainfall and the numbers of grazing mammals.-

Figure 1Map of Africa showingthe location of the SerengetiFigure 2A wildebeast, one of the manytypes of grazing mammal foundin the Serengeti

Grazing can increase the growth rate of many grass species. This is called compensatory growth. The grazing mammals remove the upper parts of the grass leaves and this increases the amount of light reaching the rest of the plant. The smaller leaf area reduces transpiration and this decreases the uptake of water by the roots. Researchers investigated compensatory growth of grasses in a region of the Serengeti. They fenced off several areas to prevent mammals from grazing the grass. During the annual migration, thousands of wildebeest moved into the study region where they grazed intensively for 4 days and then moved on. The researchers recorded the changes in the fresh biomass of the grasses in the grazed and ungrazed areas over the next 32 days (Figure 3). A further investigation studied the effect of grazing intensity on primary productivity (Figure 4).Figure 3Table showing the fresh biomass over a 32 day period on grazed and ungrazed grassland. Day I is the first day after the wildebeest moved on.

Figure 4Graph to show the effect of grazing intensityon the primary productivity of Serengeti grassland

(a)Define the terms biomass and productivity.Biomass...............................................................................................................................................................................................................................................................................................................................................................................................................Productivity..........................................................................................................................................................................................................................................................................(2)

(b)(i)Using the data in Figure 3, calculate the mean rate of change of fresh biomass per day, for both the grazed and ungrazed grassland, between days 1 and 32. Show your working.

Grazed grassland.........................................Ungrazed grassland.....................................(4)(ii)Suggest why the fresh biomass of grass in the ungrazed area decreased. .............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(c)Study the information provided in Figure 4(i)What is the optimum grazing intensity for Serengeti grassland? ..........................................................................................................................(1)(ii)Suggest reasons for the very low levels of primary productivity at the highest grazing intensity (0.8 arbitrary units). ..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)

(d)In the Serengeti, lions are the top consumers. Suggest and explain how a fall in the number of lions could affect the Serengeti food chain and grassland productivity...............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(3)(e)The grazing mammals have an important role to play in the recycling of nutrients on the grassland. Their dung is rich in organic nitrogen-containing compounds. Describe how these compounds are converted to nitrates.........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................(4)(Total 20 marks)