Ecology and Ecosystems

ORGANISMSO AND THEIR ENVIRONMENT

OBJECTIVES living organisms – and some are abiotic – the ■ To understand that living organisms require non-living components of the habitat. Ecology certain conditions for their survival is the study of living organisms in relation to ■ To understand that living organisms interact with one their environment. The interactions between the another, and with their non-living environment organism and its environment are summarised ■ To deﬁne population, community and ecosystem below. ■ To realise that available resources change through the year Changing with the seasons The ability of the habitat to supply living organisms with their requirements may vary Environmental survival kit at different times of year. The ecosystem in the All living organisms depend upon their photograph opposite will only exist for a certain environment for three ‘survival essentials’. period of time – as food or water becomes These are a supply of food, shelter from exhausted some animals may leave. These will undesirable physical conditions and a breeding then be followed by the predators which feed on site. The living organism interacts with its them. The great animal migrations seen in East environment – for example, a living plant: Africa result from the changing conditions in the ■ removes carbon dioxide, water and light animals’ environment, for example: energy from its habitat ■ poor rain means little growth of grass ■ may be eaten by an animal or a parasite ■ herbivores leave for areas of fresh growth ■ depends upon soil for support. ■ carnivores follow herbivores Factors in the environment affect the growth of ■ (then scavengers follow carnivores!). the plant. Some of these factors are biotic – other Living together Carbon dioxide Living organisms normally exist in groups. The names given to these groups, and the way they interact with the abiotic environment, are explained opposite.

A living organism interacts with its environment

Abiotic Biotic (‘living’) ‘Fertiliser’ Food (‘non-living’) factors e.g. in faeces factors predators food Water and mineral mates ions from soil

A giraffe feeds on a thorn tree. The tree requires water, mineral ions, carbon dioxide and light to grow. The giraffe Physical e.g. Climatic e.g. may provide carbon dioxide from respiration, and ions oxygen and carbon temperature from decomposition of its faeces. dioxide concentration humidity light intensity water availability 222 ORGANISMS AND THEIR ENVIRONMENT

A population is all of the members of the same species (e.g. wildebeest) in a particular area.

Air, water and soil A community is all of the make up the abiotic populations of living organisms environment. in one area (e.g. acacia trees, zebra, wildebeest and grass). The community is the biotic environment.

An ecosystem is all the living organisms and A habitat is a part of the environment that the non-living factors interacting together can provide food, shelter and a breeding site in a particular part of the environment. for a living organism (e.g. a patch of grassland).

Organisms exist in groups within an ecosystem

Q 1 Define the terms population, community and The students measured the amount of sunlight ecosystem. reaching each layer at different times in the year. 2 Name two abiotic factors that might determine Their results are shown on the graph. whether or not a habitat is suitable for a living organism. 3 Suggest two ways in which a plant and an animal in the same habitat may interact. 4 What must a habitat provide? Light reaching 5 How are the following observations related? tree layer ■ Very few flying insects are found in Britain during Light intensity the winter. Light reaching ground layer ■ Swallows migrate to Africa when it is winter in the UK. ■ Hobbies (small bird-eating falcons) leave Britain in Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec late autumn. Month 6 What is meant by the term ecology? 7 a A group of students were studying a forest.They i During which month did most light reach the noticed that the plants grew in two main layers. They tree layer? called these the tree layer and the ground layer. ii During which month did most light reach the ground layer? iii Suggest why the amount of sunlight reaching tree layer the ground layer is lower in mid-summer than in the spring. b The pupils found bluebells growing in the ground layer. Bluebells grow rapidly from bulbs. They flower in April and by June their leaves have died. i Suggest why bluebells grow rapidly in April. ii Suggest why the bluebell leaves have died by June. ground layer

223 ORGANISMSO AND THEIR ENVIRONMENT

4.2 Flow of energy: food chains and food webs OBJECTIVES lost but is converted into some other form. ■ To know that the feeding relationships in an ecosystem During respiration, some energy is transferred can be expressed as food chains to the environment as heat. The flow of energy ■ To understand why energy transfer through an through a food chain, and the heat losses to ecosystem is inefficient the environment, are illustrated in the diagram ■ To understand why complex food webs are the most stable opposite. Food webs Food chains Since so little energy is transferred from the base The most obvious interaction between different to the top of a food chain, a top carnivore must eat organisms in an ecosystem is feeding. During many herbivores. These herbivores are probably feeding, one organism is obtaining food – energy not all of the same species. In turn, each herbivore and raw materials – from another one. Usually one is likely to feed on many different plant species. All organism eats another, but then may itself be food these interconnected food chains in one part of an for a third species. The flow of energy between ecosystem can be shown in a food web. different organisms in the ecosystem can be The more complicated a food web, the more stable shown in a food chain, as in the diagram below. the community is. For example, in the forest food web shown opposite, if the number of squirrels Energy transfer is inefficient fell, the owls could eat more worms, mice and rats. The amount of energy that is passed on in a The mice and rats would have less competition for food chain is reduced at every step. Since energy food from squirrels, and so might reproduce more can be neither created nor destroyed, it is not successfully.

Sunlight provides the Producers, usually green plants, Consumers are organisms that obtain Secondary consumers energy to drive the make their own organic nutrients, food energy by feeding on other organisms. are carnivores. They obtain food chain. usually using energy from sunlight their energy by eating Primary consumers are herbivores. through photosynthesis. other animals. They obtain their energy in food compounds obtained from producers (i.e. plants).

Arrows point in the direction of Secondary consumers may be energy ﬂow along eaten by tertiary consumers. the food chain. These longer food chains are Decomposers, fungi and many bacteria, more common in aquatic obtain their energy and raw materials habitats. The ﬁnal consumer from the wastes (e.g. faeces) and remains in the food chain is called the (e.g. dead bodies) of other organisms. top carnivore.

Food chains show energy ﬂow through an ecosystem. The position of each organism in the food chain represents a different trophic (feeding) level. 224 ORGANISMS AND THEIR ENVIRONMENT

Energy transfer

Less than 1% of the Respiration losses occur from each trophic energy released from level. Respiration is not 100% efﬁcient and the Sun falls onto leaves. eventually all of this energy is lost as heat. Because of this, food chains rarely have more than 4 or 5 trophic levels.

R Energy transfer to Energy transfer to R R primary consumer R secondary consumer is only 5–10%: is between 10% and 20%: much of plant body animal material has a higher Energy ‘ﬁxed’ by producers is indigestible energy value is only 5–8% of the energy consumer rarely eats animal material is more that falls onto their leaves, whole plant – roots digestible. because: or stems may be some is transmitted left behind. (passes right through) some is reﬂected some is not the correct wavelength (only red or blue light is absorbed – see page 44).

Key

R respiration

Note that energy ﬂow is not cyclic! chemical sunlight bond heat energy Energy transfer to decomposers is very As a result there must be a variable, but eventually the entire energy content continuous input of light energy to of the animal and plant remains will be released ‘drive’ life in an ecosystem. as heat from inefﬁcient respiration.

It is more efﬁcient for humans to eat plants than to feed plants to animals, and then to eat the animals. Every step in the chain loses 80–90% of the available energy as heat, so THE FEWER ‘STEPS’ THE BETTER!

A simple forest food web Q Trophic level 1 Deﬁne the terms producer, Tertiary Hawk consumer consumer and decomposer. Which of these could be omitted from an ecosystem? Explain your answer. Secondary Starling Owl consumers 2 Write out a food chain from a named ecosystem which you have studied. Primary 3 Why are food chains usually Snails Woodlice Worms Rats Mice Squirrels consumers restricted to three or four trophic levels?

Producers Fallen leaves Living leaves Grasses Seeds

225 4.2 Flow of energy: food chains and food webs

More examples of feeding relationships Food chains and food webs in aquatic (watery) environments can be longer than those on the land. This is because this type of environment has space and ideal growth conditions for many producers. Even with energy losses at every stage there is enough ‘trapped’ energy for more steps in the chain. Many of these food chains begin with phytoplankton (tiny green plants) or algae.

A freshwater food chain

Algae Water Guppy Bass (small green fleas (small fish) (large fish) plants)

Green heron

A seashore food chain

Algae on rocks Crabs Gull (seaweed) Limpets

The seashore is an excellent environment for animals, at least as far as food is concerned, because fresh supplies are delivered with every tide! Some of the top predators on seashores need so much food that they need to travel between different parts of the habitat. A gull, for example, might have to ﬂy to several different parts of the same shore. Q 1 Look at the three aquatic feeding relationships shown on this page and the next. Make a table like this one:

Producers Herbivores Carnivores Top carnivores

2 Shark ﬁshing is a popular sport. Explain what might happen if all of the sharks living around a section of reef were captured by ﬁshermen.

226 ORGANISMS AND THEIR ENVIRONMENT

Coral reef food web The most complex food webs are found in the ocean. Catching crabs for human food could: ■ increase number of anemones, which could… ■ reduce amount of zooplankton, which could… ■ increase the amount of phytoplankton, which could… ■ increase the number of jelly ﬁsh... Shark ... OUCH!

Octopus

Sea Turtle

Crab

Butterﬂyﬁsh

Parrotﬁsh

Anemone

Jellyﬁsh

Coral

Zooplankton

Phytoplankton Q 3 Use words from the following list to complete the paragraph about ecosystems. You may use each word once, more than once or not at all. decomposition, producer, chemical, carnivore, consumer, photosynthesis, energy, light, elements, decomposers, herbivore. In each ecosystem there are many feeding relationships. A food chain represents a ﬂow of through an ecosystem, and always begins with an organism called a which is able to trap energy and convert it to energy. An organism of this type is eaten by a , which is a kind of that feeds only on plant material. This type of organism is, in turn, eaten by a (an organism that consumes other animals). 227 ORGANISMSO AND THEIR ENVIRONMENT

4.3 Feeding relationships: pyramids of numbers, biomass and energy OBJECTIVES Food chains and food webs provide qualitative ■ To be able to describe pyramids of numbers, information about an ecosystem – they show which biomass and energy organism feeds on which other organism. How do ■ To understand how data can be gathered to make we show quantitative information, for example ecological pyramids how many predators can be supported by a certain number of plants at the start of the chain? We can use a pyramid of numbers or a pyramid of Pyramids of numbers biomass, as shown in the diagram below. Look at the food chain on page 224. Two things should be clear: Pyramids of energy ■ The organisms tend to get bigger moving along A pyramid of biomass describes how much the food chain. Predators, such as the owl, biomass is present in a habitat at the time the need to be large enough to overcome their sample is taken. This can be misleading, because prey, such as the mouse. different feeding levels may contain organisms that ■ Energy is ‘lost’ as heat on moving from one reproduce, and so replace themselves, at different trophic level to the next, so an animal to the right rates. For example, grass in a ﬁeld would replace of a food chain needs to eat several organisms itself more quickly than cattle feeding on the grass, ‘below’ it in order to obtain enough energy. For so when the pyramid of biomass is constructed example, a rabbit eats many blades of grass. there would be more ‘cattle biomass’ than ‘grass

Pyramid of numbers – a diagrammatic representation Top carnivore of the number of different organisms at each trophic level Small carnivore in an ecosystem at any one time Herbivore Note 1 The number of organisms at any trophic level is Producers represented by the length (or the area) of a rectangle. 2 Moving up the pyramid, the number of organisms But generally decreases, but the size of each individual wait! increases.

Problems Bird lice a The range of numbers may be enormous – 500 000 Tawny owl grass plants may only support a single top carnivore – so that drawing the pyramid to scale may be very Blue tits difﬁcult. Insect larvae b Pyramids may be inverted, particularly if the Oak tree producer is very large (e.g. an oak tree) or parasites feed on the consumers (e.g. bird lice on an owl). So …

Biomass expressed as Pyramid of biomass – which represents the biomass Bird lice units of mass per unit (number of individuals × mass of each individual) at Tawny owl area (e.g. kg per m2) each trophic level at any one time. This should solve the scale and inversion problems of the pyramid of Blue tits numbers. Insect larvae Oak tree

Ecological pyramids represent numerical relationships between successive trophic levels. The pyramid of biomass is useful because the biomass gives a good idea of how much energy is passed on to the next trophic level. 228 ORGANISMS AND THEIR ENVIRONMENT biomass’ and the pyramid would be inverted. To overcome this difﬁculty a pyramid of energy can be constructed. This measures the amount of energy ﬂowing through an ecosystem over a period of time. The time period is usually a year, since this takes into account the changing rates of growth and reproduction in different seasons. It is even possible to add an extra base layer to the Pyramid of energy: energy values are expressed as units 2 pyramid of energy representing the solar energy of energy per unit area per unit time (e.g. kJ per m per year) entering that particular ecosystem.

GATHERING DATA FOR ECOLOGICAL PYRAMIDS

To construct a pyramid of numbers or of biomass, Wing nut organisms must be captured, counted and (perhaps) weighed. This is done on a sample (a small number) of the organisms in an ecosystem. Counting every individual organism in a habitat would be extremely time-consuming and could considerably damage the Wire

environment. 1 m

The sample should give an accurate estimate of the total population size. To do this: ■ The sampling must be random to avoid any bias. For 20 cm 20 cm Metal or example, it is tempting to collect a large number of wooden organisms, by looking for the areas where they are frame most common. To avoid this, the possible sampling 1 m sites can each be given a number and then chosen using random number generators on a computer. A quadrat is a square frame made of wood or metal. It is simply ■ The sample must be the right size so that any laid on the ground and the number of organisms inside it is counted. ‘rogue’ results can be eliminated. For example, a A quadrat is used most commonly for estimating the size of plant single sample might be taken from a bare patch of populations, but may also be valuable for the study of populations of sessile or slow-moving animals (e.g. limpets). earth, whereas all other sites are covered with vegetation. The single sample from the bare patch should not be ignored, but its effects on the results will be lessened if another nine samples are taken. A mean value can then be used. Sampling plants and sessile animals Once the organisms in a sample have been identiﬁed and counted, the population size can be estimated. For example, if 10 quadrats gave a mean of 8 plants per quadrat, and each quadrat is one-hundredth of the area of the total site, then the total plant population in that area is 8 3 100 5 800.

229 ORGANISMSO AND THEIR ENVIRONMENT

4.4 Decay is a natural process

OBJECTIVES

■ To understand that nutrients in dead organisms Decomposition by microorganisms are recycled The remains that are left are decomposed by the ■ To know that the process of decay often begins with the feeding activities of microorganisms. These fungi activities of scavengers and bacteria feed by secreting enzymes onto the ■ To know how saprotrophic nutrition is responsible for remains and absorbing the digested products. decomposition This form of nutrition is called saprotrophic feeding. Recycling nutrients The diagram on the opposite page illustrates some of the features of the decomposition process. The Humans have an unusual skill – they can decay process provides energy and raw materials modify their environment to suit themselves. for the decomposers. It also releases nutrients from For example, we cut down forests and plant the bodies of dead animals and plants, which can crops, and we build houses. Many building then be reused by other organisms, for example: materials are natural, such as wood and straw, and the environment treats these materials as the dead remains of once-living organisms – the sugar in dead rabbit environment reclaims the nutrients and returns respiration by microorganisms them to the ecosystem. carbon dioxide Starting with scavengers photosynthesis in plant sugar in plant When an organism dies, the nutrients in its body are returned to the environment to be reused. The nutrients are recycled by a series of processes In this way substances pass through nutrient carried out by other living organisms. The ﬁrst cycles as microbes convert them from large, ones to appear are usually the scavengers which complex molecules in animal and plant remains break up the dead bodies into more manageable to simpler compounds in the soil and the pieces. Scavengers eat some of the dead body, but atmosphere. The next sections describe the leave behind blood or small pieces of tissue. recycling of the elements carbon and nitrogen.

i Importance of decomposition processes to humans ■ Organic waste in sewage is decomposed and made ‘safe’ in water treatment plants (see page 275). ■ Organic pollutants such as spilled oil may be removed from the environment by decomposing bacteria (see page 233). ■ Food is spoiled due to decomposition by fungi and bacteria. Many food treatments alter physical conditions to inhibit enzyme activity. ■ Wounds may become infected by saprotrophs, leading to tissue loss or even to death. Many medical treatments inhibit the multiplication or metabolism Scavengers such as the vulture feed on of saprotrophs. dead bodies

230 ORGANISMS AND THEIR ENVIRONMENT

Saprotrophs cause decay.

Cytoplasm Cells of saprotrophs (bacteria and fungi) nourish themselves by secreting enzymes onto ‘food’ Complex organic compounds include fats, proteins and starches. Cell wall and absorbing the products.

Lipases fats fatty acids + glycerol

Environmental Metabolism inside the bacterial Amylase factors may or fungal cells uses the absorbed starch maltose affect products for respiration or for cell decomposition* growth and division. glucose

Absorbed Proteases simple compounds protein amino acids

Absorption by diffusion and/or by active transport. Antiseptics and disinfectants kill the living organisms that carry out the decay process. Simple compounds Good news! in hospitals and include fatty acids, for food preservation glucose, amino acids but and mineral salts. Bad news! in compost heaps and in sewage works.

*Oxygen is required for aerobic *Water – many decomposition *Heat – for rapid decomposition, respiration, which releases energy reactions are hydrolysis reactions, need to maintain an optimum in bacteria and fungi to drive their i.e. they use water to split chemical temperature for the activity of metabolism. In the absence of bonds. Water is also necessary to enzymes. Heat is generated by oxygen decomposition is slow and dissolve the breakdown products the respiration that occurs during very smelly, as methane and before they can be absorbed by the the decomposition process. hydrogen sulﬁde may be produced. saprotrophs or other organisms.

Q 1 Copy and complete the following paragraph. 2 Gardeners often place vegetable waste on a compost During the process of decay, and convert heap. Over the course of time the waste will be complex chemicals into ones. For example, decomposed. proteins are converted to , and to fatty a What do gardeners gain from the decomposed acids and glycerol. These decay processes involve the waste? biological catalysts called , and so the processes b Why do gardeners sometimes spray water over the are affected by changes in and . Humans heap in warm summer weather? exploit decay, for example in the treatment of to c Why do gardeners often build compost heaps on a provide drinking water, and may deliberately limit pile of loose-ﬁtting sticks or bricks? decay, for example in the preservation of . 231 ORGANISMSO AND THEIR ENVIRONMENT

4.5 The carbon cycle

OBJECTIVES ■ Sometimes conditions are not suitable for ■ To recall why living organisms need carbon-containing respiration by decomposers, and carbon compounds dioxide remains ‘locked up’ in complex carbon ■ To appreciate that carbon is cycled between complex compounds in the bodies of organisms. and simple forms by the biochemical processes of photosynthesis and respiration For example, anaerobic, low pH or extreme ■ To understand that formation and combustion temperature conditions will inhibit of fossil fuels may distort the pattern of the carbon cycle decomposition – this is how fossil fuels have been laid down in environments where decomposition is not favoured. Carbon-containing nutrients – ■ Over millions of years the formation of fossil a reminder fuels has removed carbon dioxide from the The Sun keeps supplying energy to food chains. environment. Humans have exploited fossil However, the supply of chemical elements to living fuels as a source of energy over a relatively organisms is limited, and these elements must short time, and the combustion of oil, gas, be recycled. The nutrient elements are cycled coal and peat has returned enormous volumes between simple forms in the non-living (abiotic) of carbon dioxide to the atmosphere. As a environment and more complex forms in the result carbon dioxide concentrations are bodies of living organisms (the biotic component increasing (see page 265). of an ecosystem). Living organisms require ■ The burning of biomass fuels such as wood carbon-containing compounds as: and alcohol uses up oxygen also returns carbon ■ a source of energy, released when carbon- dioxide to the atmosphere, and can have a very containing compounds are oxidised during severe local effect although worldwide it is less respiration (particularly carbohydrates and signiﬁcant than the combustion of fossil fuels. fats) The way in which these different processes ■ raw materials for the growth of cells contribute to the cycling of carbon is illustrated (particularly fats and proteins). opposite. Recycling carbon compounds Plants, and some bacteria, manufacture these compounds from carbon dioxide during the Q process of photosynthesis (see page 38). Animals 1 Refer to the carbon cycle opposite. obtain them in a ready-made form by feeding a Name the simple carbon compound present in the on other living organisms (see page 52), and abiotic part of the ecosystem. decomposers obtain them as they break down the b Name two compounds present in the biotic part dead bodies or wastes of other living organisms. of the ecosystem. These processes of feeding, respiration, c Which processes raise the concentration of carbon dioxide in the atmosphere? photosynthesis and decomposition recycle the d Which process reduces carbon dioxide carbon over and over again. Theoretically, the concentration in the atmosphere? amount of carbon dioxide ﬁxed by photosynthesis e Name the process that distributes carbon dioxide should equal the amount released by respiration. throughout the atmosphere from places where it As a result the most accessible form of carbon is released. in the non-living environment, that is carbon f Suggest a reason why some fossil fuels were dioxide, remains at about the same concentrationn formed as sediments at the bottom of ancient seas. year after year after year (about 0.03% of the atmosphere). Other processes may affect this regular cycling of carbon. 232 ORGANISMS AND THEIR ENVIRONMENT

The processes of photosynthesis, feeding, death, excretion and respiration lead to the cycling of carbon between living organisms and their environment. Fossil fuel formation and combustion affect the concentration of carbon dioxide in the atmosphere.

Photosynthesis – uses light energy Organic compounds in plants to convert carbon dioxide into organic compounds in plants. Carbohydrates and fats

Proteins

The amount of respiration in Carbon dioxide the different groups of living (CO2) in air organisms varies: and water overall, plants respire less Feeding than they photosynthesise (otherwise they would not grow) Respiration – converts carbohydrates decomposer respiration can to carbon dioxide with the release of be very high in some energy environments, e.g. the warm, moist conditions on the ﬂoor of a rainforest, and Organic compounds in animals decomposers can contribute Carbohydrates and fats 80% of the CO2 in that environment.

Combustion – releases carbon dioxide by the burning of fossil fuels. This increases the Proteins concentration of CO2 available in the environment.

Organic compounds Death and in fossil fuels e.g. excretion peat, coal, oil provide plant and animal material for decay

Organic compounds in POLLUTO decomposers – bacteria – lead and fungi free

Some conditions e.g. low temperature, low oxygen concentration and low pH prevent action of decomposers. This leads to carbon compounds being ‘locked up’ in fossil fuels. Fossil fuel formation lowers the concentration of CO2 which is available in the environment, as it continues to be removed by photosynthesis.

The carbon cycle 233 ORGANISMSO AND THEIR ENVIRONMENT

4.6 The nitrogen cycle

OBJECTIVES scheme to avoid having to use so much nitrogen- ■ To recall why nitrate is an essential mineral for plant containing fertiliser. This saves money, and also growth limits pollution of water (see page 268). ■ To know how nitrate is made available in the soil ■ To understand that a series of biochemical processes Nitrification results in the cycling of nitrogen between living In nitrification, ammonium ions produced by organisms and the environment the decomposition of amino acids and proteins ■ To appreciate the part played by microorganisms are oxidised, first to nitrite and then to nitrate. in the cycling of nitrogen The process is carried out by nitrifying bacteria which live in the soil. Nitrification only happens Plants need nitrate if oxygen is present. In the absence of oxygen the Plants need nitrogen for the synthesis of proteins process is reversed, and denitrifying bacteria and other compounds, including DNA and vitamins. obtain their energy by converting nitrate to Nitrogen gas makes up about 80% of the Earth’s nitrogen gas. This is why waterlogged soils, for atmosphere, but plants do not have the enzymes example, tend to lose nitrate as nitrogen gas. necessary to use the nitrogen directly – instead they Recycling nitrogen must absorb it as nitrate. Nitrate is formed by two Once nitrate has been formed by either nitrogen sets of processes carried out by microorganisms – fixation or nitrification, it can be absorbed by plants nitrogen fixation and nitrification. through their roots. Eventually the plant dies, and Nitrogen fixation its body is added to the animal wastes and remains In nitrogen fixation, nitrogen and hydrogen in the soil. Decomposers break down the nitrogen are combined to form ammonium ions and then compounds in these wastes and remains and the nitrate. The process depends upon enzymes that formation of nitrate can begin again. are only possessed by certain bacteria called In a typical ecosystem the processes shown nitrogen-fixing bacteria. Some of these bacteria opposite recycle nitrogen between living live free in the soil, but a very important species organisms and the environment. However, some called Rhizobium leguminosarum lives in swellings processes cause the loss of nitrate from the called nodules on the roots of leguminous plants environment. This happens naturally as a result such as peas, beans and clover. Nitrogen fixation of denitrification (see above), and less naturally only happens if oxygen is present. It also occurs when crops are harvested and removed from naturally in the atmosphere when the energy from the site where they have grown. These losses of lightning combines nitrogen directly with oxygen. nitrate can be made up either by nitrogen fixation Farmers can plant legumes in a crop rotation or by adding nitrate in the form of fertilisers.

Q 1 Use your knowledge of the nitrogen cycle to explain c planting peas or beans every third year how the following farming practices might improve soil d adding NPK fertiliser fertility. e adding well-rotted compost a ploughing in stubble rather than burning it 2 Explain why farmers drain waterlogged ﬁelds. b draining waterlogged ﬁelds

234 ORGANISMS AND THEIR ENVIRONMENT

The nitrogen cycle

The processes of nitrification, absorption, feeding, death, excretion and decay lead Some plants, called legumes (beans and to the cycling of nitrogen between living organisms and their environment. In a natural peanuts are examples), have swellings ecosystem nitrogen fixation can ‘top up’ the cycle and make up for losses by denitrification. on their roots. These root nodules contain bacteria, which can convert nitrogen gas to nitrate ions. These plants reduce the need for artificial fertilisers.

Organic compounds in plants Nitrogen gas (N2) Proteins in the atmosphere

Nitrogen ﬁxation

Denitriﬁcation

Absorption by diffusion and active transport Farmers drain and plough ﬁelds to improve oxygenation of soil and so reduce denitriﬁcation. Feeding They also add nitrogen-containing fertilisers to directly increase the nitrate content of the soil.

Organic compounds in animals

Proteins, amino acids and urea – Nitrate ions (NO3 ) in soil solution

Nitriﬁcation Death and excretion

Organic compounds in decomposers – bacteria Farmers are encouraged and fungi Amino acids Ammonium to plough roots and + and urea ions (NH4 ) stalks of harvested crops back into the soil. This provides raw material for the action of Decay – enzymes digest decomposers. organic molecules to simpler forms

235 ORGANISMSO AND THEIR ENVIRONMENT

4.7 Water is recycled too!

OBJECTIVES ■ the most important factor in water recycling ■ To know that all living organisms are largely is heat energy from the Sun. This evaporates water, and that biological reactions always take place in an aqueous (watery) environment water, and also creates the temperature gradients which lead to winds. ■ To understand that the biological properties of water result from the structure of the The steps involved in the water cycle are shown in water molecule the diagram opposite. ■ To list some of the biological functions of water The special properties of water The picture of the kangaroo shows the importance Water and life of the properties of water to living things. Life first evolved in water for a number of reasons: ■ The molecules that were used by living organisms, and that made up their structure, The high specific Evaporation of Because water is heat capacity water from incompressible, were dissolved in the first seas. of water means a surface allows it provides ■ In the muddy estuaries and shallow seas of the that cells or bodies loss of heat. excellent support. with a high water Water has a high Water helps primitive Earth, the molecules could become content tend to latent heat of support a whole concentrated enough to react together. resist heating up vaporisation. organism (e.g. a ■ Water acted as a protective shield for the or cooling down, fish), or part of even when the an organism first living organisms against the damaging temperature of (e.g. the eyeball, ultraviolet rays from the Sun. their environment or the erect penis changes. of a mammal). Recycling water Life continues on this planet because water has special properties. In particular, all three states of water – solid ice, liquid water and gaseous water vapour – exist at the temperatures found on the Earth’s surface. The temperature varies at different times and at different places on the planet, but the average temperature over the Earth’s surface is about 16.5 °C. This means that ice, liquid water and water vapour are all present and are continually interchanging. Water is recycled between different parts of the environment, as shown in the water cycle opposite. The water cycle

All of the elements that make up living organisms, Water is an Water can be a Water is an excellent not just carbon and nitrogen, are recycled. The excellent biological reagent, transport medium water cycle is different to the cycles of carbon and lubricant, for example in the for many biological for example in processes of molecules, such as nitrogen because: saliva or in the photosynthesis oxygen, glucose, ■ only a tiny proportion of the water which is synovial ﬂuid of and digestion. amino acids, sodium movable joints. ions and urea. recycled passes through living organisms

236 ORGANISMS AND THEIR ENVIRONMENT

Precipitation Condensation Droplets collect vapour water droplets, and fall as snow, which collect to form clouds rain and hail.

Evapotranspiration liquid vapour

Evaporation Melting/refreezing liquid vapour solid liquid

The water cycle is maintained by heat energy from the Sun

Q 1 a Explain how nitrogen in the muscle protein of a prey. They will also eat larger herbivores such as kudu herbivore may be recycled to form protein in that have been killed by larger predators such as lions. another herbivore some years later. A farmer in South Africa found that a number of his b Explain how the activities of some bacteria form a sheep, while feeding on grassland, were being killed by part of both the carbon and nitrogen cycles. jackals. He noted that jackals always kill sheep by Cambridge IGCSE Biology 0610 Paper 2 Q8 June 2004 attacking their necks. He designed a plastic collar for 2 Use words from the following list to complete the the sheep that covered their necks. None of his sheep paragraphs about ecosystems. You may use each word have been killed since fitting these collars. Other once, more than once or not at all. farmers are now buying the collars to protect their respiration, decomposition, producer, chemical, sheep from jackal attack. carnivore, consumer, photosynthesis, energy, light, a The prey species of the jackal are usually primary elements, decomposers, herbivore consumers. State the type of food that all primary consumers eat. In each ecosystem there are many feeding relationships. b Name the two carnivores identified in the text. A food chain represents a flow of through an c Construct a food chain for the jackal to show its ecosystem, and always begins with an organism called relationship with sheep. a which is able to trap energy and convert it d Suggest a reason why jackals survive better when to energy. An organism of this type is eaten by a they hunt in packs. , which is a kind of that feeds only on plant e When the farmer started to use collars on his sheep, material. This type of organism is, in turn, eaten by a although none of his sheep were being killed, the (an organism that consumes other animals). population of jackals did not decrease. Suggest why The process in which light energy is transferred into a the number of jackals did not decrease. chemical form is called – eventually the energy is f Name two structures, found in the neck of a sheep, released from its chemical form during the process of that could be damaged when jackals attack it. This process provides energy for all living g Some of the protected sheep die of old age and organisms, including which are microbes that their remains are eaten by other animals. Suggest feed on the remains of animals and plants. and explain why the collars of the dead sheep could 3 In Africa, mammals called jackals are quite common. create an environmental problem. They feed on small herbivores such as young Cambridge IGCSE Biology 0610 springboks and dik-diks, hunting in packs to catch their Paper 3 Q1 June 2004

237 ORGANISMS AND THEIR ENVIRONMENT

Questions on ecosystems, decay and cycles

1 Over a period of several months, a student each buffalo. These birds eat ticks that are recorded some activities of the wildlife in a parasites on the buffalo’s skin. particular habitat. The following observations i Draw a pyramid of numbers to represent appeared in her notebook. these feeding relationships. Label the pyramid with the names of the organisms. 1. Young shoots of a crop of bean plants covered with greenflies ii Draw a pyramid of biomass to represent (aphids) sucking food from the stems. the same feeding relationships. Lable the 2. Saw a large bird (hawk), which usually catches mice, swoop to take a small bird visiting the bean field to eat some of trophic levels on this pyramid. the aphids or butterflies. b Explain how the nutrition of consumers 3. Flowers of beans being visited by many different species of butterfly. differs from that of producers. 4. Mice seen nibbling at some dispersed bean seeds. Cambridge IGCSE Biology 0610 5. Spider’s web constructed between two bean plants Paper 2 Q6 November 2008 with 5 large black flies caught in it. Rotting body of a mouse nearby attracting similar flies. 3 The figure shows parts of some natural cycles

in the environment.

a Copy and complete the ﬁgure by ﬁlling in S the names of the organisms to show the feeding relationships in this community. R

a With reference to the carbon and Bean plants nitrogen cycles, explain what is happening at Q and R. b i What name is given to a chart of feeding b Identify two gases that may be released at relationships as shown in the ﬁgure? S and describe the possible harmful effects ii Name two top carnivores observed by they may have on the environment. the student. Cambridge O Level Biology 5090 c i Draw and label a pyramid of biomass for Paper 2 Q6 May 2007 the hawks, mice and bean plants in this 4 Caribbean farmers sometimes: habitat. a Plant peas and corn together to ensure that ii Draw and label a pyramid of numbers for the corn plants could grow well. Use your a bean plant, small birds and aphids. knowledge of the nitrogen cycle to explain Cambridge O Level Biology 5090 this practice. Paper 2 Q3 May 2008 b Use manure from farm animals as fertiliser 2 a Cape buffalo graze on grass. While the for their crops, which are sold in the organic buffalo are grazing, two or three oxpecker produce section at the greengrocer’s store. birds are often seen standing on the backs of Comment on this practice. 238 ORGANISMS AND THEIR ENVIRONMENT

5 a The ﬁgure below shows the carbon cycle. b Over the last few decades, the carbon i Name the processes that cause the dioxide concentration in the atmosphere has changes shown by the arrows labelled been rising. A–D. Suggest how this has happened. ii Name one type of organism that brings Cambridge IGCSE Biology 0610 about decomposition. Paper 2 Q7 November 2008

Carbon dioxide in air A

Decomposition C by BB microorganisms

Fossil fuels Carbon compounds D Carbon compounds in animals in plants

Death Death Fossilisation over millions of years Carbon compounds in dead plants and animals

6 The ﬁgure below shows the water cycle. c A logging company wants to cut down the a i The arrows labelled P represent forest area. evaporation. Which type of energy is i Suggest what effects this deforestation needed for this process? might have on the climate further inland. ii State what causes the formation of Explain your answer. clouds at Q. ii State two other effects deforestation b i What process is represented by the could have on the environment. arrows labelled R? Cambridge IGCSE Biology 0610 iii Name three factors that could alter the Paper 2 Q4 May 2009 rate at which process R happens.

land Q R forest area

river sea

239