4.1 Ecology and ecosystems - beck-shop.de · PDF file2 Name two abiotic factors that might ......

4.1

O R G A N I S M S A N D T H E I R E N V I R O N M E N T

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Environmental survival kitAll living organisms depend upon their environment for three ‘survival essentials’. These are a supply of food, shelter from undesirable physical conditions and a breeding site. The living organism interacts with its environment – for example, a living plant:■ removes carbon dioxide, water and light

energy from its habitat■ may be eaten by an animal or a parasite■ depends upon soil for support.

Factors in the environment affect the growth of the plant. Some of these factors are biotic – other

living organisms – and some are abiotic – the non-living components of the habitat. Ecology is the study of living organisms in relation to their environment. The interactions between the organism and its environment are summarised below.

Changing with the seasonsThe ability of the habitat to supply living organisms with their requirements may vary at different times of year. The ecosystem in the photograph opposite will only exist for a certain period of time – as food or water becomes exhausted some animals may leave. These will then be followed by the predators which feed on them. The great animal migrations seen in East Africa result from the changing conditions in the animals’ environment, for example:■ poor rain means little growth of grass■ herbivores leave for areas of fresh growth■ carnivores follow herbivores■ (then scavengers follow carnivores!).

Living togetherLiving organisms normally exist in groups. The names given to these groups, and the way they interact with the abiotic environment, are explained opposite.

OB J EC T I VES

■ To understand that living organisms require certain conditions for their survival

■ To understand that living organisms interact with one another, and with their non-living environment

■ To define population, community and ecosystem■ To realise that available resources change through

the year

Ecology and ecosystems

Carbon dioxide

‘Fertiliser’in faeces

Water and mineralions from soil

Food

A giraffe feeds on a thorn tree. The tree requires water,mineral ions, carbon dioxide and light to grow. The giraffemay provide carbon dioxide from respiration, and ionsfrom decomposition of its faeces.

Climatic e.g.temperaturehumidity

A living organisminteracts with its

environment

Abiotic(‘non-living’)

factors

Biotic (‘living’)factors e.g.

predatorsfoodmates

Physical e.g.oxygen and carbondioxide concentrationlight intensitywater availability


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Q1 Define the terms population, community and

ecosystem.2 Name two abiotic factors that might determine

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?5 How are the following observations related?

■ Very few flying insects are found in Britain during the winter.

■ Swallows migrate to Africa when it is winter in the UK.

■ Hobbies (small bird-eating falcons) leave Britain in late autumn.

6 What is meant by the term ecology?7 a A group of students were studying a forest.They

noticed that the plants grew in two main layers. They called these the tree layer and the ground layer.

The students measured the amount of sunlight reaching each layer at different times in the year. Their results are shown on the graph.

i During which month did most light reach the tree layer?

ii During which month did most light reach the ground layer?

iii Suggest why the amount of sunlight reaching 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.

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An ecosystem is all the living organisms andthe non-living factors interacting togetherin a particular part of the environment.

A community is all of thepopulations of living organismsin one area (e.g. acacia trees,zebra, wildebeest and grass).The community is the bioticenvironment.

A population is all of themembers of the same species(e.g. wildebeest) in aparticular area.

Air, water and soilmake up the abioticenvironment.

A habitat is a part of the environment thatcan provide food, shelter and a breeding sitefor a living organism (e.g. a patch of grassland).

� Organisms exist in groups within an ecosystem

tree layer

ground layer

Month

Ligh

t in

tens

ity

Light reachingground layer

Light reachingtree layer

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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Food chainsThe most obvious interaction between different organisms in an ecosystem is feeding. During feeding, one organism is obtaining food – energy and raw materials – from another one. Usually one organism eats another, but then may itself be food for a third species. The flow of energy between different organisms in the ecosystem can be shown in a food chain, as in the diagram below.

Energy transfer is inefficientThe amount of energy that is passed on in a food chain is reduced at every step. Since energy can be neither created nor destroyed, it is not

lost but is converted into some other form. During respiration, some energy is transferred to the environment as heat. The flow of energy through a food chain, and the heat losses to the environment, are illustrated in the diagram opposite.

Food websSince so little energy is transferred from the base to the top of a food chain, a top carnivore must eat many herbivores. These herbivores are probably not all of the same species. In turn, each herbivore is likely to feed on many different plant species. All these interconnected food chains in one part of an ecosystem can be shown in a food web. The more complicated a food web, the more stable the community is. For example, in the forest food web shown opposite, if the number of squirrels fell, the owls could eat more worms, mice and rats. The mice and rats would have less competition for food from squirrels, and so might reproduce more successfully.

OB J EC T I VES

■ To know that the feeding relationships in an ecosystem can be expressed as food chains

■ To understand why energy transfer through an ecosystem is inefficient

■ To understand why complex food webs are the most stable

Flow of energy: food chains and

food webs

Secondary consumers may beeaten by tertiary consumers. These longer food chains are more common in aquatic habitats. The final consumer in the food chain is called the top carnivore.

Sunlight provides the energy to drive the food chain.

Decomposers, fungi and many bacteria,obtain their energy and raw materials from the wastes (e.g. faeces) and remains(e.g. dead bodies) of other organisms.

Producers, usually green plants, make their own organic nutrients,usually using energy from sunlightthrough photosynthesis.

Consumers are organisms that obtainfood energy by feeding on other organisms.

Arrows point in the direction of energy flow along the food chain.

Primary consumers are herbivores.They obtain their energy in food compounds obtained from producers(i.e. plants).

Secondary consumersare carnivores. They obtaintheir energy by eatingother animals.

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


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Because of this, food chains rarely havemore than 4 or 5 trophic levels.

Note that energy flow is not cyclic!

As a result there must be acontinuous input of light energy to‘drive’ life in an ecosystem.

heatsunlightchemicalbondenergy

Less than 1% of theenergy released fromthe Sun falls onto leaves.

Energy ‘fixed’ by producersis only 5–8% of the energythat falls onto their leaves,because: some is transmitted (passes right through) some is reflected some is not the correct wavelength (only red or blue light is absorbed – see page 44).

R RR

R

Energy transfer to decomposers is veryvariable, but eventually the entire energy contentof the animal and plant remains will be releasedas heat from inefficient respiration.

It is more efficient for humans to eat plantsthan to feed plants to animals, and then toeat the animals. Every step in the chain loses80–90% of the available energy as heat, so THEFEWER ‘STEPS’ THE BETTER!

Energy transfer toprimary consumeris only 5–10%: much of plant body is indigestible consumer rarely eats whole plant – roots or stems may be left behind.

Energy transfer tosecondary consumeris between 10% and 20%: animal material has a higher energy value animal material is more digestible.

Respiration losses occur from each trophiclevel. Respiration is not 100% efficient andeventually all of this energy is lost as heat.

R respiration

Key

Producers

Primaryconsumers

Secondaryconsumers

Tertiaryconsumer

Trophic level

Hawk

Owl

Fallen leaves Living leaves Grasses Seeds

Rats Mice Squirrels Worms Woodlice Snails

Starling

Energy transfer

A simple forest food web

Q1 Define the terms producer,

consumer and decomposer. Which of these could be omitted from an ecosystem? Explain your answer.

2 Write out a food chain from a named ecosystem which you have studied.

Why are food chains usually restricted to three or four trophic levels?

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Algae on rocks (seaweed) Limpets

Crabs Gull

4.2 Flow of energy: food chains and food webs

Algae (small green plants)

Water fleas

Guppy (small fish)

Bass (large fish)

Green heron

More examples of feeding relationshipsFood 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

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.

A seashore food chain

Q1 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

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.

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Coral reef food webThe most complex food webs are found in the ocean.

Shark

Sea Turtle

Parrotfish

Phytoplankton

Zooplankton

Anemone

Crab

Butterflyfish

Jellyfish

Octopus

Coral

Q 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).

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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...

... OUCH!

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Pyramids of numbersLook at the food chain on page 224. Two things should be clear:■ The organisms tend to get bigger moving along

the food chain. Predators, such as the owl, need to be large enough to overcome their prey, such as the mouse.

■ Energy is ‘lost’ as heat on moving from one trophic level to the next, so an animal to the right of a food chain needs to eat several organisms ‘below’ it in order to obtain enough energy. For example, a rabbit eats many blades of grass.

Food chains and food webs provide qualitative information about an ecosystem – they show which organism feeds on which other organism. How do we show quantitative information, for example 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 biomass, as shown in the diagram below.

Pyramids of energyA pyramid of biomass describes how much biomass is present in a habitat at the time the sample is taken. This can be misleading, because different feeding levels may contain organisms that reproduce, and so replace themselves, at different rates. For example, grass in a field would replace itself more quickly than cattle feeding on the grass, so when the pyramid of biomass is constructed there would be more ‘cattle biomass’ than ‘grass

OB J EC T I VES

■ To be able to describe pyramids of numbers, biomass and energy

■ To understand how data can be gathered to make ecological pyramids

Feeding relationships: pyramids of

numbers, biomass and energy

Pyramid of numbers – a diagrammatic representationof the number of different organisms at each trophic levelin an ecosystem at any one time

Note1 The number of organisms at any trophic level is

represented by the length (or the area) of a rectangle.2 Moving up the pyramid, the number of organisms

generally decreases, but the size of each individualincreases.

Producers

Top carnivore

Small carnivore

Herbivore

Butwait!

Problemsa The range of numbers may be enormous – 500 000

grass plants may only support a single top carnivore –so that drawing the pyramid to scale may be verydifficult.

b Pyramids may be inverted, particularly if theproducer is very large (e.g. an oak tree) or parasitesfeed on the consumers (e.g. bird lice on an owl).

So …

Pyramid of biomass – which represents the biomass(number of individuals × mass of each individual) ateach trophic level at any one time. This should solvethe scale and inversion problems of the pyramid ofnumbers.

Biomass expressed asunits of mass per unitarea (e.g. kg per m2)

Bird lice

Tawny owl

Blue tits

Insect larvae

Oak tree

Bird lice

Tawny owl

Blue tits

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.


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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 representing the solar energy entering that particular ecosystem.

� Pyramid of energy: energy values are expressed as units of energy per unit area per unit time (e.g. kJ per m2 per year)

GATHERING DATA FOR ECOLOGICAL PYRAMIDS

To construct a pyramid of numbers or of biomass, 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 environment.

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

example, it is tempting to collect a large number of

organisms, by looking for the areas where they are

most common. To avoid this, the possible sampling

sites can each be given a number and then chosen

using random number generators on a computer.

■ The sample must be the right size so that any

‘rogue’ results can be eliminated. For example, a

single sample might be taken from a bare patch of

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 animalsOnce 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.

Wire

1 m

1 m

20 cm 20 cm

Wing nut

Metal or wooden frame

� A quadrat is a square frame made of wood or metal. It is simply laid on the ground and the number of organisms inside it is counted.

A quadrat is used most commonly for estimating the size of plant populations, but may also be valuable for the study of populations of sessile or slow-moving animals (e.g. limpets).

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Recycling nutrientsHumans have an unusual skill – they can modify their environment to suit themselves. For example, we cut down forests and plant crops, and we build houses. Many building materials are natural, such as wood and straw, and the environment treats these materials as the dead remains of once-living organisms – the environment reclaims the nutrients and returns them to the ecosystem.

Starting with scavengersWhen 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 carried out by other living organisms. The first ones to appear are usually the scavengers which break up the dead bodies into more manageable pieces. Scavengers eat some of the dead body, but leave behind blood or small pieces of tissue.

Decomposition by microorganismsThe remains that are left are decomposed by the feeding activities of microorganisms. These fungi and bacteria feed by secreting enzymes onto the remains and absorbing the digested products. This form of nutrition is called saprotrophic feeding.

The diagram on the opposite page illustrates some of the features of the decomposition process. The decay process provides energy and raw materials for the decomposers. It also releases nutrients from the bodies of dead animals and plants, which can then be reused by other organisms, for example:

In this way substances pass through nutrient cycles as microbes convert them from large, complex molecules in animal and plant remains to simpler compounds in the soil and the atmosphere. The next sections describe the recycling of the elements carbon and nitrogen.

OB J EC T I VES

■ To understand that nutrients in dead organisms are recycled

■ To know that the process of decay often begins with the activities of scavengers

■ To know how saprotrophic nutrition is responsible for decomposition

Decay is a natural process

sugar in dead rabbit respiration by microorganisms

carbon dioxide photosynthesis in plant

sugar in plant

� Scavengers such as the vulture feed on dead bodies

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 of saprotrophs.

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Antiseptics and disinfectantskill the living organisms thatcarry out the decay process.Good news! in hospitals andfor food preservation butBad news! in compost heapsand in sewage works.

Cell wall

Cytoplasm

Lipasesfats fatty acids + glycerol

Amylasestarch maltose

glucose

Proteasesprotein amino acids

Simple compoundsinclude fatty acids,

glucose, amino acidsand mineral salts.

Absorption bydiffusion and/orby active transport.

Absorbedsimple compounds

Environmentalfactors mayaffectdecomposition*

Metabolism inside the bacterialor fungal cells uses the absorbedproducts for respiration or for cellgrowth and division.

Cells of saprotrophs (bacteriaand fungi) nourish themselvesby secreting enzymes onto ‘food’and absorbing the products.

Complex organic compoundsinclude fats, proteins and starches.

*Oxygen is required for aerobicrespiration, which releases energyin bacteria and fungi to drive theirmetabolism. In the absence ofoxygen decomposition is slow andvery smelly, as methane andhydrogen sulfide may be produced.

*Heat – for rapid decomposition,need to maintain an optimumtemperature for the activity ofenzymes. Heat is generated bythe respiration that occurs duringthe decomposition process.

*Water – many decompositionreactions are hydrolysis reactions,i.e. they use water to split chemicalbonds. Water is also necessary todissolve the breakdown productsbefore they can be absorbed by thesaprotrophs or other organisms.

Saprotrophs cause decay.

Q1 Copy and complete the following paragraph. During the process of decay, and convert

complex chemicals into ones. For example, proteins are converted to , and to fatty acids and glycerol. These decay processes involve the biological catalysts called , and so the processes are affected by changes in and . Humans exploit decay, for example in the treatment of to provide drinking water, and may deliberately limit decay, for example in the preservation of .

Gardeners often place vegetable waste on a compost heap. Over the course of time the waste will be decomposed.

a What do gardeners gain from the decomposed waste?

b Why do gardeners sometimes spray water over the heap in warm summer weather?

c Why do gardeners often build compost heaps on a pile of loose-fitting sticks or bricks?

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Carbon-containing nutrients –

a reminderThe Sun keeps supplying energy to food chains. However, the supply of chemical elements to living organisms is limited, and these elements must be recycled. The nutrient elements are cycled between simple forms in the non-living (abiotic) environment and more complex forms in the bodies of living organisms (the biotic component of an ecosystem). Living organisms require carbon-containing compounds as:■ a source of energy, released when carbon-

containing compounds are oxidised during respiration (particularly carbohydrates and fats)

■ raw materials for the growth of cells (particularly fats and proteins).

Recycling carbon compoundsPlants, and some bacteria, manufacture these compounds from carbon dioxide during the process of photosynthesis (see page 38). Animals obtain them in a ready-made form by feeding on other living organisms (see page 52), and decomposers obtain them as they break down the dead bodies or wastes of other living organisms. These processes of feeding, respiration, photosynthesis and decomposition recycle the carbon over and over again. Theoretically, the amount of carbon dioxide fixed by photosynthesis should equal the amount released by respiration. As a result the most accessible form of carbon in the non-living environment, that is carbon dioxide, remains at about the same concentration year after year after year (about 0.03% of the atmosphere). Other processes may affect this regular cycling of carbon.

■ Sometimes conditions are not suitable for respiration by decomposers, and carbon dioxide remains ‘locked up’ in complex carbon compounds in the bodies of organisms. For example, anaerobic, low pH or extreme temperature conditions will inhibit decomposition – this is how fossil fuels have been laid down in environments where decomposition is not favoured.

■ Over millions of years the formation of fossil fuels has removed carbon dioxide from the environment. Humans have exploited fossil fuels as a source of energy over a relatively short time, and the combustion of oil, gas, coal and peat has returned enormous volumes of carbon dioxide to the atmosphere. As a result carbon dioxide concentrations are increasing (see page 265).

■ The burning of biomass fuels such as wood and alcohol uses up oxygen also returns carbon dioxide to the atmosphere, and can have a very severe local effect although worldwide it is less significant than the combustion of fossil fuels.

The way in which these different processes contribute to the cycling of carbon is illustrated opposite.

OB J EC T I VES

■ To recall why living organisms need carbon-containing compounds

■ To appreciate that carbon is cycled between complex and simple forms by the biochemical processes of photosynthesis and respiration

■ To understand that formation and combustion of fossil fuels may distort the pattern of the carbon cycle

The carbon cycle

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Q Refer to the carbon cycle opposite. a Name the simple carbon compound present in the

abiotic part of the ecosystem. b Name two compounds present in the biotic part

of the ecosystem. c Which processes raise the concentration of carbon

dioxide in the atmosphere? d Which process reduces carbon dioxide

concentration in the atmosphere? e Name the process that distributes carbon dioxide

throughout the atmosphere from places where it is released.

f Suggest a reason why some fossil fuels were formed as sediments at the bottom of ancient seas.

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Organic compounds in plants

Proteins

Carbohydrates and fats

Carbon dioxide(CO2) in airand water

Organic compounds in animals

Proteins

Carbohydrates and fats

Organic compounds indecomposers – bacteria

and fungi

Some conditions e.g. low temperature,low oxygen concentration and low pHprevent action of decomposers. Thisleads to carbon compounds being ‘lockedup’ in fossil fuels. Fossil fuel formationlowers the concentration of CO2 whichis available in the environment, as itcontinues to be removed by photosynthesis.

Organic compoundsin fossil fuels e.g.

peat, coal, oil

POLLUTO– leadfree

Respiration – converts carbohydratesto carbon dioxide with the release ofenergy

Photosynthesis – uses light energyto convert carbon dioxide intoorganic compounds in plants.

The amount of respiration inthe different groups of livingorganisms varies: overall, plants respire less than they photosynthesise (otherwise they would not grow) decomposer respiration can be very high in some environments, e.g. the warm, moist conditions on the floor of a rainforest, and decomposers can contribute 80% of the CO2 in that environment.

Feeding

Death andexcretionprovide plantand animalmaterial fordecay

Combustion – releasescarbon dioxide by theburning of fossil fuels.This increases theconcentration of CO2available in theenvironment.

� The carbon cycle

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.

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Plants need nitratePlants need nitrogen for the synthesis of proteins and other compounds, including DNA and vitamins. Nitrogen gas makes up about 80% of the Earth’s atmosphere, but plants do not have the enzymes necessary to use the nitrogen directly – instead they must absorb it as nitrate. Nitrate is formed by two sets of processes carried out by microorganisms – nitrogen fixation and nitrification.

Nitrogen fixationIn nitrogen fixation, nitrogen and hydrogen are combined to form ammonium ions and then nitrate. The process depends upon enzymes that are only possessed by certain bacteria called nitrogen-fixing bacteria. Some of these bacteria live free in the soil, but a very important species called Rhizobium leguminosarum lives in swellings called nodules on the roots of leguminous plants such as peas, beans and clover. Nitrogen fixation only happens if oxygen is present. It also occurs naturally in the atmosphere when the energy from lightning combines nitrogen directly with oxygen. Farmers can plant legumes in a crop rotation

scheme to avoid having to use so much nitrogen-containing fertiliser. This saves money, and also limits pollution of water (see page 268).

NitrificationIn nitrification, ammonium ions produced by the decomposition of amino acids and proteins are oxidised, first to nitrite and then to nitrate. The process is carried out by nitrifying bacteria which live in the soil. Nitrification only happens if oxygen is present. In the absence of oxygen the process is reversed, and denitrifying bacteria obtain their energy by converting nitrate to nitrogen gas. This is why waterlogged soils, for example, tend to lose nitrate as nitrogen gas.

Recycling nitrogenOnce nitrate has been formed by either nitrogen fixation or nitrification, it can be absorbed by plants through their roots. Eventually the plant dies, and its body is added to the animal wastes and remains in the soil. Decomposers break down the nitrogen compounds in these wastes and remains and the formation of nitrate can begin again.

In a typical ecosystem the processes shown opposite recycle nitrogen between living organisms and the environment. However, some processes cause the loss of nitrate from the environment. This happens naturally as a result of denitrification (see above), and less naturally when crops are harvested and removed from the site where they have grown. These losses of nitrate can be made up either by nitrogen fixation or by adding nitrate in the form of fertilisers.

OB J EC T I VES

■ To recall why nitrate is an essential mineral for plant growth

■ To know how nitrate is made available in the soil■ To understand that a series of biochemical processes

results in the cycling of nitrogen between living organisms and the environment

■ To appreciate the part played by microorganisms in the cycling of nitrogen

The nitrogen cycle

Q1 Use your knowledge of the nitrogen cycle to explain

how the following farming practices might improve soil fertility.

a ploughing in stubble rather than burning it b draining waterlogged fields

c planting peas or beans every third year d adding NPK fertiliser e adding well-rotted compost Explain why farmers drain waterlogged fields. 2


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Nitrogen gas (N2)in the atmosphere

Amino acidsand urea

Organic compounds in plants

Proteins

Organic compounds in animals

Feeding

The processes of nitrification, absorption, feeding, death, excretion and decay leadto the cycling of nitrogen between living organisms and their environment. In a naturalecosystem nitrogen fixation can ‘top up’ the cycle and make up for losses by denitrification.

Death andexcretion

Organic compounds indecomposers – bacteria

and fungi

Farmers drain andplough fields toimprove oxygenationof soil and so reducedenitrification.They also addnitrogen-containingfertilisers to directlyincrease the nitratecontent of the soil.

Farmers are encouragedto plough roots andstalks of harvested cropsback into the soil. Thisprovides raw materialfor the action ofdecomposers.

Ammoniumions (NH4

+)

Decay – enzymes digestorganic molecules tosimpler forms

Nitrate ions (NO3–)

in soil solution

Nitrification

Denitrification

Absorption by diffusionand active transport

Nitrogen fixation

Proteins,amino acidsand urea

Some plants, called legumes (beans and peanuts are examples), have swellings on their roots. These root nodules contain bacteria, which can convert nitrogen gas to nitrate ions. These plants reduce the need for artificial fertilisers.

The nitrogen cycle

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Water and lifeLife first evolved in water for a number of reasons:■ The molecules that were used by living

organisms, and that made up their structure, were dissolved in the first seas.

■ In the muddy estuaries and shallow seas of the primitive Earth, the molecules could become concentrated enough to react together.

■ Water acted as a protective shield for the first living organisms against the damaging ultraviolet rays from the Sun.

Recycling waterLife 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 cycleAll of the elements that make up living organisms, not just carbon and nitrogen, are recycled. The water cycle is different to the cycles of carbon and nitrogen because:■ only a tiny proportion of the water which is

recycled passes through living organisms

■ the most important factor in water recycling is heat energy from the Sun. This evaporates water, and also creates the temperature gradients which lead to winds.

The steps involved in the water cycle are shown in the diagram opposite.

The special properties of waterThe picture of the kangaroo shows the importance of the properties of water to living things.

OB J EC T I VES

■ To know that all living organisms are largely water, and that biological reactions always take place in an aqueous (watery) environment

■ To understand that the biological properties of water result from the structure of the water molecule

■ To list some of the biological functions of water

Water is recycled too!

Because water isincompressible,it providesexcellent support.Water helpssupport a wholeorganism (e.g. afish), or part ofan organism(e.g. the eyeball,or the erect penisof a mammal).

Water is anexcellentlubricant,for example insaliva or in thesynovial fluid ofmovable joints.

Evaporation ofwater froma surface allowsloss of heat.Water has a highlatent heat ofvaporisation.

Water is an excellenttransport mediumfor many biologicalmolecules, such asoxygen, glucose,amino acids, sodiumions and urea.

Water can be abiological reagent,for example in theprocesses ofphotosynthesisand digestion.

The high specificheat capacityof water meansthat cells or bodieswith a high watercontent tend toresist heating upor cooling down,even when thetemperature oftheir environmentchanges.


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PrecipitationDroplets collectand fall as snow,rain and hail.

Condensationvapour water droplets,which collect to form clouds

Evaporationliquid vapour

Melting/refreezingsolid liquid

Evapotranspirationliquid vapour

� The water cycle is maintained by heat energy from the Sun

Q1 a Explain how nitrogen in the muscle protein of a

herbivore may be recycled to form protein in another herbivore some years later.

b Explain how the activities of some bacteria form a part of both the carbon and nitrogen cycles.

Cambridge IGCSE Biology 0610 Paper 2 Q8 June 2004

2 Use words from the following list to complete the paragraphs about ecosystems. You may use each word once, more than once or not at all.

respiration, 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).

The process in which light energy is transferred into a chemical form is called – eventually the energy is released from its chemical form during the process of

This process provides energy for all living organisms, including which are microbes that feed on the remains of animals and plants.

3 In Africa, mammals called jackals are quite common. They feed on small herbivores such as young springboks and dik-diks, hunting in packs to catch their

prey. They will also eat larger herbivores such as kudu that have been killed by larger predators such as lions.

A farmer in South Africa found that a number of his sheep, while feeding on grassland, were being killed by jackals. He noted that jackals always kill sheep by attacking their necks. He designed a plastic collar for the sheep that covered their necks. None of his sheep have been killed since fitting these collars. Other farmers are now buying the collars to protect their sheep from jackal attack.

a The prey species of the jackal are usually primary consumers. State the type of food that all primary consumers eat.

b Name the two carnivores identified in the text. c Construct a food chain for the jackal to show its

relationship with sheep. d Suggest a reason why jackals survive better when

they hunt in packs. e When the farmer started to use collars on his sheep,

although none of his sheep were being killed, the population of jackals did not decrease. Suggest why the number of jackals did not decrease.

f Name two structures, found in the neck of a sheep, that could be damaged when jackals attack it.

g Some of the protected sheep die of old age and their remains are eaten by other animals. Suggest and explain why the collars of the dead sheep could create an environmental problem.

Cambridge IGCSE Biology 0610

Paper 3 Q1 June 2004


238

1 Over a period of several months, a student recorded some activities of the wildlife in a particular habitat. The following observations appeared in her notebook.

1. Young shoots of a crop of bean plants covered with greenflies (aphids) sucking food from the stems.2. Saw a large bird (hawk), which usually catches mice, swoop to take a small bird visiting the bean field to eat some of the aphids or butterflies.3. Flowers of beans being visited by many different species of butterfly.4. Mice seen nibbling at some dispersed bean seeds.5. Spider’s web constructed between two bean plants with 5 large black flies caught in it. Rotting body of a mouse nearby attracting similar flies.

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

Bean plants

b i What name is given to a chart of feeding relationships as shown in the figure?

ii Name two top carnivores observed by the student.

c i Draw and label a pyramid of biomass for the hawks, mice and bean plants in this habitat.

ii Draw and label a pyramid of numbers for a bean plant, small birds and aphids.

Cambridge O Level Biology 5090

Paper 2 Q3 May 2008

2 a Cape buffalo graze on grass. While the buffalo are grazing, two or three oxpecker birds are often seen standing on the backs of

each buffalo. These birds eat ticks that are parasites on the buffalo’s skin.

i Draw a pyramid of numbers to represent these feeding relationships. Label the pyramid with the names of the organisms.

ii Draw a pyramid of biomass to represent the same feeding relationships. Lable the trophic levels on this pyramid.

b Explain how the nutrition of consumers differs from that of producers.


Paper 2 Q6 November 2008

3 The figure shows parts of some natural cycles in the environment.

S

R

R

Q

Q

a With reference to the carbon and nitrogen cycles, explain what is happening at Q and R.

b Identify two gases that may be released at S and describe the possible harmful effects they may have on the environment.

Cambridge O Level Biology 5090

Paper 2 Q6 May 2007

4 Caribbean farmers sometimes: a Plant peas and corn together to ensure that

the corn plants could grow well. Use your knowledge of the nitrogen cycle to explain this practice.

b Use manure from farm animals as fertiliser for their crops, which are sold in the organic produce section at the greengrocer’s store. Comment on this practice.

Questions on ecosystems, decay and cycles


239

6 The figure below shows the water cycle. a i The arrows labelled P represent

evaporation. Which type of energy is needed for this process?

ii State what causes the formation of clouds at Q.

b i What process is represented by the arrows labelled R?

iii Name three factors that could alter the rate at which process R happens.

5 a The figure below shows the carbon cycle. i Name the processes that cause the

changes shown by the arrows labelled A–D.

ii Name one type of organism that brings about decomposition.

b Over the last few decades, the carbon dioxide concentration in the atmosphere has been rising.

Suggest how this has happened.


Paper 2 Q7 November 2008

c A logging company wants to cut down the forest area.

i Suggest what effects this deforestation might have on the climate further inland. Explain your answer.

ii State two other effects deforestation could have on the environment.


Paper 2 Q4 May 2009

Carbon dioxidein air

Carbon compoundsin plants

Carbon compoundsin animals

Fossil fuels

Carbon compoundsin dead plants and

animals

Death DeathFossilisationover millions

of years

Decompositionby

microorganismsB B

D

C

A

areaforest

P

RQ

seariver

land

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