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Disrupting Food Chains and WebsA food chain is a linear sequence of organisms, each of which feedson the preceding organism. A food web is a series of interconnectedfood chains.

An understanding of food chains and webs has helped humanscontrol pests in agriculture. Unfortunately, more often than not, ithas been our lack of understanding of the interconnections betweenspecies that has led to ecological damage.

This Factsheet describes some of these interconnections and theexpected and unforeseen consequences of our interventions.

Each species to its own nicheThe niche of an organism is the role that a species plays in acommunity. Niche and habitat are not the same. Many species mayshare a habitat but every species has a unique niche. The niche ofan organism includes:• Its habitat• Its food sources• Its use of abiotic resources – light, CO

2, oxygen etc

• The way in which it is influenced by abiotic factors – themaximum and minimum temperatures in which it can survive, forexample

• The way in which it interacts with other individuals of the samespecies and with individuals of other species.

The key point to take on board is that two species cannot occupythe same niche. Two species attempting to occupy the same nichewould be competing for identical resources such as food, water,nest sites etc at the same time. One of the species will always bebetter adapted to exploit these resources than the other speciesand will eventually eliminate the other species. This is known ascompetitive exclusion.

This concept is important when we consider ways in which humanshave disrupted natural populations and food webs.

Introduced species

An introduced species is one that is not native to an area – ithas been brought to the area by humans.

Table 1 shows when some tree species first came to Britain and thenumber of species of lichens and plant-eating insects that theysupport. Native species were here before 1600.

Table 1 Date of introduction v wildlife importance

Tree species Date of introduction Number of species of lichen Number of species of plant-eating insects

Pedunculate oak native 324 423

Hazel native 160 106

Common alder native 105 141

Common lime 1600 13 57

Red oak 1700 16 21

Italian alder 1800 11 12

The trend is clear; the longer a tree species has been here, the morewildlife it supports. This is one reason why new introductions – ofany plant or animal species - can have dramatic effects.

Fig 1 shows reasons for the decline in bird populations in the U.K.

Fig 1 Reasons for decline in bird populations

other23%

Habitatdestruction63% Introduced

species 23%

Hunting6%

Typical Exam QuestionHow have introduced plant and animal species contributed tothe decline in UK bird populations?

• Competition for food, space and nesting sites• Predation or feeding on eggs• Toxic plants have poisoned birds• The introduced organisms have brought disease that has

spread to native birds

Introduced bird species often impact most seriously on nestingsites, rather than via direct or indirect effects on food chains, butthis isn’t the case with other animal species.

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Exam Hint: Do not confuse niche with habitat!

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Zebra musselsZebra mussels (Fig 2) are filter feeders.

Reaching densities of >10,000/m2 they are capable of removingalmost every microscopic aquatic plant (phytoplankton or algae)and animal (zooplankton). Since invading the Great Lakes in N.America they have had a huge ecological and economic impact (Fig3).

Fig 2

Zebra mussel population ↑1

2

↑ Colonisation of clams

3

Clam population ↓Ingestion of and competitionwith plankton

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Plankton population ↓

4

↓ Food & O2 for native

mussels. Smotheringof native mussels

↑ Food for northernpike and yellow perch

Water clarity ↑Aquatic plants ↑

↓ Food for somefish species

Averagefish size ↓

↓ Ability to withstand starvation↓ Ability to avoid predators

↓ Fish population

Native mussel population ↓

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Fig 3. Ecological and economic impacts of zebra mussels on the Great Lakes

Typical Exam QuestionsMake sure you understand each of the consequences. In theexam, you may be told that the zebra mussels, for example, havecaused both an increase and a decrease in some fish populationsand you could be asked to suggest possible mechanisms.

The most common, and straightforward questions simply askyou to interpret food webs.

The diagram shows a food web from a river.

Birds

Trout Pike

Water beetlesDragonflynymphs Water boatmen

Small crustaceaWater snail Mayfly nymphs

Algae/aquatic plants

Q1. What impact might a drop in the population of water beetleshave on:

(i) small crustacea?(ii) pike?

Answers(i) increased population;

reduced consumption/predation:

(ii)less water beetles may lead to less trout (because theywould have less food);therefore pike population may fall too;

or

it may lead to less dragonfly nymphs;hence birds;which might lead to reduced predation by birds on pike;so pike population increases

Make sure you consider different possibilities in the exam.

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Are whales eating all the fish?Japanese scientists argue that it is essential that Japan be allowedto kill whales in order to protect their fish stocks. The diagrambelow shows a simplified food web containing three whale species.

Fish

Squid

SealPenguin

Krill

Algae

Baleen whale

Killer whaleSperm whale

Q1 What evidence is there that the Japanese scientists are correct?Q2 Suggest why reducing the numbers of whales may not lead togreater fish catches.

Answers1. Two whale species feed directly on fish;

The baleen whale competes with fish for krill;

2. Fewer whales leads to increased populations of squid, penguinsand seals;Which feed on fish;

Once you are confident with these type of knock-on impactquestions, you are ready for something a little more complex!

American crayfish• ‘American Red’ signal crayfish (Pacifastacus leniusculus)• imported from the US in the 1970s and reared commercially for

London restaurants• Some farms abandoned in mid-70s recession• Crayfish that survived spread rapidly

Characteristics• large claws• armour plating• can grow up to 12 inches in length• live out of water for months• excellent climbers• lays more eggs and can reproduce in a shorter cycle than the

native white-clawed crayfish• aggressive predator

Given these characteristics, it is easy to see why this species hasdisrupted the food webs of our rivers. Unfortunately, it also seemsto have spread a disease known as crayfish plague, which hascontributed to the dramatic decline in the population of our nativespecies.

One introduced species of American crayfish is capable ofreproducing both asexually ie, without the need for a partner, andsexually. Asexual reproduction is more energy efficient than sexualreproduction and produces clones.

Q Suggest why this species is considered a serious danger toEuropean species of crayfish. (5)

Exam Hint: Examiners don’t tell you things in the Question thataren’t important. The emphasis and short explanation of asexualreproduction is vital. You need to think about the environmentalsignificance of this process

Answeronly one parent needs to be introduced ;no wasted energy producing gametes / finding a mate ;faster reproduction than native species ;clones will be successful in same environment as parent ;so can quickly colonise new environment ;preserves successful genes ;outcompete native species;may mate with the native species ;hybrid sterility ;may prey on European species ;no natural predators ;competitive exclusion ;

Instead of giving you food chains and webs and asking what theimplications of one species dying out might be, you may be askedto work from first principles, for example:

20 individuals (10 male, 10 female) of the same species of aherbivorous (plant-eating) fish were introduced to an artificial pond.The pond contained a variety of water-plants, but had previouslyheld little animal life.

Q1 Using sketch graphs, suggest what might happen to the fishand plant populations over the succeeding months if there isno further interference with the pond.

Q2 6 months after the introduction of the herbivorous fish, abreeding pair of carnivorous fish were introduced to the pond.Outline possible changes to the pond ecosystem.

AnswersQ1 fish population might increase rapidly;

“J” curve of population growth;no interspecific competition;decline in plants as they are eaten by expanding fish population;fish population may crash as plant food supply exhausted;possibility of extinction of fish;but low populations may survive, allowing plant recovery;cycle restarts;equilibrium between two populations may be established;reflecting carrying capacity of pool;Any other reasonable points.

Num

ber

of f

ish

Abu

ndan

ce o

f pl

ants

Pop

ulat

ion

Time Time

Time

crash

recovery

plants

fish

equilibrium

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Q2 carnivorous fish population increases rapidly with abundantfood supply;decline in herbivorous fish;recovery in plants;herbivorous fish might be eliminated;especially likely in small pool;if this occurs, extinction of carnivorous fish will follow;unless alternative food sources exist;dead organisms will decompose;this may contaminate water;dead organisms provide basis for decomposer food-chains;increases in number of trophic levels/diversity could increasestability of ecosystem;correct references to positive or negative feedback mechanisms/ density dependence;

BiomagnificationMany lakes in the United States have beenaffected by large swarms of midges (small bitinginsects) in the summer.Lakes were sprayed with insecticide to kill themidges. The effect of spraying on the followingfood chain in the lake was investigated.

Plant plankton sunfish western grebe

Shortly after spraying, the concentration of insecticide in the waterof the lake was 2.5gdm-3

One month later the concentration in the plankton was equivalent to600 g dm-3. Six months later it was discovered that the concentrationin the grebe was six times the concentration in the sunfish.

The plankton bioaccumulated the insecticide. Thus the sunfish,when consuming the plankton, received huge doses of the chemical.The insecticide has now become biomagnified (increasinglyconcentrated up the food chain). The grebes eat many fish andthus receive many doses of the insecticide which is not excreted /biodegradable and builds up in the bird’s fat stores.

Red and Grey squirrelsOver the last century, the population of the red squirrel, Sciurusvulgaris,has fallen dramatically

The introduced grey squirrel, Sciurus carolinensis, has becomeextremely common.

There have been many suggested reasons for this trend, including:• that red squirrels prefer native coniferous woodland and grey

squirrels prefer deciduous woods. Conifer cones (which redsprefer) take two years to ripen, whilst acorns and beechnuts areavailable annually. Thus, the grey squirrels may, over time, haveaccess to more food, particularly over winter

• that grey squirrels can digest acorns more efficiently than redsquirrels who do eat acorns but they cannot digest themproperly.

Practice Questions1. Suggest why, in a particular ecosystem, two species do not

occupy the same niche (4)

2. The diagram shows part of a marine food web.

White whale

Cod

Bearded seal

shrimps

Small fish(e.g Capelin)

Arctic char

Shellfish CrabsZooplankton

Phytoplankton Detritus

Scientists have reported that many fish species, including cod,are being overfished. This is a result of:• the use of highly sophisticated sonar for detecting shoals• huge fishing boats and fleets• too many days at sea• larger nets• young fish removed from populations

Cod stocks may be nearing extinction. Outline the possibleconsequences for the marine ecosystem if this occurs. (6)

Answers1. species would be competing for identical resources;

food, water, nest sites etc at the same time;one of the species will be better adapted;will eventually eliminate the other species;competitive exclusion;

2. population numbers at each trophic level are balanced;controlled by, feeding / predators ;some population(s) / named species, will increase (out of control) ;other population(s) / named species, will decrease / becomeextinct;removal of cod, upsets relationships throughout web / has a‘knock-on’ effect ;credit 2 marks for named knock-on effects;credit 2 for suggested effects outside the diagram eg sea birds;ref to fishing industry;

Acknowledgments: This Factsheet was researched and written byKevin Byrne. Curriculum Press, Bank House, 105 King Street,Wellington, Shropshire, TF1 1NUBio Factsheets may be copied free of charge by teaching staff orstudents, provided that their school is a registered subscriber. ISSN1351-5136

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