Energy and ecosystems

Question 1

Define a ‘producer’

Answer 1

Photosynthetic organism that manufactures organic substances using light energy, water and carbon dioxide
6CO₂ + 6H₂O + energy > C₆H₁₂0₆ + 6O₂
Carbon dioxide + water + light > glucose + oxygen

Question 2

Define a ‘consumer’

Answer 2

An organism that obtains energy from consuming (feeding on) other organisms rather than directly using the energy of sunlight

Question 3

Primary consumer

Answer 3

Those that directly eat producers (green plants)

Question 4

Secondary consumer

Answer 4

Those that eat primary consumers

Question 5

Tertiary consumer

Answer 5

Those that eat secondary consumers

Question 6

Define a ‘decomposer’

Answer 6

When producers and consumers die, the energy they contain can be used by a group of organisms that break down these complex materials into simple components again and release valuable minerals and elements which can be absorbed by plants
Often fungi/bacteria called decomposers, or certain mammals such as earthworms called detritivores

Question 7

Define ‘food chain’

Answer 7

Describes the feeding relationship in which producers are eaten by primary consumers, who are eaten by secondary consumers, who are eaten by tertiary consumers
The arrows on food chain diagrams represent the direction of energy flow

Question 8

Trophic level

Answer 8

Each stage in a food chain is known as a trophic level

Question 9

Food web

Answer 9

As most animals do not rely upon a single food source, many food chains within a habitat will be linked together to form a food web
The relationships within a food web change depending on the time of year, age and populations sizes of the organisms

Question 10

Most of the Sun’s energy is not converted to organic matter by photosynthesis because…

Answer 10

• Over 90% of the Sun’s energy is reflected back into space by clouds/dust or absorbed by the atmosphere
• Not all wavelengths of light can be absorbed and used for photosynthesis
• Light may not fall on a chlorophyll molecule
• There may be a factor limiting the rate of photosynthesis

Question 11

Define ‘gross production’

Answer 11

The total quantity of energy that the plants in a community convert to organic matter

Question 12

Define ‘net production’

Answer 12

The rate at which plants in a community store energy

Question 13

Calculate the net production (equation)

Answer 13

Net production = gross production - respiratory loss

Question 14

Why is there a low % of energy transferred at each stage?

Answer 14

• Some of the organism is not eaten
• Some parts are eaten but cannot be digested so are lost in faeces
• Some energy is lost in excretory materials eg, urine
• Some energy lost as heat from respiration and from body due to environment - high in mammals/birds due to energy needed to maintain constant body temperature

Question 15

What does the relative inefficiency of energy transfer between trophic levels explain?

Answer 15

• Most food chains only have 4/5 trophic levels due to insufficient energy available to support a large enough breeding population at trophic levels higher than these
• The total mass of organisms in a particular place (biomass) is less at higher trophic levels
• The total amount of energy stored is less at each level as one moves up a food chain

Question 16

Calculate energy transfer (equation)

Answer 16

Energy transfer = (energy available after the transfer / energy available before the transfer) x 100

Question 17

Why can ecological pyramids be more informative than food chains/webs?

Answer 17

Ecological pyramids provide quantitative information such as the mass/number/amount of energy stored by organisms at each trophic level

Question 18

Describe pyramids of numbers

Answer 18

The numbers of organisms at lower trophic levels are greater than the numbers at higher trophic levels - shown by drawing bars wit lengths proportional to the numbers present at each trophic level

Question 19

Drawbacks of pyramids of numbers

Answer 19

• No account is taken of size (eg, a tree is treated the same as grass so sometimes it does not form a pyramid as there is a greater number of grass than trees)
• The number of individuals can be so great that it is impossible to represent them accurately on the same scale as other species in the food chain (eg, one tree may have millions of greenfly living off it)
• Only organisms present at a particular time are shown - seasonal differences are not apparent - Only organisms present at a particular time are shown - seasonal differences are not apparent

Question 20

Describe pyramids of biomass

Answer 20

Instead of counting the number of organisms at each level, their biomass (A = grams per square metre/ V = grams per cubic metre) is measured (the total mass of the plants and/or animals in a particular place)

Question 21

Pros and cons of pyramids of biomass

Answer 21

Pros:
- More reliable, quantitative description provided than pyramids of numbers
Cons:
- The presence of varying amounts of water makes the biomass unreliable, however, this is overcome by measuring the dry mass which involves killing the organisms therefore there is only a small, unrepresentative sample
- Only organisms present at a particular time are shown - seasonal differences are not apparent

Question 22

Describe pyramids of energy

Answer 22

Measuring the energy stored in organisms (kJ-2year-1) the most accurate representation of energy flow through a food chain

Question 23

Pros and cons of pyramids of energy

Answer 23

Pros:
- Results are much more reliable than those for biomass as two organisms of the same dry mass may store different amounts of energy (eg, fat stores twice as much energy as carbohydrate and so an organism with more fat will contain more energy than one with less fat although they have the same dry mass)
Cons:
- Collecting data for pyramids of energy is difficult and complex

Question 24

Define ‘agricultural ecosystem’

Answer 24

‘Channelling the energy flowing through a food web into the human food chain and away from other food chains to increase the productivity of the human food chain’ Made up largely of domesticated animals and plants used to produce food for mankind
Humans are often the third/fourth trophic level so energy consumed is often a tiny proportion of that available from the Sun at the start of the food chain and agriculture tries to ensure that this is increased

Question 25

Define ‘productivity’

Answer 25

The rate at which something is produced
Gross productivity = the rate at which plants convert light energy from the Sun into chemical energy (food) measured in kJm-2year-1

Question 26

Define ‘net productivity’

Answer 26

Net productivity = gross productivity - respiratory loss

Some chemical energy is used during respiration and the remainder is known as the net productivity

Question 27

What is net productivity affected by?

Answer 27

• The efficiency of the crop at carrying out photosynthesis

- The area of ground covered by the leaves of the crop

Question 28

Comparison of natural and agricultural ecosystems

Answer 28

Natural:
- Solar energy only
- Lower productivity
- More species diversity
- More genetic diversity within a species
- Nutrients are recycled naturally within the ecosystem with little addition from the outside
- Populations are controlled by natural means eg, competition/climate
- Natural climax community
Agricultural:
- Solar energy plus energy from food (labour) and fossil fuels (machinery/transport)
- Higher productivity
- Less species diversity
- Less genetic diversity within a species
- Natural recycling is limited and supplemented by the use of artificial fertilisers
- Populations are controlled by both natural means and pesticide/cultivation
- Artificial community prevented from reaching its natural climax

Question 29

Differences between natural and agricultural ecosystems in terms of energy input

Answer 29

To maintain an agricultural ecosystem we must prevent this climax community (the organisms that make up the final stage of succession). We do this by excluding most of the species in a community leaving only the particular crop that we are trying to grow.
To remove unwanted species and maximise growth requires an additional input of energy (used to plough fields, sow crops, remove weeds, supress pests/ diseases, feed/house animals, transport materials and other tasks carried out by farmers)
Additional energy comes in 2 forms:
- Food - farmers/people working on farms use energy (from the food eaten) as they work
- Fossil fuels - energy has come from the fuel used to plough, harvest and transport crops as well as to produce and apply fertilisers/pesticides and house/feed/transport live stock

Question 30

Differences between natural and agricultural ecosystems in terms of productivity

Answer 30

Productivity in natural ecosystems is relatively low and the additional energy input in agricultural ecosystems increases the productivity of a crop by reducing the effect of limiting factors on its growth. Energy used to exclude other species means the crop has little competition for factors needed for photosynthesis eg, light/CO2, water and minerals. The group is covered almost exclusively by the crop and fertilisers added provide essential ions and pesticides used destroy pests and prevent diseases. All of which increases the productivity compared to a natural ecosystem

Question 31

Define ‘pest’

Answer 31

An organism that competes with humans for food or space or could be a danger to health

Question 32

Define ‘pesticides’

Answer 32

Poisonous chemicals that kill pests
Herbicides - kill plants (herbs)
Fungicides - kill fungi
Insecticides - kill insects

Question 33

What should an effective pesticide be like?

Answer 33

• Specific - only toxic to organisms at which it is directed (harmless to humans and other organisms (especially natural predators to the pest), earthworms and pollinating insects eg, bees
• Biodegrade - once applied it will break down into harmless substances in the soil but needs to be chemically stable so that it has a long shelf-life
• Cost-effective - development costs are high and new pesticides remain useful for only a limited time as pests can develop genetic resistance
• Not accumulate - does not build up, either in specific parts of an organism, or as it passes along food chains

Question 34

Why is it ideal that the control agent and the pest exist in balance with one another?

Answer 34

Otherwise it would be counterproductive if the pests were reduced so much that it was insufficient food for its predators which would therefore cause them to die. This would mean that there is less predators to consume the pests and so the pests would be able to multiply uncontrolled.

Question 35

Disadvantages of biological control methods instead of chemical pesticides?

Answer 35

• They do not act as quickly - often a period of time between introducing the control organism and a significant reduction in the pest population
• Control organism may become a pest itself - its population may increase where there are few natural predators and as the pest population is reduced, the control organism may use alternative sources of food eg, crops

Question 36

Define ‘biological control’

Answer 36

Control pests using organisms that are either already predators of parasites (live in/on the pest) of the pest organism

Question 37

Define ‘integrated pest-control systems’

Answer 37

Aim to integrate all forms of pest control rather than being reliant upon one type. Emphasis is on deciding the acceptable level of the pest rather than eradicating it (getting rid of it altogether) which would be costly, counterproductive and almost impossible to achieve

Question 38

What does integrated control involve?

Answer 38

• Choosing animal/plant varieties that suit the local area and are as pest-resistant as possible
• Managing the environment to provide suitable habitats close to crops for natural predation
• Regularly monitoring the crop for signs of pests so that early action can be taken
• Removing the pests mechanically (hand-picking, vacuuming, erecting barriers) if the pest exceeds an acceptable population level
• Using biological agents if available/necessary
• Using pesticides as a last resort if pest population gets too out of hand

Question 39

How can controlling pests affect productivity?

Answer 39

• Weeds compete with crop plants for water, mineral ions, CO2 and light which are all often in a limited supply so may become the limiting factor of photosynthesis thus reducing the productivity
• Insect pests may damage the leaves of crops, limiting their ability to photosynthesise and therefore reducing their productivity
• There may be direct competition with humans who eat the crop
• Crops are often grown in monoculture which enables insect and fungal pests to spread rapidly
• Pests of domesticated animals (eg, cows) may cause disease so the animals may not grow as rapidly, be unfit for human consumption or die which all lead to a reduction in productivity

Question 40

What is the aim of pest control?

Answer 40

To limit the effects of pests on productivity to a commercially acceptable level - to balance the cost of pest control with the benefits it brings

Question 41

What is the problem with pest control?

Answer 41

There are two different interests involved:

• The farmer who has to satisfy our demand for cheap food while making a living
• The conservation of natural resources which will enable us to continue to have food in the future

Question 42

Define ‘monoculture’

Answer 42

A large area of land in which only one type of crop is grown

Question 43

What is the aim of intensive rearing of livestock?

Answer 43

To produce the maximum yield of meat, eggs and milk at the lowest possible cost - often in cows, pigs, chickens and turkeys
Only a small percentage of energy is passed on to the next trophic level in a food chain and so intensive rearing aims to ‘convert the smallest possible amount of food energy into the greatest quantity of animal mass’ by minimising the amount of energy loss by domesticated animals during their lifetime
More of the food energy taken in by the animals will be converted into body mass ready to be passed on to the next level in the food chain (humans)

Question 44

How does intensive rearing of domesticated animals (aka factory farming) increase the energy-conservation rate?

Answer 44

• Movement is restricted so less energy used in muscle contraction
• Environment kept warm to reduce heat loss from body
• Feeding can be controlled so that animals receive the optimum amount and type of food for maximum growth with no wastage
• Predators are excluded so that there is no loss to other organisms in the food web

Question 45

What are other means of improving the energy-conservation rate?

Answer 45

• Selective breeding of animals to produce varieties that are more efficient at converting the food they eat into body mass
• Using hormones to increase growth rates

Question 46

Main features of intensive rearing

Answer 46

• Efficient energy conservation (restricts wasteful loss of energy so more energy is passed to humans in the food chain)
• Low cost (food such as eggs, milk and meat can be produced more cheaply)
• Quality of food (often argued that the taste of food produced by intensive rearing is nicer)
• Use of space (intensive rearing uses less land while efficient production means that less of the countryside is needed for agriculture and so can be left as natural habitats)
• Safety (easier to regulate and control - high density animal populations are more vulnerable to rapid spread of disease but it is easy to prevent infections)
• Use of drugs (over-use of antibiotics in animals to prevent disease has lead to the evolution of antibiotic resistance and can be transferred to humans/other drugs can be used to improve growth or reduce aggressive behaviour but can alter food taste and affect human health when consumed)
• Animal welfare (animals are kept unnaturally and can cause stress/aggressive behaviour, restricted movement can cause joint pain and wellbeing of animals can be sacrificed for financial gain)
• Pollution (intensively reared animals produce large concentrations of waste in small areas causing ground/water to become polluted and smell/be dangerous)
• Reduced genetic diversity (selective breeding is used to develop animals with high energy-conservation rates and tolerance of confined conditions - can result in loss of genes which could later be beneficial)
• Use of fossil fuels (used to heat buildings that house the animals/production of materials in the buildings eg, cement/production and transportation of animal feeds) - Co2 emitted increases global warming

Question 47

Economic issues concerned with intensive food production

Answer 47

• Create a highly competitive market as we want consumers to supply a wide range of foods at minimum cost which puts farmers under pressure to cut costs and supply cheap food
• There is a conflict between the desire for cheap food and our desire to conserve the environment

Question 48

Environmental issues concerned with intensive farming by directly removing habitats and reducing species diversity

Answer 48

• Removal of hedgerows/woodland
• Creation of monocultures eg, replacing natural meadows with crops
• Filling in ponds/draining wetlands
• Over-grazing of land (thus preventing regeneration of woodland)

Question 49

Why has food production doubled in the past years?

Answer 49

Improved genetic varieties in plants and animal species, greater use of chemical fertilisers/pesticides, greater use of biotechnology and changes in farm practices leading to larger farms - leading to intensive farming
The main effect of intensive food production is the reduction in the variety of habitats within an ecosystem thus leading to a reduction in species diversity

Question 50

Environmental issues concerned with intensive farming by indirectly removing habitats and reducing species diversity

Answer 50

• Use of pesticides/inorganic fertilisers thus reducing species diversity/polluting watercourses
• Escape of farm wastes into water courses thus killing fish and other organisms
• Absence of crop rotation leading to poor soil structure

Question 51

Management techniques used to increase species/habitat diversity without raising food costs or lowering yields

Answer 51

• Maintaining existing hedgerows to a beneficial height/shape to provide better habitats
• Planting hedges rather than erecting fences as boundaries
• Maintaining existing ponds and creating new ones
• Leaving wet corners of fields rather than draining them
• Planting native trees on land with low species diversity rather than in species-rich areas
• Reducing use of pesticides - using biological control where possible/genetically modified organisms resistant to pests
• Using organic rather than inorganic fertilisers
• Creating natural meadows/using hay rather than grasses
• Leaving the cutting of verges/field edges until after flowering and after seeds have dispersed

Question 52

How are farmers encourages to adopt conservation methods?

Answer 52

They are offered a number of financial incentives