13. Energy and Ecosystems

Question 1

What is a producer?

Answer 1

Producers are photosynthetic organisms that manufacture other substances using light energy, water, carbon dioxide and mineral ions.

Question 2

What is a consumer?

Answer 2

Consumers are organisms that obtain their energy by feeding on other organisms rather than using the energy from sunlight directly.

Question 3

What is a primary consumer?

Answer 3

Those that directly eat producers (green plants), fits in the chain of consumers.

Question 4

What are secondary consumers?

Answer 4

Animals that eat primary consumers (usually predators, or scavengers or parasites).

Question 5

What are tertiary consumers?

Answer 5

Animals that eat secondary consumers (usually predators, or scavengers or parasites).

Question 6

What are saprobionts (decomposers)?

Answer 6

A group of dead organisms that break down the complex materials in dead organisms into simple ones. In doing so, they release valuable minerals and elements in a form that can be absorbed by plants and so contribute to recycling. The majority of this work is carried out by fungi and bacteria.

Question 7

What is a food chain?

Answer 7

Food chain describes a feeding relationship in which the producers are eaten by primary consumers. These are eaten by secondary consumers, which are then eaten by tertiary consumers. In a long food chain the tertiary consumers may be eaten by quaternary consumers.

Question 8

What is meant by a ‘trophic level’?

Answer 8

Each stage of the food chain.

Question 9

What do the arrows show in a food chain?

Answer 9

The direction of energy flow.

Question 10

What is a food web?

Answer 10

Animals relying on multiple food sources, within a single habitat many food chains are linked together to form a food web.

Question 11

Define biomass

Answer 11

The total dry mass of living material in a specific area at a given time. Measured in grams per square metre (g m-2).

Question 12

How can the chemical energy store in biomass be estimated?

Answer 12

Calorimetry.

Question 13

What is an ecosystem?

Answer 13

All the living and non-living components of a particular area.

Question 14

Why is most of the Sun’s energy not converted to organic matter by photosynthesis?

Answer 14

• Over 90% of the Sun’s energy is reflected back into space by clouds or absorbed by atmosphere.
• Not all wavelengths of light can be absorbed and used for photosynthesis.
• Light may not fall on a chlorophyll molecule.
• Factors, such as low carbon dioxide levels, may limit the rate of photosynthesis.

Question 15

What is Gross Primary Production?

Answer 15

The total quantity of the energy store in plant biomass, in a given area or volume, in a given time.

Question 16

How much energy do plants use in respiration?

Answer 16

Around 20-50% of the gross primary production.

Question 17

What is the net primary production?

Answer 17

The chemical energy store left when respiratory losses have been accounted for.

Question 18

Give the equation for Net Primary production

Answer 18

net primary production = gross primary production - respiratory losses

NPP = GPP - R

Question 19

What is the net primary production used for?

Answer 19

• Respiration
• Photosynthesis
• Passed through other organisms in the ecosystem

Question 20

Why is the percentage of energy transferred through each tropic system so low?

Answer 20

• Some of the organism is not consumed.
• Some parts are consumed but can’t be digested and are therefore lost in faeces.
• Energy lost in excretory materials, such as urine.
• Heat from respiration is lost to the environment, high in mammals and birds due to high body temp.

Question 21

Give the net production of consumers.

Answer 21

N = I - (F + R)

N= net production 
I = energy store of ingested food
F = energy lost in faeces & urine
R = energy lost in respiration

Question 22

Why do most food chains rarely have more than 4 trophic levels?

Answer 22

Insufficient energy is available to support a large enough breeding population at trophic levels higher than these.

Question 23

An area of vegetation 5m by 5m produces 4 x10^4 KJ of potential energy in a year.
Calculate the gross primary production of this area

Answer 23

40000/25 = 1600 KJ m^-2 year ^-1

Question 24

Give the equation for percentage efficiency

Answer 24

Percentage efficiency= energy available after the transfer/ energy available before the transfer x100

Question 25

If the energy available before the transfer is 250 KJ m-3year-1 and energy available after the transfer is 50 KJ m-3year-1, what is the percentage efficiency?

Answer 25

Percentage efficiency = 50/250 x100 = 20%

Question 26

Why are animals kept in confined spaces to increase the energy-conservation rate?

Answer 26

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

Question 27

How much of the atmosphere is nitrogen?

Answer 27

78%

Question 28

What are the 4 main stages in the nitrogen cycle?

Answer 28

Ammonification
Nitrification
Nitrogen fixation
Denitrification

Question 29

Describe ammonification

Answer 29

The production of ammonia from organic nitrogen containing compounds (urea, proteins, nucleic acids, vitamins). Saprobionts feed on faeces and dead organisms, realising ammonia, which then forms ammonium ions in the soil.

Question 30

Describe Nitrification

Answer 30

Nitrifying bacteria convert ammonium ions into nitrate ions. This is an oxidation reaction so releases energy. Occurs in 2 stages:

1. oxidation if ammonium ions to nitrite ions (NO2-)
2. oxidation of nitrite ions to nitrate ions (NO3-)

Question 31

How can farmers increase productivity for nitrification?

Answer 31

Nitrifying bacteria require oxygen to carry out reactions so farmers keep soil structure light and well aerated by ploughing. Good drainage also prevents the air spaces from being filled with water.

Question 32

Describe nitrogen fixation

Answer 32

The process by which nitrogen gas is converted into nitrogen containing compounds. Can be carried out industrially and naturally when lightning passes through atmosphere.
Carried out by 2 types of microorganisms:
-Free-living nitrogen-fixing bacteria- reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids. Nitrogen-rich compounds are released when they die & decay.
-Mutualistic nitrogen-fixing bacteria- These bacteria live in nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria.

Question 33

Describe denitrification

Answer 33

When slips become waterlogged, and have a low oxygen conc, the type of microorganism present changes. Fewer aerobic nitrifying and nitrogen-fixing bacteria are found, and there is an increase in anaerobic denitrifying bacteria. These convert soil nitrates into gaseous nitrogen. This reduces the availability of nitrogen compounds for plants.

Question 34

How can farmers prevent the build up of denitrifying bacteria and increase productivity?

Answer 34

Keep soil well aerated and drained of water.

Question 35

Describe the phosphorous cycle

Answer 35

Phosphorous exists mostly as phosphate ions in the form of sedimentary rock deposits. There have their origins in the seas but are brought to the surface by the geological uplifting of rocks. The weathering and erosion of these rocks helps phosphate ions to become dissolved and so available for absorption by plants which incorporate them into their biomass. The phosphate ions pass into animals which feed on plants. Excess phosphate ions are excreted by animals and may accumulate in waste material such as guano formed from the excretory products of sea birds.

On the death of plants and animals decomposers release phosphate ions o to the water/soil. Some phosphate ions remain in parts of animals (bones/shells) that are slow to breakdown. Phosphate ions in excreta, released by decomposition and dissolved out of rocks are transported by streams into lakes/oceans where they form sedimentary rocks, completing the cycle.

Question 36

Describe the role of mycorrhizae in nutrient cycles

Answer 36

Mycorrhizae are associations between certain types of fungi and the roots of the vast majority of plants. The fungi act like extensions of the plant’s root system and vastly increase the surface area for absorption of water and minerals. The mycorrhiza acts like a sponge, holding water and minerals in the roots. This enables the plant to better resist drought and take up inorganic ions more readily, such ad phosphates.

Question 37

Why is the mycorrhizal relationship between plants and fungi mutualistic?

Answer 37

The plants benefit from improved water and inorganic ion uptake while the fungi receives organic compounds such as sugars and amino acids from the plant.

Question 38

Why are fertilisers needed?

Answer 38

To achieve maximum yield from the crops and animals grown on them.

Question 39

What are the 2 types of fertiliser?

Answer 39

• Natural/organic fertilisers

- Artificial/inorganic fertilisers

Question 40

Describe natural fertilisers

Answer 40

Consists of the dead and decaying remains of plants and animals as well as animal waste such as manure, slurry and bone meal.

Question 41

Describe artificial fertilisers

Answer 41

Mined from rocks and deposits and then converted into different forms and blended together to give the appropriate balance of minerals for a particular crop. Compounds containing nitrogen, phosphorous and potassium are always present.

Question 42

How do nitrogen-containing fertilisers increase plant productivity?

Answer 42

Nitrogen is an essential component of amino acids, ATP and nucleotides in DNA, needed for plant growth. Plants are likely to develop earlier, grow taller and have greater leaf area. This increases the rate of photosynthesis and crop productivity.

Question 43

Give negative effects of nitrogen-containing fertilisers

Answer 43

• Reduced species diversity: favour growth of grasses, nettles and rapid growing species, which outcompete other species which die.
• Leaching: which leads to pollution of watercourses.
• Eutrophication: caused by leaching of fertiliser into watercourses.

Question 44

Describe leaching

Answer 44

Leaching is the process by which nutrients are removed from the soil.
Rainwater will dissolve any soluble nutrients, such as nitrate ions, and carry them deep into the soil, eventually beyond the reach of plant roots. The leached nitrate ions go into watercourses, that drain into freshwater lakes. Here they may have a harmful effect if the lake is the source of human drinking water, and harmful to the environment by eutrophication.

Question 45

Describe eutrophication

Answer 45

1. In most lakes and rivers there is a naturally low concentration of nitrate and so nitrate ions limit plant/algal growth.
2. As the nitrate ion concentration increases as a result of leaching, it ceases to be a limiting factor for growth of plants/algae, whose populations grow.
3. As algae grow at the surface, there is an algal bloom
4. This dense surface layer of algae absorbs the light and prevents it from penetrating to lower depths.
5. Light becomes the limiting factor for growth of plants/algae at lower depths so they can’t photosynthesise and die.
6. The lack of dead plants and algae is no longer a limiting factor for saprobiontic bacteria and so these populations grow.
7. The saprobiontic bacteria require oxygen for respiration, increasing oxygen demand.
8. The concentration of oxygen in the water is reduced and nitrates are released from decaying organisms.
9. Oxygen limits the population of aerobic organisms, such as fish. These organisms die and oxygen is used up.
10. Without aerobic organisms, there’s less competition for the anaerobic organisms whose populations rise.
11. The anaerobic organisms further decompose dead material, releasing nitrates and toxic wastes, such as hydrogen sulphide, making the water putrid.

Question 46

Suggest why after a certain point, the addition of more fertiliser no longer improves crop productivity.

Answer 46

Some other factor is limiting photosynthesis, e.g. light/CO2, and only the addition of this factor will increase photosynthesis and hence productivity.

Question 47

Why are fertilisers important in agricultural ecosystems?

Answer 47

Crops are grown repeatedly and intensively iron the same area of land. Mineral ions are taken up by the crops, which are transported and consumed away from the land. The mineral ions they contain are not returned to the same area of land so the levels in the soil are reduced, which can limit the rate of photosynthesis. Fertilisers need to be applied to replace them if productivity is to be maintained.