13.2 - Energy Transfer And Productivity

Question 1

The ____ is the source of energy for ecosystems. However, as little as ___ of this light energy may be captured by green plants and so made available to organisms in the food chain.

Answer 1

1. Sun
2. 1%

Question 2

Plants normally convert between ___% and ___% of the Sun’s energy available to them into organic matter.

Answer 2

1. 1%
2. 3%

Question 3

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

Answer 3

• over 90% of the Sun’s energy is reflected back into space by clouds and 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
• a factor, such as low carbon dioxide levels, may limit the rate of photosynthesis.

Question 4

What is the gross primary production (GPP)

Answer 4

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

Question 5

What is a plant’s NPP

Answer 5

• plants use 20-50% of the energy from its GPP in respiration.
• The chemical energy store which is left when these losses to respiration have been taken into account, is called net primary productivity (NPP).

Question 6

What is the equation for working out the NPP

Answer 6

net primary production = gross primary production - respiratory losses

Question 7

Usually less than ___% of this net primary production in plants can be used by primary consumers for growth. Secondary and tertiary consumers are slightly more efficient, transferring up to about ___% of the energy available from their prey into their own bodies.

Answer 7

1. 10%
2. 20%

Question 8

The low percentage of energy transferred at each stage is the result of:

Answer 8

• Some of the organism is not consumed,
• Some parts are consumed but cannot be digested and are therefore lost in faeces.
• Some of the energy is lost in excretory materials, such as urine.
• Some energy losses occur as heat from respiration and lost to the environment. These losses are high in mammals and birds because of their high body temperature. Much energy is needed to maintain their body temperature when heat is constantly being lost to the environment.

Question 9

The low percentage of energy transferred at each stage is the result of:

Answer 9

• Some of the organism is not consumed,
• Some parts are consumed but cannot be digested and are therefore lost in faeces.
• Some of the energy is lost in excretory materials, such as urine.
• Some energy losses occur as heat from respiration and lost to the environment. These losses are high in mammals and birds because of their high body temperature. Much energy is needed to maintain their body temperature when heat is constantly being lost to the environment.

Question 10

The net production of consumers can be calculated as:

Answer 10

N=I-(F+R)

where:
N represents the net production
I represents the chemical energy store of ingested food
F represents the energy lost in faeces and urine
R represents the energy lost in respiration.

Question 11

It is the relative inefficiency of energy transfer between trophic levels that explains why:

Answer 11

• most food chains have only four or five trophic levels because insufficient energy is 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 available is less at each level as one moves up a food chain.

Question 12

Answer 12

Question 13

State three reasons for the small percentage of energy transferred at each trophic level.

Answer 13

• some of the organism is not digested and so are lost as faeces
• some energy is lost as excretory materials
• some energy is lost as heat

Question 14

Explain why most food chains rarely have more than four trophic levels

Answer 14

• The proportion of energy transferred at each trophic level is small (less than 20%)
• After 4 tropics levels there is insufficient energy to support a large enough breeding population

Question 15

An area of vegetation 5 m by 5m produces 4 x 10*kJ of potential energy in a year. Calculate the gross primary production of this area.

Answer 15

40000 divided by 25 = 1600 kJ m^-2 year^-1

Question 16

How do you calculate percentage efficiency of energy transfer between trophic levels

Answer 16

percentage efficiency = (energy available after the transfer DIVIDED BY energy available before the transfer) x 100

Question 17

What farming practice achieves maximum yield and decreases energy loss in order to do this

Answer 17

‘Intensive rearing of domestic livestock’

Question 18

What farming practice achieves maximum yield and decreases energy loss in order to do this

Answer 18

‘Intensive rearing of domestic livestock’

Question 19

What is intensive rearing of livestock

Answer 19

• converting the smallest possible quantity of food energy into the greatest quantity of animal mass
• achieved by minimising the energy losses from domestic animals during their life time
• done by ensuring as much energy from respiration goes into growth rather than other activities or other organisms
• they do this by keeping animals in a confined space, barns or cages

Question 20

How does intensive rearing of domestic livestock increase the energy-conversion rate

Answer 20

• movement is restricted = less energy used in muscle contraction
• environment is kept warm and constant = reduce energy lost through heat transfer
• feeding is controlled so 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 21

Suggest a reason why keeping animals in the dark for longer periods might improve the energy conversion rate

Answer 21

longer dark period = more time spent resting = less energy expended = more energy converted into body mass

Question 22

Pesticides are used to increase productivity by simplifying food webs, how might it sometimes reduce productivity

Answer 22

• The pesticide may kill beneficial organisms (e.g. ones that prey on organisms that are harmful to the farmed organism)
• if the pesticide kills most of the pests then the population of organisms (predators) feeding on it will fall
• with no predators controlling it = pest population increases again (possibly to a level higher than before)
• the crop will be even more affected by the pest, leading to lower productivity

Question 23

What is the aim of pest control and why is this a problem

Answer 23

• Simplify the food web and so limit the effect of pests on productivity to a commercially acceptable level
• problem is that at least 2 different interests are involved:
  1) the farmer who has to satisfy our demand for cheap food while still making a living
  2) the conservation of natural resources which will enable us to continue to have food in the future

Question 24

What is the definition of productivity

Answer 24

The amount of light energy converted to chemical energy (organic compounds) by autotrophs in an ecosystem in a given time period

Question 25

What is the definition of gross primary production

Answer 25

The total fixation of energy by photosynthesis in a given area or volume

Question 26

What is the definition of respiration

Answer 26

Chemical energy converted from organic compounds for maintenance and growth in producers

Question 27

What is net primary production

Answer 27

• NPP = GPP - R
• this represents the actual rate of production of new biomass that is available for consumption by heterotrophs