3.5.3 Energy and ecosystems (A-level only)

Question 1

Define ecosystem.

Answer 1

= All the organisms living in a particular area along with all the abiotic conditions.

Question 2

Outline how energy is “produced” and transferred throughout an ecosystem.

Answer 2

1. In an ecosystem there are producers - organisms that make their own food - plants / algae, via photosynthesis.
2. Some sugars produced during photosynthesis used in respiration, to release energy for growth.
   - –> rest used to make other biological molecules such as cellulose. These molecules make up the plant’s biomass = mass of living material / stored chemical energy of plant.
3. Energy is transferred through the living organisms of an ecosystem when organism eat other organisms —> producers eaten by primary consumers, eaten by secondary consumers, eaten by tertiary consumers = food chain.

Question 3

List 2 methods of measuring biomass.

Answer 3

1. Using dry mass.

2. Using a calorimeter.

Question 4

How can we measure biomass as dry mass?

Answer 4

• Measured in terms of the mass of carbon that an organism contains or the dry mass of its tissue per unit area per unit time.
  1. Dry mass is the mass of the organism with the water removed.
  2. To measure dry mass, sample of the organism dried, often in an oven set to a low temperature. Sample then weighed at regular intervals (every day). Once mass remains constant, all water removed.
  3. If necessary, result from sample can be scaled up to give dry mass (biomass) of the total population or the area being investigated. Typical units for dry mass => kg / m2
  4. Mass of carbon present is generally taken to be 50% of dry mass.
  5. Biomass changed over time —> deciduous trees lose their leaves in winter, for example.
  => useful to give biomass over a particular time period —> typical biomass units kg / m2 / yr.

Question 5

How can we measure biomass using calorimeter.

Answer 5

• Can estimate the amount of chemical energy stored in biomass by burning biomass in a calorimeter.
  => heat given off tells you how much energy is in it.
  1. Sample of dry biomass burnt and energy released to heat a known volume of water.
  2. Temperature change of water is calculated the chemical energy of the dry biomass.

Question 6

Define Gross Primary Production (GPP). Units?

Answer 6

= Total amount of chemical energy converted from light energy by plants in a given area, in a given time.
kj / m2 / yr.

Question 7

Define Respiratory Loss (R). Units?

Answer 7

= (Approx 50% of) GPP lost to the environment as heat when the plants respire.
kj / m2 / yr.

Question 8

Define Net Primary Production (NPP). Units?

Answer 8

= Remaining chemical energy - energy available to plant for growth and reproduction, i.e. energy stored in plant’s biomass.
= Energy available to organisms at the next trophic level.
kj / m2 / yr.

Question 9

Give the equation for NPP.

Answer 9

NPP = GPP - R.

Question 10

Comment on the energy transfer between producers and consumers / consumers and consumers.

Answer 10

1. Consumers get energy by eating plant material or animals that have eaten plant material.
2. Not all chemical energy stored in consumer’s food is transferred to the next trophic level (only around 10%).
3. Firstly, not all food is eaten - plant roots / bones etc. so the energy these things contain is not transferred as they are not ingested.
4. Of the parts that are ingested,
   - Some are indigestible, so are egested in faeces => chemical energy stored in these parts is therefore lost to the environment.
   - Some energy lost to the environment through respiration or excretion of urine.
5. Energy left after all this is stored in the consumers’ biomass and is available to the next trophic level. Energy = consumers net production.

Question 11

Give the equation for the net production of consumers.

Answer 11

N = I - (F + R).
Where:
N = Net production.
I = Chemical energy in ingested food.
F = Chemical energy lost in faeces and urine.
R = Energy lost through respiration.
All units in = kj / m2 / yr.

Question 12

How would we calculate the efficiency of energy transfer?

Answer 12

(NP / total energy received) x 100.

Question 13

How can food chains / food webs show how energy is transferred between organisms?

Answer 13

Food chains - show simple lines of energy transfer between trophic levels.
Food webs - show many food chains in an ecosystem, and how they interact and overlap.
Decomposers (such as fungi) are also part of food webs - break down dead or undigested material, allowing nutrients to be recycled.

Question 14

Farming practices aim to increase the efficiency of energy transfer by? (2 factors).

Answer 14

1. Reducing energy lost to other organisms.

2. Reducing energy lost through respiration.

Question 15

Outline how simplifying the food web reduces the energy loss to other organisms.

Answer 15

Pests reduce the productivity of crops by reducing the amount of energy available for crop growth —> reduces energy available for humans.
=> By removing food chains that don’t involve humans, energy losses reduced and crop productivity increased.

Question 16

1. Chemical pesticides:

Answer 16

• Insecticides kill insect pests that eat and damage crops —> killing pests means less biomass lost - crops grow larger so productivity (rate at which chemical energy is stored) is greater.
• Herbicides kill weeds (unwanted plant species) - killing weeds removes direct competition for light and resources etc. Also removes habitats for insect pests helping to further reduce their numbers and simplify the food web.

Question 17

2. Biological pesticides:

Answer 17

• Parasites live in or lay their eggs on a pest insect. Parasites either kill insect or hinder its ability to function.
• Pathogenic (disease-causing) bacteria and viruses are used to kill pests.
  => Farmers can use integrated systems that combine both chemical and biological methods. Combined effect of using both can reduce pest numbers even more than one method alone, increasing productivity even more.

Question 18

Outline how farmers can reduce respiratory losses to increase the efficiency of energy transfer.

Answer 18

1. Can control conditions livestock is kept in —> more energy used for growth and less is lost through respiration (and activities that increase the rate of respiration):
   - Animals kept in pens where movement is restricted.
   - Pens often kept indoors and kept warm so less energy wasted by generating body heat.
2. Therefore, more biomass produced and more chemical energy can be stored, increasing productivity and efficiency of transfer to humans:
   - Benefits are that more food can be produced in a shorter space of time, often at a lower cost.
   - However enhancing productivity by keeping animals in pens raises ethical issues —> intensively reared animals kept in conditions cause animals pain, distress or restricts their natural behaviour, so it shouldn’t be done.