Describe what is meant by a food chain and a food web.

Explain the relationship between rises in concentration of atmospheric gases and the enhanced greenhouse effect.

Accept examples of the points below, provided that the terms underlined are clearly identified.

Accept only named examples (latin or common names) from natural ecosystems only.

Do not award marks for general names such as “fish” or “tree”.

a. food chain shows transfer of nutrients/energy in an ecosystem / arrows from one trophic level to the next in examples;

b. between different trophic levels / shown in a correct chain or web;

c. starting with a producer;

d. followed by at least two levels of consumers / shown in a correct chain or web;

e. food web is the (branched) interaction of multiple food chains / cross arrows in examples;

f. using (multiple) producers as a source;

g. transferring nutrients/energy to consumers from different food chains;

h. same consumer could be at different trophic levels in a food web;

Award [2 max] from the following list of greenhouse gases:

a. water vapour;

b. carbon dioxide;

c. methane;

d. oxides of nitrogen;

e. all (of these gases) occur naturally;

f. and human activity has increased the normal level of these gases in recent years;

g. incoming shorter wave radiation from the Sun;

h. is re-radiated as longer wave radiation/infrared;

i. (mainly) in the form of heat;

j. captured by greenhouse gases;

k. which increases the atmospheric/ocean temperature;

l. at a higher rate than normal / creating a positive imbalance;

m. which threatens ecosystems/climatic patterns/ocean patterns;

n. Earth’s history had many fluctuations in gas levels/global temperature / some scientists are skeptical about enhanced greenhouse effect;

Food chains and webs seem to be concepts that candidates had come across, but had great difficulties explaining them. Examples given should be realistic and contain specific animals and plants (5.1.4 to 5.1.8), not just “insect” and “bird”.

An alarming number of candidates still think that the greenhouse gases are kept in by the ozone layer or are part of the ozone layer. Most could identify some greenhouse gases, but few were able to explain the difference between the short wave solar radiation and the much longer re-radiated waves from Earth which are trapped by the gases.