1. Marine conservationists are concerned that ﬁsh stocks in the sea are decreasing. Drastic measures will have to be taken to stop the extinction of many ﬁsh species.
The ﬁgure below shows a marine food web. Tuna are large carnivorous ﬁshes that are an important human food. Dolphins may be caught in ﬁshemien’s nets and die.
(a) State the names given to organisms occupying trophic levels 1 and 3.
(b) (i) Draw a food chain with four links from the food web shown, involving herbivorous ﬁsh.
(ii) Draw and label a pyramid of energy for this food web.
(iii) Explain why it is more energy efficient for humans to eat herbivorous ﬁsh rather than tuna.
(c) Explain why it is necessary to conserve animals, such as tuna and dolphins, which are at trophic level 4.
(d) A toxin was introduced to the ecosystem through an accident involving a cargo ship. The toxin is directly assimilated into plankton and photosynthetic algae and remains in the organisms throughout their lifetime.
Describe its effect on the tuna population.
2. (a) The figure below shows a food chain. The figures show the relative numbers of organisms at each level.
(i) Draw a pyramid of numbers for this food chain.
(ii) How would the shape of a pyramid of biomass for this food chain be different from this pyramid? Explain this difference.
(iii) What is the importance of sunlight in this food chain?
(iv) Suggest why this food chain could not have another trophic level.
(b) A farmer sprays insecticide on his crops. The insecticide washed off into a lake where it is absorbed by the producer to enter the food chain shown below.
The numbers give the levels of insecticide in the organisms in parts per million (ppm).
(i) Describe the difference in the levels of insecticide in the organisms in the food chain.
(ii) Suggest a cause for this difference.
(iii) The fish-eating birds are the only organism in this food chain that dies from insecticide poisoning. Suggest a reason why.
1. (a) Producer and secondary consumers
(b)(i) Any food chain with 4 trophic levels
(ii) Lowest trophic level producers, highest trophic level are tertiary consumers. Decreasing amount of energy down trophic level
(iii) As you go up the trophic level, energy is lost. As herbivorous ﬁsh is of a lower trophic level, more energy will be available to the humans.
(c) If tuna and dolphins are removed, the ecosystem will be imbalance / upset.
Any reference to the increase in secondary consumer/ producers
This causes the ecosystem to be untenable l unsustainable / humans will not be able to carry out ﬁshing
(d) The tuna population will decrease
As secondary and primary consumers consume plankton with toxins, the toxins will accumulate in these trophic levels with increasing concentration.
The tuna will receive toxins at a greater concentration / amount/ called bioaccumulation.
2. (a)(i) 4 levels shown and labelled clearly; width shows relative numbers at each level, narrow at bottom
(ii) pyramid of biomass would be widest at the bottom (rain tree) and narrowest at the top (tree sparrows); the single rain tree is the largest organism and thus has the most
carbon compounds. Organisms that are smaller in size have a smaller amount of carbon compounds in total. The total amount of carbon compound decreases as we move down the food chain
(iii) Sunlight is the source of energy for the food chain; green plant photosynthesizes to convert light energy to chemical energy for the living organisms / all living organisms are dependent on the this energy to survive
(iv) As energy is lost / used up at each trophic level, the last trophic level does not have enough energy to support another population of organisms
(b)(i) The unicellular green plants had the lowest level of insecticide, 0.05 ppm. The levels increase down the food chain and is the highest in the fish eating birds at 25. ppm
(ii) The insecticide is not broken down or excreted and thus
accumulates in the bodies of the organism/ At each trophic level, the organisms feed on many organisms from the previous trophic level. Thus a higher amount of insecticides
enter the bodies of these organisms
(iii) The concentration of the insecticide is the highest in the fish eating birds. The concentration of insecticides in the other organisms are lower and does not cause death.