The crocodiles in the Olifants River are dying. Scientists are not too sure why this is so, but they think it is because of the pollution that runs into the water. Crocodiles eat catfish and tiger fish in the dams and in the river. Catfish eat fish such as carp and river bream. Fishermen in the dam have noticed that the numbers of these fish have been decreasing, while the numbers of catfish are increasing. This is a problem for them as they rely on the carp and bream they catch to eat and to sell. What do catfish eat? What eats the catfish? Is the catfish a herbivore or a carnivore? Why are the catfish numbers increasing? Do you think the decrease in the number of crocodiles has anything to do with this? Why is this change a problem for the fishermen? What could people do to overcome this problem?

1. Look at the desert ecosystem on the right. a) Identify two non-living things that are important to this ecosystem. Sun* Sand/Soil* b) Choose one of these and describe why this is important to the ecosystem. The sun* is important because it provides light energy for the plants (producers) to carry out photosynthesis, which is the basis of the food web in the ecosystem. c) Draw and label a biological diagram of one of the plants. A diagram of a cactus should be drawn. Labels should include: stem, spines, roots*. d) Draw a food web showing the names of the plants and animals. Include arrows to show the feeding relationships. Cacti → Rabbit Rabbit → Snake/Lizard Rabbit → Bird Snake/Lizard → Bird 2. Identify a carnivore and an herbivore in this picture. Carnivore: Bird* (or Snake/Lizard) Herbivore: Rabbit* 3. Read the passage below and answer the questions. What do catfish eat? Catfish eat carp and river bream*. What eats the catfish? Crocodiles* eat the catfish. Is the catfish a herbivore or a carnivore? The catfish is a carnivore* because it eats other animals (fish). Why are the catfish numbers increasing? Do you think the decrease in the number of crocodiles has anything to do with this? The catfish numbers are increasing because the number of crocodiles, their predators, is decreasing. Yes, the decrease in crocodiles directly contributes to the increase in catfish numbers due to reduced predation. Why is this change a problem for the fishermen? This change is a problem for fishermen because they rely on catching carp and river bream, whose numbers are decreasing. The increasing catfish population likely contributes to the decline of carp and river bream, making it harder for fishermen to catch their target fish. What could people do to overcome this problem? People could work to reduce pollution* in the Olifants River, which is suspected to be causing the decline in crocodile numbers. Restoring the crocodile population would help control the catfish population, allowing carp and river bream numbers to recover.