Pond Population Puzzle
A pond contains algae, small water fleas that eat algae, minnows that eat water fleas, and herons that eat minnows. A nearby farm accidentally adds fertilizer to the pond. The fertilizer causes the algae population to grow rapidly, covering much of the surface. Within two weeks, the water fleas increase in number. Then the minnows also increase. A month later, dissolved oxygen in the pond drops sharply, and many minnows and water fleas die. Students are asked to explain what happened and to predict the effect on the herons.
1. Explain how the fertilizer could lead to a drop in dissolved oxygen, using the food chain in the pond.
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Fertilizer adds nutrients that help algae grow. More algae means more food for water fleas, so their numbers rise. More water fleas feed more minnows, so minnows increase too. When these organisms die or produce waste, decomposers break down the material and use oxygen. A large increase in algae and animals leads to more decomposition, which removes oxygen from the water. So the fertilizer starts a chain: nutrients, then algae, then consumers, then decomposition, then lower oxygen.
2. Predict how the heron population might change over the next few months and justify your prediction.
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Herons eat minnows. If many minnows die because of low oxygen, herons have less food in this pond. Some herons may leave, and fewer may raise young, so the local heron number could fall. However, herons can fly to other ponds, so the whole population may not drop as much as the pond population. The best prediction is a decrease in herons using this pond, with the size of the decrease depending on nearby food sources.