Interpreting a Predator-Prey Graph
A class monitors a closed terrarium containing grass, rabbits, and foxes over 12 months. The rabbit count rises from 20 in January to a peak of 120 in May, then falls to 30 by September before rising again. The fox count lags: it is low (2) in January, peaks at 12 in July, and falls to 4 by November. The teacher asks students to explain the pattern and to predict what happens to the grass if the foxes are removed in the next year.
1. Explain why the fox peak occurs after the rabbit peak, using the graph data.
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Rabbits are the foxes' food. More rabbits (peaking in May at 120) allow more foxes to survive and breed, so fox numbers rise afterwards, peaking in July at 12. This lag is typical of predator-prey cycles: the predator response depends on the prey population that existed some time earlier, because reproduction and growth take time. When rabbits then fall (to 30 by September), fox food is scarce, so fox numbers decline afterwards (to 4 by November).
2. Predict the effect on the grass if all foxes are removed, and explain the chain of reasoning.
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Removing foxes removes the main predator of rabbits, so rabbit numbers would likely rise above the previous peak of 120. More rabbits would eat more grass, so grass cover would fall, possibly sharply. With less grass, rabbits would eventually run out of food and their numbers would crash, and the whole terrarium could become unstable. So removing the top predator does not simply help the rabbits; it risks overgrazing and a later population crash.