Population Growth Factors | Worksheet - Free Printable

Here's the complete solution to the “Population Growth Factors” worksheet, based on the graph and ecological principles:

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1. Which animal is the predator, and which is the prey?

✔ Predator: Fox
✔ Prey: Rabbit

Why?
In predator-prey relationships, the predator population typically lags behind the prey population. When prey (rabbits) increase, there’s more food for predators (foxes), so foxes increase later. When prey decrease, predators eventually starve or reproduce less, causing their numbers to drop. The graph shows this classic cyclical pattern — rabbit peaks come before fox peaks, confirming rabbits are prey and foxes are predators.

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2. When the fox population was at a peak, was the rabbit population increasing or decreasing? Why?

✔ The rabbit population was DECREASING.

Why?
At the peak of the fox population, there are many foxes hunting rabbits. This high predation pressure causes the rabbit population to decline sharply. The rabbit population usually starts falling *before* the fox population peaks, but by the time foxes reach their peak, rabbits are already in steep decline due to over-predation.

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3. When the rabbit population was at a peak, was the fox population increasing or decreasing? Why?

✔ The fox population was INCREASING.

Why?
When rabbits are at a peak, there’s an abundance of food for foxes. This leads to higher survival rates and more successful reproduction among foxes, causing their population to rise. The fox population follows the rabbit population with a time lag — it increases after rabbits peak because it takes time for foxes to respond to the increased food supply.

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4. Notice that the fox population was always smaller than the rabbit population. Why do you think that happened?

✔ Energy transfer in ecosystems is inefficient — only about 10% of energy moves from one trophic level to the next.

Why?
Rabbits are primary consumers (eat plants), while foxes are secondary consumers (eat rabbits). Because most energy is lost as heat or used for metabolism at each level, there isn’t enough energy to support as many foxes as rabbits. This is why predator populations are always smaller than prey populations — it’s a fundamental rule of ecology called the “10% Rule.”

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5. If there were no foxes in the ecosystem, could the rabbit population grow indefinitely? Why or why not?

✔ No, the rabbit population could NOT grow indefinitely.

Why?
Even without predators, rabbits would still face limiting factors like:
- Food scarcity (grass, shrubs, etc.)
- Competition with other herbivores
- Disease and parasites
- Space and habitat limits
- Waste accumulation

These are called density-dependent factors — they become stronger as the population grows. Eventually, the rabbit population would hit its carrying capacity (the maximum number the environment can support) and stabilize or crash due to resource depletion.

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6. What do you think would happen to the populations of both organisms if their ecosystem was affected by a drought? Why?

✔ Both populations would likely DECREASE.

Why?
- Rabbits: Drought reduces plant growth → less food → rabbit population declines due to starvation, lower birth rates, or increased vulnerability to disease.
- Foxes: With fewer rabbits (their main food source), foxes would also starve or reproduce less → their population declines too.

This is an example of a density-independent factor — drought affects populations regardless of their size. It disrupts the entire food chain starting from the producers (plants) up to the top predators (foxes).

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✔ Final Tip for Students:
This graph illustrates a classic predator-prey cycle, which is a key concept in ecology. Remember: Prey populations drive predator populations, but both are ultimately limited by resources and environmental conditions.

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Let me know if you’d like a printable version or visual annotations for the graph!