- Set the starting fish and pond conditions.
- Press Next generation, or run ten generations automatically.
- Watch the fish population, dissolved oxygen, and graph.
- For a controlled investigation, change only one condition between trials. Changing a control starts a fresh Generation 0 trial.
- Open the data table for exact evidence.
Generation: one simplified cycle in which fish reproduce, survive, die, or are eaten. It is not a calendar month or a claim about one particular fish species.
Choose one question. Complete three trials of ten generations and change only the named condition.
How does food supply affect the fish population?
Test food at 2, 5, and 10. Keep starting fish, predators, pollution, and temperature the same.
How does water temperature affect the fish population?
Compare cooler-than-usual, typical, and warmer-than-usual conditions. Keep every other condition the same.
How does pollution affect the fish population?
Test pollution at 1, 5, and 9. Watch both dissolved oxygen and the fish population.
How do predators affect the fish population?
Test 0, 2, and 5 predators. Keep every other condition the same.
Claim: the pattern shown by the graph. Evidence: exact numbers from the data table. Reasoning: how the limiting factor caused the pattern.
Limiting factors and food supply
Food, predators, pollution, and temperature can limit population growth. The food supply limit is the largest population the selected food level can support in this simplified model. As the population approaches that limit, fewer young survive.
Temperature and dissolved oxygen
Fish take in oxygen dissolved in water. Cooler water can hold more dissolved oxygen than warmer water. The model uses one average temperature condition throughout each trial.
| Condition | Model temperature | Oxygen without added pollution |
|---|
| Cooler than usual | 16 °C | about 9.8 mg/L |
| Typical | 24 °C | about 8.3 mg/L |
| Warmer than usual | 32 °C | about 7.1 mg/L |
Temperature can also affect reproduction and metabolism. Warmer conditions increase stress, especially when pollution has already lowered dissolved oxygen.
Pollution, predators, and variation
In this model, pollution is directly harmful and also lowers dissolved oxygen. Predators remove fish, but each predator can catch only a limited number during one generation.
Two identical trials may produce slightly different numbers because births and deaths vary by about 8 percent. Scientists repeat trials to determine whether a pattern is consistent.
Fish population data recorded by generation| Generation | Fish | Born | Died | Eaten | Food | Predators | Pollution | Water condition | Oxygen mg/L |
|---|
Copy pastes the table into a spreadsheet as separate columns. The table clears when a new trial begins.
This learning model shows how several limiting factors can affect a population. It does not predict a real pond or represent one particular fish species.
A real pond also includes:
- fish of different ages and species with different generation lengths
- overlapping generations rather than neat, separate steps
- seasonal and day-to-night changes in temperature and oxygen
- disease, parasites, shelter, nesting habitat, rain, and drought
- a complete food web with plants, decomposers, prey, and predators
- fishing, stocking, and invasive species
Scientists collect field data and repeat investigations before drawing conclusions. Use this model to explore patterns, then compare those patterns with evidence from real ecosystems.
© 2026 Virtual Science Teachers. All rights reserved.