Science project

The Effect of Overpopulation on Guppies

Research Question:

  • What behavior changes are produced by overpopulation?
  • What happens when animals compete for limited resources?
  • What are the health effects of overpopulation?
  • What is a population curve?
  • What happens to a population of yeast when resources are exhausted?


  • 32 guppies
  • Fish food
  • Three small identical fish bowls (less than 5 gallons
  • Dry yeast
  • Two-cup measuring cup
  • Measuring spoon
  • Sugar solution
  • Eyedropper
  • Microscope

Experimental Procedure:

Experiment #1

  1. Fill each fish bowl 75 percent full with water. Use the same amount of water in each bowl.
  2. Let the water sit for three days before purchasing the fish so that the chlorine leaves the water.
  3. Add two guppies to the first bowl, 10 guppies to the second bowl and 20 guppies to the third bowl. Estimate the average size of the fish in each bowl. They should all be approximately the same size.
  4. Observe each bowl for 15 minutes per day. Record breathing patterns such as whether the fish are breathing at the surface of the water and how fast their gills are moving. Record how many aggressive movements, such as tail-flicking and chasing occur in 15 minutes.
  5. After three weeks compare how much the fish in each tank have grown. Which has the biggest fish?

Experiment #2

  1. Prepare a sugar water solution consisting of one cup of water and two tablespoons of sugar.
  2. Put one teaspoon of yeast in the sugar water solution. Wait one hour.
  3. Stir the water so the yeast is equally distributed. Use the eyedropper to place a drop of the solution on a microscope slide. Count the number of yeast cells.
  4. Repeat step two every day for two weeks. Graph your results. This is a population curve. Consider what conclusions can be drawn when yeast grows unchecked and when the population outgrows it’s resources.

Terms/Concepts: Malthus, overpopulation


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