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Guide to the study

Lab part 1

Lab part 2

 
 

 

Experiments with Pond Cultures

Characterizing the freshwater "zoo" alone is an eye-opening experience, and may keep you busy for a long time. However, it is easy to set up experiments to observe the effects of environmental degradation on biological diversity.

The importance of sunlight

What would happen to life on earth if the sun disappeared? It has been proposed that one or more ancient meteor impacts may have raised so much dust that the sun was blocked for years, destroying most of the life on earth. A similar, localized, situation takes place in water sources that are polluted with materials that prevent the penetration of sunlight. To see the effects of blocking sunlight simply set up an identical tank (same size, same samples) in a dark part of the room, with an opaque cover. It doesn't have to be completely dark, since room light is much less intense than sunlight. Characterize the tank, collecting samples from the same areas as you collect in the bright tank.

Effects of pollutants

There is a great variety of materials that pollute our waters. Sewage, chemical wastes, pesticides, herbicides, detergents, etc. all pollute our lakes and streams. You can try to "kill" a healthy tank by gradually adding a pesticide to the water. Chances are many organisms will survive, but the tank will be different, and diversity - the number of different types of organisms - will be reduced. You can add a pollutant that encourages abnormal growth of algae, such as phosphate detergent. You will probably see a reduction in diversity of photosynthetic organisms. Since animals depend on plants and different animals live in specific microenvironments, you will probably see a drop in animal diversity as well.

Try adding excess nutrients such as an overdose of fish food, which will upset the balance by providing for considerable bacterial growth. Excessive organic material such as fertilizer or fish food may temporarily increase the number of organisms in the tank, but the system will eventually "crash," especially if the supply of organic material is suddenly cut off. Again, expect a dramatic decline in biological diversity.

Competition

This type of experiment is rather sophisticated, and might require well characterized, stable cultures containing only a few interdependent species. Try taking two stable cultures containing completely different organisms, but kept under the same conditions of light and nutrient availability. Mix the cultures and follow their progress. Can the different organisms share the same habitat? Do the species out-compete each other? Is there mutual destruction?
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Created by David R. Caprette (caprette@rice.edu), Rice University Dates