Bacterial decomposition converts the organic materials in the medium into food which will support the growth of a variety of protozoa.

Each different form of protozoa will make their appearance in stages as environmental conditions become favorable.

The disappearance of the various organisms and appearance of others points out that as the food supply of each kind of organism becomes progressively depleted, their numbers decrease.
Reductions in the bacterial population, accumulation of organic wastes, and changes in the pH of the water are some of the factors which can cause the decline of protozoan populations.

If the culture is kept in a lighted area, in time, green algae will appear and if maintained in light will multiply rapidly. The algae, an autotroph, provides a continuous food supply for the animal population, thus stabilizing the community.

As the culture ages, protozoans will be present but in very small numbers. Often , the appearance of rotifers indicates the rapid decline of the protozoan forms. If crustacean eggs, such as those of the freshwater shrimp were present in the soil, then the larvae and adult forms of these will be present at the end of three weeks.

Your findings from the rehydrated desert soil will vary with the type of culture started. However, it can be expected that bacteria will appear first, then flagellates (e.g., euglena). As the flagellates decline in numbers, ciliates multiply.

The climax community contains a few bacteria and protozoa, algae, rotifers, and some crustacean forms.

Some generalizations you should try to make while doing the activity include:

1. In which culture dish (activity B) did the greatest changes in numbers take place? Why?
2. Which organisms from both activities appeared to rise and decline the most? The least?
3. Which environmental factors were necessary for the survival of most organisms? What is the ideal temperature for survival with or without food?
4. Do photosynthesizers survive without food? If so, why?
5. Who is eating whom? Construct a food chain or web illustrating this.
6. Assuming that no new organisms were added each week when food was added,
7. account for the presence of more organisms in the culture dishes.
8. Did "new" types of organisms appear in later examinations of the cultures? If so, from where did these organisms come? From where did the bacteria come?
9. What causes successional changes to occur in a community?
10. Which physical and biological factors could have caused the succession in the culture dishes?
11. Why did we use a sampling technique (activity B) to determine the population numbers of the various species?