The River Tank

A Versatile Tool

There are five environments in the River Tank - underwater in the slow currents, underwater in the fast currents, a bog, the water's edge, and high above the water on the ledges. A class might consider how many different types of organisms can live successfully in the tank and populate each of these environments. Turtles, leeches, crayfish, snails, (Fig. 3) worms, fish, salamanders, American chameleons (Fig. 4) newts, insects, bacteria, algae, higher plants, and many protists all can be successfully raised in the River Tank, but questions remain as to which ones can coexist, and for how long, before being eaten by another inhabitant.

Likewise, by describing the characteristics of the plants that grow in these different environments, students gain an understanding of the concept of niches. They can predict which plants will thrive and can test their predictions by establishing a viable plant community in the tank.

Figure 2 This rear view of a 30-gallon tank shows the large, gravel-filled cavity, which provides an area for bacterial growth and increases the filtering capacity of the tank.

Establishing the fish collection poses a similar challenge for students. For example, the 20-gallon tank holds about 12 gallons of water and can support 12 to 16 fish. A class might consider the following questions when determining which fish to include:

1. Which species are considered community fish, and which are considered aggressive?
2. Do all fish do well in a moving-water environment?
3. How many herbivores and scavengers does a tank this size require?
4. Do streams have bottom, mid-level, and surface feeders, as ponds do?

Once the fish colony is established, students can investigate additional questions:

1. Do fish orient themselves in predictable ways?
2. Does the water level (and thus waterfall size) affect fish migration between the pools?
3. Are certain species territorial?
4. Does crowding affect certain species more than others?
5. Since territorial fish often establish themselves near caves, does creating caves and overhangs change the behavior of certain fish?

Figure 3 Aquatic snails act as scavengers and window cleaners for the tank system.

Modeling an Ecosystem

By determining the roles that plants, animals, and microorganisms play in the River Tank system, students gain insight into the relationship between producers, consumers, scavengers, and decomposers. Furthermore, by studying its chemistry, students realize how utterly dependent the biotic community is on the abiotic components of the system (i.e., light, temperature, inorganic compounds, and oxygen and carbon dioxide levels).

A class could synthesize its understanding of all that is required to create a balanced ecosystem by modeling a particular type of stream. Since aquarium fish come from all over the world, a class could set up a tank to represent a particular continent or biome and choose plants and animals accordingly. Students also could contrast various steam ecosystems by setting up multiple tanks or by making picture essays or oral presentations.

Such projects would raise important questions about the extent to which one can accurately model a natural ecosystem, and about how the River Tank differs from such a natural system. With such questions answered, the River Tank serves as an ideal introduction for a field trip to a local stream or river, enabling a class to maximize the value of its visit.

The questions raised by establishing a balanced ecosystem in the tank can be the basis for sparking class discussion and for generating research projects. By reflecting on what they learned, students become sensitive to the effects of destroying part of an ecosystem or of removing a particular organism. They also consider more complex issues, such as whether endangered species are worth saving even if doing so severely impacts the local economy, or whether a closed system, such as an aquarium or space station, can really be self-sustaining.

Finally, a tremendous amount of supplementary information about organisms, habitats, niches, and ecosystems is available on CD-ROM, laser disk, and video. Viewing some of these images or using them to create a multimedia presentation gives students exciting opportunities to connect what they are learning in class to the world around them.

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