Key Points and Steps for Making a Bottle Ecosystem

■ Step 1: “Prepare a clear plastic bottle and cut off the top.” Can a bottle like this be used directly to make a bottle ecosystem? Students generally find bottles at a dump or in a bin, or save their own empty drinks bottles. In either case, wash the bottle before using it for the first time. Do not use soap or detergent, though (children often reach for these). These chemicals leave residues such as soap on the plastic, which can harm aquatic life. I recommend washing it with hot water, adding a little baking soda if necessary. If possible, you can also place a filter at the bottom of the bottle. Of course, you can do without one, but without a filter, bacterial growth will quickly make the water cloudy, and you will have to change it frequently.

■ Step 2: “Put a layer of mud from the bottom of a body of water into the bottle, then fill it more than halfway with water from a natural source.” For children in cities, both bottom mud and water from a natural source are difficult to obtain. Even in rural areas, sending children out after class to find them can be dangerous. My experiments also show that mud greatly affects the water quality in an ecosystem bottle. It can easily make the whole bottle dirty and messy, leaving many aquatic plants and animals unable to survive. I suggest taking some coarse sand or small stones, placing them in a large sieve, and rinsing them with tap water to remove dust and dirt. Spread them across the bottom of the bottle instead of mud. The sand or stones provide a home for bacteria that feed on decaying plants and animal waste. If water from a natural source is unavailable, use water left to stand overnight. To prepare this, let tap water stand for at least 24 hours beforehand, allowing enough time for chlorine-containing chemicals to break down and dissipate. Most aquatic organisms are sensitive to these substances and can easily die after exposure. Once you've filled the bottle, mark the water level on the outside with tape. As water evaporates, the concentration of minerals increases. To maintain a balanced environment, replenish the water to keep its volume constant. Any water added should also have been left to stand overnight. ■ Step 3: “Plant a few aquatic plants in the bottle and put some duckweed on the surface.” Note that the plants usually added to an ecosystem bottle are algae. They have no roots and are essentially held in place by the sand. Fill the bottle with water before adding the plants so the incoming water does not uproot them. One child missed this small detail and spent an entire afternoon planting aquatic plants without success. After filling the bottle and planting it, generally wait a day or two for suspended material to settle before introducing aquatic animals. ■ Step 4: “Add freshwater snails, small fish, small shrimp, little loaches, and so on.” Many students find that fish are especially likely to die on their first day in the bottle. The water quality seems good on the first day, so why do they die so easily? The cause is that the fish cannot adapt to their new surroundings. Most fish are very sensitive to sudden changes in water temperature, so abruptly transferring them to water at a different temperature can easily kill them. There is therefore a proper way to introduce fish. Put a plastic bag into their original tank and transfer the fish into it. Move the bag, with both the water and fish inside, into the ecosystem bottle. After about 1.5 hours, you can release the fish into their new home. Students often ask their teacher how many fish should go in one bottle. The number is directly related to the volume of water and the amount of oxygen available to the fish. For goldfish, two or three adults per four liters of water is enough. Another way to judge whether there are too many fish is to watch for fish floating at the surface and constantly “gasping.” If this happens, there is not enough dissolved oxygen: either there is a water-quality problem or there are too many fish. Freshwater snails, small fish, and small shrimp can be hard for city children to obtain, so aquatic snails can be used instead. Generally, one or two aquatic snails are enough for a 20-liter bottle. They reproduce quickly. Before long, you will see gelatinous eggs on the bottle walls or at the surface, and within two weeks you and your students can observe them hatching into small snails. Aquatic snails mainly eat green algae—the thin green film that grows on the walls of the bottle. Note that the amount of sunlight reaching the bottle directly affects how quickly the algae grow. In a balanced ecosystem bottle, the amount of new algae growing each day should roughly match what the snails can eat. Once the bottle is established, organize a weekly discussion based on the students' observation notes. Ask them to suggest reasons for the changes they see or predict what might happen next in this aquatic ecosystem. The activity should last at least two weeks, which differs from the schedule in the textbook, so adjust the teaching plan accordingly.