If you were to simply let the ecosystem go, only providing it with water on a regular basis, you would see that as the plants matured, different populations of organisms would erupt. The key is that as a change happens in one part of an ecosystem, there is a corresponding response by all the other organisms that are present. In many cases, elements like pesticides are harmful to an ecosystem - although they were originally designed to be beneficial. Introducing them into an ecosystem often sets off a chain reaction of events that devastates the entire system. In the case of industrial pollution like acid rain, the problem is even worse, causing widespread destruction over a large geographic region.
Students conduct a simple investigation.
Students employ simple equipment and tools to gather data and extend the senses. Students use data to construct a reasonable explanation of how an ecosystem works. Students communicate investigations and explanations. Students discover that all animals depend on plants. Some animals eat plants for food.