The goal of all the environmental fate and effects testing at P&G is to make sure products will not affect the environment. So, the best place to test the toxicity of P&G ingredients would be in the real environment. Because of the value of the environment and the possibility of effects, this would not be an appropriate approach. Instead of using the real environment, small parts of the environment that have been isolated are used. These are called "model ecosystems" or "mesocosms."
A model ecosystem is the highest tier of ecotoxicity testing available. Definitions of model ecosystems vary, but they are usually fairly large (>10 meters long for streams, >10,000 liters for ponds), can be indoors or outdoors, contain a diverse array of species and are sustainable for long periods of time. Basically, these are slices of the environment that have been moved into the laboratory for experimental work.
The advantages for using model ecosystems are many and include: