The effects of intraspecific competition and waste products are taken into consideration in this model. yt means the amount of waste at the time t.In this model, the newly created wastes have a stronger effect than the wastes created previously. In other word, there is the cleanup of wastes. This cleanup speed is parameterized as the value of c and the greater value means faster cleanup speed. The a value parameterizes the strength of intraspecific competition.