This model describes an interspecific competition. In this explanation, let assume that 2 species X & Y are in a competitive relation and the population of X is x, the population of Y is y. If one of these species is absent (x or y =0), this model is same to the logistic growth model. The value b represents the effect species X has on the population of species Y. When they are harmful to each other, the value a & b are positive number