As species composition and number of each species change, adaptive life-styles of individual species also change. If predator ignore less prey and use more prey, food web structure also change. This shows typical change of predatory swiching. there is two species coexisting relationship except living different places or eating different food.We construct a mathematical model applying Lotka-Volterra equation. If the fraction of prey and predator species decrease,link in food web break.We discuss about food web structure in this model.