A quantity f¯ denoting the average fitness of an ecosystem is introduced in the Bak-Sneppen model. Through this quantity, a different hierarchy of avalanches, f0 avalanche, is observed in the evolution of Bak-Sneppen model. An exact gap equation, governing the self-organization of the model, is presented in terms of f¯. It is found that self-organized threshold fc can be exactly obtained. Two basic exponents of the new avalanche τ, avalanche distribution, and D, avalanche dimension are given through simulations of one- and two-dimensional Ba...
