ROBUST PERMANENCE FOR ECOLOGICAL MAPS

We consider ecological difference equations of the form x(t+1)(i) = x(t)(i) A(i)(x(t)), where x(t)(i) is a vector of densities corresponding to the subpopulations of species i (e.g., subpopulations of different ages or living in different patches), xt = (x(t)(1), x(t)(2),..., x(t)(m)) is the state of the entire community, and A(i)(x(t)) are matrices determining the update rule for species i. These equations are permanent if they are dissipative and the extinction set {x : Pi(i) parallel to x(i)parallel to = 0} is repelling. If permanence persists under perturbations of the matrices A(i)(x), the equations are robustly permanent. We provide sufficient and necessary conditions for robust permanence in terms of Lyapunov exponents for invariant measures supported by the extinction set. Applications to ecological and epidemiological models are given.