Press perturbation in lattice ecosystems: parity law and optimum strategy

Press perturbation, in which the reproduction rate r of a target species is altered and held at higher or lower levels, is studied by computer simulation. We apply a lattice Lotka-Volterra model to idealized ecosystems, and obtain the optimum strategy roptdefined as follows: the steady-state density of the target species takes a maximum value at r=ropt. It is found that ropttakes a finite value, and it usually locates at a phase transition point. Moreover, we find that the long-term response to applied perturbation can be significantly predicted by parity law, where the parity is defined by whether the system contains an even or odd number of species. If the parity is even (odd), the density of the target species increases (decreases) with increasing r. An evolutionary meaning of roptis discussed in connection with the origin of sexual reproduction. Copyright 1999 Academic Press.