Noise-induced changes in the critical and oscillatory behavior of a prey-predator system are studied using power spectrum density and spectral amplification factor (SAF) analysis. In the absence of external noise, the population densities exhibit three kinds of asymptotic behavior, namely: absorbing state, fixed point (FP) and an oscillatory regime with a well defined proper (natural) frequency. The addition of noise destabilizes the FP phase inducing a transition to a new OR. Surprisingly, it is found that when a periodic signal is added to the control parameter, the system responds robustly, without relevant changes in its behavior. Nevertheless, the coherent stochastic resonance phenomenon is found only at the proper frequency. Also, a method based on SAF allows us to locate very accurately the transition points between the different regimes. © 2001 Elsevier Science B.V.
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