  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 (OR) 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.
