  A new model to investigate environmental effects of genetically
distinguishable predators is presented. The Holling type II response function,
modelling feeding satiation, leads to persistent system's oscillations, as in
classical population models. An almost complete classification of the cases
arising in the Routh-Hurwitz stability conditions mathematically characterizes
the paper. It is instrumental as a guideline in the numerical experiments
leading to the findings on the limit cycles. This result extends what found in
an earlier parallel investigation containing a standard bilinear response
function.
