“…Using Sotomayor's Theorem [16] we prove the analytical conditions required for a transcritical bifurcation between E 2 and E 3 to occur. Also, using well known conditions (see, for example [8,17]) we have shown that a Hopf bifurcation occcurs in E 3 at the threshold value µ * , giving rise to sustained population oscillations [14,15,20]. Point E 2 cannot give rise to oscillations, since both eigenvalues are always real.…”