  Identifying early warning signs of sudden population changes and mechanisms
leading to regime shifts are highly desirable in population biology. In this
paper, a two-trophic ecosystem comprising of two species of predators,
competing for their common prey, with explicit interference competition is
considered. With proper rescaling, the model is portrayed as a singularly
perturbed system with fast prey dynamics and slow dynamics of the predators. In
a parameter regime near singular Hopf bifurcation, chaotic mixed-mode
oscillations (MMOs), featuring concatenation of small and large amplitude
oscillations are observed as long-lasting transients before the system
approaches its asymptotic state. To analyze the dynamical cause that initiates
a large amplitude oscillation in an MMO orbit, the model is reduced to a
suitable normal form near the singular-Hopf point. The normal form possesses a
separatrix surface that separates two different types of oscillations. A large
amplitude oscillation is initiated if a trajectory moves from the "inner" to
the "outer side" of this surface. A set of conditions on the normal form
variables are obtained to determine whether a trajectory would exhibit another
cycle of MMO dynamics before experiencing a regime shift (i.e. approaching its
asymptotic state). These conditions can serve as early warning signs for a
sudden population shift in an ecosystem.
