Species decline and extinction: synergy of infectious disease and Allee effect?

Host-parasite models with density-dependent (mass action) incidence and a critical Allee effect in host growth can explain both species decline and disappearance (extinction). The behaviour of the model is consistent with both the novel pathogen hypothesis and the endemic pathogen hypothesis for chytridiomycosis. Mathematically, the transition from decline to disappearance is mediated by a Hopf bifurcation and is marked by the occurrence of a heteroclinic orbit. The Hopf bifurcation is supercritical if intra-specific host competition increases with host density at a large power and subcritical if the power is small. In the supercritical case, host-parasite coexistence can be at equilibrium or periodic; in the subcritical case it is only at equilibrium.