Allee effects cause source-sink dynamics between homogeneous habitat patches

Background/Question/Methods

In the classic source-sink models, net immigration from a population with a pre-dispersal growth rate > 1 (source) subsidizes the density of a population with a pre-dispersal growth rate < 1 (sink). When the source population is absent, the sink population deterministically either goes extinct (true sink) or decreases to a lower non-zero equilibrium (pseudo-sink). A tenet of classic source-sink theory is that habitat heterogeneity facilitates the dynamic. That is, the difference in population growth rates derives from a difference in the extrinsic environment. The objective of this study is to explore whether an Allee effect could create source-sink dynamics among homogeneous patches of habitat. Allee effects are depressed population growth rates at low population densities and the Allee threshold is an unstable equilibrium. While Allee effects are difficult to detect, accumulating evidence suggests that they may be prevalent in ecological systems. Simulations of a multi-patch model with logistic growth, stochastic growth, and inter-patch dispersal were used to address the objective. We investigated the roles of initial conditions, disturbance, and density-dependent vs. density independent dispersal on source-sink dynamics and metapopulation persistence across patches of identical quality.

Results/Conclusions Simulations of a two-patch metapopulation model demonstrate that source-sink dynamics can emerge between habitat patches of identical quality when an Allee threshold is present. The system had four alternate stable states. The first, when both populations are above the Allee threshold, is referred to as a source-source dynamic. A second is when both populations go extinct after dropping below the Allee threshold. The third and fourth stable states, when one population is above the Allee threshold and the other is below, results in source-sink dynamics. Source populations were at densities above the Allee threshold and sink populations below the Allee threshold. Initial population sizes are important in determining the initial stable state. Shifts among stable states, including source-sink reversals, can result from stochasticity in the system. Density-dependent dispersal tends to promote source-source dynamics and greater persistence of the metapopulation. Temporal heterogeneity in source-sink dynamics presents challenges in identification of viable populations in habitat conservation and invasive species management. These results demonstrate that labeling populations as sources or sinks can greatly oversimplify a complex dynamic, and result in sub-optimal management decisions.  Identification of the mechanism for source-sink measures can be critical to the success of management and conservation efforts.