BALANCED DISPERSAL BETWEEN SPATIALLY VARYING LOCAL-POPULATIONS - AN ALTERNATIVE TO THE SOURCE-SINK MODEL

Analysis of long-term monitoring data on breeding collared flycatchers (Ficedula albicollis Temm.) has revealed equal numbers of immigration s and emigrations between neighboring populations of different sizes. Dispersal patterns were close to patterns simulated under a conditiona l dispersal and with populations near saturation level. Local growth r ates of the 11 sites were computed and did not support the idea that t he observed balanced exchanges could be the result of a source-sink sy stem. This is the first empirical evidence for a system of discrete ha bitat patches with component populations that exist as simultaneous so urces and sinks to their neighbors. Dispersal propensities were invers ely related to population sizes, which showed little variation in time . These results are consistent with recent modeling of dispersal as an evolutionarily stable strategy, and they demonstrate that dispersal c an be an active phenomenon requiring neither the dominance hierarchies nor the temporal instability generally invoked by ecological and popu lation genetic models. We note a parallel to the concept of Ideal Free Distributions and discuss implications for the evolution of dispersal mechanisms in fragmented populations.