THE DEMOGRAPHIC SIGNIFICANCE OF SINK POPULATIONS

Rates of reproduction and mortality undoubtedly vary across the ranges of most species. Local demographic 'sources', where reproduction exceeds mortality, may subsidize non-sustaining 'sink' subpopulations, where local reproduction fails to compensate for mortality. We examine model metapopulations of sources and sinks, connected by intrinsic dispersal of juveniles and density-dependent dispersal of adults or juveniles. Results show that a large but finite proportion of the metapopulation might reside in non-sustaining subpopulations. Although these demographic sinks are unable to persist independently, they may contribute significantly to metapopulation size and longevity. Our analysis leads to several recommendations for the conservation of natural populations: (1) demographically productive subpopulations need to be identified and protected; (2) conservation of buffer habitats and marginal subpopulations should be a part of comprehensive conservation plans; and (3) the long-term status of even seemingly secure metapopulations should be carefully monitored.