Conservation challenges of a sink: the viability of an isolated population of the Snowy Plover

Source-sink dynamics are easily overlooked when formulating recovery objectives for threatened species. This could lead to unrealistic criteria imposed on sink populations, which in turn might restrict an entire metapopulation from being delisted. Therefore, an understanding of the viability of subpopulations within the context of a metapopulation is needed to develop appropriate recovery objectives. Consequently, we used 11 years of mark-recapture, productivity, and movement data to analyse the viability of a small, geographically isolated population of the Snowy Plover Charadrius nivosus, a shorebird listed as threatened under the U.S. Endangered Species Act. Simulations confirmed that the focal population in northern California is a sink that relies upon immigrants from neighbouring populations. Furthermore, these source populations will increase within the next 50 years and are likely to achieve the delisting requirements. However, the northern California population is unlikely to reach the delisting criteria given the current vital rate estimations. Management scenarios demonstrated that lethal predator removal and reducing human disturbance facilitate population recovery and may partially alleviate the reliance upon immigration. However, the use of nest exclosures reduced population growth because they are known to compromise adult survival. These results highlight the importance of maintaining viable source populations and re-evaluating the recovery objectives of metapopulations with active sinks.