Abstracts Biological Sciences

Population viability of snowy plovers in coastal northern California

by Luke J. Eberhart-Phillips

For endangered metapopulations, the roles of source-sink dynamics are easily overlooked or not fully understood when formulating delisting requirements. This could lead to unrealistic recovery criteria imposed on sink populations, which in turn might restrict the entire metapopulation from being delisted. Therefore, an understanding of the viability of sink populations within the context of source populations is needed to develop appropriate conservation objectives. Consequently, I conducted a population viability analysis on the small geographically isolated northern California population of the snowy plover (Charadrius nivosus), a shorebird that is listed as threatened under the federal Endangered Species Act. The threatened status of the plover results from three factors that limit population recovery: 1) predation by native and introduced vertebrates, 2) encroachment of invasive vegetation in breeding habitats, and 3) human disturbance. I utilized 11 years of mark-recapture, productivity, and movement data to explore population viability under various management scenarios that addressed the three limiting factors within the framework of surrounding source populations. I also investigated how viability is influenced by periodic over-winter catastrophes that reduce survival. Simulations confirmed that the northern California population is a sink that relies upon immigrants originating from surrounding populations such as Oregon, San Francisco Bay, and Monterey Bay. Within the next 50 years, simulations revealed that these source populations will increase 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 reducing human disturbance through the use of symbolic fencing and the restriction of recreational vehicle use on beaches occupied by plovers provides benefits to the local population that may partially alleviate the reliance upon immigration. Lethal predator management was also found to be effective at growing the population; however the use of nest exclosures reduced current population growth because they are known to compromise the survival of incubating adults, the most “elastic” vital rate in northern California. The dependence of northern California upon immigration highlights the importance of continued plover management in source populations. A cold winter weather catastrophe was an influential phenomenon affecting population growth in northern California, and is likely a major factor contributing to its status as a sink because these catastrophes trigger large stochastic shifts in adult and juvenile survival. Sink populations such as northern California may be important for the viability of the greater metapopulation. Therefore I recommend that the protection and maintenance of productive source populations should be a priority for snowy plover management, however the preservation of the northern California population…