Apparent survival and population growth of Western Snowy Plovers (Charadrius alexandrinus nivosus) in Humboldt County, California
The Western Snowy Plover (Charadrius alexandrinus nivosus) is a federally listed threatened species as well as a state listed “species of special concern” in California. Accurate survival estimates are required to determine the best conservation strategies for threatened and endangered species. The population located in Humboldt County, CA was studied for five breeding seasons (2001-2005) and apparent survival for adult and juvenile plovers was estimated using program MARK. In addition, the population structure over this time period was described to determine the importance of immigration. The growth rate of the population was also estimated using the Pradel model and site specific birth and death rates to quantify the importance of immigration. Apparent survival for adult males (φ = 0.64) was greater than that for adult females (φ =0.57) while apparent survival for adults was greater than that for juveniles (φ = 0.30). The estimated annual growth rate using Pradel models demonstrated a stable population (λ = 0.99); however, this included both local recruitment and immigration as sources of new individuals. When calculated algebraically, based on estimates of local recruitment and survival, the growth rate demonstrated a declining population (λ = 0.71), clearly indicating the importance of immigration to the persistence of the population. Source/sink dynamics probably occur in the spatially disjunct populations of Snowy Plovers along the Pacific coast due to differences in habitat quality. However, the Humboldt County population should not be defined as a sink population across all breeding sites. At a smaller scale, sites along gravel bars of the Eel River were closely approaching the recruitment requirements to be classified as source areas whereas beach breeding sites were sink areas. Continued management activities, especially those that were designed to increase productivity (e.g. exclosures) are recommended. Because productivity is a major problem for plovers in this area and due to the success observed in Oregon, population control of predators should be considered. Increased management, such as curtailing the access of off-road vehicles along the more productive gravel bar areas during the breeding season is also recommended.