Metapopulation dynamics of the California least tern

The California least tern (Sterna antillarum browni) is federally listed as an endangered species. Its nesting habitat has been degraded, and many colony sites are vulnerable to predation and human disturbance. We examined a metapopulation model for the California least tern that can be used to predict persistence of populations along the Pacific coast and the effects of various management actions. We used the model to estimate the effect of reducing predation impact, an important source of reduced fecundity, in various populations. Apart from restricting human access to nesting sites, most management efforts have concentrated on predation. In the model, each cluster of nearby colonies is defined as a population. Within each population, the model includes age-structure, year-to-year changes in survival and fecundity, regional catastrophes (strong El Niño/Southern Oscillation [ENSO] events), and local catastrophes (reproductive failure due to predation). The model predicted a continuing population increase and a low risk of substantial decline over the next 50 years. However, this prediction was sensitive to assumptions about survival and fecundity. Under a pessimistic scenario, the model predicted a high risk of decline, although a low risk of extinction. We simulated the effect of predator management by reducing the probability of reproductive failure due to predation. The improvement in viability ranged from 1% to 4% for single populations and up to 8% when all populations were included. Results indicated that the number and location of populations selected for focused management influenced the effectiveness of management efforts.