Recovering marbled murrelets via corvid management: A population viability analysis approach

The expansion of human activities into rural areas and natural landscapes has resulted in widespread increases in the abundance of synanthropic species that threaten rarer native species. Quantitative assessments of how much impacts need to be reduced to reach acceptable levels of risk to the affected species are rarely conducted prior to the implementation of control measures, and it is perhaps not surprising that many efforts have not yielded the desired outcome. Here, we used matrix-based population viability analysis models to show that reducing predation by rapidly growing corvid populations on marbled murrelet (Brachyramphus marmoratus) nests likely constitutes an effective means for recovering a declining murrelet population in central California. For example, a modest 40% reduction in predation reduced extinction risk dramatically from 95.8% to 4.6% over 100 years and a 60% reduction resulted in a stable population (λ = 1) when the proportion of breeders, renesting rates, and corvid predation rates were assumed to be 0.77, 0.13, and 0.69, respectively. However, nest predation would only need to be reduced by 40% to produce a stable population if corvid management was coupled with a modest increase in after-hatch-year survival from 0.896 to 0.910. Corvid control resulted in greater gains in murrelet population size when the maximum number of breeders was allowed to increase over the projection period, as might be expected if the amount of old-growth nesting habitat increased over time, but extinction risk was insensitive to the presence of a carrying capacity. Approximately half of known murrelet nests in central California are within 1 km of heavily used campgrounds in a single state park, indicating that significant gains in viability could be achieved by targeting efforts in small areas providing corvid food subsidies. Risk assessments such as ours can provide quantitative prioritization rationale for efforts intended to mitigate the impacts of synanthropic species on threatened species.