A population-viability-based risk assessment of Marbled Murrelet nesting habitat policy in British Columbia

Abstract

The Marbled Murrelet (Brachyramphus marmoratus Gmelin) is a small threatened seabird of the Pacific coast of North America. Through simulation modelling we varied the long-term minimum amounts and quality (nesting density) of old-forest nesting habitat to examine effects on murrelet population viability, our measure of population resilience. Applying diffusion approximations we estimated population longevity and persistence probability under uncertainties of at-sea demography and onshore edge effects affecting nesting success, time scale, spatial scale, and subpopulation structure. We cast our analysis in a Bayesian belief and decision network framework. We also applied the framework to spatially explicit land-use and murrelet inventory data for the northern mainland region of the British Columbia coast. We found a diminishing expected value of persistence probability (EVP), for a single independent population, below a nesting capacity of ≈5000 nesting pairs (≈15 000 birds), accelerating below 2000 pairs. A strategy of multiple semi-independent subpopulations provided a higher joint EVP across a wide range of total nesting capacity. There was little improvement in EVP, for any number of subpopulations, above 10 000 – 12 000 pairs (≈36 000 birds, 45%–60% of coastwide population estimate in 2001). Depending on estimates of nesting density, 12 000 pairs would require between 0.6 and 1.2 million ha of potential old-forest nesting habitat.