What Marbled Murrelets Need to Avoid Extinction
Scientists conclude that maintaining between 0.6 to 1.2 million hectares of coastal old-growth forest could ensure that marbled murrelets survive in British Columbia. The exact amount of forested area needed by the threatened seabirds depends upon how well-suited the forest is for their nesting.
The lower end of the range provides for 0.02 nesting pairs per hectare, which forests near the ocean with tall, old trees can accommodate.
The area should support 12,000 nesting pairs, totalling 36,000 birds, or about 45 to 60 percent of the number estimated in the province for 2001. Any more murrelets does not appreciably improve chances of the bird not becoming extinct.
The forecasts do imply that murrelets will become extirpated in some areas. The remaining birds would be distributed among 3 to 6 subpopulations spread along the coast, with 1000 to 4000 birds in each region. Maintaining several separate groups provides greater resilience in the face of a catastrophe hitting the birds.
To capture all the variables that significantly influence murrelet population viability, scientists modelled population dynamics using a Bayesian network. The calculations take into account changes in the marine environment and chicks lost through predation of nests along forest edges.
J. Douglas Steventon, Glenn D. Sutherland and Peter Arcese. 2006. A population-viability-based risk assessment of Marbled Murrelet nesting habitat policy in British Columbia. Canadian Journal of Forest Research. 36(12): 3075-3086.