Applying the Declining Population Paradigm: Diagnosing Causes of Poor Reproduction in the Marbled Murrelet

Abstract:  We identified six approaches to diagnosing causes of population declines and illustrate the use of the most general one ("multiple competing hypotheses") to determine which of three candidate limiting factors—food availability, nesting site availability, and nest predation—were responsible for the exceptionally poor reproduction of Marbled Murrelets (  Brachyramphus marmoratus ) in central California. We predicted how six attributes of murrelet demography, behavior, and physiology should be affected by the candidate limiting factors and tested predictions with field data collected over 2 years. The average proportion of breeders, as estimated with radiotelemetry, was low (0.31) and varied significantly between years: 0.11 in 2000 and 0.50 in 2001. Murrelets spent significantly more time foraging in 2000 than in 2001, suggesting that low food availability limited breeding in 2000. In 2001, 50% of radio-marked murrelets nested and 67% of females were in breeding condition, suggesting that enough nest sites existed for much of the population to breed. However, rates of nest failure and nest predation were high (0.84 and 0.67–0.81, respectively) and few young were produced, even when a relatively high proportion of murrelets bred. Thus, we suggest that reproduction of Marbled Murrelets in central California is limited by food availability in some years and by nest predation in others, but apparently is not limited by availability of nesting sites. The multiple-competing-hypotheses approach provides a rigorous framework for identifying causes of population declines because it integrates multiple types of data sets and can incorporate elements of other commonly used approaches.     

Local Sustainability Indicators

Local Environment has established a bit of a niche in local sustainability indicator studies. In 1997 it published the important debate between Jeb Brugman and Graham Pinfield, which highlighted that sustainability indicators and their role are contested issues (Vol. 2, Nos 1-3). In 1999 it produce a first special issue on the topic (Vol. 4, No. 2). As the editors identified, this special issue probed ‘deeper into aspects of indicator selection, methodology, theory and practice’. The papers presented there began to discuss a theme that is developed further in this special issue, that of the relationship between the objective and the subjective in indicator development and use. Our special issue brings together three full papers and a commentary presenting research and information from the USA, Sweden, France and Austria. In this editorial we are going to review the growth of interest in sustainability indicators and the changing way in which they are viewed and briefly comment on some recent indicator projects that we have been involved in.