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. Aplicación del Paradigma de la Población en Disminucioń: Diagnosis de las Causas de la Reproducción Deficiente de Brachyramphus marmoratus Resumen: Identificamos seis métodos utilizados para diagnosticar las causas de la disminucioń de poblaciones y damos un ejemplo del uso del más general ("hipótesis de competencia múltiple") para determinar cual de tres posibles factores limitantes (disponibilidad de alimento, disponibilidad de sitios de anidación y depredación de nidos) es responsable de la reproducción excepcionalmente deficiente de Brachyramphus marmoratus en California central. Predijimos el efecto de los factores limitantes sobre seis atributos de la demografía, comportamiento y fisiología de B. marmoratus, y probamos las predicciones con datos de campo recolectados a lo largo de 2 años. La proporción promedio de reproductores, estimada con radiotelemetría, fue baja (0.31) y varió significativamente entre años: 0.11 en 2000 y 0.50 en 2001. B. marmoratus forrajeó † Peery et al. Diagnosing Causes of Population Declines 1089 significativamente más tiempo en 2000 que en 2001, lo que sugiere que la baja disponibilidad de alimento limitó la reproducción en 2000. En 2001, 50% de los individuos radio-marcados anidó y 67% de las hembras estaban en condición reproductiva, lo que sugiere que existían suficientes sitios para nidos para la reproducción...

Conservation physiology

“…Animals soiled with crude oil are often held temporarily in captivity during rehabilitation. Much has been learned from the Exxon Valdez oil spill, for example, about the effects on cleaned sea otters Enhydra lutris, which showed about 43% survival in their first postcleaning year, a rate that was much lower than comparable populations [52]. Ben-David et al [53] determined that rehabilitation is a viable option for river otters Lontra canadensis, but that otters with low hemoglobin levels died more frequently after release.…”

Metabolic responses to adrenocorticotropic hormone (ACTH) vary with life-history stage in adult male northern elephant seals

“…Studies on the impacts of anthropogenic stressors to wildlife have begun to recognize that integration of various stressors, such as pollution, disturbance, alterations in food availability and others may have demographic impacts by increasing the allostatic load on individuals and affecting health and reproduction. This realization has led to the development of the field of conservation physiology [3,[7][8][9].…”

Genetic analyses of historic and modern marbled murrelets suggest decoupling of migration and gene flow after habitat fragmentation

“…Ecologically, a reduction in N e in central California is supported by an order of magnitude decline in reproductive rates between historic and modern sampling Hall et al 2009). In addition, approximately 90 per cent of nesting habitat was removed, nest predator populations increased dramatically, prey availability declined and significant mortalities owing to oil spills and gillnetting occurred Peery et al 2004b;. Limited gene flow between northern populations and central California could have occurred owing to the removal of large amounts of old-growth forest in the approximately 300 km long region between central and northern California populations (figure 1).…”

“…Either scenario would explain the observation that migrants were involved in proportionately fewer parent-offspring pairs than residents. Some migrants probably do breed in central California, as the only migrant for which breeding information was available (from radio-telemetry) attempted to nest in the Santa Cruz Mountains (Peery et al 2004b). If significant numbers of migrants do attempt to breed in central California, their reproductive success could be lower than residents owing to outbreeding depression (Nosil et al 2005), competition with residents for breeding sites (Fretwell & Lucas 1970) or a lack of adequate knowledge of local predators and foraging resources (Stamps 2001).…”

“…This species has declined throughout much of its range owing to the loss of old-growth nesting habitat, nest predation by synanthropic species and changes in prey resources in the coastal waters where they forage Peery et al 2004b;Piatt et al 2007). Extensive harvesting of old-growth forest since the late nineteenth century, particularly in the southern portion of the murrelet's range, has resulted in the geographical isolation of the central California population, which is now separated from the nearest substantial population to the north by several hundred kilometres (figure 1).…”