|Abstract or Summary
- Populations of organisms are influenced by both top-down (predator driven) and bottom-up (environment or resource driven) effects. Seabird research has largely focused on bottom-up factors influencing reproduction, with little emphasis on top-down. Our goal was to better understand top-down impacts on colonial nesting seabirds over a range of spatio-temporal scales. We studied the coast-wide distribution and abundance of a Common Murre (Uria aalge) metapopulation during two decades (1988-2006) of Bald Eagle (Haliaeetus leucocephalus) recovery in Oregon. Bald Eagles prey on seabirds, but were functionally absent during much of the 20th century. After eagles were protected under the U.S. Endangered Species Act in 1978, populations increased rapidly especially along the coast where eagles cause disturbance at seabird colonies. We also studied the effects of predation and disturbance in 2012 at three Common Murre breeding sites located in regions of varying Bald Eagle density, and at a single site over a seven year period from 2007-2013.
We found regional changes in the distribution and abundance of Common Murres at breeding sites in Oregon associated with increases in coastal Bald Eagle nests over 20 years of study. Coast-wide Bald Eagle nest density was not uniform. The highest Bald Eagle nest density was found on the north coast, intermediate density on the central coast, and lowest density in on the south coast throughout the study. On the north coast, counts of murres declined by 50% between 1988 and 2006. In contrast, the number of murres counted and the number of breeding sites occupied increased substantially on the central coast, where Bald Eagle nest density was lower. Changes in the number and size of murre colonies on the north and central coast were associated with the regional density of Bald Eagle nests and initial number of murres present at each site, rather than immediate proximity of eagle nests to murre colonies.
Bald Eagles were the main predators causing disturbance at individual breeding sites studied. In 2012, we found differences in disturbance frequency, colony disruption, and predator activity among three sites in regions of high, medium and low Bald Eagle abundance. Eagles caused complete reproductive failure at the north coast site (high eagle density) in 2012. In addition to Bald Eagles, California Brown Pelicans (Pelecanus occidentalis) caused disturbance at the central and south coast sites, leading to low reproductive success and failure of remaining chicks at these sites in July 2012. We found no difference in reproductive loss between disturbances caused by adult and sub-adult eagles however, secondary nest predators (gulls, corvids and vultures) had a greater overall impact on reproductive loss than primary predators. From 2007-2013, we found a
negative association between mean reproductive success of murres and mean rate of eagle disturbance.
Our observations provide evidence for top-down regulation of breeding populations of Common Murres in Oregon, mediated by recently recovered native, avian predators. These findings challenge the effectiveness of site fidelity and natal philopatry for murres in the presence of avian predators. Scientists and managers on the U.S. West Coast should expect continued impacts from Bald Eagles as the population reoccupies its former range and increases to carrying capacity. Increased disturbance is likely at more Common Murre colonies in the future, particularly in years when alternative prey is limited for eagles or pelicans.