Top-down Influences of bald eagles on common murre populations in Oregon

• Main Author: Horton, Cheryl A.
• Other Authors: Dugger, Katie M., Suryan, Robert M., Kennedy, Patricia L., Jones, Julia A., Fisheries and Wildlife, Oregon State University. Graduate School
• Format: Master Thesis
• Language: English; unknown
• Published: Oregon State University
• Subjects: Bald eagle > Ecology > Oregon; Common murre > Effect of predation on > Oregon; Bald eagle > Oregon; Common murre > Ecology > Oregon; Bald eagle > Food > Oregon; Common murre > Predators of > Oregon; Common murre > Oregon; Common Murre; Uria aalge; uria
• Online Access: https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m039k745w

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 ...