Habitat

PROMO Intro ShagRock by RL 512x219

Three Arch Rocks NWR and Wilderness includes nine islands located one half-mile off the Oregon coast, west of the community of Oceanside. Of the nine islands, three are especially prominent: Finley Rock, Middle Rock and Shag Rock. Each rises 200-300 feet from the ocean's surface; all possess arches sculpted by relentless wind and waves. The other six rocks are smaller and include Storm Rock, the westernmost rock, and Seal Rock, the easternmost rock. With the exception of Seal Rock, these five smaller rocks can become awash in high-ocean surge levels.


The rocks and islands of Three Arch Rocks are composed of Miocene-age basalt, born of volcanism 10-20 million years ago. Formerly whitewashed with Common Murre guano, today the rocks are brown and mostly unoccupied: resurgent Bald Eagle populations have driven many of the nesting seabirds elsewhere. While still majestic, the ecological character of the refuge and wilderness has shifted. 

The marine ecosystem surrounding the islands plays a key role in shaping the islands’ ecosystem and wildlife. Like the nearby Oregon Islands NWR, Three Arch Rocks is part of an ecological transition zone known as the Northern California Current Large Marine Ecoregion. This ecoregion is influenced by both the sub-arctic waters from the Gulf of Alaska and warmer waters off California. 

The ocean surrounding Three Arch Rocks experiences the seasonal upwelling effect present along the entire Pacific coastline, stretching from California to Canada. This upwelling is caused by a combination of strong winds, effects of the continental shelf break, and the Coriolis effect—the deflection of winds and weather due to our planet's rotation. The cool waters fuel phytoplankton blooms which feed zooplankton, juvenile fish, squid, and other prey for seabirds and pinnipeds. But Three Arch Rocks, located on the northern part of the Oregon Coast, experiences a more seasonally-dependent upwelling effect due to unfavorable wind patterns much of the year. Years with strong upwelling generally correlate with higher seabird and pinniped populations and reproductive success. Conversely, El Niño years are correlated with diminished seabird and pinniped populations and reproductive success as the upwelling is absent, taking with it the primary food source of many fish and wildlife species. 

Vegetation on Three Arch Rocks NWR and Wilderness is largely unstudied. The north slope of Shag Rock is carpeted with plants despite soil being shallow in most areas; much of this slope is covered by Siberian Spring Beauty (Calytonia sibirica). Semidi Islands Aleutian Cackling geese formerly used this site as a spring staging area, foraging heavily on this plant. Due to eagle predation, however, this site is no longer used by the geese. Middle Rock has very limited soil, and vegetation is confined to patches on steep slopes. Finley Rock appears to have deep soil accumulation on the inaccessible north slope, boasting dense vegetation. 

As an island chain, invasive and non-native species are of particular concern to Three Arch Rocks NWR. Invasive plant species pose serious ecological problems by forming large monospecific zones, lowering biodiversity, outcompeting native plants, and eliminating habitat for burrow-nesting seabird species. Mainland animal species also pose significant threats to wildlife habitat on the islands, though the distance between Three Arch Rocks and the mainland poses a significant obstacle for most animal species. 

The primary activity in the refuge has been, and continues to be, ongoing annual aerial photographic surveys of seabird populations, in which refuge biologists fly over the entire coast in a helicopter, taking photos of all islands with seabird colonies. As of 2013, Three Arch Rocks NWR is still being photographed despite its lack of Common Murres. In recent aerial surveys, Brandt’s and Double-crested Cormorants have been the dominant seabird species observed in photographs, apparently more resilient to eagle disturbance.