Geographically Widespread Drift Log Destruction of Intertidal Communities on Rocky Shores of Western Canada

Abstract

The destructive effect of drift logs on several rocky shore intertidal communities first identified in 1971 by Dayton in the north-east Pacific Ocean has received little further attention despite its ecological and conservational significance. Using satellite imagery from 202 sites along western Canada's coastline, we observed widespread log accumulation at the tideline, with an average of 311 logs/km (max: 2718) on sandy shores and 194 logs/km (max: 1238) on rocky shores, independent of proximity to human settlement and industrial activity. Historical analyses of archival photographs reveal a 520% increase in drift log abundance since the late 19th century and an estimated 800% (range 0%–4000%) increase since pre-European settlement, trends that correlate with the expansion of forestry operations over the past century. Populations of key intertidal foundation species, Balanus glandula, Semibalanus cariosus, which support a taxonomically diverse interstitial community, were 20%–80% lower on log-exposed surfaces compared to immediately adjacent log-protected microhabitats such as crevices, with the most pronounced reductions occurring in middle and upper intertidal zones. During high tides and storm conditions, over 90% of the drift logs stranded at the high tideline can be re-mobilized and transported to other shores, intensifying their abrasive effects on the intertidal habitats. While future higher resolution remote sensing will refine assessments of drift log impacts, our current findings indicate that ongoing log-induced abrasion has significantly degraded intertidal communities across most rocky shores in western Canada. This degradation likely has cascading negative effects on both aquatic and terrestrial species that depend on these habitats for foraging.