Drift logs destroy intertidal ecosystems, study shows

Logs are a familiar sight on the beaches along the coast of Vancouver Island and Haida Gwaii and are often viewed positively, as they can stabilize the banks, be used for firewood or act as benches by beach-goers. However, new research from the University of Victoria (UVic) shows that these logs are not as innocuous as they seem.

According to a study published by UVic biologist Tom Reimchen and two of his students, free-floating logs that wash ashore, referred to as drift logs, are causing widespread destruction of rocky intertidal ecosystems communities along the coast of Western Canada. The paper is published in the journal Marine Ecology.

“In this study, we looked at both the ecological impact of drift logs, and at log abundance and movement over time,” says Reimchen. “Both aspects of the study had worrisome results.”

While drift logs may seem rather stable to the casual observer, more than 90% of logs are displaced annually, and log movement during storms is frequent and extensive. This movement disrupts the ecological environment in the intertidal zone—the stretch of beach between the highest high tide and the lowest low tide.

Reimchen found that populations of barnacles, a key intertidal species, are 20–80% lower on surfaces that are exposed to logs compared to protected crevices. The interstitial spaces between the barnacles, which support a variety of small invertebrates, are negatively impacted as well. While new barnacles settle every year, they are removed regularly with seasonal movement of the logs.

Many shorebird species, such as black oystercatchers, surfbirds, black turnstones and rock sandpipers, rely on the rocky intertidal zones for food. Researchers suspect the loss of barnacle beds and interstitial invertebrates due to log abrasion has likely contributed to the decline of shorebird populations. Since 1970, there has been a 50% drop in some of the shorebirds.

“Drift logs cause persistent and cyclical damage to the intertidal environment along the BC coast,” says Reimchen. “And the problem has only gotten worse over the last century, as the number of drift logs along the BC coast has been steadily increasing.”

Reimchen, alongside undergraduate students Esteban Pérez Andresen and Melanie Marchant, used satellite imagery and archival photographs to quantify log abundance from inside passages and exposed outer western shores as well as the west shores of Haida Gwaii. They found a 520% increase in drift logs since the late 19th century and also discovered that the most remote shores have log abundance comparable to more developed areas.

The logging industry has played a major role in this increase, with more than 60% of logs analyzed showing signs of human origin. Even those classified as “natural” logs may originate from human influence, such as logging-related landslides.

“We need to recognize drift logs as a significant ecological disturbance, comparable to ice-scouring or heatwaves,” says Reimchen. “It’s critical that we begin reducing the number of drift logs added to the marine environment and begin introducing conservation measures to protect organisms in the intertidal zone.”