A once-minor sea snail overgrazed patches of kelp in Tasmania

Local surges in the turbinid periwinkle Lunella undulata were linked to bare-looking patches and slower kelp recovery.

Editorial illustration — not from the study.

The study examined apparent barren patches in kelp beds in eastern Tasmania associated with high numbers of the turbinid periwinkle Lunella undulata. In a 117-day caging experiment, plots that excluded the periwinkles developed substantially more fleshy algae than plots where the snails remained.

Legal-sized periwinkles were commercially harvested during the study, after which algae recovered across most treatments. Overgrazing continued in cages containing the periwinkles, while kelp recovery followed as periwinkle biomass declined over roughly 16 months.

When the snails surged

Fleshy macroalgae reached a mean cover of 47 ± 8% standard error in cages excluding periwinkles at the end of the 117-day experiment. That was five times higher than in partially caged plots, six times higher than in open plots and 10 times higher than in cages containing periwinkles. Harvesting legal-sized periwinkles during the study was followed by algal recovery in all treatments except the periwinkle-inclusion cages, where overgrazing persisted. As periwinkle biomass declined, kelp beds recovered and fleshy algae increased over about 16 months.

Why a minor grazer matters

The findings show that a grazer that is usually subordinate to other species can still reshape parts of a kelp bed when it becomes locally hyperabundant. The researchers suggest that increased harvesting of the range-extending urchin Centrostephanus rodgersii may have reduced competition and allowed the periwinkle to become locally dominant, although this proposed link remains a hypothesis. The results indicate that changes in one grazer can have effects involving other grazers and kelp recovery.

Evidence and limits

The study combined field observations with a caging experiment using periwinkle-inclusion and exclusion treatments, along with cage and open controls. It provides experimental evidence that the periwinkles can overgraze a localized kelp-bed patch. The study focused on peri-barren patches in eastern Tasmania, so the findings do not establish that the same process occurs in all kelp beds. The proposed connection between harvesting the urchin and the periwinkle’s increased dominance is presented by the researchers as a hypothesis, not a demonstrated cause.

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