05/07/2026
Kelp forests are shaped by relationships between predators, herbivores and kelp.
Sea urchins are one of the dominant kelp grazers in BC. When their populations become very large, they can remove significant amounts of kelp and contribute to major ecosystem changes.
Sea otters and sunflower sea stars can both help keep sea urchin populations in balance by feeding on them. Because they influence the broader ecosystem in this way, they are considered keystone predators.
When keystone predators are removed, a trophic cascade can occur, where changes in one part of the food web affect the entire ecosystem. In kelp forests, the loss of predators can increase sea urchin grazing pressure and contribute to kelp forest decline.
Sea otters were hunted to local extinction on the BC coast during the colonial maritime fur trade in the late 1700s and 1800s. They were later reintroduced to parts of Vancouver Island’s west coast between 1969 and 1972, but they are still absent from parts of their historic range.
In areas without sea otters, sunflower sea stars became especially important predators of sea urchins. But between 2013 and 2016, sea star wasting disease caused widespread declines in sunflower sea stars across the Northeast Pacific Ocean.
In many regions where sunflower sea stars declined, sea urchin populations increased, contributing to trophic cascades that favoured sea urchins over kelp.
Today, kelp forests in BC are shaped in part by:
• Historical loss of sea otters
• Uneven recovery of sea otters
• Recent loss of sunflower sea stars
Because these predator relationships differ by region, kelp forests do not respond the same way everywhere. Some areas have experienced greater decline than others, reflecting variable patterns of change across BC.
Come back for Part 2 where we’ll explore some of the additional regional and environmental factors influencing kelp forest change!
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