Drastic shellfish loss measured along Vancouver coast
By Summit Voice
SUMMIT COUNTY — Global warming is altering the composition of tide pools along the coast of Vancouver Island, with consequences that go far beyond impacts to single species.
Daily high temperatures during the summer months have increased by almost 3.5 degrees Celsius in the last 60 years in the area, causing the upper limits of barnacle and mussels habitats to retreat by 50 centimeters down the shore, thereby putting them within easy reach of predatory starfish.
“That loss represents 51 per cent of the mussel bed. Some mussels have even gone extinct locally at three of the sites I surveyed,” said University of British Columbia zoologist Christopher Harley, who published the results of his tide pool surveys in the most recent issue of the journal Nature.
Harley said the combination of rising temperatures and predation will add up to biodiversity impacts more severe than currently expected.
“Warming is not just having direct effects on individual species,” says Harley. “This study shows that climate change can also alter interactions between species, and produce unexpected changes in where species can live, their community structure, and their diversity.”
His study encompassed surveys of the upper and lower temperature limits of barnacles and mussels from the cool west coast of Vancouver Island to the warm shores of the San Juan Islands.
“Rocky intertidal communities are ideal test-beds for studying the effects of climatic warming. Many intertidal organisms, like mussels, already live very close to their thermal tolerance limits, so the impacts can be easily studied,” he said.
At cooler sites, mussels and rocky shore barnacles were able to live high on the shore, well beyond the range of their predators. However, as temperatures rose, barnacles and mussels were forced to live at lower shore levels, placing them at the same level as predatory sea stars.
As a control to the surveys, some areas were protected by excluding sea stars. In those areas, the prey species were able to occupy hotter sites where they don’t normally occur, and species richness at the sites more than doubled.
“A mussel bed is kind of like an apartment complex – it provides critical habitat for a lot of little plants and animals,” says Harley. “The mussels make the habitat cooler and wetter, providing an environment for crabs and other small crustaceans, snails, worms and seaweed.”
These findings provide a comprehensive look at the effects of warming and predation, while many previous studies on how species ranges will change due to warming assume that species will simply shift to stay in their current temperature range.
Harley says the findings show that the combined effects of warming and predation could lead to more widespread extinction than are currently predicted, as animals or plants are unable to shift their habitat ranges.