A UBC study shows that some mussel beds in the Salish Sea have decreased by 51 per cent over the past 52 years, a consequence of gradually rising temperatures off Vancouver Island, the Gulf and San Juan Islands and Washington’s Olympic peninsula.
The study shows that the climate change is already affecting species by not only causing stress but changing the complex relationship among the species in an ecosystem, as some species may become relatively stronger and others weaker.
University of British Columbia associate professor of zoology Christopher Harley say climate change will bring biodiversity loss caused by a combination of rising temperatures and predation – and may be more severe than currently predicted.
The study, published in the current issue of the journal Science, examined the response of rocky shore barnacles and mussels to the combined effects of warming and predation by sea stars.
Harley surveyed 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, where water temperature rose from the relatively cool of the1950s to the much warmer years of 2009 and 2010.
Map showing the area of the UBC climate change study. The squares show areas used for “spatial comparison of temperature and zonation.” The circles were used for comparison. (Science)
“Rocky intertidal communities are ideal test-beds for studying the effects of climatic warming,” Harley says. “Many intertidal organisms, like mussels, already live very close to their thermal tolerance limits, so the impacts can be easily studied.”
At cooler sites, mussels and rocky shore barnacles were able to live high on the shore and that is well beyond the range of their predators, including the sea star. As temperatures rose, barnacles and mussels were forced to live at lower shore levels, the same level as predatory sea stars.
Daily high temperatures during the summer months have increased by almost 3.5 degrees Celsius in the last 60 years, causing the upper limits of barnacle and mussels habitats to retreat by 50 centimeters down the shore. However, the effects of predators, and therefore the position of the lower limit, have remained constant.
“That loss represents 51 per cent of the mussel bed. Some mussels have even gone extinct locally at three of the sites I surveyed,” says Harley.
“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.”
The study says, “the loss of mussel beds over time has probably resulted in declines of species richness.”
When pressure from sea star predation was reduced using exclusion cages, the prey species were able to occupy hotter sites where they don’t normally occur, and species richness at the sites more than doubled.
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.
“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.”
He adds ecological change can only be anticipated if scientists understand the ways various factors “determine the distribution and abundance of species in space and time.”