Warming and predation will result in marine biodiversity loss

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Marine biodiversity loss may be more severe than currently predicted, and biodiversity loss will be as a result of increased temperatures and predation, according to a study published in the journal Science.

University of British Colombia zoologist Christopher Harley based his study on the response of rocky shore barnacles and mussels – to both warming and predation from sea stars – from the cool west coast of Vancouver Island to the warmer shores of the San Juan Islands.

"Rocky intertidal communities are ideal test-beds for studying the effects of climatic warming," says Harley. "Many intertidal organisms, like mussels, already live very close to their thermal tolerance limits, so the impacts can be easily studied."

At cooler locations, mussels and rocky shore barnacles are able to live high on the shore, out of range of sea stars.  However, as temperatures rise, barnacles and mussels are forced to live at the lower shore levels, where they are at the same level as their predators.

There has been an increase of almost 3.5°C in daily high temperatures during the summer months in the past 60 years, which has resulted in the barnacle and mussel habitats retreating down the shore by 50cm.

"That loss represents 51% of the mussel bed. Some mussels have even gone extinct locally at three of the sites I surveyed," says Harley.

Harley reduced the effect of predation by sea stars using exclusion cages, enabling prey species to occupy warmer sites where they would not typically occur – as a result, biodiversity at the protected 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."

The findings offer a more comprehensive look at the effects of warming and predation, which is important as many other studies focused on how species ranges will change due to warming have assumed that species would simply move so as to stay within their temperature range. Harley believes the findings show that the warming, coupled with predation, could lead to more widespread extinction as affected species are not able to just 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."

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