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Cold-blooded sealife runs double heat risk

Extremes of heat are twice as risky for cold-blooded sealife as for other ectotherms. A hot rock could be safer than the deep sea.

LONDON, 29 April, 2019 – When it comes to global warming, there may no longer be plenty of fish in the sea: new research suggests that cold-blooded sealife may be twice as likely to be at risk in its natural habitat as land-dwelling ectotherms.

This finding is unexpected: the ocean is, in both area and volume, the single biggest living space on the planet. Fish that feel the heat can move towards the poles when temperatures get too high.

But when US researchers took a closer look at the data available on the thermal discomfort zones – those moments when cold-blooded creatures begin to overheat and need to find a safe, cool place in which to lie low – those spiders and lizards that survive in the tropics and temperate zones actually stand a better chance of finding somewhere to hide, and thus living through heatwaves, than their marine cousins.

“New conservation efforts will be needed if the ocean is going to continue supporting human well-being, nutrition and economic activity”

“We find that, globally, marine species are being eliminated from their habitats by warming temperatures twice as often as land species,” said Malin Pinsky, of Rutgers University in New Brunswick.

“The findings suggest that new conservation efforts will be needed if the ocean is going to continue supporting human well-being, nutrition and economic activity.”

He and colleagues report in the journal Nature that they searched the literature for detailed information on 400 species, and calculated the safe conditions for 88 marine and 294 land animals. They also identified the coolest temperatures available to each species during the hottest parts of the year.

More terrestrial refuges

And they found that, on average, fish and marine animals were more likely to live on the edge of temperatures that could become dangerously high. Land animals – insects and reptiles – could disappear into the forests, seek the shade or go underground: something sea creatures could not do.

That terrestrial reptiles and amphibians and marine animals are at risk is not news: researchers have already recorded significant movements of sea species in response to heat extremes off the Californian coast.

There has been repeated evidence that rising global temperature, as a consequence of greenhouse gas emissions from fossil fuel use, has begun to affect commercial fisheries, and other researchers have made it emphatically clear that only determined human action to contain global warming and protect breeding grounds can keep fish on the family supper table.

What most would not have expected was to find that land animals were less at risk, simply because they were land-dwellers.

Limited evidence

Research of this kind tends to deliver findings that can be challenged, and the authors concede that their conclusions are limited by the available evidence. Of 159 separate studies, 153 were in the northern hemisphere and 137 were from the temperate latitudes. Of their marine ectotherms, only 7% were pelagic: these are the fish – among them cod and tuna – that can swim to deeper, cooler layers when surface temperatures soar.

The remaining 93% included slow-moving bottom-dwellers such as lobsters, horseshoe crabs, abalone and snails, which may have nowhere left to go when life locally gets too hot to handle. The researchers make it clear that they are not talking about complete global extinctions of species: they choose the phrase “local extirpations”.

And they make it clear that land-dwelling cold-blooded animals are by no means safe from increasingly frequent, intense episodes of heat extremes driven by climate change: they would continue to be vulnerable to loss of what the researchers call “local refugia” – for example woodland cover – which “would make habitat fragmentation and changes in land use critical drivers of species loss on land.” – Climate News Network