Cold-blooded sea-life living on the edge in a warming world

Twice as many marine species as terrestrial species have already experienced local extinctions due to a warming climate.

The conclusion emerges from a massive analysis of previously published data that appeared April 24 in Nature.

Scientists have debated which habitats and regions of the globe are likely to be most affected by human-caused climate change. The new study is the first to comprehensively compare marine and terrestrial species’ sensitivity to warming.

An international team of researchers combed through previously published literature to assemble data on the highest temperatures that 88 marine and 294 terrestrial species can tolerate.

The species included in the current study are all ectotherms, meaning creatures that do not have internal mechanisms for regulating their body temperature. Among the species studied were molluscs, crustaceans, insects, spiders, other invertebrates, fish, amphibians, and reptiles.

Next, the researchers used various modeling approaches to estimate the body temperatures reached by each species during the hottest hours of the year. They took into account animals’ ability to escape from the heat in underground burrows, patches of shade, or deeper water.

The difference between an animal’s thermal tolerance and the highest body temperature it experiences is its thermal safety margin – that is, how close the animal lives to its heat-tolerance limits.

The researchers found that marine species have narrower thermal safety margins than terrestrial species – on average, the thermal safety margin for a marine ectotherm is about 80% as wide as that of a terrestrial one.

Marine species generally have less ability than terrestrial ones to take refuge in cooler spots within their environment. But terrestrial species depend on such refugia: The researchers calculated that the body temperatures terrestrial ectotherms would reach in full sun on hot days often outstrip their thermal tolerance.

In the oceans, thermal safety margins are narrowest around the equator. On land, thermal safety margins are narrowest around the edges of the tropics – at 30° latitude in the southern hemisphere and 22° latitude in the northern hemisphere.

“Moreover, average [thermal safety margins] for tropical marine species were at least 3 °C narrower than at any latitude on land,” the researchers write.

Species in regions where current thermal safety margins are the slimmest may be most vulnerable to future warming, making tropical oceans a literal hotspot for future risk.

The researchers again combed through the scientific literature to find out whether warming is already causing local extinctions. They found that twice as many marine species as terrestrial species are known to have disappeared from the warmest parts of their range due to climate change.

Finally, the researchers used global climate models to determine how wide species’ safety zones would be under different climate change scenarios. They found that if we manage to decrease emissions sharply and keep 21st century warming to 1 °C, thermal safety margins will be about 50% wider than if emissions continue on their current trajectory.

Different processes are likely to drive species loss due to climate change in the oceans and on land. In the ocean, slimmer thermal safety margins will probably result in more local extinctions. But marine species tend to have greater dispersal and colonization abilities, so if appropriate habitat is available species could hang on in one spot even if they disappear from another.

Terrestrial species, meanwhile, could be vulnerable if habitat fragmentation and changes in land use destroy the cooler microhabitats that they depend on for refuge from the heat, the researchers say.

Source: Pinsky M.L. et al.Greater vulnerability to warming of marine versus terrestrial ectotherms.” Nature 2019.

GIF: NOAA

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