New research by the University of Nebraska-Lincoln suggests that temperature can largely explain why the greatest variety of aquatic life resides in the tropics but also why it has not always and, amid record-fast global warming, soon may not again.
The bulging, equator-belted midsection of Earth currently teems with a greater diversity of life than anywhere else – a biodiversity that generally wanes when moving from the tropics to the mid-latitudes and the mid-latitudes to the poles.
Published in the journal Current Biology, the study estimates that marine biodiversity tends to increase until the average surface temperature of the ocean reaches about 65 degrees Fahrenheit, beyond which that diversity slowly declines.
During intervals of Earth’s history when the maximum surface temperature was lower than 80 degrees Fahrenheit, the greatest biodiversity was found around the equator, the study concluded.
But when that maximum exceeded 80 degrees, marine biodiversity ebbed in the tropics, where those highest temperatures would have manifested, while peaking in waters at the mid-latitudes and the poles.
Marine life that could travel considerable distances likely migrated north or south from the tropics during periods of extreme heat, said co-author Will Gearty, a postdoctoral researcher of biological sciences at Nebraska.
Stationary or slower-moving animals, such as sponges and sea stars, may have instead faced extinction.
“People have always theorized that the tropics are a cradle of diversity — that it pops up and then is protected there,” Gearty said. “There’s also this idea that … there’s lots of migration toward the tropics, but not away from it. All of that centres around the idea that the highest diversity will always be in the tropics. And that’s not what we see as we go back in time.”
Gearty, Yale’s Thomas Boag and Stanford’s Richard Stockey went back about 145 million years, compiling estimated temperatures and fossil records of molluscs — snails, clams, cephalopods and the like — from 24 horizontal bands of Earth that were equal in surface area.
The trio chose mollusc records for multiple reasons: They live (and lived) around the globe, in large enough numbers to accommodate statistical analyses, with hard enough shells to yield identifiable fossils, with enough variation that their diversity trends might generalize to fish, corals, crustaceans and an array of other marine animals.
That data allowed the team to derive the temperature-diversity relationship across 10 geologic intervals that covered most of the elapsed time from the Cretaceous period through the modern-day.
“Temperature seems to account for a lot of the trend that we see in the fossil record,” Gearty said. “There are certainly other factors, but this seems to be the first-order predictor of what’s going on.”
To investigate why temperature might be so influential and predictive, Stockey took the lead in developing a mathematical model.
The model accounts for the fact that higher temperatures generally increase the amount of energy in an ecosystem, theoretically raising the ceiling on the biodiversity an ecosystem can sustain, at least to a point.
But it also factors in metabolism and the small matter of oxygen, which, by dissolving in water, makes aquatic life possible in the first place.
Colder waters dissolve more oxygen, meaning that elevated temperatures naturally reduce the amount available to marine life and, by extension, potentially limit the biodiversity an ecosystem can support.
Higher temperatures also raise the metabolic demands of organisms, increasing the minimum oxygen needed to sustain active marine animals. (ANI)
