A sweeping analysis of the movements of tens of thousands of aquatic species has shown that because of global warming, animals that once thrived in the ideal environment of the tropics are journeying away from home.

Mark J. Costello and Chhaya Chaudhary spoke with The Academic Times regarding their new comprehensive study tracing the transitory locations of 48,661 marine species from 1955 to 2015, published April 5 in Proceedings of the National Academy of Sciences. 

The extensive list of species ranges from open-water fish to arthropods, such as shrimp, as well as bivalves, such as mollusks. While some species were more prone to leaving than others, distribution records over time demonstrated a steep dip in species' presence in the tropics, close to the equator, and an increase in the subtropics, away from it. 

This shift, the authors say, is due to a rise in water temperature, indicating that global warming will continue to force the rearrangement of marine life in the coming decades.

"Sometimes, the media thinks that the species is going extinct," said Costello, a professor at the Faculty of Biosciences and Aquaculture at Nord University. "But it's not, really. They're just moving; they're going; they're disappearing."

"It's not entirely a catastrophic story," he continued, "but it means they are changing. And we need to understand how that's happening."

A postdoctoral researcher at the University of Auckland, Chaudhary relayed her own concerns about lessening biodiversity in the tropics. Particularly, she notes that it might remove necessary competition among different species vying for the same limited resources.

"Species which are more dominant and can sustain the increasing temperatures might end up having more population than the other ones," she said. "We don't know how things will change, but, for sure, the change is much faster than what we expect."

Chaudhary also stressed that traveling species would consequently invade the subtropics, intruding on species that are native to the area.

Costello added that animals living in open ocean water, called pelagic species, at least have the option of relocating as the water temperature increases because they can swim away. On the other hand, benthic species, such as clams, can't move on, because they rely on the seabed. Species like these seabed-dwellers could die out due to the rising temperature; marine life is already known to be struggling in warm waters. 

"The source populations around the equator are probably dying out due to overheating, as people have seen in the Great Barrier Reef or these other heat-wave events," he said. "We have a massive loss of species at those particular locations."

Despite the fact that, historically, some research has similarly indicated trends of marine species rearrangement, the researchers explained that there has been doubt among scientists about whether those correlations are reliable. Some have hypothesized that such results in marine data were victim to sampling bias or merely reflect greater habitat availability in higher latitudinal regions. 

Aiming to put an end to this discussion, Costello and Chaudhary have been scrutinizing heaps of data on marine species over several years. They published an initial review in 2016 after screening approximately 65,000 species from already published studies, as well as the Ocean Biodiversity Information System.

In that work, statistics from around the globe and in a variety of taxa concluded that the dip in species' residence at the equator was indeed consistent.

"We went to every other paper we could find on the subject," Costello said, emphasizing that every single one demonstrated a decrease in marine species' presence at the equator.

In a second paper, published 2017, Chaudhary noted, the duo also showed that "It is not actually sampling bias, because we adjusted for sampling bias for almost 50,000 species, and we still found the same dip."

Then, incorporating temperature changes spanning decades in their most recent study, the researchers found a striking connection: As temperature levels started to rise above a mean annual surface temperature of 20 degrees Celsius, species richness began to either decline or plateau for most taxa.

In a trend that began around the 1980s, Chaudhary relayed, there has been a consistent rising temperature on par with the progression of climate change.

Climate change has already led to a 1 degree Celsius increase in temperature worldwide since 1880. Although seemingly incremental, one or two more degrees could be catastrophic. To mitigate such devastating effects, experts have been analyzing how best to realize the goals of the global Paris Agreement, which aims to limit temperature rise to a maximum of 1.5 or 2 degrees Celsius above preindustrial levels.

"Biodiversity is responding exactly as people have predicted at a global scale in the ocean over time," Costello explained. "Even if we look back in the fossil record, you'll find that there were previous decreases in the tropics due to it getting too warm."

A potential limitation in the new study is that the team had access to data only through 2015, which Costello attributed to the rate at which marine data becomes available. 

In the future, the team hopes to collect and analyze abundance data, which would refine the exact population per species headed away from the tropics. They also hope to address how, when water temperature increases, the level of oxygen lowers, often at the expense of marine organisms.

"This could be another sort of compounding factor. Most studies really only looked at temperature so far; oxygen has hardly been looked at at all," Costello said. "And we know [that in] the past, global mass extinctions are largely caused by lack of oxygen in the oceans."

The paper, "Global warming is causing a more pronounced dip in marine species richness around the equator," published April 5 in Proceedings of the National Academy of Sciences, was authored by Chhaya Chaudhary, University of Auckland; Anthony J. Richardson, University of Queensland; David S. Schoeman, University of the Sunshine Coast; and Mark J. Costello, Nord University.